Under Cut Stems VS Back Cuts (Flow Bench Test) Vortec Heads
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- เผยแพร่เมื่อ 27 ต.ค. 2021
- Vortec Intake valve testing
Do Under Cut "Skinny" Stems add Flow? Or just a reduction in valve train weight?
What about back cutting the valves?
Here are some flow bench test results you may find interesting
Flow testing done at 28" 4.030 Bore with stock style 3 angle valve job
forgot to mention in the video
The backcuts are approx .050 wide
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Very smart to make a special valve to test a theory. That’s hot rodding at its best.
Heck yeah!
AWESOME TECH, as usual!!!!
THANK YOU!!!
Thank you for another awesome video! Always referring to these for help. Looking forward to the next one.
Buy a couple guides from afr and an intake and exhaust valve. You can get the 2.02 intake valve from their 180 eliminator and the exhaust valve they use in that head. Probably less than 80-90 bucks. Be interesting to see for testing purposes, they are 8mm stems and backcut. Keep in mind it's not just about flow, by lightening the valve train you can get more rpm before valve float with a lighter spring which is a total win win situation
I was sitting here thinking I'd like to see content on testing of back cuts , your vid came up first and glad it did .
Great work and documentation!
YAY! its Pat back in the shop :)
Great info..thank you for sharing!
the stem encourages directional change before the valve. And, that’s important before the valve parts the air into many different directions
I have some observations to add 🙂Sometimes a flowbench will mask what really happens in a running engine. Particularly with wet flow, although you may lose a few cfm with a backcut, the vaporisation ridge improves atomisation and will gain you power even if the flowbench says otherwise.
Also, if the engine is undervalved. Then increasing the Min area at the throat with the undercutting will improve power as the cfm deficiency increases with rpm. Same goes for throat %, what works on a suitably valved combo won't if it's severely undervalved. Was the port minx at the pushrod pinch or the throat?
Awesome vid, love these discussions. Thanks for taking the time 🙂
I've heard from bow tie owners talking about necking down valve stems and I always ask why and they say better flow. I'm like ok what ever suits you. I am mopar and the mopar engine bible says to grind off for the back cut. My wonder is if they say do it, why isn't it factory. But it does help plain and simple.
Very informative video! I've been trying to find some good info on the aluminum version of the vortec fast burn heads, but its been difficult to find. I'm assuming the info you gave on the steel version would also apply to the aluminum GM performance heads also. I'll need to freshen up the set I have on a ZZ383 soon, and I'm trying to decide if I'm going to keep them, or spend some more money and go with a set of AFR's. Not sure at this point if the HP to money ratio would be worth making the switch. Keep up the great video's!
I haven't done any porting on the aluminum fast burns but they are a good head!
But if you do switch up those heads, shoot me a email if you want to sell those fastburn's haha
Pcperformance1@outlook.com
@@cuttersperformance Will keep you in mind! Keep up the great work!
To break the dyno that thing turned out great!
It did make good power but just a fluke that that joint broke haha this dyno has had some powerful engine on it
Stock forged rods with ARP bolts, 6,500 RPM should be no problem. Those aren't sbb cast iron rods or bb Caddy cast iron rods, those are forged sbc factory rods. Even with the factory bolts, 6,000 RPM shouldn't be a problem. Those old sbc 302 engines were over revved regularly to 7,000 RPM back in the day.
@@itseithergonnaworkoritaint7852 haha thats what said too. I get it though...they dont want issues
@@cuttersperformance
Yeah but your sbc broke it, you'll always have those bragging rights because it broke when your engine was on it. LOL
@It's Either Gonna Work Or It Ain't Garage i made that comment. I dont think he liked that haha 😆 they are drag racers in their 70s. Legends, good guys
did you ever chuck your valves in a drill press with a thin wall guide liner to protect the stem.. swirl polish the back of the valve and the 30 degree back cut.. come back and lightly grind the 45 to verify you have not damaged the sealing area..
about 25 years ago. somebody did up a super speedway head.. that had a wedge shape bowl .. so there was less area when the valve was closed to build up volume which means its more to restart flow as the valve opens for the next intake event.. this created a high pressure do to the ram effect at the far side of the bowl... with less air to restart.. it did not flow as good on a conventional flow bench.. but on a dyno.. it made more top end power.. i would love to check them on a spin tron.. but i have no use for a 6500 RPM+ race engine currently..
what the heck am i talking about.. with the intake valve closed 3/4 of the time.. the end of the airflow is blocked.. if you have less volume to restart flow it should make better power when you are up in the RPMs and really have the ram effect going on..
i miss the old shop i worked in back in the 80s and 90s.. it was open 24-7... had a massive 10HP compressor that allowed me hours and hours of fun in the evenings with my die grinders and heads.. i still dabble around with several electric die grinders once in a while..
To properly swirl polish an 8 intake valve set you need a lathe or a reversible drill mounted in a vise because 4 of the valves need to be spun in the opposite direction.
Air never stops flowing in the port, past the valve yes but i think it behaves more like a high frequency pulse wave with frequency being rpm dependant.
Your wedge bowl design sounds like an attempt to tune the port to a specific rpm range, a super speedway motor would be the ideal environment where rpm doesn't change a whole lot, sounds like something Smokey Yunick would have been playing with.
I think small block heads in general and perhaps the vortec more so tend to flow more air higher up in the port, so yeah a necked down stem would be under that stream so to speak and not really aiding flow.
