Let's Settle This. Bore vs Stroke - It's Not That Simple
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Here we have two engines. This one is oversquare it's bore is twice as large as it's stroke giving is a bore to stroke ratio of 2. The other engine is undersquare it's stroke is twice as large as it's bore giving a bore to stroke ratio of 0.5. And in this video we will answer the question of which one of these makes more torque and why and because horsepower is essentially torque times rpm we will also answer the question of which one makes more power. And we will also apply the lessons we learn onto real engine examples to see if theory and practice match up.
Simple physics tells us that these two engines make the same torque. Torque after all, is a product of the force and the leverage applied. If we imagine a hand turning a wrench our leverage is the length of the wrench and our force is how hard we push on the lever. If I push twice as hard on half the lever length the torque will be the same as if we pushed half as hard on twice the lever length. So with this logic in mind, if these two engines have the same displacement, they make the same torque? No, they do not. Here’s something that’s often overlooked when it comes to bore and stroke. An increase in stroke is a guaranteed increase in torque. It’s guaranteed because the connection between the rod, crank and piston is a fixed, constant, mechanical connection.
But an increase in bore is NOT a guaranteed equivalent increase in torque because doubling the bore does not necessarily double the force acting on the piston and that’s because combustion is a variable that constantly changes with RPM and engine load.
To create combustion we need air and fuel. We need somewhere between 11ish to 14ish parts air to just one part fuel. So the struggle is to bring air into the engine. There are many factors that determine how much air comes in and at which rpm. Throttle body diameter, intake manifold size and shape, intake manifold runner diameter and runner length, intake port shape, length and diameter, the number of intake valves, the angle of the valves against the centerline of the engine, intake valve size, and then we have camshaft duration and lift which determines how much and how long the valve opens.
Now some of these we can continuously control throughout the rpm range and partially compensate for different engine breathing requirements at different rpm, some have 2 or maybe 3 different settings, but many of these are fixed. The fixed nature of some engine parts and the partial control of others means that they can only be truly optimized for a certain rpm range.
Now let’s see how bore and stroke impact air quantity and air velocity. If we observe our two engines side by side at the same rpm we can see that the undersquare piston travels much faster. It travels faster because it must cover the much greater stroke distance within the same period of time. This results in greater piston velocity which means that we can have decent air and fuel mixing even at low rpm.
But here’s something else that works in favor of increased air velocity in undersquare engines and that is that the reduced bore diameter forces the engine to have smaller valve diameters. These small valve sizes than dictate the rest of the orifices of the engine because the intake ports, runners, and throttle bodies must all match. If we have vastly different airflow capacities at any one point in the system we create airflow inefficiencies, in other words air struggles to get past such points and engine performance suffers noticeably.
So the small valve diameters result in reduced diameters throughout the system which further improves air velocity. But as we know this reduces maximum air quantity which means that maximum power potential at high rpm suffers and torque starts falling off as the engine struggles to breathe through the small orifices.
But an undersquare engine isn’t particularly concerned with high rpm performance and that’s because it can’t even reach very high rpm anyway. It can’t do it because of the increased piston speeds. To achieve high speeds we need high acceleration and high acceleration produces high force, which means that at high rpm the piston forces in an undersquare engine can actually damage which means that we must reduce our redline in order to preserve the engine.
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#d4a #bore #stroke
00:00 Force and Leverage
02:38 Air Quantity vs Air Velocity
07:14 Piston Velocity
10:37 Rod Ratio
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Mass air density higher equals higher horses
hi, D4A. could you make a video about star engines, these old aeroplane engines wich interestingly also had odd piston numbers like 3,5 and 7. why did this work out for those, but not for inline engines. and why does nobody still use this layout it even seems extinct in aeroplanes.
learning a little about those would be great.
@d4a Can you tell us about Honda's dual stage intake and why it works? Such as found in the old A20A3 injected engines. It's like having a small & large intake all in one.
I really liked your video. Could you do a video of the difference between V engines V.S. inline engines and the difference between Diesel V.S. Gasoline expanding on the concepts in this video?
You need more cats.
Hey man, just wanted to say thanks for the way you do videos. No yelling, no gimmicks, no filler. That is really appreciated. Peace.
this was my comment: WHAT AN AMAZING CHANNEL these people, who are this good, deserve every single penny they make
Couldnt agree more, I skip the countless dumb intro’s and when someone starts yelling I’m out even if i wanted to hear content
Yeah, just for now
I use a soundbox to hear a stupid loud ”MO POWA!” or ”HRRS PRRS!” every now and then. But I want to choose the timing for best comedic effect and I appreciate D4A for providing a blank canvas for this purpose.
so if someone yells "HEY GUYS" in a loud voice, you don't watch that video?
Engines are fascinating, from sound to power, you can never really know how it is from a data sheet alone.
True! Many feelings can't be put into numbers
Word ends in “s”? Better use an apostrophe 🤦♂️
@@Lozzie74nobody cares nerd
Apostrophes are like fuel additive: Don't put it in if you don't need it.
@@Lozzie74depend's
"Suck, squeeze, bang, blow" was about all I knew before I found your channel. You have taught me a ton more since!
Damn, from the streets to the science
I'm a mechanical engineer
And I found this channel 2days ago
I learnt more from this channel then 4.5 years In college
Then your college sucked... and you should ask for your money back.
Damm your college must have been terrible
*than
Very well done. I design race engines and assumed you would leave key points out. But you checked all the boxes. Great explinations.
Please tell me which one is powerful I don't understand video
@@chaturgamer1120 Both have pros and cons. Short stroke engines have higher horsepower and are generally faster but at the cost of low RPM power. To use all of their power you have to rev them high. Long stroke engines on the other hand produce more power at low RPM, and lots of it, but cannot keep it up when you rev them up. So you have impressive torque figures on relatively wide RPM range but horsepower rating is usually lacking. The engine simply "runs out of steam" when you try to push it higher.
