Exhaust Valve Lift: How Much is Too Much, and Why?

[MadBill]

A trend has been developing over the last few years away from EVL often being greater than IVL. (although it's still seen in many catalog cams, mostly in split duration applications) In fact for custom grinds, it is now sometimes up to 0.100" less. Work I have done with the Dynomation computer simulation backs this up, as does data I have seen from The Engine Analyser. Allan Lockheed, who created The Engine Expert program, told me about a Super Modified racer he was helping at the track. Allan recommended less EVL, so the guy switched from a 1.7 to a 1.5 ratio and gained so much power he had to change gearing to stay off the limiter. Back at the shop, he found the engine had picked up 40 HP vs. the pre-weekend numbers.
So here's the question: What are you guys seeing in this regard, and if there is a case for limiting EVL, what are the theories re the underlying reasons?

[SStrokerAce]

I'd like to see more of your data on this...

I think it really depends on the cam design. It's esentially lowering the exhaust duration and opening the exhaust valve later. To me that seems like the reasons why it works so well.

If you are going stupid high lift on the intake it doesn't make sense to do the same thing on the exhaust. The intake valve wants the max lift at the max port depression and velocity, so it makes sense to give it more there. The exhaust on the other hand has peak pressure at low lifts when the valve is open so high lift exhaust flow really doesn't do too much for you.

Bret

[bill jones]

-I think there is several reasons for having too much exhaust lift.
-1 is that with common SBC and BBC flat tappet type cams is that duration and lift seem to need to go with each other.
-An BBC example is if you want a low lift at the valve for a common street driven engines say .480" max lift but you want 230 degrees of duration at .050" you are likely to get at least about .530" lift or more "just because that's the way it is".
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-Of the applications I've seen---mostly SBC's with mildy ported heads you can get pretty good exhaust flow real easy up to about .600" lift and the relation of the intake to exhaust flow get close to .85 or .90 meaning the exhaust flows 85 to 90% of what the intake flows.
-So you wouldn't need a lot of exhaust lift in that sort of situation.
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-I have basically never seen a SBC ever really respond to high exhaust lift yet with a flattappet cam you can still see more power with intake rockers up to 1.75:1---and you don't get anything worth talking about on the exhaust side.
-I have seen cars virtually always increase in speed with more intake lift and just leave the exhaust alone.
-We have seen some restricted sbc oval track cars actually pleasantly surprise us when we tried some low ratio 1.3:1 exhaust rockers for cam break-in purposes and left them on for a race or two---then ended up running them the rest of the year.
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-Since the exhaust opens under serious pressure that the port probably hits peak valve curtain flow and then peak port flow from EVO to BDC and may even stay there well past BDC---so exhaust port speed is always high compared to the intake having to be sucked in under with less than 15psia in a normally aspirated engine.
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-There are two totally different scenarios happening between the intake and the exhaust ports and we tend to forget about the high pressure that the exhaust has to play with.
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-Roller cams can be made to get the exhaust valve open to the max total curtain area so quick that maybe the cam companies are seeing that why put stress into the exhaust spring with all that high lift when we reallyt don't see an power benefit from it.
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-Maybe this isn't a very good explanation but Larry and Darin are likely to set us straight about the subject.

[Rick360]

I am no expert and this is just my theory as to why this works.

Exhaust velocity is only useful at one point in the cycle, the end. If you lift the valve high for better flow when the piston isn't even half way up, you will lose velocity at the end of the stroke to help fully evacuate the cylinder.

Getting the ex valve opened at the right time to blow down enough pressure before the piston starts rising in the cylinder and just open enough to to keep pumping losses down as the piston rises, leaving enough velocity at the end of the stroke to be useful in savenging.

