Exhaust...

[VehiGAZ]

I replaced my OEM muffler with a Super-50 Flowmaster and 2.5" piping from the cats back. This was done after I supercharged.

I will swear up and down that my off-the-line performance improved noticeably with the new exhaust setup. The dyno graphs listed with the supercharger kit that Tone used to have on his site confirmed the performance enhancement of a 2.5" exhaust setup (PowerVault in that case) with a supercharger.

Since the supercharger does not affect low-rpm off-the-line performance at all, I would imagine that this upgrade would benefit a non-S/C'd VX just as well.

That said, it is brutally loud inside and out with the Flowmaster. It has a great gurgle at idle, but the rest of the time it's obnoxious - the stereo doesn't go loud enough to drown it out. I am waiting to hear if the Cherry Bomb muffler someone was going to try out was any better.

[twistedsymphony]

Basic Exhaust Design 101...

With a turbo charger 99% of the time a less restrictive exhaust = better
a turbos performance improves, the larger the pressure differences the better the turbo works... removing your ehaust completely after the turbo would give you the largest gains (though obviously it's a bad thing to dump all that hot air in the engine bay, and listen to all that noise)

With a Normally Aspirated vehicle this is not the case... there is actually an optimal diameter for the exhaust. Obviously if you restrict the exhaust too much it will cause pressure working against the motor. However since the motor works in "pulses" the dead air between those pulses can also work against you.

Tuning the diameter of the exhaust to tune the dead space between the pulses, while at the same time not restricting the flow will actually create momentum in the exhaust system... this creates a slight vacuum which has the effect of pulling the exhaust fumes along. This is call the "scavenging effect"

Obviously a certain size is only good for a certain RPM range, since when you change the RPMs you change the spacing between the pulses. Because of this you tune the size based on your goals, tuning the diameter for the lower RPM ranges will produce better low end torque and better fuel economy but you'll be too restrictive on the high end and reduce your high RPM power, tuning the exhaust for the higher peek HP rpm range will improve your peak power but you may actually lose some low end torque as well as MPG in the lower RPM ranges (ie: anything but hard acceleration). Most modern factory exhausts are tuned for the low RPM ranges already which is why on some cars owners experience a drop in RPM, and a drop in low end torque even thought they see a boost in peek power with a larger exhaust.

With a Super Charger you have a scenario that is half like that of a Turbo and half like that of a NA car... the intake is pressurized like that of a turbo, but there is no turbo restricting the exhaust, meaning that the benefits of the scavenging effect can still be realized...

the difference here is that the exhaust is coming from a pressurized state in the cylinder to atmospheric pressure in the exhaust, so it's expanding as it's released and as a result needs a much larger diameter pipe across the whole RPM range... in most cases the exhaust would need to be so large to create scavenging even at low RPM that in general with a super charger bigger is better.

In General
These rules are different from vehicle to vehicle and really the only way to know what's going on is to test various system and see how it effects fuel consumption and power output through the RPM range... even still these things can't reliably be determined from just one or two pulls on a dyno with some monitoring equipment since the ECU "learns" and adjust and it can be difficult to tell if the effect your seeing is the result of the ECU over compensating for new equipment (that would pass over time as it "learns") or if it's actually a result of the new equipment...

This is really what you're paying for when you buy a name brand exhaust... presumably they've already done the math, and the testing to figure out the right combination to fit whatever it is they're selling the exhaust to improve.



"scavenging" is what people are TRYING to talk about when they talk about back-pressure... Really this is something that should be "tuned" in the headers, where the individual cylinder tubes come together is where the diameter needs to be sized to induce scavenging.... beyond that point you can and should relieve any "back pressure" since the pulses should be normalized by that point. Some cars even have a valve to adjust this on the fly making the engine perform better across the whole RPM range, there are a few newer production vehicles that include this feature but it's actually illegal to install one aftermarket in the US (why, I don't know)

If the OE designed the headers well, upgrading to a larger exhaust will improve things, if the OE designed the headers poorly, or relied on the restrictions present downstream in the exhaust to produce the scavenging effect then upgrading to a larger exhaust might actually hurt your performance.