exhaust question Options

[eldrewado]

So I made my own exhaust on my GT. After the stock resonator i upped the diameter of the pipe to 2.25 then it goes into a long glasspack muffler then a downpipe just before the rear axle. I seem to be getting better gas mileage, so i want to keep my flow through design, but its so LOUD. I ordered 2 vibrant resonators I heard one is good and two is better and they are flow through. The question is....Should I build the system resonator-resonator-glasspack,
resonator-glasspack-resonator, glasspack-resonator-resonator, or just leave the glasspack out?

[Galcobar]

Reduced backpressure leading to a loss of torque, well, not exactly a myth but certainly incorrect.

Backpressure is always bad. There are no circumstances in which allowing burnt fuel and oxygen-deprived air to remain in the combustion chamber improves performance. Fouling the air-fuel mix is bad for performance. So is making it harder for the engine to get the exhaust out of the combustion chamber, as energy spend forcing the exhaust gases out is energy not going to the wheels.

Reducing the barriers in an exhaust system will always improve performance. Generally, this means fewer bends in the piping, or switching from baffled mufflers which are designed to slow exhaust flow to straight-through mufflers which are designed to allow it to move unimpeded.

The other issue, which is where the mythology of backpressure arises, is the bigger-is-better belief when it comes to pipe diametre.

At a given flow speed, a bigger pipe will accommodate more exhaust flow. Naturally the bigger the engine, the more exhaust it will produce at a given RPM. A similar rule applies to engines with forced induction, since the compression allows a larger amount of fuel and air to be combusted at a time -- false displacement, if you will. This is why bigger engines, or engines with forced induction, benefit from larger exhaust piping.

Too small a pipe and the exhaust gases cannot exit the engine quickly enough; this is what is typically cited as backpressure. However, too big a pipe and the exhaust gases have room to spread out, cool off and slow down. Think of a river in winter. In the narrow sections the water flows quickly and stays liquid. Get into a wide section and the river slows down enough to freeze, which can cause flooding upstream. Cold exhaust gas acts just like ice in a river: it dams the flow and prevents the hot gases from quickly exiting the engine.

Too small a pipe and you get backpressure. Too big a pipe and you get ... backpressure. Same problem for different reasons.

The problem is that we don't use exhaust piping which can adjust to the amount of exhaust the engine is producing. If we did, then we could keep backpressure at a minimum value regardless of RPM while maintaining efficient/fast exhaust gas flow out of the engine.

Instead, we have to pick when (at what RPM) we want the engine to be most efficient, and match the exhaust pipe size to the amount of exhaust produced. Car manufacturers know that most drivers want good acceleration from a stop light, so they target low RPM efficiency and thus choose small piping to match. Enthusiasts who prize performance at high speeds will sacrifice engine efficiency at low RPMs for efficiency at high RPMs.

You don't lose torque from increasing your exhaust piping size. Rather, you move the peak efficiency point of your engine higher up the RPM scale. Or to put it another way, you're choosing where to have the least backpressure.

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TLDR: Straightening out your exhaust path and removing barriers to flow will improve engine performance at all engine speeds. Change the size of your exhaust piping only if you want to change the RPM at which your engine is most efficient.