Exhaust Sizing calculations 1

[Korichnevijgigant]

I am trying to get some help in sizing my exhaust pipe for my 1997 Pontiac Trans Am WS-6

Can anybody point me in the right direction for equations?

I pulled out my Marks handbook hoping it would help, but I couldnt find anything

Currently, I have long tube headers with a 1.75 inch primaries 3 inch collectors then going into a 2.5 inch y-pipe that has 3 inch cut-outs which then has regular catalytic converters then into a 3" Flowmaster catback.

I want to redo the y-pipe because it was cheaply made (in college)
I eventually want to go to a true dual exhaust system.

If you could please help me out I would appreciate it

Attached is a thread I posted on another forum

 

[NormPeterson]

You may be able to find something under "gas flow in pipes" in various piping references (the Piping Design and Engineering book by ITT Grinnell did - an older piping guy may still have a copy).


I doubt you'll need more than 2.5" for a street 5.7-ish liter motor anywhere but through the converters. The SBC I have in one of the cars is a little more serious than yours and doesn't.

Strictly speaking, an X-pipe system isn't "true duals". An "X" has some advantages especially if the mufflers themselves are somewhat restrictive (Flows not necessarily being all that great in this respect). It certainly won't sound the same even if the rest of the piping and the mufflers are identical.


Norm

 

[Korichnevijgigant]

NORM!

well they call it true duals because the Trans Am (and Camaros) have a y-pipe after the manifolds and then a single pipe leading to the muffler then it has 2 outlets, so you get the appearance of dual exhaust, but its not true, hence the name.

Anyways, I have been doing so leg work trying to get to the answer eventually, right now I am working on the exhaust flow rate out of the valve. I want to get the flow rate out and the pressure that its at, so I can get the optimal flow area ratio so I dont have choked flow, because I think Im gonna reach sonic flow out of the valve.

Well thanks for the help.

 

[thundair]

http://www.maxracesoftware.com/pipemax36xp2.htm

This will work its easier then reading the same page over and over.

Cheers

I don't know anything but the people that do.

 

[Korichnevijgigant]

Great thanks

 

[dbecker]

Hello, there is a plethora of aftermarket for your application. It would seem a little overkill to try and calculate with any certainty the size of the primaries for your car. The aftermarket companies (at least the good ones) have already done this for you; just buy a set of headers and you'll be fine especially for a street WS6 with stock displacement motor.

Now if you intend to build a motor that revs higher, has larger/smaller displacement and uses custom ported heads/valvejob than you may need to look into getting with a custom cam designer and tune your header length to that.

Sorry for the long reply, good luck.

 

[Korichnevijgigant]

Hi dbecker

did you read my post?

I dont need headers, I already have some, which in fact are not the best design (from an area ratio and gas dynamics standpoint Ar=5.94 which is <6) which goes to prove what many already know about the aftermarket, ESPECIALLY the exhaust side, they dont exactly know what they are doing. Which is evident by them sizing mufflers by pipe diameter and not CFM


Anyways, I already crunched the numbers and the 3" will be optimal, but the 2.5" would be satisfactory, but depending on the wave reflection by the catalytic converter the 2.5 could choke the system.

 

[dbecker]

Hello Korichnevijgigant,

So you basically calculated what was already available to you through off the shelf parts.

 

[Korichnevijgigant]

off the shelf parts?

I think you missed the point of the thread.

I was trying to correctly size the exhaust system from the header back, in order to see if I would lose anything with the 2.5" or if I would significantly gain anything with the 3"

since the engine produces a certain flow rate, I was seeing which flow area would be more efficient

 

[MikeHalloran]

I suspect the 2.5" was used (vs. 3") for ground clearance.
Isn't it also flattened a little in some areas?
Doesn't it snake through a tunnel in the floorpan, next to the axle torque arm?

True duals should be better, if you can make them fit.



Mike Halloran
Pembroke Pines, FL, USA

 

[Korichnevijgigant]

Mike:

Yes the original Y-pipe is 2.5" and junctions to a 3" I pipe all the way back where 2 outlets on a crossflow muffler lead to the exits

if you see the link in the original post you can see they did a great job