exhaust systems

[jester]

I need to install a new exhaust system in my 64 Biscayne. I have the 400/425 hp High perf , headers and thats where it ends. I understand that the factory installed 2 1/2 inch that ended after the mufflers. From there the system was 2 inch. My question is: should I replace the pipe with 2 inch from the muffler back or should I have a 2 1/2 inch made?

 

[dq409]

2 1/2 all the way back,,,Thats my vote !! ,,dq

 

[bobs409]

I would say 2 1/2" also. I never could understand why the factory did that. : A big engine like that with 2" tailpipes? Seems odd to me.

 

[jester]

Thanks guys, I've been reading the posts on the subject and for the life of me, I can't figure why they did that either. I can by the stock pipes in 2 inch over the counter but I guess the right thing to do is have it custom made. I seen a picture on the old sight that showed a pipe cut and welded into the header going to the muffler. I like that idea. I need to see if I can get that done, Thanks again.
Congrats on the new sight ! I'll be here often...

 

[dq409]

Jester, go to my website and you will see that photo. I would post it but for some reason it won`t take my photos today,,,BOB?,,,dq

 

[SteveD409]

Maybe they did it for the same reason that they put two head gaskets on these engines-to stifle that brute power just a little!:

SteveD

 

[TomO]

I have read some atricles written by Jim Hand (Pont. guy)on testing of exhaust sytems. I believe that one of the things that he found was that a smaller dia of pipe could be placed down stream and not affect the flow. Going from the 2.5 to 2.00 pipe. If I remember he found that as the exhaust travel down the system cooled down and became more dense. Kinda like putting a inflated ballon in the freezer....it deflates. I know that the article is on the web somewhere and I will look for it tonight. Jim is a rather ledgenary guy around theses parts. I have seen his station wagon at the drag strip in the past and have always been astounded at the times that the thing ran. This guy has written some great artiles on how to build a combination and how to tune it.
TomO

 

[dq409]

TomO,,,Thats a fair assumtion,,,,,,On my 327 mild build I had a 2 1/4" through the mufflers then down to 2". I think if you were to step down on a 409 powered car,your choice but I would only step down from 2.5 to 2.25,,,, JMO I think 2.5 would be OK to run the length with the 409/425 though.,,,dq

 

[CDNpontiac409guy]

2 1/2 to 2 1/4

That's IT !
Yes, I agree. I don't think the 2 1/2" tail pipe is at all necessary. but the 2" is too small. Out of 1 7/8" tube headers / 3" collector into 2 1/2" - 3" mufflers, into 2 1/4" tail pipes.

 

[TomO]

I found it!!! The article tested 2 1/2 pipes with 2 1/4 tail pipes the result was a .02 second difference in the 1/4 mile. The testing was done with a station wagon w/ a 455cid pont. The compaired 3" pipe all the way to 2 1/2 - 2 1/4 pipe. Of all of the mufflers tested the Walker/dynomax 11749 tested with the least back pressure. Also noted in the article was to mount the mufflers as far back in the system as possible. Mine are under the rear seat floor wells.

 

[CDNpontiac409guy]

It's just a Pontiac 455 !

TomO... that makes sense. That engine gets shifted at the same RPM that my 409 idles at ( OK, I'm exagerating.... a little ). I would bet that if they tested with a carefully selected combination of 2 1/2", they would have MADE MORE power.

About tubing.
Headers.
BIG headers !
I just ordered all the parts to build my headers for the Belair.
In addition to my own non-mainstream inclinations ( you may have noticed I get them some times: ) to run 2" primary tube headers... I was speaking with some rather ( understatement ! ) knowledgeable Stock / Super Stock engine builders at a machine shop in Red Dear. They STRONGLY recommended that I step-up the tubing size to 2 1/8" about 8-10 inches from the port. Then feed them around 34" down stream into a 12" long 4" collector.

This is my NHRA Stocker engine. Cubic inch will peg at about 423 ( .060" over D**** 440 piston diameter / .015" stroked crank ), compression at around 11.4:1... 260-268 dur @ .050 Stocker cam... approx 7500 RPM shift.
Well, I WILL be dyno testing this engine. I will have on hand, a set of 1 7/8" Belanger, a new set of Wilson 2", and my set of 2" / stepped to 2 1/8". Won't even try the 1 3/4":evil

Any bets ?

GAWD!.... I LOVE these "smilies" !!!!!!
HEE HEEE HEE !

Bob... this is good, man. The more I use this board, the more fun it is !

Aubrey

 

[dq409]

Jester, Here is a different view,,,,Right click it and save it to your hard drive,,,dq

 

[jim_ss409]

Aubrey, I'll be really interested to see the results of your dyno tests. As you know I'm building a farely mild 474 CI stroker, 238/248 @.050 roller cam and I got a set of 1 3/4" Ballengers because the 1 7/8" Hookers hang down below the frame and they won't work too well on my lowered car. I don't mind giving up some top end power to gain the ground clearance but I'm worried that I might be giving up more than I think.
When I was talking to Curt Harvey he said that there was less hosepower loss than you might expect, I think he said about 15 to 20 HP difference with the 1 3/4 compared to the 1 7/8" but it later occured to me that he was refering to the 1 3/4 Thorley Tri Y style headers that step up to about 2 1/8 at the first Y which is only about a foot from the head. So even though us guys with lower reving engines probably won't need headers quite as large as you will it would still be a good indication of what works.

 

[roaminchariot]

I think part of the trick of matching headers is not just how big they can be. The dynamics of resonance are related to the diameter verses the length of a tube, to time the exhaust pulse so that it exits into a low pressure spot and scavenges the cylinder. The diameter vs tube length sets the frequency or engine rpm at which this occurs. A larger diameter for the same length would raise the rpm at which the pulses flow best--which is good for a race engine or power at higher rpm. So it depends on what you want and at what rpm you want it at! I'd like to see the dyno test results with the whole exhaust system in place!