I was taught that you want about one mm of squish so put a piece of solder in there and turn the engine over by hand so the piston smashes the solder then measure the flat piece of solder and if it's more or less than one mm fix accordingly. Some people put two gaskets at the base of the cylinder or you can sand down the head a little bit. Depending on if you have too much or too little
What a nice clear video! Don't exactly know why I got it recommend to me lol! But I watched it and it's really good and clear! Should help lots of people with the question!
The quicker the burn (the better then chamber design) the less advance you need, so the less time is spent with the piston in a position that is less than ideal (or actually detrimental) to power production. That is to say if you have a head design that needs 38° btdc of spark advance (to develop peak cylinder pressure at the ideal time after top dead center (usually around 20° atdc) then you will have 36° of crank rotation where you are actually pushing the piston in the wrong direction. In addition the fuel charge will burn more slowly and generally be more likely to have uneven flame propagation. If you can reduce the needed timing to 20° btdc then along with having a shorter period where you are trying to push the piston back down the bore, you will have a much greater portion of the peak cylinder pressure occurring during the ideal area of crank rotation to deliver maximum power.
i dont mean to be so off topic but does someone know of a way to get back into an instagram account..? I stupidly lost the password. I appreciate any tricks you can give me!
@Turner Saint I really appreciate your reply. I found the site on google and Im waiting for the hacking stuff now. Seems to take a while so I will reply here later with my results.
been runing 17BAR cold measured 250cc engine with 0.5mm squish and never had a hole in piston or any detonation mark... I do not really know what do you mean about getting detonation from tight squish, as everybody runs it as low as he can
What happens if your squish is to large. I have had problems with AM cylinders and pistons on chainsaws. They seem to set the squish in the .040-.050 range. This creates a real problem when you add a cylinder gasket to the cylinder. You then end up with a squish of .060-070, making the saw a pita to crank. It will pop and try to yank the starter handle out of your hand. I cannot tell you what this does to your shoulder and wrist.I have just built an ms 460 and it is not a saw you can pull through. At least, I cannot. I checked the squish on a factory Stihl 461. It was .038 and I cranked it in three pulls. I can get my squish to .040 by building a no base gasket build. What do you think?
Can I ask if you have seen some the VHM heads that have Squish adjustable. Thinking of getting some for my 2 stroke, mainly because they are O ring and not gasket types.
(paraphrasing)"If squish is too-tight, your worry is pre-detonation" Ok I'm reallly hoping you reply to this... My understanding (maybe from one of your videos actually) is that optimizing power is "on the edge of knock", and - so far as I can tell - both knock and pre-ignition are caused by comparable phenomena IE too tight/hot a chamber during the TDC phase, causing ignitions of charged-air pockets that are *not* part of the actual flame-front.... Why, then, do other types of engines (picturing a big race-boat engine now, not my chainsaw engines!) talk of 'quench' and optimize to the point of precision wherein there's literal evidence of piston//band touching? Obviously such touching is not desirable, but I *would've* thought that that level of tightness would be so far beyond knock &pre-ignition that it'd be an engine-destroyer, yet I'm watching a boat-designer explain how tight that area is kept (IE 20thou would be seen as imprecise, sloppy) Would have thought that such a position would be impossible to reach, because knock/ignition would cripple you first, but that builder spoke of it like it's no biggie (maybe it's different when the chamber has valves?) Do you think that setting too-tight a squish, and testing it carefully, is still "very VERY likely to destroy the top-end" or do you think there's a chance you could get the cylinder to nearly-slap/slap the band and, immediately, shut it down? Obviously if you worked-up to that level of tighteness, IE that tap occurred because you lowered squish another 4thousandths, on your way to that too-far point (IE if you just went tried running it when it had negative squish, not that you could even start it of course lol, but of course that'd theoretically destroy the engine, if it could actually operate that way :P ) Thanks for advice on that, honestly in chainsaws all the tuners just say "20thou tightest squish on 60cc and below, then maybe 25cc squish lowest for up to 100cc engines", I HATE this I mean squish should be a measurement taken to determine piston-distance NOT compression, yet it's used in-place of real/true compression-ratio testing (which is all the more obnoxious when guys on a forum are both talking about the same saw, but one has machined his base 30thou lower dropping the chamber and the other hasn't, if those two talk about squish# as a rubric for compression they will be talking apples&oranges, because cutting squish band reduces comb.chamber volume ergo same level of piston-to-band squish would now equal higher compression (because of smaller comb.chamber)
Wish I knew what angle is best for the band, making it dead-flat seems inferior to a slight angle (like 1-3 degree inward slope to 'aid' squishing effect, obviously tapering the opposite way would *prevent* proper function of a squish band), all pro porters I know simply "cut the band" which leaves them w/ a true-flat squish band, but OEM cylinders tend to be 1-3deg of positive slope from cylinder wall upwards to the chamber (which makes sense considering *how* squish bands work) but in doing a new project here I'm literally in a spot where I'm better off IF I can keep the current ~2.5deg inward slope, instead of going flat, but am watching videos and nobody talks about angles :/
Hi good sir, question here. if you remove the head and barrel gasket that would increase the piston height hence decreasing the squish band right? now, do you need to grind the piston so that it will match the curve of the combustion chamber? cuz if you increase the piston height it will hit the cylinder head. Thanks in advance.