Yet both have their places. For extreme examples of the two think of Formula 1 car and a tractor. Formula 1 car is the epitome of a short stroke engine, very big pistons that are (a little exaggeration) barely vibrating up and down, revving to 20000 RPM and going really, really, really fast. Versus a tractor with slowly spinning long stroke engine but with a lot of momentum with each engine rotation. Both are powerful, but only one you would take to a race track and only one you would use to pull a tree from the ground.
And about more realistic examples like a sports motorcycle vs a Harley. The former, much like Formula 1 car, is obviously faster but if you want to travel around and especially with a lot of heavy luggage, you might actually be happier with a Harley. And I am not talking about usability but how they feel in real life, relaxed riding. Harleys long stroke engine is always on the optimal power band when you are cruising at normal speeds and you do not need to play with gears so much if you need to pass a car in front of you, just twist the throttle and the bike obeys. Sports bike can do it faster but it usually involves punching the gears down to get the revs up and then it goes like a lightning.
TLDR Long stroke engine = instant power from low but cannot keep it up to high RPM's. Short stroke engine = Not so much power initially but keeps getting better and better as the RPM increases and beyond what the long stroke engine can offer as far as speed goes.
*edit* This is of course an over simplification and there are variables. Engineers are creative bunch and can improve the short comings of each engine design. Like the Corvette LS2 engine mentioned in this video, it is a short stroke engine but it feels like a long stroke engine until you notice that it revs quite high. Or the BMW engine that is even more short stroke but because of the mechanically complex tricks it has it does produce nice amount of power down low even if the engine is still happier up high.
@@MaaZeus bro thankyou so much for your valuable time i really appreciate it ☺️ 👏👏 now i understand this
Your channel is criminally underrated. When it comes to the actual physics of cars and their components, this is by far the best channel on all of TH-cam. You explain these concepts in a way that even an idiot could follow, however, at the same time you are so thorough with your explanations that someone could write a damn college thesis on the concept after watching your video 😂 Really hope you get more recognition moving forward.
This channel is okay, but the LOUDMOUTHTIM youtube channel is better.
Alas. People largely want want mindless entertainment because of their uneducated upbringing.
We enjoy it because we have an inquisitive mind. I certainly was not raised in front of a screen instead largely outdoors, left to my own devices....''Go outside and play'', so I did.
But yes. I love this channel. I could listen to him explain the working of a basic Meccano set and be enthralled, in fact, @d4a should.
Opinions are like arseholes@@Wadley225, everyone has one.
We don't need to see yours.
Not sure I've ever put it that way but agree with you 100%.
I've been a subscriber about 6 years but didn't have a lot of interest in 'How to' as I have been a motorcycle mechanic 40+ years. The more technical video's I've always found a lot more interesting.i think the technical video's started when he was doing his engineering degree?
Perfect channel if your interested in engines but confused by the plethora of information out there.
Not just simple and concise but also complete.
"Complete" is an impossible goal, but yes. I'll add that D4A applies far beyond the engine building realm and even beyond the design realm: I watch it to help me visualize the engine I'm inventing's various sub-systems and how they interact and fit together.
It's cool how his videos often don't just speak to an issue I'm currently pondering, but actually point to the solution I'm seeking, especially when the kid says something is impossible, such as this video's water hose thumb thing.
@@RichardLewisCaldwell Perhaps a form of inflatable intake or iris mechanism that can evenly control the diameter and length of an intake to optimize fuel delivery at all rpms?
@@RichardLewisCaldwellFunny thing, I'm also working on an engine concept for use in hybrids, also scalable for use in planes, ships, trains, and trucks. The intent is to remove mechanical energy conversion and just go thermoelectric. This concept, if it proves viable, will simplify all vehicles, remove a LOT of fuel weight from cargo and passenger vehicles, and encourage further research into thermoelectric materials to improve power output and efficiency. The best part of it though is the range my system will allow, which is many magnitudes greater than current vehicles. In addition, this system can be used in stationary power stations to provide grid power with a smaller footprint in terms of both emissions and infrastructure size. It would only need a steady water and electrolyte supply to run the electrolysis plant which will supply the generator with h² and HHO, which will be directly burned in a sealed environment pressurized with Argon to prevent carbon and nitrous emissions and recover the water produced. Operating pressure will be around 85 psi and the operating temperature will be around 2350°F at the chamber and 434°F at the radiator. The thermoelectric modules responsible for converting that heat into electricity will be specialized versions meant to withstand this temperature gradient and produce proportional power to that gradient, or about 200 watts of electricity per module at 12v, which is about 17 amps. With 48 of these modules attached, that's 9.6MW of electricity. This is enough to drive a vehicle and charge the battery packs at a highway cruise. The electrolysis pack will produce about 50psi of hydrogen and oxygen and consume about 400-600 watts at 240v. This system will be able to be mounted in all current vehicles with the tank and electrolysis pack mounting where the gas tank is and the entire generator where the engine is. Since this replaces the engine, an electric motor will be provided to mount to the existing transmission so the overall operation of the vehicle remains unchanged, except then you could start in 5th gear. My intent for this is to provide a viable hydrogen based stepping stone between gas and electric vehicles and between fission and fusion based grid power while we work on generating fusion supplied electricity and build the infrastructure needed to support fully electric vehicles.
@@BoycottTH-cam289 Good idea. I've thought about variable-length induction systems that ensure the pressure rebound hits the intake valves at the optimal time. I'm sure designers have done both, varying length and cross sectional area. But no. I'll post the solution here next week.
