Originally Posted by Jose-aka-Pedro

3inch is Way Over Kill. You Will lose Power.

2.5 inch exhuast on a da. er anyhonda Is good for 500whp.
And then its time to upgrade.

But on Hondas you need some back pressure.

2.5 is most. n/a Id say 2.25

imo

Originally Posted by TeamIntegra

3. Myth 3: I Need A Little Bit of Backpressure For Midrange Power

THE MIGHTY BACKPRESSURE MYTH:

You want zero backpressure not some backpressure as you may sometimes hear from a salesman or an oldtimer domestic V8 hot rodder.

Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.

In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.


This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.

So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.

Regarding the backpressure issue:

Many people use backpressure to get midrange driveability at the sacrifice of lower power potential at the upper powerband rpms. Using back pressure is the wrong way to build a high performance exhaust system. The exhaust system should extract the exhaust gas from the header, to minimize parasitic pumping pressures.

The proper way to make an exhaust system that will act as an extractor is to properly size the tubing so that the the exhaust gas' flow velocity creates a "vacuum" behind the header.

Also, you have to realize that making a sytem which provides the best performance at all throttle positions and all powerband rpm ranges is next to impossible. There's always going to be a compromise and giving up some optimal power potential in one area of the rpm range.

You must tune the exhaust size/length for the throttle positions and rpm ranges where you want the most performance knowing that you'll sacrifice performance at the other end of the rpm range.

C. SUMMARY

If the exhaust has the design characteristics you want and is cheaper, get it. Please try not to be hooked by a salespitch or brand name hype.There's not much separating exhausts these days in terms of performance and design features for 2.25 to 2 3/8 in. straight-through designs. They are all pretty much identical.

The trick these days is finding a 2.5 in. diameter exhaust for the longer hybrid 4-2-1 and traditional 4-1 headers with bigger JDM-style 2.5 in. collectors.


In summary, plan where you want your peak torque will be and how wide your power band will be along the rpm range. Then choose a header-cat-exhaust system with the design characteristics that facilitates that goal.

You may get more midrange power but give something up at the top rpms.

Or the opposite, you can plan that you want more power in the upper rpms with some compromise losses at the midrange rpms.

Remember, if you get more midrange with some exhaust backpressure (the old backpressure myth) in a "bolt-ons only" engine package, ask yourself: what is the loss in hp at the upper rpms with more backpressure? And will this loss in power up top be acceptable to you?