mark kibort

I spent a lot of time doing this, so its been done before and everyone saw the results of devek level I headers and the stock exhaust system. 12hp. with the 3.5" exhaust, it gained an almost equal 12hp. so, 24hp can be expected for both. that was on top of the stock 177 rwhp dyno run.
I can post the dyno runs if you want to see it.

mark kibort

the dyno shows 26hp from the cross over pipe, but it also shows the same torque gains from 2000rpm all the way to redline. cross over and headers, big exhaust, usually only helps from max torque range to redlne, not down in idle. if you have been around the dyno a lot, you can see these types of gains from just showing up on a different day. you need to run about 3 runs. make the change and do it again. btw, this is what a lot of folks see with a cross over and usually not much gain with headers on an S4 vs the stock exhaust, which is pretty good. PLUS, 290 rwhp is what we have seen with an S4 and just the cat removed.

55hp is a not really fair to advertise as a potential gain for headers and exhaust. again, realistically, 25 to 30 would be more like it, and you should be very happy with a 10-12% increase for any and all exhaust work.

GlenL

If the OP is going to add capacity to the intake side then making a large change on the exhaust is warranted.

Predicting "what'll I have after..." is always a challenge. People rarely change one thing at a time and, after the first change, are comparing to a different baseline. Looking at different engines is a whole other set of uncomparable situations.

Single-point changes are often a disappointment. That said, the older "lima bean" exhaust manifolds are terrible. But, without up-stream changes the net effect of dropping, oh, $2000 on headers and custom pipes will be a lot of noise and a dozen or so horsepower.

I think this is why the kids stick on NOS. Or raise the pressure of the blow-off valve.

Carl Fausett

jshuberg - now I see where you got that impression.

http://www.928motorsports.com/parts/xpipe.php

I think you mis-understood what I wrote. I was encouraging the use of an X-pipe as opposed to headers and an X-pipe and a complete exhaust. It gets most of the HP available, for a lot less time and money.

We have documented that our headers, our Header-x (this one http://www.928motorsports.com/parts/headerxpipe.php ) and mandrel bent dual exhaust on a supercharged car and the gain was 55 HP. I believe with extended merge collectors, it can even be higher.

This is why I also state on that page that to get this type of benefit (50+ HP) requires a lot of money and work.

That was not a promise of headers providing 55 HP. That was a statement that gains as high as 55HP from the ENTIRE EXHAUST SYSTEM are possible. Big difference.

Carl Fausett

Ketchmi - Dave: thanks for your input but be sure to mention that your X-pipe does not fit headers.
It is not meant to, but I run into a lot of guys who assume it does.

This is the only X-Pipe that bolts up to MSDS-brand headers, and we are the only manufacturer of it:
http://www.928motorsports.com/parts/headerxpipe.php

So - we offer both. X-pipes for standard exhaust manifolds (yours) and X-pipes for headers (ours)

Our Header-X also is designed to connect to the SS performance exhaust you make, or a true racer can just go all straight pipe on it if they wish.

jshuberg

I absolutely realize that. I wasn't under the impression that headers alone would result in 55HP gains, but that an entire exhaust replacement might get you up there. It didn't occur to me this was also on a supercharged car, that fact makes a big difference.

I'm not expecting anything approaching 55HP when I replace my exhaust, although I am cautiously optimistic that 25-30HP (12-14%) gains may be attainable. As I said my exhaust is 30 year old, and not exactly in "like new" shape to begin with. I also expected a little more than a 20HP gain from the cam upgrade (and the timing advance), so if it's the exhaust that's currently choking the system (as I suspect), replacing it should realize some additional gains from the cams as well.

123quattro

I have stock Euro heads and cams with a US Ljet setup on top of that. Yes. I don't doubt the power numbers. It runs dead even from 50-100mph with a stock C5 Vette.

mark kibort

270rwhp? just heads cams and headers on Ljet. wow.... care to post the dyno sheet? what did it baseline at? lets see. I had a 4.7 us that was about 20hp below normal at the start and ended up at 242rwhp, so I guess its in the believable range. Although, I don't think anyone has seen those kind of gains for just the euro heads and cams with headers. I think if you put on the euro plenum, runners and TB , that would be good for more HP. maybe flow theory is right here, in that once you have a restriction (Ljet flappy inlet) it doesn't matter if things are bigger down stream, you get what you get! this could also be why, the CIS version of what we are talking about , gets near 320rwhp, vs the repeatable Ljet version of the 5 liter.