@@BoycottTH-cam289 As promised: one of any pair of input valves will have better performance, especially with regard to swirl. So limit your variable valve tech to the crappier valve. I call it winking.
Hmm, the whole engine description barfs. I'll try parts:
The near-Carnot Piston Engine
Introduction
The near-Carnot puts 2/3 real-world efficiency in reach through several interwoven innovations, including insulated tungsten combustion chambers (see 'Cascading thermal expansions and simultaneous combined cycles'). Said chambers' natural hot wall ignition means that any liquid fuel will work at any air:fuel and compression ratio without a spark plug. This also eliminates quenching, which is the source of most deposits, degraded oil, and unburned hydrocarbon emissions.
Hot wall ignition combined with the well-shaped combustion chamber, serious swirl, and toroidal overturning enables the use of lower pressure fuel injection systems that cause less parasitic loss and are far cheaper (fuel injectors can represent ¼ of a diesel's cost).
Cheap and efficient Compression Adjusters (CAs) replace both the plainly inefficient throttle and the expensive and inefficient turbo. A pair of CAs vary the example engine's compression ratio from 8:1 to 32:1 with negligible vacuum and no retrograde gas flow. The example engine's expansion ratio is a constant 32:1.
We've got lots to cover. Let's start with a pair of simple innovations with impossible-sounding functionality. The first simulates time travel:
The Flywheel-replacing Temporal Torque Transfer Device (TTTD)
Flywheels are heavy and must change speed in order to store and provide torque. A TTTD adds and subtracts torque magnetically instead of via angular momentum, so it can distribute its (net zero) contribution to torque through the combustion cycle as desired: adding torque during final compression and subtracting torque when engine torque peaks. And unlike a flywheel, a TTTD's rotor's relatively low mass doesn't significantly affect engine response.
TTTDs use a horseshoe magnet rotor (legs out, oriented perpendicular to rotation) and a horseshoe magnet stator (legs in, same orientation but with poles reversed) so that as the rotor magnet's legs approach the stator magnet's oppositely-poled legs, the TTTD pulls its shaft through final compression and dwell. The resulting energy debt is repaid by the subsequent power stroke as said pairs of legs are separated. Functionally, torque is sent “Back to the Future” (and yes, I resemble “Doc Brown”).
A TTTD has only one moving part, is essentially 100% efficient, and needs no controls. However, significant functionality can be added via either an electromagnetic stator or a physically-spreadable stator that misaligns each half-stator in opposite directions so torque transfer is reduced but net stator-to-rotor alignment is maintained. The width and shape of the legs' tips determines the shape of the positive and negative blocks of torque applied by a TTTD to its shaft.
The example engine's TTTD adds torque from perhaps 25 degrees BTDC to about 10 degrees ATDC and subtracts torque from about 40 degrees ATDC to 75 degrees ATDC on each rotation of the primary crankshaft, thus serving both CCs in a twin-CC engine. Note that since multiple stator and/or rotor magnets can be used, a TTTD can replace the flywheel of any two-stroke and most four-stroke conventional engines.
Well, that crosses “time travel” off the impossible-but-not-untrue bucket list, which still leaves:
The Inverted Crosshead (IC) - slashing piston friction with perpetual motion
Traditional engines route lateral forces from the connecting rod to the wrist pin to the piston to the cylinder wall. This engine enlarges each connecting rod's rounded upper end sufficiently so that the outer surface of said end fits between and so can and will alternately bear laterally on each of a pair of vertical plates cast into or attached to the engine block below each cylinder, resulting in a flat linear contact (or a centering shallow “V” contact) that's thermally isolated from both the piston and cylinder wall, and never stops moving: a rod's rotation is at maximum when its sliding stops (at TDC and BDC) and its sliding is at maximum when its rotation stops (near mid-stroke). This drastically cuts lateral bearing friction and wear while removing the piston head and the cylinder wall from the process: lateral forces go directly from the relatively cool connecting rod and wrist pin to the relatively cool engine block via said pair of vertical plates. Additionally, this “inverted crosshead” elevates the piston rings well above the wrist pin, greatly improving piston orientation, which, combined with the lack of lateral forces to affect the piston/cylinder wall interface, enables tighter tolerances and the elimination of out-of-round machining and the need for a second pressure ring while reducing the piston skirts to a circular miniskirt or just a coating on the ring lands. Piston ring friction represents almost half of a traditional engine's friction, so just eliminating one of the pressure rings represents perhaps a 20% reduction in engine friction.
Note that the example engine's CCs use a 2:1 bore to stroke ratio in order to further reduce friction and pumping losses while providing plenty of room for big valves and inverted crossheads. The expected increase in thermal losses doesn't happen because of the CCs' insulated tungsten combustion chambers.
A CC's diminutive size, 2:1 bore to stroke, and 8:1 compression ratio combine to drastically reduce said CC's speed, countering the increase in mass that results from incorporating a tungsten combustion chamber into a piston.
Next, lets throttle pumping losses:
The Compression Adjuster (CA)
The engine uses pairs of four-stroke combustion cylinders (CCs). Each CC pair feeds a central two-stroke re-expansion cylinder (RC), a well-known technique that balances much like a four-stroke four-banger. Each CC is fed by its own CA, which operates at half speed. A CA's four (technically half-)strokes are: Intake, Intake, Compress, and Compress&Transfer, with the Compress&Transfer stroke coinciding with the associated CC's intake stroke.
CAs are low temperature and low pressure devices, so they can be made of various materials, including plastic. Since a CA operates at half-speed and utilizes its entire cylinder head for intake (see 'Valving and Swirl' below), very little intake vacuum is required.
