Oversized valves: good or bad?

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daliant

daliant

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It seems like every performace s6 build uses oversized valves, my theory on the subject (just a theory) is that big valves in a slant might actually hinder performance due to the small bore size. Even with a bore notch to unshroud the valve 1/3 of the valve is crammed up against the cyl wall blocking air flow. Also the bigger the valve the more of an obstuction it becomes to air flow into the cyl.
Has anyone ever put a slant 6 head on a flow bench and did a before and after test with stock valves vs oversized?
What are your thoughts on this?
 
daliant said:
It seems like every performace s6 build uses oversized valves, my theory on the subject (just a theory) is that big valves in a slant might actually hinder performance due to the small bore size. Even with a bore notch to unshroud the valve 1/3 of the valve is crammed up against the cyl wall blocking air flow. Also the bigger the valve the more of an obstuction it becomes to air flow into the cyl.
Has anyone ever put a slant 6 head on a flow bench and did a before and after test with stock valves vs oversized?
What are your thoughts on this?
Click to expand...

I am fascinated by this subject, and I can see it going either way, frankly.

I am also wondering if the elevated cylinder pressures in a forced induction motor might behave differently from a normally-aspirated motor under these circumstances.

Veddy interesting!!!!


Thanks for bringing it up!
 
Bill Dedman said:
I am fascinated by this subject, and I can see it going either way, frankly.

I am also wondering if the elevated cylinder pressures in a forced induction motor might behave differently from a normally-aspirated motor under these circumstances.
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I'm interested as well and would like to see results of test OP proposed.

IMO, a forced induction motor would have to behave differently than a naturally aspirated one. At a given rpm, there are X lbs of boost that is going to try to fill the cylinder as soon as a valve is cracked open. A naturally aspirated engine on the other hand will gain volume drawn in as piston approaches half-way down and will slow down its draw as the piston approaches BDC of the stroke.

This is assuming valve timing remains constant. You know better than I how a cam that is optimized for forced induction is different than one for a naturally aspirated engine.
 
The same "good or bad" arguement can be made for anything. 12:1 compression pistons are "bad" if you wanna run pump fuel. A cam with .260* [email protected]" is "bad" without other things that make it work well. A loose converter is "bad" without deep gears. Likewise, without good flowing headers, a well thought out intake and carburetor and maybe even some head porting big valves can be "bad" too. But all that stuff that can be "bad" can be just as "good" if used with the correct combination of parts. Are big valves "bad" as a stand alone modification in an otherwise stone stock engine? Probably, but I don't know anybody who would do that .....unless perhaps in a turbo application. Then it might have some merit. It's just like the car as a whole. If the induvidual parts don't match the overall combination, anything can be "bad".
 
Oversize valves are somewhat useless unless the head is properly ported to take advantage of them. Indeed, some mild porting, particularly in the pockets, will help the stock valves emmensly in both naturally asperiated and forced induction. What 2 Darts mentioned about forced induction is true, but mild porting will help here as well.
 
Yes OS valves in a number of configuration have been flow tested. IN A WELL PORTED head with proper induction, exhaust, and bore the increase is dramatic. For a STOCK engine and drive train its not so.
Frank
 
baseline flow of stock intake valve 106 cfm with a 1.70 intake valve it was 130cfm 23% gain

the baseine flow of the stock exhaust valve was 77 cfm with a 1.44 valve was 103 cfm 34% gain
numbers taken from the Mopar performance six cylinder engnes manual. valve seats were enlarged( of course ) and blended
 
Think of an engine as a well-oiled sewing machine.Carb has to be correct for; compression and cam. Intake the same. Cam has to be degreed for what the rpm is. Then, exhaust has to be tuned. And we haven't talked about piston weight, balancing, dissy, timing, etc.
It all works together when correct. this is how national records are won.
 
one thing that oversize valves offer is bring the valve job back to the top...

after several valve jobs ...the valves are usually sunk...the larger valves bring them back to the top of the seat....
 
66aCUDA said:
Yes OS valves in a number of configuration have been flow tested. IN A WELL PORTED head with proper induction, exhaust, and bore the increase is dramatic. For a STOCK engine and drive train its not so.
Frank
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How about on a turbo motor?
 
Like was said.. If the only change is the size of the valves, there will be a small gain due to the fresh seat and the seat being moved out to the edge of the bowl. There will also be a very small gain in very low-lift flow. How much and for how far in terms of lift that gain helps is a function of the intake system that feeds it (which includes the head port, the intake manifold port, the plenum, and the carb) plus the rate of lift of the cam. I always go oversize because of those gains. There is generally no percieved loss by going slightly larger unless you go too large on the port, intake, carb, or cam, in which case the engine will still make more power, but the rpm range of that peak will move up to where the intake system is at peak efficiency. I say percieved loss because the bench might show it, but with the rest of the components a dyno doesnt seem to note any loss.
In terms of a boosted engine, the idea is bigger is always better. At least in my circles which are primarilly V8. Boost is only a reading of resistance to flow. You're measuring how hard it is to ram air into the engine. So a ported head at 5psi will move a bit more air than an unported head at 8psi. You'll also really need to examine the turbo you want to use if porting is being done. Because it's pretty easy to undersize the turbo's compressor side when the airflow of the intake side is increased by 15-25%.
 