Rick

[MadBill]

I'd like to see more of your data on this...Bret

On a 364 road race SB2.2 for which I just completed the Dynomation simulations, power rose by 40 HP going from 0.700" to 0.750" IVL. It continued to rise past 0.800", but with ever smaller gains. (we'll likely settle on something in between, to maintain some spring life) On the exhaust however, power peaked at 0.625" and steadily dropped off with both higher and lower lifts.
Similarly, a 7.0 L. LS2 with ET C5R heads, Harrop IR manifold, Burns collectors, etc. showed rapid gains up to 0.750" IVL, but again, power maxed out with 0.630" EVL.
Even a 2.0 L Alfa Romeo twin cam 'wanted' 13 + mm IVL, but no more than ~ 9 for the exhaust.(in this case a trend mirrored by the more recent factory cams)
A lot of good data and thoughts presented here! I think "the sky's the limit" IVL situation makes sense, based on the comments re flow/lift requirements at max piston velocity, but although the case is developing for why an engine doesn't need more than 'X' EVL, I haven't heard anything yet to explain why it actually chokes if you give it too much.

[SWR]

Another engine that in stock form run this cam setup,is the SAAB 900 Turbo 2-litre... it has a "turbo cam" on the intake,i.e. high lift and fairly short duration,but it has very little lift but loads of duration on the exhaust,when compared to "normal" turbo grinds.

A stock engine makes 12 psi or so at 1800 rpm....and if you tune it with more boost you'll break the gearbox at 250-ish hp. Put a "normal" high-lift,short-duration turbo grind on the exhaust,and you'll have spool about 1000 - 1200 rpms later,and you can get 400+ hp from it with no 'box problems,because it has moved the torque/spool WAY up in the rpm's... Turbo engines need addressing exhaust velocity more than huge flow,just like NA exhaust's do.

[SWB]

If you look at L/D ratio for both inlet and exhaust it's obvious that the exhaust is usually getting lifted far higher than the inlet relative to the diameter. That could have something to do with it.

Usually too you see the longer exhaust period is just an inlet profile with 10 degrees more duration. That's easier for the cam company to produce.

SWB

[dbusch]

exactly the things i have seen on my 398 SBF.

Also, one time i tried an aggressive ex profile vs. the high seat duration lobe i had been using. the engine slowed down SIGNIFICANTLY at the track. i am talking about 1 tenth in the 1/8th mile! goes to show the ex side wants the exact opposite than the intake side. little lift and a ton of duration via a nice slow ramp. i am tempted to take more rocker off the ex side on my current cam to see what happens...

[beth]

"I haven't heard anything yet to explain why it actually chokes if you give it too much."


I think it has more to do with exhaust gas inertia timing rather than choking.

beth

[shawn]

"I think it has more to do with exhaust gas inertia timing rather than choking. "


I think i'm missing it. Could you explain what the lift has to do with intertia timing?
thanks,
shawn[/quote]

[beth]

I think It is possible to overlift the exhaust valve causing over exhausting of gasses too soon in the exhaut cycle so the remaining gasses don't have enough inertia to drawn down and refill the cylinder as well as they should later in the cycle.


beth

[MadBill]

Hmmm.. I have heard it claimed that the piston needs to just 'chase' the exhaust up the cylinder, too much pressure causing excessive pumping losses. Perhaps too little pressure saps its momentum as you suggest and degrades its scavenging capability.
Must study the exhaust velocity graphs in Dynomation and see if this is detectable.
Maybe if, by way of a test, I open the valve way late the simulation will want more lift to compensate...

[shawn]

I think It is possible to overlift the exhaust valve causing over exhausting of gasses too soon in the exhaut cycle so the remaining gasses don't have enough inertia to drawn down and refill the cylinder as well as they should later in the cycle.

doesn't that have to do more with opening/closing times and ramp speed than actual lift?

shawn

[beth]

I don't know which parameter is most important but lift matters IMO as long as the head flow increases with more lift.



beth

[Erland Cox]

I see this playing with my Dynomation. This thread is a few years old, what is your thought today?
I have tried every single parameter but only lowering the exhaust valve lift will lower the cylinder pressure at EVO.
Just as Mad Bill writes there is a maximum exhaust lift, higher or lower is worse.
Cam timing, port size headers, nothing else helps.

Erland