Reducing the distance between the piston crown and the head increases the compression ratio, which need to be calculated so you don't suffer from knock. But yes it increases the squish. A lot of modern pistons have flat crowns, so you don't need to 'follow' the shape of the combustion chamber. But yes in most most applications the piston needs to be machined to match. Its all about keeping the compression ratio in check.
are you running race gas I am told around 0.030 is when you start needing race gas I use 0.035 for now on pump gas its a really small cc head its making a lot more power right off idle its a older cr250 stock was 0.060 I cut the head down 0.030 but im using a thicker base gaskets hopes to get it to rev out more to but I got to rejet for the top end then ill know. I awlays use 0.020 or less on chainsaws that I mod 100cc and under and pump gas
Good info and clear explanation, might be helpful to others to also explain the tuning of the powerband that can be accomplished by having more or less of the total area of the head dedicated to squish..i.e. larger percentage of area giving more low end and less helping with top end over rev
Roger Miller Thanks for the comment, I wish you'd all stop jumping the gun lol That's a subject for a later video and I'm building a mock up example model to make it easier for people to understand - thanks for the comment - matt
Also interested in your follow up videos on head design Matt. One thing I have really never seen is how the dome shape effects the loop scavenging flow. Thanks again, Frank.
What a crock! Running a tight squish band gap will not over pressurize the fresh charge that sits between the squish band and the piston and therefore drive that mixture into pre-ignition. The charge as an escape route into the main combustion chamber, that's the whole point of a squish band. Skimming the head in order to close the gap could push the CR up high enough to drive the engine into detonation if the combustion chamber bowl is not opened up in order to bring the CR down to a level that suits the octane rating of the fuel being used.Its a simple fact that if the squish gap is too big then the squish band does not function and further more the fresh charge that is 'hanging around' in the gap is not exposed to the flame front until well after TDC by which time it is too late for it to help produce any useful power, this is why the gap needs to be as small as possible in order to reduce the charge waste. The limiting factor here is the engine design and the RPM that it is expected to run at. If the engine has weak cases, a stretchy con-rod and a 'whippy' crank then this sort of stuff can stack up and cause the piston to hit the head, it won't hold together for very long if that happens.There are theories with regards to the gas velocity and this will be effected by the squish band gap, the width of the band and the angle that the squish band and piston form, possibly the idea is that if the velocity is too high there will be too much turbulence in the chamber but I think that this is beyond the scope of your video and is stretching my knowledge to the limit. (Even if that was true then this high velocity charge would still not detonate or be subject to pre-ignition).You talk about the squish band forcing the charge towards the plug and that is all very good but the primary reason for having a squish band is keep the engine away from detonation, it is the end gases that detonate AFTER the plug as fired but BEFORE the flame front has reach them and by forcing the end gases into the burning flame front they do not get a chance to detonate.
