are they there for a reason?

so i take that as the lower the backpressure, the more hp and tq?

 

Are WHAT there for a reason?

Less back pressure will NOT NECESSARILY give more HP or more torque. The S2000 already has a pretty good exhaust system. Replacing it with a less restrictive system may even reduce your power, unless you are adding an S/C, turbo, etc.

 

My understanding is that little or no backpressure always offer some topend gains, it will however screw with the drivability of the car due to a drop in low end torque.

 

More beckpressure makes more tq at low rpm and less hp at high rpm.

Less backressure gives low tq at low rpm and more hp at high rpm.

=Don

 

I just posted this a few days ago in another thread:

Backpressure is Never what you want, what the goal is to have . . . is the proper diameter and length exhaust in order to achieve the highest gas velocity.

The higher velocity helps pull a spent charge out of another cylinder.
The charges have to accelerate out of the valves, if there is no velocity from another cylinder, it has to do all the pushing. That is why a good set of headers is better than no manifold at all, and why you can usually take a car and increase performance with better manifolds . . . But the S2000 is pretty damn good to begin with. So what has happens most of the time is we go and make it worse . . .

The problem with the straight pipe is that some people have removed the O2 sensor, and the ECU goes nuts - hampering performance. So this is a case of hurting performance by removing the cat, because the computer is wondering what's up. Change the ECU to that Mugen one, or a programmable one and you be better off.

 

so how about the the length of the pipe vs performance (hp & tq in low & high rpm)?

it's like why spoon and other single outlet exhaust runs their muffler on the right hand side instead of running a little longer pipe to the left? (or running at a angle to the left)

 

Backpressure is not the objective, what you are trying to optomize is the exhaust velocity. Too large a pipe and the gasses move slowly, too small and gasses need to move Warp 10 to get out of the system.

Think of a foot long water pipe that holds one liter of stationary stationary atmosphere. Now think of a foot long pipe much larger in diameter, wide enough to hold 10 liters of stationary atmosphere. Which one will flow 500 liters of atmosphere at the highest velocity?

The old rule of thumb that smaller pipe provides more low end power, etc... is rather flawed. A better metric would be two fold: explain that more absolute power requires a larger end to end pipe diameter and the pipe diameter should be properly sized to the desired power band. In other words, Z-sized pipe for greatest effiency at X volume of flow. Too little flow = efficiency loss from too high an exhaust velocity. To great a flow = efficency loss from too little exhaust velocity. Exhaust gas flows best at a certain velocity, and this is the last part of the BASIC equation. The reason why there appears to be a correlation in power gains for large pipe diameters and high RPM operation is because you are trying to move more exhaust gasses. The pea shooter exhaust on the average stock Civic provide the greatest flow efficiency at low RPM operation. A Civic DX is meant to a commuter car, and the engineers most likely designed the exhaust for commuter engine speeds (very little airflow) and noise reduction.

Adding a turbo to the mix basically shifts the optimum pipe diameter larger. This is almost another topic altogether though, because you've stuck a hunk of metal into the middle of the exhuast flow.

What size is the right one for a given application? Ugh. Two parts: Where do you want the most exhaust efficiency? And some Math. I'll defer the calculations to somebody who knows how to use a calculator.

Very good point on the collector diameter Skogshuggarn!

Perhaps one of the more physics/engeering savy board members will carry the torch from here.

 

My speculation would be: Less pipe = cheaper/lighter exhaust!

 

what's the relation to pipe length to gas velocity and backpressure then?

 

You need all the factors, such as gas volume, temperature, cam profile, AND what RPM window you're shooting for when dealing with an engine.

Im sure you dont want to get into propagating waves/harmonics (because I dont) created by valves and how you can get those to help out. . . get this, a properly designed intake manifold can in effect 'supercharge' a NA engine at a given RPM and load.

And you can forget some of this if you want to talk turbo's. Then you want some pressure BUT remember, most people will say hey that helps spool up . . . but no, it's linked but pressure is still bad - the more air you can flow the more power you will make.