Prodigious

My 92 Z28 has hooker 1-5/8" long tube headers that merge into a 4" resonance chamber, and then dumps. This setup was on the car when I bought it, and it sounds nice, but I really don't know how well it flows. Like, what HP numbers would it be able to support?

And yes, this is my first post here

EDIT: The headers might actually be 1-3/4", I'm not certain though.

ASE doc

From the sounds of it, not knowing how well the install was done or what total length is, I would say it will support something 550+ hp, based just on the 4" pipe size and assuming 1 3/4" primaries. There is a simple formula that I found online for calculating HP capacity by pipe size. It goes something like 115 CFM per sq inch of CSA and 1.1 CFM per HP. Keep in mind though that there are so many variables that this formula doesn't take into account. Like any calculator, it's only intended as a guide to help you get close.

Understand though that exhaust HP "capacity" is based on the flow curve of a given pipe size assuming full length exhaust. The length of the exhaust from header to tailpipe has a big effect on the flow curve. Shorter pipe is going to flow more.

There is a big downside to having huge high flowing exhaust. While it may support tons of power up top, it will cost you torque down low due to loss of velocity. Forget back pressure. Backpressure is largely misunderstood and is all bad. The key here and the benefit to smaller pipe is greater velocity. As the exhaust moves through the pipe it transmits the energy that started it in motion, the rising of the piston with the exh valve open, through the tube and out the tailpipe. This energy is reflected back to the exhaust ports and creates scavenging, and improves cylinder filling. This all means more power.

Any size and length exhaust is going have a flow curve through which it reaches optimal velocity, best scavenging and on to where it becomes restrictive. The trick is to match this curve to your motor.

As engine power band moves upward in the rpm range, we tend to see shorter higher flowing pipes. A motor that makes peak torque at 5,000 rpm has less need of low end scavenging and more need of high flow. The exhaust flow curve should always be matched to the engine torque curve.

All in all, in doing alot of exhaust research, I have seen a wide range of installs and results. Some people making alot more power than you would think possible through what seems like small pipe. One case is a guy running 11 seconds at something like 110mph with a single 3" catback. Doesn't seem to make sense but it's true.

There are people on this forum that know alot more than me about this stuff. Do a search and see what you find. BTW welcome to TGO!

EDIT: That should read 2.2CFM per HP instead of 1.1CFM.

Prodigious

Thanks for the response and warm welcome to the site! But basically, I need to modify the system to have some back pressure to yield maximum results, although my current system is capable of flowing high horsepower numbers, it hurts the low end torque? Also, the tailpipe ends about a foot before the back of the car if that helps any. You can't see it at all unless you're under the car.

novaderrik

no... backpressure is bad... always bad... never good...

you want to find that sweet spot where the exhaust flows as much air as the engine can breathe, but not bigger or you will lose velocity and overscavenge the exhaust, which kills power..

Prodigious

Okay, so my exhaust flows too much air therefore reducing the velocity and overscavenging it? How could I alter the setup to hit the "sweet spot" for a lightly modded 355 TPI?

novaderrik

put a single 3" or dual 2.5" pipes on it with good flowing mufflers, which will be good to somewhere just north of 400hp without killing velocity below that..

Prodigious

Okay, I'll do that setup when I get around to doing my exhaust. What mufflers would you recommend? I don't want any quiet ones, a camaro is a muscle car and it should sound like one IMHO.

ASE doc

There is a pretty good right up on exhaust and the function of velocity vs. flow. You can find it in the sticky list on this very same Exhaust tech board. The guy put alot of thought and research into it. It's like I said, back pressure is always bad. I learned that a long time ago when learning to test catalytic converters for restriction. Most people misunderstand this. At 1 psi, you are losing alot of power. By 2 psi, your engine struggles to accelerate.

What you're trying to achieve is velocity of the gases through the system to help move more exhaust and pull exhaust out of the open exhaust valves.

A single 3" with a good y pipe is a great system for up to 350-400hp. I will disagree with novaderrik on one point though. While single 3" out flows dual 2.25, dual 2.5 out flows single 3" ,enough so that dual 2.5" should support about 500hp. In fact I think Magnaflow's website ballparks dual 2.5" to the 450-550 range. Of course you get different answers from different sources. The point is to get you close.

EDIT: Using the formula that I posted above(which I got from Hotrodders.com), the capacity for dual 2.5" pipe works out to 512hp. Again, just a general idea.