I sortof agree - plus that a good squish is also a little tapered/wedge-shaped, very little but it's there, the greater height towards the middle of the cylinder (this aids in aiming the squeezed charge into the actual combustion chamber). The outer region must be as low as you can make it (or dare to lol) for the factors mentioned above. The squishband is polished/shiny also, and on a good squish you'll not see any carbon deposits (on the head) due to the cleaning effect the fast escaping charge has. The cooling effect of both the head and the piston crown prevents detonation of the charge trapped in the squish, and theory is that when the piston descends the flamefront reaches the squish uniformly circular, and will burn this bit then too, aided by the then already very high pressure. This is also the reason why the sparkplug is best positioned dead-center, it in fact allows for the highest CR possible. Oh, holes in 2T crowns are most often caused by a too-deep (warm/hot) sparkplug, the burning gas within the plug acting like a torch aimed at always the same spot. I'm old enough to remember the socalled surface plugs (noseless) which did away with this pest, but you needed an electronic ignition to drive them. Sure, detonation screws up anything, but this will likely not be in the middle of the crown but more towards the outer fringes. Mind though, all this is for racing 2T's, probably not your average scooter you use to go buy beer with.
sidecarbod, you are correct. The description for deto in the video is way out. Your last sentence is spot on, deto happens AFTER the plug has fired with the end gases self combusting due to the radiated heat from the combo of the head, liner and piston, which is pushed over the self ignition temp by the UV energy (all electric sparks produce UV, ask any TIG/MIG welder) from the ignition spark which reaches the end gases way before the flame front (speed of light vs. 80 m/s ) The idea of squish is to remove as much end gas as possible, because if there is no gas to deto, it wont deto.
Wow, thanks that made my day, easily the best youtube-comment I've found in like 1/2 a year of learning porting (for my chainsaws...have done 3 of them so far, though I consider all 3 'stage 1 of X' builds :P ) I replied to someone else the same way saying how I've seen guys w/ boat engines talknig about 'quench' and how their engines DO see piston-to-band touching (not desirable or anything, but more a 'it happens sometimes and isn't a big deal') SO, in my world, guys don't talk of compression ratios they simply measure squish with solder and go by the rules of thumb: 20thou tightest for 60cc saws, 25-30thou for up to 100cc saws. This is repeated to the point of dogma. But I'm newer to the scene and the one thing I cannot stop thinking is "These guys are never talking about how they got a lil piston-slap, would bet 95% of them did NOT lower their bands as far as they could have" (ergo they're wasting power, the charged air in the band area that doesn't get ignited until way too late to be of much help, especially in chainsaw engines where porting usually includes raised exhaust roofs) I didn't know better and I put like 8deg of inward slope onto a squish band (54mm cyilnder for 92cc saw), was told the cylinder is now ruined -- do you concur with that? It seems 1-2 is the norm recommendation for slope but I can't get it through my head why you wouldn't wanta lil more slope (to a point...maybe my 8deg is past that point!) Again thanks so much for putting that reply you did, I literally copied it to a word-doc in my "mods" folder lol :P
Flying 2-stroke model engines - I just heard of this for the first time this week - and your video taught me much. Thanks for the share...
I was taught that you want about one mm of squish so put a piece of solder in there and turn the engine over by hand so the piston smashes the solder then measure the flat piece of solder and if it's more or less than one mm fix accordingly. Some people put two gaskets at the base of the cylinder or you can sand down the head a little bit. Depending on if you have too much or too little
thanks for the vid, i will do this to my lawnmower hopefully I'll get the lawn done in 30 minutes instead of 60 minutes
What a nice clear video! Don't exactly know why I got it recommend to me lol! But I watched it and it's really good and clear! Should help lots of people with the question!
The quicker the burn (the better then chamber design) the less advance you need, so the less time is spent with the piston in a position that is less than ideal (or actually detrimental) to power production.
That is to say if you have a head design that needs 38° btdc of spark advance (to develop peak cylinder pressure at the ideal time after top dead center (usually around 20° atdc) then you will have 36° of crank rotation where you are actually pushing the piston in the wrong direction. In addition the fuel charge will burn more slowly and generally be more likely to have uneven flame propagation.
