3 inch exhaust too much?

[Chlsnk]

The machine shop told me my engine sould be running about 10:1 compression with my 2.02 double hump heads .40 over 350 with a mild port job. They also told me a 3 inch exhaust with an H pipe will be the ideal exhaust set up for me.

I'm all for the H pipe but 3'' inch exhaust just seems like alittle much for the said engine you know not enough back pressure?

[sdelmar]

I would think so too. I would figure 2.5" would be sufficient with true duals. Dual 3" would be enough for a really "built" 350!

[ProStreet68SB]

+1 for 2.5in. You'll be happy with the torque gains of 2.5 over 3 also.

[bigsnookmaster]

I'd use 2.5" with an x-pipe. I use a 3" mandrel bent with an x-pipe, but I have a pretty healthy 406 with a mechanical roller cam. You will be much happier with 2.5" Good luck

[ripdog28]

2.5inch

[BossHogg69]

I agree on 2.5" at the most, with an H or X. I talked to one of the guys at Pypes a little while back when I was doing research for my exhaust and was told a 2.5" setup with an X will support a 500-525hp. I wouldn't go to 3" unless I was pushing 550hp or more, especially in a street driven cruiser. I have a friend with a '77 Cheyenne swb with a 450hp 454 and he switched from a set of 2.5" duals to a 3" setup and claimed he lost some torque, but that's just a seat of the pants observation from him.

[swb85]

Originally Posted by Chlsnk

but 3'' inch exhaust just seems like alittle much for the said engine you know not enough back pressure?

Biggest.....myth.....EVER! Back pressure is a bad thing, and you never want it in your exhaust. I do agree that 2.5" is plenty for the motor in question, but you wouldn't lose any power with 3".....just be a little noisier.

[Wild83C10]

Originally Posted by swb85

Biggest.....myth.....EVER! Back pressure is a bad thing, and you never want it in your exhaust. I do agree that 2.5" is plenty for the motor in question, but you wouldn't lose any power with 3".....just be a little noisier.

Agree 100% backpressure only hurts you, it doesn't help.

2.5" is plenty for that combo, but 3" would be bada$$. It did on my 355. Flowmaster sells a universal exhaust kit that works well on our trucks. That is what I made mine with. I am actually selling mine and you can see pictures in the classified section.

[leddzepp]

2.5" for a small block, 3" for a big block.

[Chlsnk]

When It comes to gettings exhaust done I may go with the 3 inch exhaust if it isn't going to hurt me.

[QKENUF4U]

from what i read a long time ago in a hot rod/or similar mag..... use what the ENGINE NEEDS to the muffler and after the muffler WHAT EVER YOU WANT...... a engine is a big air pump, ya just gotta figure out how big is needed to get all the air OUT thats been sucked IN.....
3" would def. sound good..

[Super73]

Hmmm.. I must have too big of an exhaust for my 347ci motor..

BTW, bellow are a couple dynograph overlays showing a mild cam and bolt on 347ci 3rd gen motor (stock heads). The lower numbers are going through 3" exhaust to the cut out then necks down to the 2.5" exhaust to the mufflers. The higher numbers are with the cut outs open IE dual 3" straight open..

I have done several exhaust similar to this, some 3" all the way through, some necked down to try and quiet it down a little.. All of them showed gains in both TQ and HP under the curve with less restriction. I was proving a lot of people wrong back in 01 about back pressure. Even with stock cams, the gains were all over..

I think a well thought out set of headers make the biggest difference with exhaust scavenging.

[SCOTI]

Originally Posted by Super73

I have done several exhaust similar to this, some 3" all the way through, some necked down to try and quiet it down a little.. All of them showed gains in both TQ and HP under the curve with less restriction. I was proving a lot of people wrong back in 01 about back pressure. Even with stock cams, the gains were all over..

I think a well thought out set of headers make the biggest difference with exhaust scavenging.

That's how it should be viewed IMO. Get the correct headers for the application & design the exhaust from there. If the headers have large primary pipe (>1.625") w/3-3.5" flanges use 3" tubing. If they're smaller primary pipe (1.625 or less) w/a 2.5-3" flange, go w/2.5" tubing.

I'm planning to do 3" from the collector through the mufflers w/some 2.5" tail-pipes for a slightly warmed over 350 (and I do mean slightly....) because eventually it will get a 408/AFR combo sitting on the stand & I want to spend my money wisely. I figure that's a good all around compromise.

[Marv D]

Originally Posted by Super73

I think a well thought out set of headers make the biggest difference with exhaust scavenging.

I was waiting for someone to bring that up. If you have a set of super cheepie 1-1/2" long tube blackjack headers dumping into a 2-1/2" collector,, ,,, dumping that into a 3" exhaust pipe and muffler just doesn't make any sense to me. THINK the system all the way through. Good pieces here and there just don't get the job done.

[mickman77]

My BBC has 1.625 headers w/ 3" collectors. The old exhaust system was 2.5" thru cherry bombs and out behind the tires. All of that was cut out and 3" thru flowmasters dumping in front of the rear axle made a helluva difference. Another bonus is that it isn't really that much louder.

[petey-pablo]

Im running a 3" exhaust setup my mechanic recomended for the motor he build for the truck. Others would disagree but I like it.