My experience with back cutting the intakes on vortec is that it preferred an angle to a radius, some people say just throw a radius on there but that didn't work for me.
I found the same. I was even trying double back cuts, and all different angles
Also forgot to mention in the video, i found a .050" wide backcut worked the best overall
Has to do with fuel shear. A radius doesn't promote as much shear as multiple angles. Darin Morgan talks about it in his vids on wetflow testing.
@@b.c4066 Fuel shear is definitely something to consider when chasing flow numbers on a flow bench ... for me im looking more at what effect velocity change is going to have in various areas to charge density, if thats gonna cause fall out.....
@@luckyPiston I know just about enough that I can kind of grasp some of the basic concepts, and to listen to the experts that get results. All of them say a radius is not a good thing for intake valves or seats, at least within the context of a 23° sbc intake port/seat/valve. Wish I had more time and access to a pro, I would give my time and labor in return for being taught how to port heads, it fascinates me.
@@b.c4066 You can build a flowbench, mine uses leaf blowers mounted to a 45 gallon drum surge tank, i use an actual venturi instead of a restrictor plate and even went digital readout , when its going full tilt it sounds like a jet taking off , LOL but you can keep it simple too, i saw a kit at one time that used a shop vacuum.
Yup like electricity air likes to take the path of least resistance, yes trying to figure out something you cant see traveling at high velocity, what it's doing , pretty fascinating.
Omg God I became confused! Which one hade the better result?
Hey I have a pair of vortec 906 heads and I wanted to know if the valves have to be all the same height on the top where the rocker arm sits on them there was work done to them for a 280H cam 230/230 things done
Degrease / hot tanked
Magna-flux /pressure test
Installed hardens spiral guides
Machined guides down for .600 lift
Grind int seats to 2.02
Grind exh seats to 1.060
Both 3 angle grind
Surface cylinder heads
Machine guides for .500 posi valve seals
Assemble both cylinder heads with 2.02 /1.060 -new stainless steel valves
So after this was all done I got a local shop to install them on my 96 vortec engine in my 96 chevy Silverado it ran amazing till one of the cheap rocker arms that were put on slipped off one of the exh valves on number 2 cylinder and damaged the valve so I took out the valve and took the head to another local head shop and the guy noticed the Int and exh valves are at a different height and said it shouldn’t be like that ,my question is ,it that true even with the work that was done because it ran good before this happened and he said it shouldn’t have ran good at all ,hopefully you can give me some insight , I don’t want to spend more money again
Seems like there’s one more variable besides back cut angle-how close does the back cut come to the actual seat location on the valve? Really interesting that there’s a big difference in angle effect between stock bowls and pocket ported. Thanks!
I totally forgot to mention in the video
What i found is a .050 wide backcut seem to work the best. Tried going wider and bring it right to the sealing edge, and also going very slight.
.050" wide seemed to be the overall best
It has been shown that a venturi shape leading up to the seat is ideal with a poppet type valve, generally speaking pocket or bowl porting attempts to create that shape and back cutting the intake is tuning the main obstruction to air exiting that venturi.
I Just tore apart my 906 heads and mine looked like they were back cut from the factory?.....id love to show you a pic. I also had some other questions for you if a an email would be appropriate?
@@AR_420 yes thats correct, like I say in the video the factory valves are cast with a back cut (but not a 30 deg back cut) a 20 to 22 deg back cut is more like the factory valve
@@cuttersperformance my apologies. I missed it. Haven't had a chance to re-watch it. I also had a pic of my intake gasket to port match. Wanted your advice if at all possible. Thank you for your time either way. 😊
What do undercut stems do on the normal 70s 80s iron heads 487, 993, 882, and 305 416 heads I won't get into all the numbers if they actually help or hurt flow? Been wanting to know. The backcuts are worth it.
Putting a set of vortec heads on my small block 350 in my 77 chevy will the vortec intake and exaust valves work with my application
As in to reuse the stock valves?
Hey I bought a spring kit from you last year I was wondering if I could get ahold of you and see if I could get some more?
Unfortunately i am currently back ordered on my kits
Sorry man!
Interesting. My machinist wanted me to try 8mm stems from an ls. This kinda makes seem like an unattractive cost haha
The only real benefit is a lighter valve train
I did it for the lighter valve train since my valve springs for my 496 were so heavy. Plus I found the stuff on deal
So I have a set of vortec heads. You are saying that larger 1.6 exh valve is the way to go. I have a basic seat grinder can I open the factory seats to the 1.6 valve or do I need to change seats?
A 1.6 valve is the way to go, but only if your porting or atleast blending the bowls. Adding a 1.6 exhaust valve without blending or porting will do nothing or sometimes hurt flow.
The factory seats can be cut for the 1.6 valve
@@cuttersperformance
I was going to unshroud the valve's, pocket port and clean up the exhaust. I really liked your video's very helpful. I'm not new to engine building but this is my first sbc it's for a 88 iroc. Done tons of ford and Mopar tho
Is that Toronto motorsports park in the intro?
Good eye!
@@cuttersperformance nice! I've been watching your videos alot lately and it's really helping me out with my build, didn't realize you were local to me
Can you back cut used valves?
Yes sir!
Could I send you a set of vortec heads to be reworked? Thank you
Sorry man but I actually only do vortecs for engines im building. I find its not worth the costs to the customer, by the time its done and shipped back its more than a set of aftermarket heads typically
Splitting hair