If you can reduce the needed timing to 20° btdc then along with having a shorter period where you are trying to push the piston back down the bore, you will have a much greater portion of the peak cylinder pressure occurring during the ideal area of crank rotation to deliver maximum power.
i dont mean to be so off topic but does someone know of a way to get back into an instagram account..?
I stupidly lost the password. I appreciate any tricks you can give me!
@Kamden Reyansh instablaster ;)
@Turner Saint I really appreciate your reply. I found the site on google and Im waiting for the hacking stuff now.
Seems to take a while so I will reply here later with my results.
@Turner Saint It did the trick and I finally got access to my account again. I'm so happy:D
Thanks so much, you really help me out!
@Kamden Reyansh you are welcome xD
Good way of explaining it pal just swapping out a 3bn for a 3mb on my dt125r at the moment just hope i dont need no shim spacer plate at the base
1:25 he explains the squish but the beginning shows you the parts of the engine. i`m tuned in for the stroke ratio formula but not here.
been runing 17BAR cold measured 250cc engine with 0.5mm squish and never had a hole in piston or any detonation mark... I do not really know what do you mean about getting detonation from tight squish, as everybody runs it as low as he can
What happens if your squish is to large. I have had problems with AM cylinders and pistons on chainsaws. They seem to set the squish in the .040-.050 range. This creates a real problem when you add a cylinder gasket to the cylinder. You then end up with a squish of .060-070, making the saw a pita to crank. It will pop and try to yank the starter handle out of your hand. I cannot tell you what this does to your shoulder and wrist.I have just built an ms 460 and it is not a saw you can pull through. At least, I cannot. I checked the squish on a factory Stihl 461. It was .038 and I cranked it in three pulls. I can get my squish to .040 by building a no base gasket build. What do you think?
Large squish just means the fuel doesn't burn as well and you lose a bit of power
Can I ask if you have seen some the VHM heads that have Squish adjustable.
Thinking of getting some for my 2 stroke, mainly because they are O ring and not gasket types.
I have one question...if i have piston 0.25size or 54mm size piston..how much i must cut my head and how i want use degree head
(paraphrasing)"If squish is too-tight, your worry is pre-detonation" Ok I'm reallly hoping you reply to this... My understanding (maybe from one of your videos actually) is that optimizing power is "on the edge of knock", and - so far as I can tell - both knock and pre-ignition are caused by comparable phenomena IE too tight/hot a chamber during the TDC phase, causing ignitions of charged-air pockets that are *not* part of the actual flame-front.... Why, then, do other types of engines (picturing a big race-boat engine now, not my chainsaw engines!) talk of 'quench' and optimize to the point of precision wherein there's literal evidence of piston//band touching? Obviously such touching is not desirable, but I *would've* thought that that level of tightness would be so far beyond knock &pre-ignition that it'd be an engine-destroyer, yet I'm watching a boat-designer explain how tight that area is kept (IE 20thou would be seen as imprecise, sloppy) Would have thought that such a position would be impossible to reach, because knock/ignition would cripple you first, but that builder spoke of it like it's no biggie (maybe it's different when the chamber has valves?)
Do you think that setting too-tight a squish, and testing it carefully, is still "very VERY likely to destroy the top-end" or do you think there's a chance you could get the cylinder to nearly-slap/slap the band and, immediately, shut it down? Obviously if you worked-up to that level of tighteness, IE that tap occurred because you lowered squish another 4thousandths, on your way to that too-far point (IE if you just went tried running it when it had negative squish, not that you could even start it of course lol, but of course that'd theoretically destroy the engine, if it could actually operate that way :P )
Thanks for advice on that, honestly in chainsaws all the tuners just say "20thou tightest squish on 60cc and below, then maybe 25cc squish lowest for up to 100cc engines", I HATE this I mean squish should be a measurement taken to determine piston-distance NOT compression, yet it's used in-place of real/true compression-ratio testing (which is all the more obnoxious when guys on a forum are both talking about the same saw, but one has machined his base 30thou lower dropping the chamber and the other hasn't, if those two talk about squish# as a rubric for compression they will be talking apples&oranges, because cutting squish band reduces comb.chamber volume ergo same level of piston-to-band squish would now equal higher compression (because of smaller comb.chamber)
lovely top youve got there 😂😂
if you turn the headbolts to tight..is it possible to have problems ?
yes. piston will seize, the seize marks on the piston will be at the same location where the studs are. respect the torque settings.
