Obviously depends on the lobe shape but its does indicate something that many many people forget. There is an optimum amount of overlap in any tunned runner system exhuast and intake manifold and the head influences these as well. The problem is finding it. Runner length should be from the seat to the end of the tube on both sides intake/exhuast. Aparently in prostock applications they are moving away from overlap to get power up so there must be a very good reason.There has to be a point in any system where more overlap becomes less. But there are an absolute ton of factors.

 

I'm not aware of the valve timing trends in Prostock, but I do know a place
where I can verify this information from professionals in the Pro Stock engine
tuning business.

I'll see if I can get a direct response and post it with his permission.

 

again this will depend on the lobe sizes. But 30yrs ago prostock cams were in the 100-110 range from what i have been told. With advances in cylinder heads,Manifolds,headers exhuast and carbs and the newer lobe designs they have been getting wider and wider. that duration and RPM are much higher. Its just something a few yrs ago that made me go Hmm why did they do that. So i tried playing with wider LSA's to see if there was trend. It turns out that we started finding power on the smaller cube engines by going wider then everybody else was. The duration was going up but the overlap was being held constant. It was interesting to see the engine make alot more TQ and HP at the same RPM. there are in fact alot of vairiable and if this thread is going to distill down to best LSA it totally negates the conversation. there is no best overlap. There is Best overlap with each camshaft cylinder head header exhuast bore and stroke combination. What would be nice is to build a good mathmatical formula to determine what the best overlap would be for a given combination of parts.

 

That's what I'm talkin bout!

 

Yes, of course. If you increase overlap and hold EVO and IVC constant, then you will change duration. Otherwise, you adjusting many other variables all at the same time and can't tell which one affected power the most.

 

Ummm yeah.... there is a set overlap that every motor likes.

Pro Stock went wider LSA for a few reasons.... most of it is RPM. You need to close the intake valve, later and later in a PS motor due to the RPM range that they run in. 10,000rpm (or more) dicates a late IVC point, plus as AdrenilineZ mentioned this much duration means lots of overlap no matter what the LSA. PS heads also run canted valves, which will always widen your LSA, and extreme example is the Hemi, and a mild example is a BBC. One more thing that people never look at is the pressure differences between the cylinder and the intake port at the RPM they are turning... Now put the valve lift and duration that is needed to get the air moving into the cylinder and you can see that 130° of overlap can cause problems.... PS wants scavenging but those motors run more on inertia/wave tuning than anything else out there, and that means you have to get everything right.


As for the early EVO, it will always hurt power, but when you have a smaller exhaust valve to fit more intake valve in the head you just can't get enough time(duration) to clean out the cylinder unless you do this. Now balance this out with the rest of the valve events, and if you need time (duration) to empty the cylinder and you have to keep overlap down, your ECL needs to be higher and now the valve opens eariler. On top of that PS motors have very good burn, but very high leads relative to that burn due to the bore and stroke ratio and the shorter and shorter rods they are running. It's not as bad as F1, but it's not very good either. The burn still happens in the same place in the crank rotation.

Bret

 

Go back and read my post. I never said you could over-scavenge an LS1. That phenomenon only occured on a full-race effort Hemi engine. The guys building wedge-head motors have not been able to achieve over-scavenging. It may be possible on an LSX, but I really doubt you'll have the exotic combination of parts necessary for this to occur.

I suppose you could theoretically/mathematically model the overlap/scavenging process, but to do so with any semblance of accuracy would require a non-linear Finite Element Analysis and, for the effort required to do this, you might as well run some experiments, which have already been done and I posted the results earlier (how convient!).

Furthermore, some engines have been built with extra overlap added in by using a small lobe before the intake lobe and another one after the exhaust lobe. In other words, the intake valve starts opening during the end of the exhaust stroke, then closes a little to make room for the piston, then opens like normal. The exhaust valve is also opened more after TDC to increase overlap. At least in the test that I saw, this improved power. I imagine this isn't used more because a) most people get a poor enough idle without resorting to this and b) the valvetrain dynamics are likely outside the accepted reliability envelope that we normally operate in.

Mike

 

Yep the dynamics to run that at more than about 2000-3000rpm would pretty much make an impossible situation.

Bret

 

I have seen LSA changes from 114 to 116 pickup power. Obviously there are many facotrs in play. Saying gernerically however that wedge heads can't overscavenge is fairly rediculous. There could be more to the 114 to 116 scenario for which i can't give out cam specs could have alot more to do with intake then exhuast dynamics. But making a genral statment that More overlap is always better is ignorant.

 

Ummmm I was talking about the double lobe deal, not LSA when I said that to Mike?

Bret

 

Well I'm glad we had this conversation then. I can't wait to relay this information to the 2 said highly competitive and record-holding engine builders so they can start making real power. . . And, yes, your example of changing LSA from 114 to 116 and gaining power is almost totally irrelevant because of the changes to IVC and EVO.

 

So Mike's theory of max overlap seems a little controversial, but I don't see anyone arguing against as much exhaust duration as possible. Which brings me to an obvious question.

What's the latest you would want an EVC?

 

right before it hits the piston!

 

From the man's finger tips:

 

It changes the behavior of the camshaft completely. But you can just as easily overscavenge a wedge head as you can a Hemi head. In fact it might be even easier to overscavenge a wedge head given the fact that the valve are right next to each other. With a very good header/collector you could easily siphon out a portion of the incoming intake charge. look at the valve placement. My personal opinion is that a canted valve engine is less sensitive to overlap then a wedge becuase a canted valve engine moves the valve seat surfaces away from each other.

 

Well yes they are runing 65 degree of overlap but they have also realized that A there is an optimum number for a specific intake exhuast combo B that to achive that optimum number they have to move the lobe forward or backward.

 

I would argue against too much exhaust duration...

Bret

 

You don't know how scary that is right there..... those specs make sense for a 10,000-10,500rpm max PS motor.

Bret

 

You don't have to get defensive, I'm just supplying information about an
engine class I have little knowledge about.

The only thing I'm disputing is the direction of overlap toward a decreasing
value. Overlap is critical for any N/A engine. The amount of overlap doesn't
concern me.

I think many people here are getting too wound up on the exact figures, instead
of understanding the basics. I personally don't give a $hit if my overlap is
a degree off, or my IVO is not optimized. I had a quarter mile goal in mind
and I've achieved that goal. That makes me happy and the fact that I
can visualize and understand the effect of a later IVC or a tighter LSA using
similar lobes.

This discussion has gone from street engine to Prostock overnight and it's
totally confusing the conversation as far as I'm concerned. Nobody is going
to get the perfect cam numbers from this thread, or any other thread for that
fact. What's right for Giga's engine, is not right for your engine, or Darin's
engine. Even from city to city, and season to season those specs are going to change.

 

Just wondering what is the conversation about. Its about Overlap and examples of such should be talked about prostock or otherwise. An engine is an engine and has to be thought of as a complete system.