Apparently, it's the overlap when both intake and exhaust valves are open at the same time that causes the problem. If the intake to exhaust cam timing could be changed to close that overlap it should make a difference. We can't do that. Sterling Gee can do it with his Variocam setup. Time for a SC Sterling so we can test this theory.

 

Is there any source for custom ground cams to eliminate/reduce overlap?

 

I have seen considerable horsepower increases in supercharged cars with our crossover, personal experience as well as customers. Tony showed a significant drop in boost but ran a quicker quarter mile. Increased exhaust flow will reduce the amount of boost due to efficiency but will increase horsepower per pound of boost at the same time. Granted these were on S4 cams which do not have as much overlap as the 85/86' cams.

The 89' that I built is running 545rwhp with only 12.5psi of boost. Prior to supercharging it did 267rwhp with stock exhaust and 460rwhp with 10psi. Changing to the crossover and adding 2.5psi of boost got us to a safe 545rwhp. There is no way that 2.5 psi would yield a gain of 85rwhp alone. We are seeing approx. 22-23 horsepower per pound of boost with a crossover whereas we were getting 20-21 horsepower per pound of boost with stock exhaust.

Not disputing other's ideas or experience, just stating my own results.

 

Valve overlap? Is there any on an S2-S4? My understanding is that at no time are the exhaust and intake valves open at the same time. You can pressurize the intake and it should hold.

 

I think all the intake valves are closed at 45 deg BTDC. The valve timing info in the WSM is with 1mm of valve lift and it does show overlap even at 1mm. I did some EAP work and made some screen prints of the results. These are showing a NA engine with stock exhaust and a full header type exhaust vs. the same engine but boosted at 10psi and the same exhausts. Don't take these numbers as absolutes, but as indicators. Engine operation is a quite complex balance of attributes right from the air intake to the end of the tailpipe. These examples are only an indication of possibilities using two differences. The main point is that exhaust changes that may yield X% on a NA engine don't necessarily yield even close to the same % increase on a boosted engine.

https://rennlist.com/forums/attachme...d=179403&stc=1
Header diff NA.jpg
https://rennlist.com/forums/attachme...d=179404&stc=1
Header diff boosted.jpg

 

OK so what happens when you put a restrictive exhaust driven air compressor on the pipes ? suddenly you have back pressure !

 

Well, yeah but turbos are reported to have 2 to 3 times as much back pressure as the boost pressure. The valve overlap will kill you there too with reverse flow of exhaust going back into the intake during the overlap period. Of course we all know turbos work even with cams designed for NA engines so it isn't an immediate killer. Not sure how a turbo at 20 psi boost would work with cams designed for NA though. probably some loss in efficiency. It would be like a EGR with quite a percentage of exhaust gas left in the chamber.

 

Hi Dave,
I think we are at the point of agreeing that the stock manifolds with 2.5" X crossover is good whether boosted or not. Have you went further beyond that with increased exhaust flow to see if it helped appreciably on boosted engines? Does a 2.5" rear exhaust help boosted engines over the stock rear exhaust? How about headers instead of the stock manifolds? On a 545hp NA engine, you'd want (need) headers and something like a 3" dual exhaust all the way back. How much improvement would that type system make over your stock manifolds and X crossover on your engine? That's what we're trying to figure out.

 

Hi Sterling,
I'm finishing an ITB system now for a guy in Canada who'll put it on a 7.0L, dual turbo, 928. Not sure how he plans to keep the rest of it together.

 

I guess I don't get the overlap. You can pressurize a manifold that has no seal leaks at ANY crank angle and it should hold. Cylinders with the exhaust valves open will have closed intake valves and vice versa. Maybe there is no physical overlap, but there are dynamics of airflow that would account for these findings. Maybe it is different with GT cams.

 

Hey Louie,
I understand what you are discussing, just throwing in my experience. The crossover/supercharger performance gains will not be proportional to the n/a crossover gains but they do definately help. I believe it was worth 40ish rwhp on the supercharged 89' over the stock cats which is higher than any n/a gains we have seen. (It is cat equipped btw)

I have yet to see if headers or a 2.5" rear system would be any additional help. I do know that they are not as critical to power production as they would be on a n/a engine but I'm pretty sure that they will not hurt peak performance as was suggested. A dual 3" exhaust on a supercharged car that is normally not on boost will kill low end/mid range power, that's a given.
The balance of drivability when off boost to maximum peak horsepower is a fine line and different for every combination. GT's seem to respond better than S4's to supercharging as well as 85/86's primarily due to the cams as far as I can tell. Opening up their exhausts may not see the gains an S4 will but only to a point. I believe their balance point will be lower/smaller than the S4 cams. I would suggest maximizing exhaust flow to the limit of the supercharged engine when off boost, maybe just a touch on the overkill side.

If you are just trying to post a maximum peak power level, then by all means size the exhaust to the engine under full boost. Your low/midrange driveability will suffer but you will have a big number to fall back on.

I would love to have the time/energy/money to find out where the point of diminishing returns is and develop an exhaust for specific horsepower levels but I don't see that happening anywhere in the near future.

 

would it be easily possible to place variable restrictors (circular)like another car I know, at the end of the exhaust to maximize the low and mid range torque while at the same time, while following rpms opening progressively until wide open at the higher rpms to maximize the higher torque and hp.
I think it has been done one the zo6 and others....what do you think ,,it sounds doable.

 

So would a 2.5 Dual Be Suggested? What about a single 4"?

Devek LII headers, IIRC, are 1 5/8 ID, right? Those, combined with some good high flow cats, currently available ">< over" and some good muffling would sound aggressive, not too brackish, and would have enough back pressure to keep the system happy right?

 

Heres what I found. 89S4, Twin screwed, intercooled

Stock manifold, X-pipe, RMB, 8.5psi boost = 460rwhp

DEVEK Level 2, dual 3" exhaust and X-pipe, dual Borlas = 435-440rwhp

At high RPM I had major blowby with fuel and gas milage was worse.
I purchased some 3" baffles that slide in the end of my exhaust, it is a 1 1/2" pipe with 3 3" plates attached. All have holes that allow airflow. They are adjustable by means of a cap that can be placed inside at the end of the pipe, one solid and one with an inch hole in it. By changing the cap, or removing it alltogether, you can adjust the backpressure. I currently have the cap with the hole tig welded on. Car feels much faster with less blowby. My dyno shop is working on two cars for Barrett Jackson next week, so I can't get in to see where I'm at. When they get back, I will dyno with the current setup, pull out the insert and install the solid cap and dyno again. This should give a clear indication on the backpressure needed.

Hope that helps.
Later, Woody

 

Something very wrong here. Stock cams have so little overlap due to the fact that they are MAF regulated and the MAF is unforgiving of any overlap.

Backpressure does not help any engine. it is a pumping loss.

Blowby has nothing to do with exhaust. sounds like another issue.

Woody, looking forward to your redynoing.

good luck,
Marc