Wish I knew what angle is best for the band, making it dead-flat seems inferior to a slight angle (like 1-3 degree inward slope to 'aid' squishing effect, obviously tapering the opposite way would *prevent* proper function of a squish band), all pro porters I know simply "cut the band" which leaves them w/ a true-flat squish band, but OEM cylinders tend to be 1-3deg of positive slope from cylinder wall upwards to the chamber (which makes sense considering *how* squish bands work) but in doing a new project here I'm literally in a spot where I'm better off IF I can keep the current ~2.5deg inward slope, instead of going flat, but am watching videos and nobody talks about angles :/
Hi good sir, question here. if you remove the head and barrel gasket that would increase the piston height hence decreasing the squish band right? now, do you need to grind the piston so that it will match the curve of the combustion chamber? cuz if you increase the piston height it will hit the cylinder head. Thanks in advance.
Reducing the distance between the piston crown and the head increases the compression ratio, which need to be calculated so you don't suffer from knock. But yes it increases the squish. A lot of modern pistons have flat crowns, so you don't need to 'follow' the shape of the combustion chamber. But yes in most most applications the piston needs to be machined to match. Its all about keeping the compression ratio in check.
thanks man, you're vids are very helpful, nice to see vids like these help other wanna be mechanics like me. lol, keep it up, more power!
No worries mate, thanks for the comment - matt
Tighter the gap, more power. Tighter gap, better fuel needed to prevent detonation. I run 0.020" squish on YZ250
Thinner the gap more power, until you start to get detonation - then bad times
are you running race gas I am told around 0.030 is when you start needing race gas I use 0.035 for now on pump gas its a really small cc head its making a lot more power right off idle its a older cr250 stock was 0.060 I cut the head down 0.030 but im using a thicker base gaskets hopes to get it to rev out more to but I got to rejet for the top end then ill know. I awlays use 0.020 or less on chainsaws that I mod 100cc and under and pump gas
Or start getting metal to metal contact when the metals expand and that never ends well. Don't get greedy boys!
Good info and clear explanation, might be helpful to others to also explain the tuning of the powerband that can be accomplished by having more or less of the total area of the head dedicated to squish..i.e. larger percentage of area giving more low end and less helping with top end over rev
Roger Miller Thanks for the comment, I wish you'd all stop jumping the gun lol That's a subject for a later video and I'm building a mock up example model to make it easier for people to understand - thanks for the comment - matt
Also interested in your follow up videos on head design Matt. One thing I have really never seen is how the dome shape effects the loop scavenging flow. Thanks again, Frank.
Can anyone explain why there is a spark plug in the diagram? I believe CI engine does not have a spark plug
LOL its a spark ignition engine - hence the spark plug............
vespa px polini 177 spark plug hole threaded going try a picaso 177 head
What a crock! Running a tight squish band gap will not over pressurize the fresh charge that sits between the squish band and the piston and therefore drive that mixture into pre-ignition. The charge as an escape route into the main combustion chamber, that's the whole point of a squish band. Skimming the head in order to close the gap could push the CR up high enough to drive the engine into detonation if the combustion chamber bowl is not opened up in order to bring the CR down to a level that suits the octane rating of the fuel being used.Its a simple fact that if the squish gap is too big then the squish band does not function and further more the fresh charge that is 'hanging around' in the gap is not exposed to the flame front until well after TDC by which time it is too late for it to help produce any useful power, this is why the gap needs to be as small as possible in order to reduce the charge waste. The limiting factor here is the engine design and the RPM that it is expected to run at. If the engine has weak cases, a stretchy con-rod and a 'whippy' crank then this sort of stuff can stack up and cause the piston to hit the head, it won't hold together for very long if that happens.There are theories with regards to the gas velocity and this will be effected by the squish band gap, the width of the band and the angle that the squish band and piston form, possibly the idea is that if the velocity is too high there will be too much turbulence in the chamber but I think that this is beyond the scope of your video and is stretching my knowledge to the limit. (Even if that was true then this high velocity charge would still not detonate or be subject to pre-ignition).You talk about the squish band forcing the charge towards the plug and that is all very good but the primary reason for having a squish band is keep the engine away from detonation, it is the end gases that detonate AFTER the plug as fired but BEFORE the flame front has reach them and by forcing the end gases into the burning flame front they do not get a chance to detonate.