With relative nominal cylinder volumes of 4:1:4 (CA:CC:RC), the example engine's maximum working compression ratio is 32:1 (CAvol/CCvol * the CC's compression ratio). I say nominal because the CA's function is significantly degraded by non-swept volume, such as exists in the CA2CC transfer passage. Fortunately, this doesn't significantly affect actual efficiency and is easily compensated for by enlarging the CA. In fact, it hints at a way to adjust the engine's compression ratio on the fly without increasing pumping losses:
When a lower compression ratio is desired, the Compression Adjuster (CA) simply raises its ring of intake flaps away from its piston, increasing the dead space between said flaps and piston, and so reducing the CA's pumping power, which reduces the engine's working compression ratio while decreasing pumping losses. When the CA is fully extended it functions as a perfectly tuned intake system and the engine's working compression ratio approaches the CC's native compression ratio. CAs can be driven electrically or via a half-speed shaft.
The CA eliminates the need for throttles and turbochargers: size the CA for maximum compression and raise the CA's intake flaps as desired to increase the CA's dead-space and so functionally shrink the CA on the fly. The example engine efficiently adjusts from an 8:1 to a 32:1 compression ratio while maintaining a constant 32:1 expansion ratio. The working compression ratio can be further reduced by use of the steam throttle, as described in the next section. An anti-vacuum valve in the example engine's RC's cylinder head mitigates over-expansion issues in very low torque situations. Alternatively or additionally, variable valve timing on the RC's exhaust valve(s) can mitigate over-expansion without retrograde gas flow.
In a twin-CC engine the two CAs are phased 180 degrees apart, causing two issues:
First, they are single-cylinder devices whose vibrations combine to form a rocking couple. Compliant mounts can mitigate the problem. Or the CAs can be balanced along with the rest of the engine, which would generally be done with a pair of counterweights, one on each end of the engine.
And second, air should be able to travel freely between the CAs' bottom ends. This is handled either through the rest of the engine's bottom end or via a tuned connecting tube, perhaps of variable length. The connecting tube solution lets the CAs use their own lubrication system.
The example engine's CAs are cast with the rest of the engine and share its lubrication system. This configuration has less variation in bottom end pressure and utilizes the CAs as oil coolers.
I watch other technical channels yet nobody else seems to be able to strike this perfect balance of information, coherence, engagement, approachability and no BS attitude. Thank you for your dedication and work you put into your amazing content.
As always, thank you for a fantastic video. I've been playing around with engines for 45 years, in all sorts of applications, including 2 stroke marine (outboard), marine inboards, petrol (benzene) and diesel car engines. I learn a lot of new things with every one of your videos. Love your work!
Yet another fascinating & extremely well presented video. Even for a career mechanic like myself you provide a commendable level of description. It must also be said, a highly professional level of video editing must take a considerable amount of time to produce. Well done indeed.
Again I can't thank you enough I'm 43 and I built my first engine over the summer and I couldn't of achieved this without your knowledge. Everything from bearing clearance to piston ring gap all learned from you. It started right up and runs like a champ, a.c.l. bearings, k-1 rods, Manley pistons, stuffed turbo, theta 2, again thank you
What engine?
@@adrianzmajla4844 Theta 2
@@adrianzmajla4844I'm guessing Hyundai with the mention of theta 2
Also, don’t forget that swept volume varies linearly with stroke, but exponentially with bore (by the square of bore). So increasing stroke and decreasing bore in the same proportion will lower swept volume, while increasing bore and decreasing stroke in the same proportion will increase swept volume.
the only time i feel lucky is when youtube recommend me this channel, dude is literally the best auto teacher, not just simply talking with a white board but also show us how it works
So enjoyable to be positively caught by surprise on topics I think I know a bit of each time you release a new video. Not even mentioning the format that is very suited to the purpose without click bait, flashy imagery or advertising. Thank you very much, mate !
I love the way you explain and guide the viewer through each topic, just the right amount of information at the right times, with needed heads up early on that are fulfilled by the end. I could listen to you explain just about anything! Awesome work! Thank you!
You're videos are great! You are a truly elite level teacher. Knowledgeable, passionate about the subject, clear speaking, and well organized sharing of info. Well done! Keep it up!
I like that the theory you present always results in practical pros and cons. And judgements on what would be best.
Great article. Back on the early 2000’s I bought a KTM 450exc. The 400 and 450 used the same small piston, but the 450 used a larger stroke. The same stroke was used in the 520, but the 520 used a larger piston. Three engines that are otherwise identical. Some people years later made a hybrid “453” where they took the larger piston of the 520 and the short stroke of the 400.
And which was the better combination for torque
@@russcole5685 the 450 and 500 are very similar up high in the rpm range. They are both much more torquey at low rpm compared to the 400. The 400 is supposed to feel like all of the power is up top and it’s a pussycat down low where it doesn’t make torque.
I've been watching your channel for a while, now I'll comment. You have a very pleasant way of explaining things to me. Even though, as a technician for vehicle technology and e-mobility, I already know most of the topics, I always learn something new from your videos and I really like that. Thank you for your work and all the best for your future. Best regards, Jan
Thank you so much Jan!
Your videos are great! And your command of the English language is also great. Thanks for your work. It is appreciated👍❤️
another great video, the key thing to me as a hobby tuner is the max possible redline determined by piston speed, so I know how high it might be able to rev if I go all out on hardware like cams, TBs, valves etc. doesnt make sense to build an engine with peak power at 9k that loses its pistons at 8k😂
So you must be racing your engines because its going to be hard to rev street engines this high often
@@msk3905 honda vtec engines like the b18 or f20 can rev as high as 9200 rpm and 9500 rpm just fine on the street
@@msk3905
Gear ratios and final drive ratio exist.
@@keijokojootti7790 so you drive around at 35 mph spinning your engine at 5000, huh does that make sense to you?
