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How does valve configuration effect a powerband?

T

Tom Wiley

Active member
Joined
May 23, 2006
So, lets say as an example, we have two nearly identical motors. Both 2 liters, both flow the same amount of air through the same motor (CR, Squish, Cam specs, etc) with the same turbo. How does the valve configuration effect the power band?

Peter Lissen's 8v racecar dyno



Ken Lanham's 2.6 16v turbo

It appears to me that the 8v has a much broader torque curve than the 16v. Granted that very well may be because of the cams, so the comparison may be apple to oranges.

What are your thoughts on the matter?
 
The valve configuration would have an effect on volumetric efficiency, which would affect the work required to flow a given quantity of air into the engine. Thus your original assumption that they flow the same amount of air would be a poor assumption.
 
BrickBaller said:
The valve configuration would have an effect on volumetric efficiency, which would affect the work required to flow a given quantity of air into the engine. Thus your original assumption that they flow the same amount of air would be a poor assumption.
Click to expand...

It is exactly what I am asking. Assuming for the example, that they flow the same, how would the powerband be affected?
 
If they flow the same at all RPM, and everything else is identical, there would be no difference. You're eliminating the key variable of how they flow at different RPM. You can't just assume that they flow the same, because they won't. The different flow rates at various RPMs is what would affect the power band.
 
how can you tell if the 8v is broader what are the rpms i don't see any just mph? whatever that is
 
increases in port volume and valve area typically* results in proportional decrease in gas speed vs flow.

Gas speed typically* has a significant effect on cylinder filling. Therefore a motor with a lower port volume/cross section/valve area will typically* have better VE at lower engine speeds where the pressure drops associated with that smaller cross section are not having a large effect on mass flow.

Therefore the ve curve tends to be fatter. The cost comes at the price of attainable maximum flow vs. pressure drop.

With a less restrictive port that has more volume, VE is poort at lower engine speeds to to low gas speed, but as flow increases gas speed does as well, and it will start to eat the head with smaller ports as the pressure drop across the smaller port increases and eventually negates it's gas speed advantage.


Obviously the name of the goal is getting the most flow at the highest gas speed possible.

That's why just hogging some ports out and jamming some huge valves in and getting some arbitrary monster flow number is largely a marketing ploy and meaningless - as higher flow numbers can actually equal lower VE.

* "typically" uses to denote horrible generalizations that may or may not be true in any given scenario.
 
The answer to your question has a lot to do with gas velocity and how it affects cylinder filling. In Two valve engines that velocity tends to be higher at points lower in the rev range (this has a benificial effect on torque production). Sure, that can be affected greatly by valve size, cam spes, etc...., but as a general rule, 2 valve engines tend to all be like this.

A lot of companies go through great lengths to try to regain some of that "bottom end" that 4 and 5 valve engines tend to be weak in. Baffles or special ports in the exhaust system, weird tricks in the plenum, dual runner setups, etc....

Note: Sorry Capt. We must have been posting near the same time.
 
both tq graphs start at 125 or so and end at 350 but the 16v one goes to over 500 foot lbs and the 8v doesn't come close. so how is the 8v broader?
 
burstcurse said:
both tq graphs start at 125 or so and end at 350 but the 16v one goes to over 500 foot lbs and the 8v doesn't come close. so how is the 8v broader?
Click to expand...

Because it flattens out instead of continuing to increase. My supposition would be that this is largely caused by boost and turbo choice.
 
Geeze its easy: the SHAPE of the camshaft determines the SHAPE of the powerband.

Your example needs to nail down several more variables.
Volvos 16v's 2 intakes vales are somewhere around 27% more valve area than a 44mm 8v intake

But the OEM 16v cams are lame ass little teardrop shaped things with extremely lazy opening rates and a tip so sharp you could prick your finger on it and draw blood.

But you should think about what happens once the new charge is in the cylinder and there the 16v ports and head design help immensely----if they admit enough air in the extremely limited column of air when the valve is open. (Not area under the valve but more the "volume" under the valve x 2)

In other words theres FLOW but there's thermal efficiency also.

A compact chamber right over the center of the piston with a central sparkie plug is extra good.

Then again theres the powerband's shape and there's the question of where that curve is on the scale, and again that's mostly independent of the valve being 8 or 16 but rather total area and camshaft's shape.

I gotta run down to the machine shop for some crap but more later.

e
 
its somewhat difficult to compare the two engines as they're both used to a fairly different end and this reflects on the design choices and ultimately what the power levels are..

Peter's car is strictly for road racing, as such he has it tuned for as broad a torque band as he can get (thats also an old dyno graph for his car)

Ken's car is a mix between a fun daily driver/occasional drag racer/occasional road racer.

off the top of my head, I *think* I know this much:
Peter's is a 2.5L, with a *lot* of headwork and a pretty serious cam
Ken's is a 2.6-2.7L with a more or less stock 16v head (I think..). I do not know if he had any special cams in it for that dyno or if he was using stock cams.

there are already some good 'generalizations' and good info so I won't add anything there (no reason to, all seems to be good info), but this particular comparison is a bit interesting.

I'll throw another one in the mix.. hockey 930's dyno graph with his 16v turbo looked an awful lot like Peter's and nothing like Ken's.
 
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