The stock exhaust piping is f*cking terrible. It’s narrow, it’s got too many bends, the bends are all kinked because they’re crush-bent and anything is an improvement. So yes, a 3" exhaust will make power over a stock exhaust, but a 2.5" will make more.
Backpressure is always bad. There’s no such thing as good backpressure. This is a myth, repeated by dumbass to dumbass that has somehow lasted through a century of advances in car modification. This lie originated because tuners would notice a loss of low-end and a gain in high-end when going to larger pipes, and the opposite when going smaller. Being ignorant Americans with the worst education system in the world, they attributed this to the only phenomenon their puny brains could remember: backpressure. In fact, the reason is quite different.
It’s all about maintaining velocity. You know how sucking a drink through a straw is really hard through a small straw, and really hard through a huge straw? The same goes for engines - intakes and exhausts.
In an N/A application, the intake, intake manifold, valves, cams, exhaust ports, header and exhaust are - under ideal circumstances - matched so that they will help make peak power together. The key to this is encouraging a velocity stack on the intake side and perfect scavenging on the exhaust side.
Your engine has four cycles - intake, compression, combustion, and exhaust. The ones we’re concerned with are naturally the intake and exhaust cycles. Each cylinder needs to make two full revolutions to produce power. In a simple theoretical model, it has to use power to suck in the air, and it has to use power to push the exhaust out. Furthermore, in this simple theoretical model, since the piston is never actually as large as the entire volume of the cylinder, and since it must clear the exhaust valves, it cannot push all the exhaust out. As a consequence:
- An engine never fills up completely on fresh air, with that all-important oxygen.
- An engine must expend some of the energy it creates to push out the exhaust, and suck in fresh air.
Now, I’ll cover a velocity stack briefly because it’s less important and helps illustrate scavenging better. Remember, air can have momentum too - this is what you perceive as wind. Once an engine gets the air in an intake going, it can generate momentum for that air to reduce the delay between which the intake valves open and the cylinder is ready to take in the air. The air will already have momentum and be pushing towards the intake valves and into the engine. More air = more power.
On the exhaust side, you have scavenging. As with the intake stroke, the exhaust stroke occurs every four times. In between, you have spaces of nothing. You can kinda (but not really) feel this when you hold your hand up to your exhaust - that pulsing is actually the combined effect of multiple cylinders, rather than just one at a time, but if you remember your physics you’ll note that nature abhors a vacuum. Where there is vacuum, gas will rush in to fill the space. That means that the next time there’s an exhaust cycle, the exhaust will come out a little on its own, with less effort from the engine. The key is to try and create a larger vacuum, and to do that you need to keep the exhaust gases moving. You might think that bigger is better, but bigger will cause the gases, and their energy, to dissipate and slow down. Too small, and the gases will be under too much pressure and they will also slow down - and worse, the engine will have the actively pump them out (hello backpressure!) The best way to imagine this is to think of blowing air bubbles into water through a straw. Again, with a small straw, it’s hard to blow a lot - but fairly easy to maintain a constant but low flow of air. With a big straw, it’s easier to blow more, but it’s much harder to do a little.
The engine also doesn’t produce the same volume of air at different RPMs - at high RPMs it needs more air and expels more exhaust, and at low RPMs you have the opposite. So what you need then, is, an ideal size for your idealized RPM range that works with the RPM range your cams and intake and IM and head are designed for. You want the velocity stack, valve lift and timing, and scavenging effect to harmonize at the same peak.
