Custom Exhaust - 3" or 2.5" ????
 

Good day to all! The time has finally come to open up my FRS and create a custom exhaust. I have pretty much everything figured out as far as components goes but i have one question.

Should I go 3" or 2.5"?

Things to consider:
-My set up will be a single exit coming out the driver's side
-I will be starting with the down-pipe all the way back
-I could potentially be boosting sometime in the future

Any opinions are greatly appreciated! :thanks:

2.5 if you're going to stay NA for a while
3 if you're going to go turbo in the next 6 months to a year (otherwise I would just go 2.5 for now, then 3 when you go FI)

2.5 will give you a bit of backpressure, which helps create low-end torque in a NA setup. With FI, you want as free of an exhaust as possible to reduce lag.

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Backpressure is a myth

Care to share your source on that?

2.5 is good for NA street car, some brands have gone with 2.75 but I would think thats better for a NA track car that spends most time in the higher RPM.

Back Pressure is not a myth. The gas velocity also needs to be considered, exhaust travels in the pipes as pulses. Each pulse leaves a low pressure behind it, this in turn draws the next pulse along. This is better known as scavenging.

To small a pipe and its restriction, to large and the effect of the pulse is dissipated. Having said that there is no magical best pipe size mainly because the pulses vary at different RPM. So one exhaust my help low end while another helps higher up. These principles are more obvious when applied to header when each primary is located right at a single exhaust port. An ideally designed header can actually help pull gas out of the chamber, often this ideal design isn't physically possible in the engine bay, but the possibility is there. There is a term of this but it escapes me at the moment. Need to finish my coffee.

Back pressure and scavenging are two different terms but are closely related. People do say with "A larger pipe you will loose back pressure", in some ways its better to say with "Too large of a pipe will loose scavenging".

These principles apply to an entire NA exhaust and FI before the turbine. Turbos disrupt these pulses and in the case of down pipes the concepts of pressure drop can be applied so thats a different story.

Just my .02

from what ive gathered through opinion (some educated) and my research. A 2.5" exhaust will most likely be a "better" route to go if I am staying N/A for the time being. Lets say i begin to flirt with a header, tune, maybe a throttle body, etc. At what point or HP range will a 2.5" exhaust begin to restrict exhaust flow and/or "scavenging" ?

Again, thank you for your input!

I really dont think 2.5 will be a restriction with the normal realm of NA bolt ons. Start getting into FI then stepping it up makes sense.

Assemble turbo ... if not, it is better to stay at 2.5? "If we put a 3" on a car without turbo is ceirto you lose power at low revs?
perrin has an article in which he says that this is not true, but that you say?

http://blog.perrinperformance.com/3-...ults-and-tune/

NA I don't think it makes much difference at all really. Some have seen gains going to 3" even NA, but to me the car is too damn loud with a 3" exhaust.

Man up.

4" or GTFO

This is not true. Backpressure is bad, hurts torque everywhere. But at low rpm a smaller-diameter exhaust isn't going to create or cause much, so no biggie.

Tuning and scavenging is a different story, but that's more to do with the primaries, not the main exhaust diameter.

3" exhaust won't hurt low-end torque even you if you're running modest power N/A, but it is just unnecessary cost and weight.

Enough of these threads already jesus