If it is a stock that wont see north of .550 lift, 30 degree face (not 45) at valve and head helps a lot in low lift situations, ie stock. Seems new motors are already going in that direction.
 
moper said:
Like was said.. If the only change is the size of the valves, there will be a small gain due to the fresh seat and the seat being moved out to the edge of the bowl. There will also be a very small gain in very low-lift flow. How much and for how far in terms of lift that gain helps is a function of the intake system that feeds it (which includes the head port, the intake manifold port, the plenum, and the carb) plus the rate of lift of the cam. I always go oversize because of those gains. There is generally no percieved loss by going slightly larger unless you go too large on the port, intake, carb, or cam, in which case the engine will still make more power, but the rpm range of that peak will move up to where the intake system is at peak efficiency. I say percieved loss because the bench might show it, but with the rest of the components a dyno doesnt seem to note any loss.
In terms of a boosted engine, the idea is bigger is always better. At least in my circles which are primarilly V8. Boost is only a reading of resistance to flow. You're measuring how hard it is to ram air into the engine. So a ported head at 5psi will move a bit more air than an unported head at 8psi. You'll also really need to examine the turbo you want to use if porting is being done. Because it's pretty easy to undersize the turbo's compressor side when the airflow of the intake side is increased by 15-25%.
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Thanks, moper. 'preciate the good info!!!:read2:
 
One thing everyone just have to keep in mind - the stock induction , head and exhaust system was designed for the 170cid slant when they stroked it to 225 they changed the jetting and that was about it
the 1.7/1.44 inch valve sets is what the 225 needs to breathe if you combine it with a decent valve angle job, bowl blending and gasket matching job.
An take it from me - if you match up the inlet - 2bbl minimum, and sort out the exhaust clean up the stock manifold - that combo with the same cam and cr is worth 30bhp and 35lbft
 
StrokerScamp said:
The same "good or bad" arguement can be made for anything. 12:1 compression pistons are "bad" if you wanna run pump fuel. A cam with .260* [email protected]" is "bad" without other things that make it work well. A loose converter is "bad" without deep gears.
Click to expand...

Disagree, I have seen several cars with high way gears and loose converters some of the funnest street cars to drive, just ask gmachinedart

green1 said:
Think of an engine as a well-oiled sewing machine.Carb has to be correct for; compression and cam. Intake the same. Cam has to be degreed for what the rpm is. Then, exhaust has to be tuned. And we haven't talked about piston weight, balancing, dissy, timing, etc.
It all works together when correct. this is how national records are won.
Click to expand...

Worst analogy I have heard in my life, when did sewing machines get engine parts?

Can some one explain why hp 383s came with large port heads and 2.14 valves but when you have a 360 or 408 it is automatic for someone to say 2.05 valves are to large? Seemed to work out ok for the 383 .

I think this is another case of people reading a lot of posts and thinking they know what they are talking about, but..but everyone online says so, overanalyzing "uncomprehended" theories again.

I agree that a good way to compensate for a sunken seat is to oversize the valve itself, but as far as "flow" the bowl will need to be opened up proportional to larger valve installed (or at least faired into the seat), if not you just put a larger wall in front of the incoming air.

Bottom line, until you try it, you won't know. Try it dude.
 
fcm42000 said:
Disagree, I have seen several cars with high way gears and loose converters some of the funnest street cars to drive, just ask gmachinedart
Click to expand...

Yeah? So? Just because you disagree doesn't mean you're right. A loose converter with highway gears (let's say 2.94) will never acheive lockup on the street. That will lead to higher than needed heat in an automatic transmission and it will eat up a lot of useful horse power. It's just an "incorrect" combination, whether you want to "agree" or not makes no difference. You're just wrong.




fcm42000 said:
Can some one explain why hp 383s came with large port heads and 2.14 valves but when you have a 360 or 408 it is automatic for someone to say 2.05 valves are to large? Seemed to work out ok for the 383 .
Click to expand...

Different port sizes and shapes plus years of experience from veteran racers and engineers like....oh i don't know......Larry Shepard?

fcm42000 said:
I think this is another case of people reading a lot of posts and thinking they know what they are talking about, but..but everyone online says so, overanalyzing "uncomprehended" theories again.
Click to expand...

I agree, so stop doin it.

fcm42000 said:
I agree that a good way to compensate for a sunken seat is to oversize the valve itself, but as far as "flow" the bowl will need to be opened up proportional to larger valve installed (or at least faired into the seat), if not you just put a larger wall in front of the incoming air.

Bottom line, until you try it, you won't know. Try it dude.
Click to expand...

Pretty much the only factual statement in your cute little rant.
 
green1 said:
Think of an engine as a well-oiled sewing machine.Carb has to be correct for; compression and cam. Intake the same. Cam has to be degreed for what the rpm is. Then, exhaust has to be tuned. And we haven't talked about piston weight, balancing, dissy, timing, etc.
It all works together when correct. this is how national records are won.
Click to expand...

I think your sewin machine analogy is a good one....whether "other folk" think so or not. lol
 
fcm42000 said:
Can some one explain why hp 383s came with large port heads and 2.14 valves but when you have a 360 or 408 it is automatic for someone to say 2.05 valves are to large? Seemed to work out ok for the 383 .

.
Click to expand...
The 383 works well with a 2.14" valve because of the bore size, its .250 larger than a 360 so there is more room for a bigger valve.
 
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