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mts 03-05-2004 09:43 AM

69 firebird exhaust
 
Hi, me and my son are re-doing a 69 fbird. It has a 400, 4 spd and prior to us working on it had lots of lame brained repairs, ie coffee cans in places coffee cans don't go, nails for cotter pins, that sort of stuff. Anyway, basically the engine has been totally redone. Other than a magnum roller and I think 280 split cam, its .30 over stock and has edelbrok intake and 750cfm edelbrock carb. The question is re: exhaust. I bought some hooker headers and plan to put either 2.5 or 3" pipes to flowmasters on. I don't understand why one would be better than the other, I mean the old american way, if one is good, 2's gotta be better, why stop at 3"? Can anyone help me understand the nuts and bolts of how to select the best exhaust for this car? Its not going to the strip, or even to the blvd. Just to school, dates, and errands for me (I'm sure thats all my son will admit to anyway), thanks!

Siggy_Freud 03-05-2004 10:32 AM

2.5 inch exhaust is a really good size to use when you are going to be using dual pipes (as with headers like you are). With a healthy 400 you COULD use a 3 inch setup but with 3 inch dual pipes you might lose a little backpressure and hurt the performance during your powerband (only by a couple HP I'd imagine).

I went from 2.25 exhaust to 2.5 on a healthy 350 and really loved it but im not sure i'd go any larger.

If you want the look of the bigger exhaust just get some flared exhaust tips that are a 3 inch design (these actually look really good in the right applications).

In any case its an awesome car to work with and a nice engine as well. Have fun with it. Im sure someone will have more info on exhaust shortly.

adryan16 03-05-2004 11:43 AM

I run a 350 pontiac with dual 2.5" pipes, but I found this after the fact. I personally think that 2.5" is more than enough, but this is something to consider I guess. If you go to this website, scroll down on the right, and you'll find where it says "Exhaust sizing for power". Just something to think about.

http://pontiacdude.cc/

36scsc 03-05-2004 12:54 PM

mts,

There is no simple answer to your question but since you state that you are going to be doing little high speed/ high rpm stuff with it you may want to consider this one general rule of thumb....Smaller diameter pipes produce low- to mid-RPM torque and larger diameters produce mid- to high-RPM torque. Low rpm + big dia. exhaust = reduced velocity and more heat loss, neither of which you want. Bigger ain't always better.

mts 03-06-2004 12:21 AM

thanks
 
Thanks for the help! I know that stock is what its all about, and yes I have a nice straight 69 with some coffee cans and other oddities but is getting upgraded. I am putting power disk brakes on all 4's, new polyurethane bushings and suspension including springs. The knucklehead before me put shackles for the leaf springs and welded them to the back bumper, go figure. Anyway, since we are in arizona I am also putting AC in the thing. I probaby will put 2.5 in it. Once I get the beast done I'll post some before/after photos. the interior is 100% stock, redone, and sharp even down to the fake wood. Im into this thousands as you guys know so I appreciate the advice so I do it right. I'm not going to cut corners at this point. I forgot to mention in case it matters on the exhaust, its all msd ignition. Thanks guys and gals. Later

Siggy_Freud 03-06-2004 01:39 AM

MSD makes some great stuff however it shouldn't affect your choice in exhaust sizing. For that motor, application etc. I'd recommend either 2.25 or 2.5 inch piping.

Cant wait to see the bird.

Dylan.

jimfulco 03-06-2004 02:01 AM

Straight 2.5" pipe will handle up to ~230hp per pipe, but mufflers will usually be more restrictive than the same size straight pipe.

http://www.pontiacstreetperformance....p/exhaust.html has some good exhaust info. The rest of the site is good, too.

6567GTO 03-06-2004 07:15 AM

Great advice given so far. If you're interested, here is a link to a CD of Pontiac exhaust sounds you could buy. There's something like 26 different combos both with and without an X-pipe crossover for a total of 52 tracks.

http://shop.sslonline.com/cgi-sz4/we...21103589450.ca

TurboS10 03-06-2004 07:33 AM

Read through this thread with all the links. It has a ton of information and dispells the backpressure bull.

http://hotrodders.com/showthread.php...t+backpressure

Chris

4 Jaw Chick 03-06-2004 07:51 AM

Hey guys, glad to hear that bigger isn't always better. :D
mts, my cousin had a '69 bird, back in the '70s. It was quite a desireable car to have in our neck of the woods, and I have to admit, I'd love to have one like it today.
Siggy, you said what I was thinking. Bigger diameter could affect the backpressure and sacrifice a bit in the performace dept.
Keeping in mind that mts' son will be the primary driver, maybe that wouldn't be a bad thing....:P

Chickie.

machine shop tom 03-06-2004 08:16 AM

I've run 2 1/4" exhaust with headers in my 69 Camaro for years. I've not felt a need for anything bigger plus there is precious little room for anything bigger (I'm running full duals to the rear, exiting behind the wheel opening). I'm running a 350, but I don't think a 400 would tax the system much more. The mufflers are Midas turbos, which were rated pretty good years ago. Not too loud but it barks pretty good up in the r's.

6567GTO 03-06-2004 08:24 AM

originally posted by 4 Jaw chick:

Quote:

Hey guys, glad to hear that bigger isn't always better.

Boy! Me too!:sweat:


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