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302 Exhaust size

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odd ball
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Joined: 13 Mar 2006
Posts: 100
Location: San Francisco, CA
302 Exhaust size

Ok, Need some advice on exhaust size for 302 that will someday be stroked to 347. Getting ready to buy exhaust pipes and only want to do it once. Is there really much noticeable difference between 2.25 and 2.5 inch exhaust? I know bigger = less torque, but is a 1/4 inch really gonna make much difference other than the $40 extra? Ultimate goal is a street cruiser with 347, 350 heads, 600cfm carb, small cam, 3.55 gears, not too crazy. Opinions Appreciated!
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69 Fairlane 500, 351w, 2200 stall C-4, 9" with 3.50 and trac-lock

Post Tue May 02, 2006 12:55 pm 
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la8ron
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Joined: 10 Mar 2006
Posts: 150
Location: New Zealand

I would go for the 2.5 inch exhaust that way when you go to the 347 you dont have to rebuild your exhaust again. The loss you would get would be bearly noticable if at all. There is meant to be some ratio for how big an exhaust for the amount of hp you have, got told it years ago but unfortunately cant remember it any more.
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Post Wed May 03, 2006 12:59 am 
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GerryProctor
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Joined: 10 Mar 2006
Posts: 660
Location: San Antonio, Tx

For sure 2.5" mandrel-bent pipes with performance mufflers. Larger pipes also add a sound component to the exhaust. The larger the pipe, the throatier the sound.

Post Wed May 03, 2006 5:23 am 
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leadfoot25
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Joined: 07 Apr 2006
Posts: 47
pipes

I don't beleive you can go too large a pipe, the faster you get rid of exhaust the faster you go, as far as torque, you tune this with your muffler choice, look at drag cars...open headers, don't think they're too woried about torque! bigger=more better LOL

Post Wed May 03, 2006 12:52 pm 
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scotta66gt
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Joined: 09 Mar 2006
Posts: 396
Location: Arlington VA
2.5" especially if muffler shop is involved!

Howdy. If you're gonna have a regular muffler shop do the work remember that the bends will neck down the pipes. I'd definately go 2.5", the bends will neck it down to something like 2". Hot Rod did an article a while back and they tested bigger and bigger exhausts and they always found more torque AND HP as the size increased.

FWIW, Scott
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Post Wed May 03, 2006 6:46 pm 
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Kyle
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Joined: 19 Apr 2006
Posts: 11
Re: 2.5" especially if muffler shop is involved!

quote:
Originally posted by scotta66gt:
Howdy. If you're gonna have a regular muffler shop do the work remember that the bends will neck down the pipes. I'd definately go 2.5", the bends will neck it down to something like 2". Hot Rod did an article a while back and they tested bigger and bigger exhausts and they always found more torque AND HP as the size increased.

FWIW, Scott


The bigger the exhaust, the more it kinks when you bend it as well. Just some food for thought.

~Good luck finding a shop with a true mandrel bender.

Post Wed May 03, 2006 9:32 pm 
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odd ball
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Joined: 13 Mar 2006
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Location: San Francisco, CA

Thanks for the thoughts, I was leaning towards 2.5 myself, just wanted to make sure I wouldn't lose too much low end torque. Im gonna order a set that Summit has on sale that fits v8 mustangs and cougars, figure I can make it work, same engines and tranneys and stuff. And it is mandrel bent, no loss through the curves.
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Post Thu May 04, 2006 3:26 pm 
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1320lane
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Joined: 17 Mar 2006
Posts: 270
Location: Central Oklahoma

Have you looked at some other vendors? I think there's a company called Pypes & somethingorother. Doug Bender (Fairlaniac) had some stuff done by them I think, maybe his car was a prototype.
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Post Thu May 04, 2006 6:53 pm 
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jswordy
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Joined: 15 Mar 2006
Posts: 19
FLOWMASTER and exhaust size by dyno

Flowmaster makes a complete 2.5" exhaust for 66-67 that is a DIRECT BOLT-ON to 68-69. Looks perfect! Why don't they list them for 68-69? Dunno. FCA members have been asking them to do so for a couple years and still no go.

I agree a 2.5 system is probably right for your car once it is stroked. But it will be inefficient at 302. So I am going to have to disagree with most of the "bigger is better" crowd. But don't take my word for it. Engine guru David Vizard did an excellent article on exhaust systems in "Popular Hot Rodding" in which he showed by dyno results that scavenging effect can be severely compromised by a too large diameter pipe and/or inadequately designed system.

