Exhaust Diamater
 

I have a bit of a dilema on my hands. Currently just have some old flow masters piped in to keep the noise down. I do not know what series they are, but it is loud.

I am wanting to install the summit x pipe and actually hang my exhaust in the next few weeks. My dilema is this, my Flowmasters are 2 1/4. I am not against replacing them if need be but wouldn't mind not spending the cash either...

With my engine I am not sure what size diameter to run (350 over .030, Comp cam XE274H-10, edelbrock 60899 heads, edelbrock performer intake, edel 600 cfm carb, HEI, full roller rockers, serpentine, etc etc) Should be a pretty healthy little 350.

So with that, should I go to a 2 1/2+ or is 2 1/4 enough? I would rather do it right, and replace the mufflers than be sorry that I didn't spend the time & cash upfront only to redo it in a year.

Re: Exhaust Diamater
 

someone is bound to have the flow knoledge you seek . i would like to know also . bump

Re: Exhaust Diamater
 

There was a lot of discussion a week ago in this thread http://67-72chevytrucks.com/vboard/s...d.php?t=506642. Lots of different opinions. I'm also trying to decide on 2-1/4" or 2-1/2" dual system. Just remember all these trucks with small blocks had single exhaust systems. I think the original diameter was 2-1/4" (not 100% sure). So, even a dual 2-1/4" system is a lot of flow compared to the original.

Re: Exhaust Diamater
 

I'd do 2.5 to the muffler, 2.25 the rest of the way. (just for the sound) 2.25 will have sort of a wraspy sound to it where 2.5 would sound more mellow like it's breathing better. 2.25 would probably flow ok, but 2.5 I think would be the ticket. I wouldn't do any more than 2.5 for the backpressure. If you go too big you'll loose some torque on the bottom end.

Just my .02.

Re: Exhaust Diamater
 

Thank ya sir, exactly what I needed. Guess it will be a good time to get the Spintech's or magnaflow, or 50 series, or... Ugh. More youtube clips in my future.

Re: Exhaust Diamater
 

Video of my exhaust.
2.5 to muffler....2.25 to rear.
With Flowmasters 40's.

Re: Exhaust Diamater
 

Ha. I have actually watched yours a few times on youtube. Sounds great. I have a FM40 on my Jeep (I6, totally different sound obviously), and had one on my F150, on the V8 ford it had a drone at highway speeds that I have heard you don't get with the Spintechs and Magnaflows. I want it to sound civil but throaty at low speeds, and nasty when you open it up.

Re: Exhaust Diamater
 

How many hp do you estimate the motor to produce?

Re: Exhaust Diamater
 

Purely a guess, but I am expecting around or just below 400.

Re: Exhaust Diamater
 

here is what mine currently sounds like with 2.25" and only about 18" of pipe before the muffler. Also on cheap summit headers I have larger chevelle headers to go on when I do the exhaust.

I assume the FM on there currently is a 40 due to the volume.

Re: Exhaust Diamater
 

This is out of an old hot rod magazine I had.

The exhaust needs 2.2cfm per hp that your motor produces.
Exhaust pipe produces 115cfm for every sq/in
At 2.25” OD pipe, 16 ga (roughly 0.06” thick) the ID is 2.13” & area would be 3.56 sq/in
so 3.56 sq/in x 115cfm = 409cfm / 2.2 per hp = 185.90 x 2 for duals = 371.8 hp for 2.25” od

At 2.50” OD pipe, 16 ga (roughly 0.06” thick) the ID is 2.38” & area would be 4.45 sq/in
so 4.45 sq/in x 115cfm = 511cfm / 2.2 per hp = 232.27 x 2 for duals = 464.5 hp for 2.5” od


I wouldn't choke it down to 2.25 if I were you.

Re: Exhaust Diamater
 

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Re: Exhaust Diamater
 

Yeah I am sold on the 2.5, now it is a matter of what mufflers to get to replace the FlowMasters

Re: Exhaust Diamater
 

For that amount of HP..I would go 2.5. Let it breath!
Be loud and proud!

Re: Exhaust Diamater
 

Agree on the 2.5". I think 420-450 hp is roughly the line for moving up to 3". I have an XE-274 with AFR heads and it is right around 500hp in my 383. You should be in the 400-420 range fairly easy although your smaller carb may or may not short you a few HP.

I went with spintech mufflers just for something different. They are a little more expensive than flowmasters though.

Just noticed you mentioned spintechs above. Mine drone right at 1500 and really scream at 2500 when accelerating. At idle they are nice and throaty. WOT they are borderline obnoxious, to evryone but me :) On the highway mine isn't too loud, and doesn't drone really. Cruising between 2200 and 2800 rpm you notice it but it doesnt bother me.

I have 3" super sport 9000's, which are about the smallest they make and dumping out before the axle.
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Re: Exhaust Diamater
 

maybe I missed it but what is the outlet size of the header? This should give you the answer you are looking for dont make it any smaller than it already is

Re: Exhaust Diamater
 

I have these guys, which are 3" http://www.summitracing.com/parts/PTE-H8021-1/

Planning to dump either just before or just after rear axle, probably after to keep some of the noise out of the cab.

Re: Exhaust Diamater
 

Nice choice on deciding on the 2.5. Fwiw, I have my exhaust dumped right before the axle and it sounds ridiculous (in a good way) but it gets obnoxious after a while. I'm going to dump it behind the rear wheels soon.
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Re: Exhaust Diamater
 

I agree, 2.5 and you should be golden

Re: Exhaust Diamater
 

Run your exhaust all the way in the same size pipe, and ditch the flow stoppers! I'd run 2.25" with your setup! When you change pipe size you create turbulence, this is also how flow masters make the sound so many people love.