However, its interesting that you are making what the euro 4.7 makes rwhp with the US intake and Ljet.

mark kibort

you wont get any gains advancing the timing. we already did that and it showed losses at any point greater than 2 degrees over stock.
the exhaust is not choking the system. it has pumping losses, and there is some scavenging that happens when you use the xpipe or good headers.

I don't think any exhaust system can guy that much. now, if you are talking about what might be possible on a 500hp set up, then yes. You mention supercharged, and before and after tests and all bets are off. you cant say 55 is possible. that's just misleading. no one can and has ever gained this much with an exhaust change alone. if so, post the dyno runs and conditions /mods.

if you get 20hp on a 200hp car, and 30hp on a 300hp car, you are doing pretty well. 55 is just dreaming.

I also think that most would agree that a straight 3.5" single with a nice merge collector and headers , will produce as much, or a little more than the crossover and stock manifolds.

SwayBar

I agree.

jshuberg

I just had a dyno tune done by a very competent shop. I'm not sure how many advance positions they tested, but they claimed that 37 degress BTDC @ 3000 (I presume with the vacuum advance connected) that the engine produced the greatest HP. My understanding is that in general that more advance equals more power, up to the point where engine knock or predetonation occurs.

If what you are saying is true, if I were to reduce the advance to 2 degrees over stock I would in fact see more power? Am I understanding you correctly? If so, after the new exhaust goes in I'll have it dyno'd at where the advance is currently set for an apples-to-apples comparison, and then have them reduce the advance and see how she performs then.

The idea that anything past 22 degrees results in power loss seems strange to me though, since I'm currently about 20HP over stock at 37 degrees advance....

ptuomov

As long as the previous exhaust isn't very undersized, it's difficult to get a big power increase from a supercharged car with tuned length headers or exhaust. I believe that's because the intake port to exhaust port pressure ratio is already favorable at overlap without any pulse tuning, as long as the exhaust is large enough. If the primaries are ridiculously small, then of course a supercharged car can show great gains from a bigger exhaust. Things like the collector length don't seem to matter that much with supercharged cars, however, and I've rationalized the overlap pressures already being in a good shape even without exhaust pulse tuning.

123quattro

I'll see if the shop still has it on file. This was 5 years ago when I did all the work. Stock it did 180-190whp I think.

Ketchmi

What I have seen and it has been experienced by a couple of folks out there with supercharged cars is that adding my crossover decreased boost between 1 and 1.5psi while producing more horsepower. Boost pressure is a function of mass vs. restriction. If you reduce some of the restriction, you reduce the pressure level while also increasing the amount of flow.

Most exhaust systems do not create horsepower, they allow it. If your exhaust is properly sized for your horsepower application, going bigger is not going to help. When it gets overly large you will lose torque as I found out with my 16v single exhaust I designed and tested. A single 3" properly bent with free flowing mufflers was just too much for early 219hp cars and actually reduced average power. The 240hp cars saw some high end gains but only held their own at lower rpm. The Euro 300 and 310 16v engines loved my 3" single and had gains across the board. It still wasn't feasible to produce one with the Borla setup available as it was a real quality piece ready to go.

As Tuomo stated, forced induction cars do not respond the same as N/A cars to exhaust modifications. Losing restriction in the exhaust will always help but not to the same degree as on an N/A car. Whereas my crossover is based on one pulse creating a vacuum on the opposite side, a boosted car is actually pushing the air through so it's only function at that point is a loss of restriction. (which it still does very well!) A simple explanation would be holding a carrot in front of a donkey or kicking it in the *** to get it to move...

Carl Fausett

You are right, forced induction cars do not react the same to exhaust modification the same as NA cars.

Boosted cars seem to "overcome" some of the restrictions in a given exhaust system, simply because the pressure delta between the cylinder pressure when the exhaust valve opens and the end of the system (tailpipe) is so much greater than on a NA car. Pressure differential moves fluids, in this case, exhaust gases. The increase in pressure before the restrictions (mufflers, bends, connections, transitions, etc) is much greater on a boosted car.

On the other hand - the laws of fluid dynamics say this: start with a tube that is flowing say 1000 cfm, (just to pick a number) and has a certain resistance to flow. Measure the resistance to flow as pressure drop before and after the restriction. Increasing the CFM in that same tube makes that same restriction much worse. The pressure drop before and after the restriction will be much greater as the CFM (flow) increases. I cannot recall if the effect is linear or exponential, but I believe I recall the increase to be near exponential. So restrictions get worse the more fluid you try to move across them.

Result: careful system sizing and design is still quite important on boosted cars. Smooth transitions, large radius bends, all the usual stuff. The cylinder scavenging effect may be gone (compared to a NA car) but the effect of restrictions in the system are still very significant. Arguably, even more so.