@@sigmamale4147 big whoop tell me how often you are at those rpms? Lets say you are driving down the street at average speed of 35mph and a mustang pulls up and references lets go by the time you get to use the power of your high revving engine he’ll be at next light already?
I love how this channel helps me understand the thought process that leads to a conclusion I already knew beforehand, but couldn't exactly describe why in detail.
The delivery of your technical explanations is unmatched anywhere on the web. Thank You.
This is another take on your informative veto !
When we are teenagers HP is what is exciting to us ! When Dad was providing the car, gas , insurance etc…. it was no big deal .lol
When we mature and go out on our own , things change and we figure out things !
Things like …….points on your license for speeding , higher insurance rates , higher gas prices for premium gas , low gas mileage, short tire life , etc……
There we realize TORQUE is not a bad thing ! 😂
Ah, yes, becoming an adult... I always wanted an old '60s muscle car, but now I dream of cars like the Suzuki Swift Sport or Civic Type R... And if it has to be an exotic, give me a Lotus Elise with the 1.8 Toyota engine - small car, small engine, big fun factor!
Yeah, nah, the fun begins at 6000rpm (from 10000rpm for a bike). There's so many run-of-the-mill 2.0-3.0L turbos or 600cc parallel twins, that's why an old-school high-revving NA engine or 600cc inline-four is something special!🙂
Besides why not have torque *and* power with the 6.5L Ferrari V12? 790hp @ 8500rpm, 720 Nm (530lb.ft) @ 7000rpm. 😉
@@Donnerwamp I like my Mercedes-Benz W123. So far, it has been pretty reliable (though right now the engine is being rebuilt) and the parts are available and not super expensive. I think that if I needed another car, part availability and price would be one of the major decision points. That and the fact that I dislike computers in cars.
For example, I would like to have a Lada 1600 or a Gaz-24 Volga (or some large American car like a 1975 Lincoln Continental), but finding parts for them would probably be major pain and cost a lot of money.
@@Pentium100MHz I totally understand that. While I'm not "afraid" of computers in cars, I'd like to limit them to only what is necessary. You can get some impressive results by only adjusting software settings, I don't think everything must be watched and controlles by a digital birdbrain. Some things like ESP and traction control are nice safety additions when done well, as soon as it interfers with me driving in construction sites or beeps more than it's not, it's just an annoyance.
@@Donnerwamp My aversion to computers in cars comes from my dislike of too much complexity. My car works quite well with no computers and whatever problems there are come from the fact that my car is 40 years old and not due to the lack of computers. Also, the computer components in cars (the computers themselves, various sensors) are expensive and an additional point of failure.
Thank you very much excellent explanation I'm very grateful for I watched your videos it's been awesome man can I learn a lot thank you for sharing that knowledge
Your videos have helped me learn tremendously. I’ve got a notebook in my office with about 18 pages of info I’ve gathered from watching just a few videos. Thanks so much for explaining things to us.
I had wondered this for a long time. As I was unable to reason it out, I'm happy to see it's complicated. Your explanation made it understandable.
Thanks.
*WHAT AN AMAZING CHANNEL* these people, who are this good, deserve every single penny they make
1:20 The force isn’t doubled, it’s quadrupled (assuming pressure is the same). This is part of why F1 cars will have super short stroke length. The drawback is more reciprocating mass, and more strain on the crank, which can be addressed with advanced manufacturing and fancy materials
I think he meant the bore to stroke ratio of sqrt(2), otherwise the displacement would not be the same between the two examples.
Been a subscriber since very early on in your channel back in the engine review days! Just realized YOU'RE ALMOST AT 1M SUBS! Congrats! You provide so much valuable knowledge for us. Very well deserved success!
Hey man, just wanted to say i learned more from you than any other person in my entire life. i cant say how much i am grateful for your videos. there is no other channels even comparable to yours.
Thank you so much man.
Great video! Also, muscle car engines are usually oversquare because a very famous modification for them is a big stroke crankshaft, that significantly increases their displacement and hence made more power and torque
Isn't it also that a 2-valve engine needs a big bore or else it won't flow very much air because of the limitations of 2-valves compared to 4- or 5-valves? 🤔 Whereas, for example, the 5.0L Coyote with a 92.2mm bore can still have more valve area than a 5.0L Windsor with a 101.6mm bore because the Coyote has two more valves per cylinder?
I highly doubt a manufacturer would design a production engine with accomodations for the small fraction of total customers that might be changing internal components in the future. More likely , they design it to function exactly the way they want and any modifications that can be done are happenstance. I'm not even sure they make engines rebuildable on purpose, they may happen to be that way simply because of how they have to be assembled and inspected.
@@NickKautz That's true, on the other hand Ford themselves have run into the limitation of the narrow bore spacing on their 4.6L Modular engine. The 5.2L Voodoo/Alluminator version of the engine needs to use a spray-on cylinder liner and the 5.4L version used in V8 Supercar and GT3 racing has quite an unfavourable rod-to-stroke ratio. 5.4L is pretty much the maximum capacity the Modular/Coyote can be taken too.
I bet Ford wished they had used the same bore spacing as the Windsor in some respects. E.g., to make a bigger capacity engine they made a 6.8L V10 based on the Modular architecture as it didn't lend itself to making a bigger V8, and now they have made a completely different 7.3L pushrod V8 for their larger pickup trucks.
[There used to be a longer stroke version of the Modular with a 25mm taller block, the 5.4L from the 2000's, but this is not made any more.]
That’s the case for older muscle cars. Today muscle cars have become under square. By the way they aren’t over square because people do stroker kits on them, they are over square because it allows the engine to rev higher creating more horsepower.