I sortof agree - plus that a good squish is also a little tapered/wedge-shaped, very little but it's there, the greater height towards the middle of the cylinder (this aids in aiming the squeezed charge into the actual combustion chamber). The outer region must be as low as you can make it (or dare to lol) for the factors mentioned above.
The squishband is polished/shiny also, and on a good squish you'll not see any carbon deposits (on the head) due to the cleaning effect the fast escaping charge has.
The cooling effect of both the head and the piston crown prevents detonation of the charge trapped in the squish, and theory is that when the piston descends the flamefront reaches the squish uniformly circular, and will burn this bit then too, aided by the then already very high pressure. This is also the reason why the sparkplug is best positioned dead-center, it in fact allows for the highest CR possible.
Oh, holes in 2T crowns are most often caused by a too-deep (warm/hot) sparkplug, the burning gas within the plug acting like a torch aimed at always the same spot. I'm old enough to remember the socalled surface plugs (noseless) which did away with this pest, but you needed an electronic ignition to drive them. Sure, detonation screws up anything, but this will likely not be in the middle of the crown but more towards the outer fringes.
Mind though, all this is for racing 2T's, probably not your average scooter you use to go buy beer with.
sidecarbod, you are correct. The description for deto in the video is way out.
Your last sentence is spot on, deto happens AFTER the plug has fired with the end gases self combusting due to the radiated heat from the combo of the head, liner and piston, which is pushed over the self ignition temp by the UV energy (all electric sparks produce UV, ask any TIG/MIG welder) from the ignition spark which reaches the end gases way before the flame front (speed of light vs. 80 m/s ) The idea of squish is to remove as much end gas as possible, because if there is no gas to deto, it wont deto.
Wow, thanks that made my day, easily the best youtube-comment I've found in like 1/2 a year of learning porting (for my chainsaws...have done 3 of them so far, though I consider all 3 'stage 1 of X' builds :P ) I replied to someone else the same way saying how I've seen guys w/ boat engines talknig about 'quench' and how their engines DO see piston-to-band touching (not desirable or anything, but more a 'it happens sometimes and isn't a big deal') SO, in my world, guys don't talk of compression ratios they simply measure squish with solder and go by the rules of thumb: 20thou tightest for 60cc saws, 25-30thou for up to 100cc saws. This is repeated to the point of dogma. But I'm newer to the scene and the one thing I cannot stop thinking is "These guys are never talking about how they got a lil piston-slap, would bet 95% of them did NOT lower their bands as far as they could have" (ergo they're wasting power, the charged air in the band area that doesn't get ignited until way too late to be of much help, especially in chainsaw engines where porting usually includes raised exhaust roofs)
I didn't know better and I put like 8deg of inward slope onto a squish band (54mm cyilnder for 92cc saw), was told the cylinder is now ruined -- do you concur with that? It seems 1-2 is the norm recommendation for slope but I can't get it through my head why you wouldn't wanta lil more slope (to a point...maybe my 8deg is past that point!)
Again thanks so much for putting that reply you did, I literally copied it to a word-doc in my "mods" folder lol :P
Wait, is this the guy who somehow things Bernoulli and Venturi's principles suddenly cease to exist inside a cylinder head? LOL
Ow - more bullshit comments - go on then... lets hear it
LOL, I thought a Squish Band is the rubber string of a G-String when you get your balls squished from it because you balls are hanging to low! LMAO!
This is pre-cussing ?
more vids like this
Cannot watch it. Getting that weird feelings in teeth due to that scratching sound of maker in paper.
Delete that nonsense!