Chris69

One of the things I don't like about the 2-1-2 single cat, is that the front Y pipes come together before the cat. That can't be a good thing. I've been searching to see if they make a 2.5" dual/in with a 3" single/out. My rear Y starts with a 3" single pipe. Seems like that would really help the flow. So far I've only been able to find 2.25" dual/in (still may not be as bad as a single 2.5" into the cat). Whatcha think? The comparison would be two 2.5" into a single 2.5" (the front Y pipes), or 5" total to 2.5" total into the cat. With the dual/in 2.25" cat, that would be 5" total to 4.5" total into the cat. with 3" out. Am I thinking right? :skep:

mapman

Chris81,

The inside diameter of my stock (1980) exhaust pipes where they attach to the exhaust manifold, is about 1.8". The i.d. of the "Y" where it enters the catalytic converter is about 2.6".

Therefore, the area of the ("Y") exhaust pipe where it attaches to the cat is just about equal to the area of the two ends where they attach to the exhaust manifold.

It's a good bet that the factory exhaust system is balanced so that no component restricts the flow of the other components in the system. If you want to significantly improve on the stock system, then I think you need to change most all the components.

Shark Racer

I calculate the internal area of both 1.8" pipes to be 5.09"(together).

Internal area of a single 2.6" pipe to be 5.3".

So just a little more exhaust area at the pipe entering the cat.

However. I've seen the pipes leaving the exhaust manifold and they're damn small. My reccommendation would be to get a pair of headers, larger cat, possibly dual in/out, and larger mufflers out the back.

Actually, I'm glad you posted this. It's inspired me to get headers sooner, considering my manifolds are pumping 5.1" of exhaust into 7.9" of exhaust pipe!

-Steve

mapman

Pacin'California,

I think the reason for the larger diameter exhaust pipes at the rear of the car is to accommodate the increased volume of exhaust gas after it passes through the catalytic converter.

If you run headers into a cat, I think the area of the exhaust past the cat needs to be greater than the area of the headers running into the cat.

The ratio (7.9" / 5.1") or post cat area of about 155% greater area than pre cat area looks like a good starting point - if you are going to run catalytic converters.


[Modified by mapman, 9:20 AM 1/10/2003]

Shark Racer

I have dual converters underneath the car. Stock manifold piping going into dual 2.25" pipe, then 2 cats, then dual 2.25" to the mufflers (Dynomax Super Turbo) and out.

-Steve

jerryp58

The exhaust manifolds on my '77 are 2".
A few inches off of the exhaust manifold, the pipes expand to 2-1/2".
Then right in front of the CC, they merge into a 3" header collector.
The 3" pipe goes into the front of the CC.
Two 2-1/2" pipes come out the back of the CC and are separate all the way to the exhuast tips.

If a dual-in/dual-out could have fit somehow (I have the crossmember with a hole on either side of the trans), I would have gone with that, but FWIW just working the geometry of it (not accounting for inner diameter):

the area of a singe 3" pipe would be 7.1"
the area of two 2-1/2" pipes would be 9.8"

Some loss, but not too bad.

Here are some pics of mine if you're interested...

http://users.erols.com/jerryp58/corvette/exhaust4.jpg
http://users.erols.com/jerryp58/corvette/exhaust3.jpg
http://users.erols.com/jerryp58/corvette/exhaust2.jpg
http://users.erols.com/jerryp58/corvette/exhaust1.jpg



[Modified by jerryp58, 6:52 AM 1/10/2003]

Ganey

Chris

You would be fine w/ the 2.25 on your setup.

Chris69

Tnx for the pics. I have 1 5/8" primary tube hedman headers into 3" collectors. What I removed had 2" collectors (shorty), then expanded to 2.5" pipes and came back together before the cat. New Y setup is similar, which is where the thought to go to a dual/in cat would improve the flow and the path. Like Ganey mentioned, even 2.25" dual/in would be a big improvement. I'm also guessing that a bigger cat rated for a heavier vehicle would also help out. I heard back from the Car-Sound cat guy and they have a dual 2.25 with a 3" out. I guess I'll give it a try. At the same time, I'm geting rid of the s-pipe (so the tips can come straight out the back). I've got some round chrome tips on it and they are curled back in for a nicer finish, but makes a weird ricer kind of flippin/flappin sound when you nail it. Ugly. :smash:

Since I'm rambling on.... the muffler shop missed a spot on the pipe at the back of a muffler (since fixed). There was a lot of air pressure getting kicked out of that spot, going back towards the front of the car. Can't be a good thing.... and they're single chamber mufflers!