@@2seep That's the case with modern muscle cars as well. Both the ls/lt engines and the hemis are oversquare engines. Only undersquare being the coyote, which is pretty close to being square, and that's because it's a dohc. The ls series and the hemis are ohv, meaning they can't rev high anyway. I'm sorry, 6500rpm isn't high. Even the ls7, with being slightly oversquare and close to being square has a 7000rpm redline. That's not a lot, regardless of displacement....
Doubling the bore quadruples the area off the top of the Piston the quadrupling the force for any given pressure
actually is the opossite, same pressure working on the same area is still the same, the point of enlarging the bore us to incresase volume of air and gasoline, more pressure more power
Quadrupling the surface area of the piston means dividing the center to center length of the crank journal (and the mechanical advantage that it provides) by 4 to maintain displacement.
Best channel for explaining engines out there on TH-cam.
Also it’s really good to understand as a non native English speaker. 👍
No matter how much I think I know about this stuff I always learn something from these. Thank you for these excellent videos!
I feel smarter for having watched this.
Years ago, I read some Germans calculated that .5 liters is the ideal cylinder size... I've always wondered if there was anything to it.
Maybe they were right? So many amazing 2 liter 4 cylinder over the years. But again... ideal for what?
@@d4a efficiency, i've heard the same. one cylinder, 500 mill square engine supposedly best efficiency.
@@SoulTouchMusic93 2 liter 4 cylinder diesel here, can confirm they're mad efficient
Fords 3.5 v6 are amazing for hp& torque . From truck into high performance GT.
ideal for the particular engine they were designing
What a great educator you are! Been watching and learning from your channel for a long time now, and just wanted to pass along my appreciation. GREAT WORK INDEED!
I forgot how i stumbled across your channel but it's definitely one of the best. The amount of information, visuals, and cohesiveness of everything is so nice. I would like a little more into bike engines since im currently trying to rebuild my 08 crf250r. Your channel has definitely helped me a lot!!
All hail the Square Engine, the superior shape!✊🏻
86 is a very interesting number. It's on many famous engines 😁
You’re my favourite channel in terms of engineering, TH-cam and person.
If you're also interested in suspensions, you might also like the channel Suspensions Explained. Highly recommended as well!
Appreciate your channel and the information you provide. I like your straightforward explanation, helpful graphics, and application. I've learned a lot from you.
This channel has the most informative videos on youtube regarding cars and engines. I have watched most of your videos at least 3 times just to get a grip on the info overdose.
Thank you so much
Just an observation: Most naturally aspirated engines have a max torque of about 100Nm/L. A HD engine is relatively under squared and have a max torque of about 80Nm/L and a over squared engine like the Ducati V4 have a max torque of 120Nm/L, that's 50% more. That is the extremes but a couple of years ago I checked about 100 motorcycles and it was kinda obvious that the over squared engines had a lower max torque per liter.
Now, the engines that are over squared are usually on the sport bikes and the others on gliders or off-road type bikes and one can imagine that the torque curve is more flat on the gliders and off-road and less flat on sport bikes where power is much more important.
Note that I only checked road bikes, and no mx bikes and also only four strokes.
Undersquare engines are usually not high performance engines, thus have relatively low torque and even lower power relative to their displacement. I think that is why you’re re seeing this difference.
I gravitate towards slightly I oversquare (American V8) to square as to me give me the best performer on the street. When an engine has a higher peak torque than HP peak it means there is a restriction somewhere so your last BMW example falls in this category
@@msk3905 Some engines are tuned for low rpm operation. This can be to optimise for a certain application, or a necessity due to limitations in the engine configuration. Aircooled Harley Davidsons (and other aircooled large capacity engines) are an example of both.
High peak torque as compared to peak power simply means that the engine doesn't perform well at high rpm, doesn't need to mean there is a restriction somewhere.
my engine felt stronger before swapping the head and cam, as a linear increase in power dont "feel" as powerful as the gulp of torque down low under the midrange, despite gaining 25% power with the new head and cam. the linear power band all the way up to 7k compared to the old flattening out after 5.5k rpm. the but dyno is quite faulty that way.
If it felt stronger before upgrading cam and head then you picked wrong cam and head
@@msk3905 it felt stronger before because with a ported head and a more radical cam it lost power below 3k rpm but as i said i gained 25% on the top end and it pulls another extra 1000rpm so considering i usually enjoy the top end and dont have a slushmatic transmition it's the right choice for me, simply drop a gear and dont lug the engine when i wanna go
Love your work. Keep it coming; I always learn something on top of a long ago undergrad in Automotive Technology. Cheers.
Thanks D4A for another class of "I go to engineering school so you don't have to" awesome insightful information presented in the simplest manner that idiots like myself can digest and understand.
As you stated here, and as I always hear elsewhere, a pushrod V8 is limited in its valve operation. What occurs to me is, why can't a pushrod V8 have variable-ratio rocker arms? Wouldn't that effectively be the same thing (more or less) as VVT in an OHC engine? Even if it's not a direct comparison, it seems to me some gains could be realized in such a design.
Variable ratio rockers would just change valve lift. The camshaft doesn't change, any extra lift just comes from the valve cycling faster. So no it wouldn't be variable valve timing, but it wouldn't be a total waste. Don't think it's been done on a production pushrod v8, but it has been done on plenty of other engines.
I was just researching this yesterday, wow. But not necessarily the power aspect, but engine wear/longevity etc. Are there any generalizations that can be made in that regard? *edit; got some answers to this in the video. The main answer being "it depends".
well, many toyotas use high stroke to bor ratio engines as well as trucks.
or in other words: build quality>engine design.
