Some exhaust info..

[EG3]

Well, the origin of this thread is at the .org, feel free to go over there and look for "exhaust theory" by GeoFx. I recently did an addition but wanted to post here as well..


Understanding Exhaust (Good info on exhaust velocity & volume, backpressure & why it DOESN'T increase torque)

Torque Loss & More.. Confusing post but very informative if you read the top one first

My basic (completely theoretical) excel calc on pipe dimensions and peak torque rpms (ie, where will you get your peak torque)

The last one is based on this: Understanding Header Design.. I basically used metric calculations but you can play with the pipe diameter and it should calc out correctly. For newbies to this, the pipe diameter is the main exhaust pipe diameter. For stock exhaust, it can be assumed to be 2.5" (but has several restrictions and differences. those are explained by others in this thread).. You can check out the calculation for correctness.. It looked fine to me but may not be..

 

[gto]

Nice addition to the IS Wiki.
Good job

 

[FxJojo]

thanks for the usefull info ..

 

[EG3]

no problem.. The original thread is very good on technical info but it's long. What I've basically came up is the info below. Sorry for copy & paste, I'm just too lazy to write it over again:

Basically what I've come up is that most restrictive parts are the cats and the Y-pipe. Replacing the cats are easy, almost all aftermarket HFCs fit except the ones that have a bigger heat shield. Almost all Z Y-pipes fit RWD FX but only Kakumei (afaik) fits AWD. For a balanced system, pipe size to go is 2.5" if you're going to use a Y-pipe and single mid. If you want a louder system, 3.0" works well. Many people who use Nismo don't feel much torque loss at low end but that may not be the case for others.. If you're going to go dual, 2.0" all the way with mandrel bents, 2 mufflers & an X pipe is focused on low end power/torque and average high end. If you're not gonna use mandrel bents, 2.25 or 2.5 dual is fine depending on where you want power and how loud you want your system.

I'm personally going with Berk ceramic HFCs, Kakumei y-pipe and APS single turbo exhaust modded to fit our NA system. It'll be a 2.5" dual cat-back system with two y-pipes joined to form an X. If I feel any torque loss/don't like the performance, I'll reduce pipe sizes to 2.25". I'm also worried about loudness as I have headers as well and the complete system will be really loud.. That may lead to pipe downsizing as well..

Many people use Nismo over here and they're happy even theoretically it should reduce low end torque but it may not adapt to your driving style or your mods. Also, you'll find two very good threads on IS mentioning Kakumei Y-pipe:

Keemo's Install
gchristopher9's review

Also, many people actually won't be reducing their pipe size because the sound will be worse. It's up to your preferences basically.. Go with a bigger pipe diameter for high end power and deeper, louder sound or go with a smaller pipe diameter for low-mid end power and close to stock sound. You can also go with different diameter piping and this will help you adjust your peak torque rpm depending on how you adjust it. I believe this is probably why Nismo doesn't feel sluggish with most as they don't replace the stock Y-pipe.

Finally, the most important points NOT to lose ANY power/torque are:
1) Don't overkill: a 3.0" dual is unnecessary if you're not FI.
2) If dual, use a balancing part such as an X-pipe or H-pipe. X-pipe is always better than H. These work to equalize exhaust flow on each pipe and remove drone.
3) The engine will lose torque each time the pipe diameter increases. Try to make it as uniform all over the system
4) If using crush bends (standard muffler shop work), avoid sharp turns. A 45 degree crush bend on a 2.25" pipe will reduce the area at the bend to about 2.0" or less. It will introduce restriction which will cause power loss.