That means that while a system may have volumetric efficiencies, those do not directly correspond with scavenging velocity. Think of water in a pipe. The bigger the pipe, the larger volume of water it can move at a set pressure over a smaller pipe, but it does so at a slower velocity.

Scavenging velocity is crucial to engines because the velocity at which gases EXIT the cylinder has a direct affect on the pulsed volume of new charge that is drawn into it, and also has a direct effect on how much of the spent charge remains to dilute the new charge.

Pipes that are too big for your engine will not create a strong pulse on intake/exhaust overlap, will dilute the new charge with leftover spent gases and will rob you of horsepower. In essence, pipes that are too big can negate everything you try to do with heads, cam, intake and carb by weakening the pulse at the carb that you just spent good cash to strengthen. It has same effect as strangling the system with pipes that are too small.

A system that is not designed to maximize the pulse wave created in it by each puff of exhaust gas into it will not pull gases in on the intake/exhaust overlap and will rob the engine of power.

Dyno charts of an engine starting with too small exhaust through perfect size and then on to too big turn out to look like a bell curve. It's the Three Bears strategy: not too small, not too big...but just right.

For a 302 without a radical cam to increase overlap, a 2" system with H or X pipe would be a much better choice than a 2.5." But since you are going to higher cubes and probably a wilder cam in the future, spend money once even though it means your engine will likely be somewhat less efficient in the short term.


Last edited by jswordy on Fri May 05, 2006 1:20 pm; edited 1 time in total

Post Fri May 05, 2006 1:15 pm 
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jswordy
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Joined: 15 Mar 2006
Posts: 19
Double post. Says it is not posting due to bad password, but

then it does!

Post Fri May 05, 2006 1:16 pm 
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odd ball
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Joined: 13 Mar 2006
Posts: 100
Location: San Francisco, CA

thanks for writing all that Jswordy, it is in line with what I have always thought. The Flowmaster kit looks real nice, but its outta my budget. Also, I failed to mention that the pipes are hooking up to Hooker comp. headers. I change my mind everyday, can't even decide if I want full length pipes to bumper or just dump it with turndowns at the axle tube. Maybe 2.25 inch pipes to the axle tube, smaller size but a short straight shot out, I dunno....
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Post Fri May 05, 2006 2:44 pm 
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edwardejv
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Joined: 10 Mar 2006
Posts: 160
Location: morrow GA

i had a shop do mine 2.25" i bought the headers and mufflers and it all cost around 525.00 out the door i also let the shop put the headers on that added 100.00 to that 425.00 if i wasnt lazy, that inclueded a H pipe to and turn downs at the axle
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Post Sat May 06, 2006 9:28 am 
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nick
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Joined: 13 Mar 2006
Posts: 40
Location: manitowoc, wisconsin

did you guys find that having the exhaust turn down at the rear axle made for a droning sound in the car? i ran 2.25 exhaust in my 69 that turned down at the axle. i was thankful i didn't have to sit in the back seat. once tailpipes were installed, everything was quiet. at that time there was a set of glass packs on the car.
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Post Wed May 10, 2006 4:40 pm 
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roger
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Joined: 09 Mar 2006
Posts: 1008
Location: ontario, canada
exhaust update

i thought you might like to know that i just went with a dual exhaust set up from Bondi;s Auto which is 2.5" & includes mandrel bends ($40.each)with Dynamax muff. ,over the axle right out the back to gentle turn downs.
Its all stainless so you can shine up the tips & they come out like the bumper
Sound is more subdued, no more drone at cruising speed as i had with the old 1.5" pipe with Flowmasters.
Of course it feels faster, but that may be physcological way of making up for $1,000.
Much throatier, until you really get on it. At Hi-RPM, sound is simply spectacular!

Post Mon Jun 12, 2006 12:33 pm 
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odd ball
Senior Member


Joined: 13 Mar 2006
Posts: 100
Location: San Francisco, CA

nice, im gonna order my pipes sometime this month before the sale at summit ends, ill let you know how it goes...
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69 Fairlane 500, 351w, 2200 stall C-4, 9" with 3.50 and trac-lock

Post Mon Jun 12, 2006 4:08 pm 
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