Longer stroke will put more force on piston sidewall so if you are looking at longevity it will wear quicker, thats not to say that you still cant see many miles on it. Rpm and vibration are what kill engines pay more attention to that
@@msk3905 The longer stroke engine spends the majority of it's time at lower RPM to do the same amount of work and lasts a lot longer for it. Check out any engine from a long haul rig, either truck or boat. You won't see many square engines...
@@pontiacg445 Not sure he is asking about long haul engines, these are diesel and no idea what the rod lengths and highway driving is less harmful then stop and go? Boat gas engine are recommended that they be rebuilt after 1,500 hours is that long lasting to you? Have you ever heard of the Square ok lets see off the top of my head, Ford 5.0L modular engine that has been produced from 2011 on, my 408ci has 4.03 bore with 4" stroke, Ford 3.7 has 3.7 bore & 3.66 stroke, VW made a 16v 3.25 x 3.3 engine, VW W16 in the veyron is square engine design.
My highlight of the day. Thank you for your great channel! 👍 ❤
That was an EXCELLENT presentation.
I've watched a lot of your videos. Very impressive work sir.
When you are comparing stroke vs bore, you have to keep the displacement equal. At least one variable in the equations has to be fix, here it's the displacement. So comparing K20 and K24 doen't make sense because they have unequal displacement...
Why do you have to keep displacement equal? I agree it's an interesting point of comparison but it doesn't mean other comparisons are invalid.
This video is about bore and stroke, so that's what he's varying between engines.
You are dropping RPM though. A 500cc engine that spins at 7,000 rpm will have the same swept area (and air throughput) in one minute as a 700cc engine revving at 5000 rpm or a 1L engine spinning at 3,500 rpm. Fuel/air mixture being the same they should use similar amounts of fuel. The lower displacement/faster spinning engine is making the similar peak HP by sacrificing torque for RPM.
Instead of quoting engine displacement as swept area for a single revolution we should consider engine displacement over a set period of time. People love to talk about American V8s making less HP per liter than high strung European L4s, but HP is a function of time and displacement is not. Torque is the unit that does not have a time component.
Regardless of the underlying mechanics, we all know that the vast majority of the greatest engines in history are oversquare. To put it another way that all F1 fans will understand:
The bigger the bore, the more the crowd roars
Not in my world! Long stroke of the under-square for a nice fat torque curve and I'm a very happy boater.
Brilliant ! Love how you manage the physics side of this so easily, you are true a great educator.
This perfectly explains why Porsche can generate tons of power and a top speed in excess of 200mph with just a small 3.6 or 3.8 liter flat 6 engine. Very interesting. Thanks.
Power? Torque? Who cares! Max bore all the way! Get that RPM sounds going 🔊🔊
No replacement for displacement kind of guy !
Me too !
Great for trailering boats , motorcycles , campers etc….. 😁
That's pretty much exactly the opposite of what's required in marine applications. If the engine doesn't produce enough torque at the low end, the boat will never 'plane', so the engine will never get the revs up high enough to take advantage of that extra power. Marine engines don't have a shift-able gearbox. The gear reduction (between engine and propeller shaft) is fixed, so if you prop for the top end, it'll never get out of the hole. Prop for the 'holeshot' and you'll blow the engine at the top (serious over-revving) or just leave a whole lot of potential power as 'unavailable'... Best engine? Under-square with as flat as possible torque curve for the win. Keep the revs in check with prop pitch and you'll have a boat that is easily drive-able and beautifully responsive.
@@Chris-hx3om can't ships adjust their propeller? Sounds like some clever electronics could make a uneven torque curve work then.
@@Chris-hx3omexplain the 2jz boats in thailand
@@Chris-hx3om Fascinating, appreciate the info
One of my favorites is that of bikes like Honda CBR250RR MC22
- 45~ hp out of 250cc (180 hp per liter N/A)
- 18,000 RPM, genuine F1 sounds from inline-4
- Top speed around 180 km/h
- The torque is a whopping... 18 ft-lb or 25 Nm
The main advantage of a larger bore is that you can increase the size of valves to get more air and burn more fuel for more power, whereas stroke is better for torque at the expense of a weaker crank that cant rev as high
You increase both you get most power 🤯
I thought the main advantage of an over-square engine was reduced piston speed, so the revs can be higher before the piston starts outrunning the combustion front... Higher revs (with associated air flow) equals more power.... There you go.
@Chris-hx3om although that is an advantage, which also puts less stress on other components by speed, it also add rotational mass. Realistically our main problem with making power is getting enough air in the engine in the first place when using gasoline since it requires more air per molecule of fuel to burn completely and efficiently. It's why top fuel dragsters make so much power. Realistically the fuel has less potency for power, but because it requires so little air to burn efficiently you can effectively cram 4 times the fuel in there, making more power anyways
@IgnitionP although true, the main problem we face is material strength and rotational mass, especially at the piston and rod which causes the components to face a large ammount of stress. The less mass you can get away with on those components and still have the same strength the easier it would be to rev up higher, but you would also need the airflow and fuel volume, cam timing, valve spring stiffness, ignition timing, and a bunch of other things to support it
@@IgnitionP Ferrari 6.5L V12 is the answer! 😊
Excellent presentation of the complexities involved! This shows very few of us have the technical education and experience to properly pick our own designs.
Great video, so many people just think about the ratio, and don't realize it's the secondary characteristics that actually give all the difference
Thank you so much. You make these concepts very easy to understand even for people that are not highly mechanically inclined. You don't have to be an engineer to understand with the way you illustrate and explain. You do a great job
Fantastic graphics and explanations as always. You should be proud of what you do.
I just made an account so that I can subscribe, as well as like and comment EVERY one of your videos.
You've given me such a great understanding of the principles that govern engine performance and dynamics. After watching your videos, I know I'll be able to choose the right parts for my budget and the requirements of my build, no matter what the application or vehicle!
