Coming from bigblockland... this never came up. The 440 with any mods just loves the 3". But now i'm messing with smallblocks... for the first time ever. Plan is to build a Magnum, likely a 360 (though possibly a 318), with all the usual stuff: Air Gap, 670 Holley, Whiplash cam, slightly higher than stock compression (possibly upwards ov a full point more... effectively... as i'll be shaving stuff, and the Hughes cam will build more pressure as well), nice headers, and down the road some ported heads. Well down the road, some Edelbrock Magnum heads and maybe a bigger single plane. Seems this plan starts out around 400HP... and goes up. Thats about what my 440 made... yes, more air out the back, but less RPM than the smallblock i'll build.

2.5" or 3"?

I'm leaning towards 3"... more room to grow, and well... it'll sound mean as hell.

I'll be building the exhaust myself, all mandrel, Dr. Gas X-pipe, from either Super Comp headers or Dougs, side-exit, T/A style... just like my Charger.


What do you non-insane smallblock guys run?

I can tell you that I had a 340 that I would turn to 7k, ran best of 11.85 117mph@3350lbs.

That car had 2.5" to the axle, and 2.25" tail pipes. It was only about .08 to 0.1 quicker when the exhaust was off of it so, so even that little exhaust didn't hurt it too bad.

You do not need 3". It weighs more, it makes the car louder and it fit's tighter. Even a 600" motor does not need 3" IMO.

FAST class cars are required to run 2 1/2.....
And look how much power they make. Some run 130+mph

Yes, but how much faster would they be w/ less restriction?
A pro stocker would probably still run 7s or 8s in the 1/4 w/ 2.5" exhaust...does that mean 2.5" is plenty for an 800 hp motor?
A 600" motor doesn't even need 3" exhaust???? Please.

If you don't mind it being a little louder (most in this hobby enjoy that) and think you'll be adding power in the future, it won't hurt a thing to go w/ 3" exhaust now.

I'm running a 3" system and have elec dumps to
help reduce back pressure on my 416... I wouldnt
have gone smaller... this was 590 HP on the dyno
but I did change to a smaller SFT cam.. was a roller
when dynoed

First off, the mufflers MUST flow as well as a 2 1/2 inch pipe to make a 21/2 system as efficient as it can be. Pretty symple, right? Easy to forget though. A very well designed 21/2 system should allow a full 500 hp with zero loss. But looking at flow charts, percentage of loss as you go above 500+ will start small, but rise rapidly, choking a 700 hp motor by about ten percent, or 70 hp. A good 3 inch system looks like it would allow 730 hp with out any loss. At 800 hp, the loss looks to be about 2 percent, or 16 hp. All this assumes a very well designed system.

Okay, more information.

I'm not worried about tight fit, this is exiting out the side. It will be a one-piece system, just like i had on my 72 Charger, only smaller. That was a very handy system actually... undo collectors and the two bolts and slide it out.

I'll be buying some pretty slick mufflers. They'll flow. My X-pipe perfectly imitates the Dr. Gas design, so i know its good (bad X-pipe designs can do more harm than good, and kill the sound).

I'll be seeing 350-400HP right off the bat. And maybe upwards ov 450-470 with the next build. I have no plans to ever see above 500HP.

I also just realized that both the X-pipes i have are 2.5". I thought i had one 3". I might have just decided. Hmmm... I do like the sound ov 3" though. My Charger sounded awesome... even with a complete junk 440. Maybe a 2.5" system with 3" past the mufflers...

A very well designed 21/2 system should allow a full 500 hp with zero loss.

Well, alot of us gear heads are not designers so we stick to the std. 3 inch system.

I myself run 3 inch magnaflows and pipes or 4 inch side pipes off the headers with modified inserts that can still be removed.

This has been rehashed over and over the years and the three inch seems better.

That dufuss on road kill, not steve D, but the guy whose always doing the redneck builds, beating on new parts, ect has a spot on the hotrod channel with back to back 2.5 to 3 inch systems (kits) I think and the 3 inch produces more overall.

I had a two and half inch system on my old 440 powered BB Dart, Hooker fender well headers into the 2.5 inch pipe back to a set of original Thrust Corvair turbo muff and then 2.5 pipes back to the front edge of the rear tires for a side exit.
I ran the car three times at the old LACR track, it ran a best of 12.23 ET at 109.+ MPH.
I removed the exhaust system completely, not very heavy either and it ran 11.39 ET at 117.+ MPH with no other changes
My Duster had a full three inch system with Magnaflow straight through 14x7 side entrance center exit oval muffs mounted behind the rear end at the rear bumper, it picked up .02 ET and.2 MPH in the 1/8 mile by removing it with no other changes. That system was a little heavier than the Dart was so maybe it was the weight that made the Duster faster and quicker
The Duster was a lot louder than the Dart at WOT
I never got stopped or warned about exhaust noise on either car by the local law enforcement

The OP wants to know at what point he needs to go up in size. It doesn't make sense to spend extra to put a 700 hp system on a motor that won't go above 475 or so. Referance points is all most of us offer, and he will have to decide if the future improvements can make use of it.
I put a 2 1/2 system behind a smallblock with a cross over and sonic turbos where the spare tire well used to be, and it is very quiet, yet will allow enough flow for 400 hp with very litle loss of power if any. Now if i decide to put my race car on the street, 3 inch may not be big enough.

Its Moparts. It is easy

300 hp = 3.0"
400 hp = 4.0"

If you think you might want 401 hp or more in the future, just put a 5" on now.

Ive ran 3 inch full exhausts since like 79-80 and so did all the other really fast street cars, looking for the pea shooter pipes or even a 2.5 into 2 behind the mufflers and since the mufflers back then were not the best it was always a factor to size up the competition.

Just like the "fast" street cars of the 80-s sporting Radial T/A tires...really not so fast.

My buddys Cutlass ran 10.60's at about 123, with slicks. Just a thrown together big block chevy, with mostly parts people had lying around. driven to the track street car. one day, after putting his street tires back on, and loading up the car with the slicks, jack and tools, he realized he could make one last pass, before the night was over...
on the 275/60 radial ta's... and the car loaded with parts and tools.... 10.90....
he didn't go back to slicks, unless it was for money. and this was 1998.

3 inch tailpipes hit the shocks and about everything else--you work and work to get them just right then you put a cooler in the trunk or worse someone in the back seat and clankity clank--so for a car you will USE I think it is better with the 2 1/2 in like already stated the mufflers play a bigger role in most applications just no fun with the 3 in 1/16th to a 1/4 in away from the gas tank and same on the shocks etc

a good read, with real world testing.
http://www.pontiacstreetperformance.com/psp/exhaust.html

My small block is around 600HP and if Pale ups his to this level in the future, is 3" optimal for us?

3 inch tailpipes hit the shocks and about everything else-

Mount the shocks vertical and snake the pipes on the inside of the shock.

Anything is better then turn downs before the axle.

Yes.. I run dumps also.. I wont drop the exhaust off when
I run.. I just open the dumps.. I run a ladder bar so
things clear.. I dont have the BEST muffs but not bad