so would doing this modification help in re-increasing back pressure with an aftermarket exhaust.


Im not sure how all of that works exactly but I figure you will have more air going in and out...

Huh?????? What are you asking and/or trying to accomplish?

from what I understand one of the many disadvantages to some aftermarket exhaust setups is that the free flowing exhaust causes a drop in back pressure. Which in turn can rob low end torque.

My question is if allowing more intake will counter act the free flowing exhaust from an aftermarket system. I suppose like balancing an equation.

Of course it may not work like that at all. That's is why its more of a question. :confused:

here's a good link regarding the 'myth' behind back pressure:

linky

in a carbeuretted engine, back pressure results in less air moving through but the same amount of fuel delivered, so you get a higher fuel/air ratio and more 'bang'. if you reduce back pressure and you get more air moving through but a lower fuel/air ratio because a carbeuretted engine can't modify the amount of fuel to react to this; less power is developed.

a computer-controlled engine should be able to react to reduced back pressure by increasing fuel flow *provided* it is smart enough and the fuel delivery system is up to the task.

one would *presume* that the boxster is up to the task, which begs the question of why some folks find reduced low-rpm torque with freer-flowing exhaust. even the dynos published by NHP show a slight low-rpm torque reduction:

linky

all i can surmise is that it is a flaw in the dme mapping which could be corrected with a proper tune. it can't be an issue with the fuel delivery system, as the system has the capacity to deliver lots of fuel at higher rpm. of course, cams and valve timing should also be included in this conversation.

with respect to the air intake, the math is 3.2 litres x 7200 rpm = 382 litres/second. this will give you the size of the intake required to deliver the air at atmospheric pressure. any smaller than that and the engine has to do work to draw the air in (accelerate it to the speed at which proper flow can be provided). i did the math before and i *think* the number was 92 mm diameter.

there are a number of bottlenecks in the intake system that will cause this work to be done - the snorkle, the airbox, prehaps the intake manifold itself, etc. the actual acceleration of the air around these bottlenecks, followed by the deceleration afterwards, is a mechanism used to sound baffle the system. so, any work done to reduce these restrictions should be of benefit.

regarding the throttle body itself, while the increased diameter is a good thing, note how cross-sectional area changes with actuation of the throttle (ie, how much air gets through); it is not linear, so what a larger throttle body does, in essence, is create a twitchier throttle - more air (fuel) sooner, better 'throttle response' - an expensive sprint booster.

thanks great post thanks for the information. So I suppose it would correct to say then the two are not related necessarily in quantity.

I wonder if the ROW dme serves a better solution for this.

The major problem with the oem exhaust system, is that it is very restrictive. Therefore, by eliminating some of these restrictive components in favor of some of the better designed aftermarket products, one can effectively increase flow. I hasten to say, however, that at some point it becomes a delicate balance between creating a less restrictive exhaust system that increases power commensurately versus one that is too free flowing and consequently loses power by eliminately the necessary degree of back pressure. In my experience, the latter of the two has almost always resulted in a loss of low end to mid range torque.

In addition, there is much more to intake/exhaust tuning than just the restriction or backpressure. The exhaust pulse pressure waves can be tuned just like a musical instrument or subwoofer port. Whole books are written on the subject.