I hope your channel only continues to grow because no one else makes videos half this good!
Thank you for your explanation, it help correct some of my technical misunderstanding of piston engines of smaller bore/higher stroke (higher RPM).
You make excellent videos! You have a knack for explaining complex concepts in ways ordinary Joe's like me can (mostly) understand. Thanks!
this is the most informative and educational video on engines i have seen on YT! Subscribed! Gonna dive into the other videos here
Incredibly clear and precise explanation of a difficult topic. Great! Thanks.
Hey d4a, just wanted to say, your videos are massively helpful for learning. They are well made, explain the concept very well, and your teaching style is phenomenal. Thanks for making these videos, they help me learn these concepts without having to read textbooks.
Happy New year man! Thank you for your videos. I have learnt so many things about the engine by watching your videos. I hope one day car manufacturers would approach you to test out their cars with interesting engines because you could really provided awesome insight about how they work, tbh more than the average car video presenter on TH-cam. 👌👍 keep it up
Great to seeing you back to doing what you do best!
This is one of your best videos... I was thinking this subject over recently with particular attention to one well known and commonly modified v8 engine, and you have confirmed many of my suspicions about common steps in tuning said engine.
In Engines masters, the guys tested two 383 cubic inch engines. The first was a 350 Chevy stroked by installing a 400 crank in it. The next was. Mopar 383 with its stock bore and stroke. The Chevy had a 4” bore and 3.70” stroke, the Mopar has 4.25” bore and 3.38” stroke. The engines were built as close to the same as possible, same cam grind, type of intake and exhaust manifolds etc. when they put them on the dyno they were shocked to get identical torque at an the same RPM. There is more to this puzzle than bore and stroke. 😊
Extremely well written, well explained, with top-notch animations and graphics.
Great video! Who would have guessed engines are the result of multiple complex systems working together and not just a few lines on a spec sheet :)
I don't know how you manage to make video after video summarizing complex topics in a way that's easier to digest and remember than any professor I've had but you have real talent my friend. Massive respect for also not succumbing to the typical TH-camr nonsense. You're a rare breed and I hope people realize that.
The channel about cars and engines that gets my full trust and attention.
A thousand thanks for all you do.
What an awesome video, dude!!! The info in this is incredible.
Bro, it's just f'ing amazing that someone figured all this out so long ago. What a time to be alive
I love car mechanics and especially engines and this guy is just what I needed to start learning about this. He explains so well and just makes it so fun to learn
I always learn something from the way you frame the question. Thanks.
I like the simple yet thorough way this gentleman explains key concepts. Definitely the smartest automotive channel on all of TH-cam (sorry Tyler). 😁
I love how this video shows us the bigger picture. Very insightful! I recently interviewed the chief engineer of Lexus GX550. On paper, the new V35A-FTS is very undersquare compared to the oversquare 1UR-FE in GX460. The new 3.4L Twin turbo V6 produces 479 lb-ft @ 2000 rpm, which is insane. I automatically assumed it was because of the undersquare design. But after talking to the chief engineer, he actually said bore & stoke was not really the "main thing". It was just 1 factor among many other factors, exactly like what you said. They didn't even "pick" long stroke to start the new engine design. Instead, it was more like a result after weighing many other parameters and trade offs.
I like how you make such informative videos and no annoying background music.
Thanks for all the great videos. Merry Christmas
Awesome video mate, very well put together
Awesome video! Keep on going man. You are the best.
Thanks for these vids they are really helpful. Im going to school to become a master tech and all of this is new to me so its really cool to see.
Oversquare for high revving, high speed motoGP bikes. Undersquare for low revving diesel trucks for transporting heavy loads. One just as amazing as the other!
Thanks for this very informative and helpful video! It really brings some of these topics into a proper perspective!
Another great video, well done 🙂👌
Loved your guest appearance on the HP Academy podcast too!
Always a tremendous amount of fairly easy to understand info. Thanks. I was gonna ask about a square engine comparuson.
Because folks like you make these educational videos, knowledge becomes much more accessible for the young generation. Thank you!
Great video! I really enjoyed the real world engines comparisons to round out the theory.
Love the real world examples, comparison and explanation. This what's set this channel apart from the rest
Contrary to your comment at around 13;00, the Royal Enfield Himalayan [air cooled 411 cc] is 'under-square' with a Bore / stroke of: 78 mm × 86 mm (3.1 in × 3.4 in) which is why it is called their 'LS', Long Stroke engine. it should also be noted that under-square naturally aspirated engines are less effected by loss of power at very high elevations, where there is less air.
An engine with a bore to stroke ratio of 2 has double the displacement of an engine with a bore to stroke ratio of 0.5 with double the stroke.
A piston with double the diametre has 4 times the area, meaning with the same cylinder pressure, it makes twice the torque when it has half the stroke.
Bore to stroke ratio does not impact displacement. An engine of 50cc and an engine of 500000cc can both have the same bore to stroke ratio.
@@d4a That is true. Still, you implied that the undersquare engine has twice the stroke of the oversquare one. This is also how the two graphics were scaled. If that is the case, the oversquare engine has twice the displacement.
He didn't specify that engine 1 had twice the bore diameter of engine 2. Engine 2 would have about 63% of the bore diameter of engine 1 to have the same displacement.
Great video, I always learn alot. Thank You for the knowledge.
Bro 25 minutes went by like it was 5 incredible content very educetional
On the remote off chance that nobody told you yet: You're a true genius when it comes to explaining stuff. I really hope you work in the education field.
Informative, entertaining and possibly my favorite new channel on Yt now
Brah. Its not even about how much you know, but how clearly you articulate everything. Master of ELI5