Exhaust...2.25 or 2.5in and over or under axle??? Options

[Davesceli]

Which is better for a stock 5sfe, 2.25 or 2.5in pipe? I've searched around and its a pretty even split on opinions between them. I've decided on the HKS Carbon TI muffler with the 60mm (just over 2.25 in) inlet, i know its expensive for a stock motor but it looks damn good and sounds sweet, my mind is made up. Carbon fiber is just so sweet!!!!

Now i just gotta decide what size piping, its gonna be catback, and whether or not to go under axle or over. What are the pros and cons of each??? If i go "under axle" will there be any clearance issues since my car is lowered???

i know this question comes up a few times, but im looking for a definative answer...i've searched alot and i still can't come up with a good answer.

[Galcobar]

Backpressure is bad. No ifs, ands or buts, or conditional possibilities. Exhaust gas being pushed back into the combustion chamber is not good for combustion, and therefore not good for power production.

The myth of backpressure being good for some engines (naturally aspirated in particular) stems from big pipes being bad for NA engines. By this logic, overeating is bad, therefore starving is good.

The point of an exhaust system is to get the exhaust gas out of the combustion chamber, since burned fuel impedes the burning of a new batch of fuel and air, just as covering ash does not help a fire burn.

A good exhaust system gets the gas out quickly. The most obvious way to increase the ability of an exhaust system to remove gas is to increase its size -- bigger pipes can move more gas, while smaller pipes get filled up and create....backpressure. Unfortunately, this doesn't hold true once you take into account what happens when you give hot gases room to expand.

In expanding, they cool down, and slow down. Now you've got cold, slow-moving gas sitting in the way of the hot gas you're trying to remove from the engine. You lose power from the engine expending energy on pushing the cold gas, which doesn't flow as quickly, and from the now-fouled air-fuel mixture.

Or put it another way. Small pipes create backpressure by lack of capacity. Big pipes create backpressure by slowing down the exhaust flow. Neither way is good.

The trouble is, the volume of gas produced varies depending on how hard an engine is working. At low revs, low volume, at high revs, high volume. Since we don't have expandable exhaust piping yet, we chose a compromise size. With luck and good design, the size is chosen to match the volume/RPM at which the best performance is desired.

Stock designs emphasize and drivability -- basically, how fast the car takes off from a stop. That means they are chosen to be most efficient at a low RPM -- and be as quiet as possible. Enthusiasts emphasize overall speed, and typically want to match their peak power with their most efficient exhaust sizing, so they go larger to accommodate the higher RPM point.

This is why people report "losing" power at the bottom end when going to larger pipes -- the engine is less efficient at lower RPMs because it can't get the exhaust out fast enough, but more efficient at higher RPMs.

And just as bigger displacement means more exhaust volume at any RPM, and therefore bigger pipes, forced induction and N2O produce more exhaust and therefore more volume.

A bend in the piping is never a good thing. It reduces the ability of exhaust to exit your system quickly, and therefore reduces the efficiency of the engine overall. It's equivalent to increasing the length of your exhaust system, about eight feet for a 90-degree bend is the rule of thumb tossed around.

So how large your piping should be is an issue of where you want your engine to be most efficient -- low RPMs, match the stock size. Higher RPMs, go up as far as 2.25" (5SFE really doesn't produce enough gas to warrant 2.5" unless you spend all day at peak). And if you don't have to worry about rough roads ripping parts of your undercarriage off, go under-axle for the smoother flow.