Exhaust 3" or 2 1/2"?

IMHO, I'd run 2.5". Here's why. The resistance and flow characteristics in the pipe itself changes dramatically for each size you move up. Using basic math you can see the difference.

3" pipe= 85.2 square inches volume in 1' of pipe 2.5" pipe= 58.8 square inches volume in 1' of pipe. Difference of 26.4 square inches. 2.25" pipe= 46.8 square inches volume in 1' of pipe. Difference of 12" and 38.4" from 3" pipe

You can see how this adds quite rapidly even though it's only a fraction of an inch change in diameter. Over the full length of an exhaust system which is...oh say...16', the 3" system ends up with a capacity almost twice that of the 2.25" system and 50% more than the 2.5" system. This may sound good, but it's a huge difference for what the engine sees in resistance. If the motor weren't blown I'd say 2.25" would be all you'd need, especially if you want to keep some reserve torque. If you had a more cubes, or a BB, then I'd say the 3" would work. I'm running 2.5" on my 400ci Windsor and it's plenty.

The other thing you might think about is the sound you'll get from the different size pipes. Acoustically there is a difference. As a brass musician and someone who has studied acoustics, I will use some simple comparisons. The French Horn and the Baritone are roughly the same length, however the Baritone has roughly twice the bore (inside diameter) of the French horn. The difference in resistance is immediately noticeable. All things being the same, the French horn plays much higher than the Baritone and the Baritone plays much lower than the French Horn. For the same amount of exerted force (flow). Hence a smaller exhaust pipe will raise the pitch of your exhaust note, and I'm not even getting into mufflers. Another thing, big exhaust tips or trumpets don't make the pitch deeper, just louder. Listen to any riceboy for proof of that and I won't have to use a musical instrument comparo.

IMHO, I'd rather have a pipe that's slightly too small than drastically too big. Even though it's only 1/2" difference, it ends up being a HUGE difference all done.

So was that WAY too much description?! ;)

Dr. Gas makes an X pipe that is 3" in and 2-1/2 out. Sounds about perfect for you. They sold me that setup for my 342 and it worked pretty good, but I wasn't running any boost.

MadDAWG

Scott,

It probably looks good to most folks, but "square inches volume"? It's square inches of cross sectional area or flow area, and cubic inches of volume. A 3" pipe has 7.07 square inches of flow area (A = pi x radius x radius) and a 16 feet length of 3" pipe has 1357 cubic inches of volume (V = Area x length).

This is a good set-up as the exhaust gas will cool and volumn decrease as it travels through the system. Expense is down also.

bradtx

That engine is passing a lot of gas. Treat it like a big block, and go 3" all the way. There's a good article in either Hot Rod or Car Craft this month. They tested a decent big block on 2.5" and 3" pipes. The 3" pipes made more power in the mid and upper range, without any loss of power at the bottom end.

Yeah, I didn't realize I had it off by one dimension until *after* I hit send. oops, but the theory is still sound.

Both of you are ignoring the fact that at 15 lbs of boost, a 331 needs to move as much air as a 662-incher. The theories of tube sizing for static displacement go out the door. From the Holley site:

Read the whole page at:

I have 2.5" exhaust the whole way for my 351C stroked to 377 CID. I get good power through the entire RPM range with max of 340 RWHP @ 5,500 RPM & 400 lb-ft torque @ 3,300 RPM.

He's right. That's why many of the Turbo Regals run dual 3" exhaust and this is with 3.8L or 231ci. Forced induction needs all the exhaust it can get.

///Mike TS #63

1993 BMW 525i of all the old cars - Newly updated

Also gotta' remember that forced induction needs less back pressure than an N/A motor. The back pressure in an N/A motor keeps the pressure up a bit during the valve overlap of the exhaust closing and intake opening, this helps with the cylinder fill. Forced induction doesn't need much help to fill the cylinder, but the back pressure is needed to assure the fresh intake air doesn't make it out the exhaust valve during the overlap period.

I was too focused on the math problems to see this, but you're damn right! Thanks!

The sales guy gave me some big long talk on how the X will make the dual

2-1/2 inch section flow more than the a 3" pipe would, but to be honest it pretty much all went over my head. However it does sound wicked. The Dr. Gas setup is much smoother sounding than my BBK X pipe I have on my shorties now. I just need to get me some 1-3/4 long tubes w/ 3 bolt collector by this coming spring.

MadDAWG

Excellent post. Too big an exhaust will slow exhaust velocity dramatically. Not a good thing if you want any power below top end.

LJH

95GT (331 stroker)

I have walked into a great deal locally. I have decided to return the hooker 1

3/4 headers I bought. I was picking up a meziere electric water pump from a guy locally when I noticed the exhaust he had in a trash can. It turned out to be a set of kooks 1 7/8 step headers x 3 1/2 with the kooks 3 1/2 in dumps. I picked up the exhaust and water pump for 900$. My plan is to cut the dumps off the headers reduce to 3" run a 3" crossover and put the dumps where the stock mufflers go. What do you think? My neighbors are gonna hate me!

$900 for a water pump and a set of headers you found in a trash can! Hmmmm . . . . why don't you have a look thru my garbage cans. Honestly, this may have been a good deal if the headers were stainless steel and were for sale, but they were in the trash can for christ sakes! You could have waited til he set them out for pick-up, drove by, and had them for free!

Will you be anybody's bitch, or are you his now that he broke it off in you?

Just because it was stored in a trash can doesn't mean that's where it was headed, dumb ass...

And if you wanted to negotiate for something out of the owners *trash can*, I bet you'd immediately agree with them when they fed you the same line you fed me, "I was just storing them there". Yeah, right!

These kooks headers were in brand new condition stored in a garage. These are stainless in perfect condition . I saw receipts for the exhaust for 1400$ from kooks. The meziere water pump lists for 400 $ and was also in perfect condition. I think I made a pretty good deal. And to the one jerk You come up my way And youll see who I make my bitch!

The headers are stainless in perfect condition and they were for sale. They were in a plastic can that had never been used for garbsge. He also showed me receipts for the exhaust which was 1400$ from kooks. The meziere water pump retails for 400$ So I think I made out pretty well. Bring your mustang up my way (if you even have one), and will see who's the bitch!