How much difference does exhaust pipe size really make?

[c4fan]

I'm finally getting ready to buy my exhaust system and was wondering if there is really that much of a difference between 2.5, 3.0 or even 3.5 inch pipe. I plan on installing the usual mods and want to make the right choice when I plunk down my hard earned cash. I am replacing the Y pipe back so any suggestions or experiences would be appreciated.

 

[tntcorvette]

if your car is stock, ide go with 2.5"

a too big exhaust system will cause a loss of torque, but mabey a smigen more hp.

 

[vettl83]

2.5" should work fine, unless your build up is going to be on the wild side. Then streetability becomes an issue. 84 CF 91 ZR-1 w/2.5" system.

 

[Raistlin]

I'm running hedman 1 5/8 long tube headers. My first exhaust was 2.5 duals, no cats. I picked up a lot of hp and tq everywhere (254rwhp, 354rwtq w/ AIR pump elim, K&N, cut air box, frisbee delete, porting to plenum only behind the throttle body)

The 2.5" pipes in the "kit" i got never fit right, so I ended up buying LT1 exhuast hangers and getting a local exhaust shop to make up a 3" dual system with an H-pipe.

I didn't notice any difference between the 2.5" and 3" dual exhaust. Some of this is probably do to the long tube headers, plus, the TPI has such strong low-mid range tq. I haven't been back to the dyno to get some hard numbers, but SOTP i'm not missing any low-end tq.

 

[EricVonHa]

The 2.5" should be fine for just over 400hp. The big thing is make sure the pipes are mandrel bent. Any of the aftermarket performance mufflers are fine. Choose your flavor. There isn't going to be any appreciable gains from one to another.

 

[c4fan]

Thanks guys. Think I will stick with the 2.5. The car is daily driven and is not going to be too wild. Definitely don't want to give up any bottom end grunt.

 

[Redbob]

Like the guys said, 2-1/2" pipes should work fine, since the vast majority of the restriction or backpressure in your system comes from pipe transitions and catalytic converter and muffler internal restriction, very little from frictional flow resistance.

I made a 2-1/4" system for a stock '91 ZR-1 that sounded better than stock, reduced the weight by several pounds, and even lowered the system backpressure by 10%, since there was less of a "step" down (or up) from the pipe diameter to the muffler and resonator inlet and outlet bushings. I forget the exact figures, but this smaller diameter system picked up several horsepower.

Regards,
R

 

[Mad-Mic]

yeah you'll be fine with 2.5" pipe.

BTW sweet looking ride! i got the same car but with the 88-90 sawblade rims on mine i just love that color!

 

[scorp508]

You'll be fine with 2.5, but if you have future plans of big modifications might as well go 3" now and forget about having to do it twice.

 

[Redbob]

Sorta depends on how far you plan to go:
2 x 2.5" will be all you need for 475 HP or more on the street.
Going bigger at lower HP just makes the exhaust sound "trashier" - but by all means, if you want it to sound like Bubba's pick'em-up, then go as big as you can.
Just keep in mind that bigger = heavier, so when you're talking about microscopic horsepower gains for large weight increases, well, you figure it out.

Regards,
R

 

[Mad-Mic]

regards to weight....

for every 100 lbs lost in weight you'll gain .10 at the track

 

[scorp508]

Guys he said he's replacing from the Y-Pipe back. That to me says he's not doing duals.

 

[Redbob]

Guys he said he's replacing from the Y-Pipe back. That to me says he's not doing duals.

Exactly.
He's not going for maximum horsepower, and it would be senseless to put an exhaust suitable for a 600 HP race engine on a Sunday-cruiser Corvette.
The cost, the weight penalty, and the crummy sound is hardly justified by... no horsepower gain whatsoever , do you think?

Of course, if you just want the big pipe look, go down to Miguel's Mufflers and have him put 4" outlets on your mufflers.
Might want to get some 20" rims at the same time, and some of those valve stem caps that light up, maybe a little neon for underneath...

 

[KTA1000]

To through a wrench into the gears,, I seen a few of the Vette catalog companies offer a 2 1/4" true dual system, eliminating the cats for "off road only",,, they claimed 30 HP gains, could this be true? has any one used these? How do you think they would compare to a Cat back system? Too bad it's not a 2 1/2 system

Paul

 

[scorp508]

Exactly.
He's not going for maximum horsepower, and it would be senseless to put an exhaust suitable for a 600 HP race engine on a Sunday-cruiser Corvette.

I hope you don't think I meant 3" duals.

 

[Redbob]

To through a wrench into the gears,, I seen a few of the Vette catalog companies offer a 2 1/4" true dual system, eliminating the cats for "off road only",,, they claimed 30 HP gains, could this be true? has any one used these? How do you think they would compare to a Cat back system? Too bad it's not a 2 1/2 system

Paul

Paul:
Take a look back to my first post on this subject, 7th post down from the top.
I mentioned that I've tested a 2-1/4" (true dual) system on a ZR-1, and got a 10% reduction in exhaust backpressure, good for a couple horsepower even though the LT-5 engine is a 4-valve.
And, this system had simple "open" bends, not mandrel-bends like so many 'feel' is necessary.
This was even before we made any changes to the muffers.
You can guess that if this worked on a 405 HP LT-5, it'll work on most street cars!

What we accomplished by reducing the pipe diameter was to have the pipes more closely match the openings in the resonator and mufflers.
{On the ZR-1 exhaust, even though the pipes are 2-3/4" OD, the muffler and resonator inlets are closer to 2"!! (2-1/8" endplates, with an inlet bushing)}
So, there was much less disturbance of the flow as it entered or exited from the resonator or the mufflers.

I know some of this sounds counter-intuitive, but I ran a "data-driven" operation: if the flow-bench, road-test, and dyno results agreed with fluid dynamics theory, we went with it.
Had a hard time explaining some of this to a couple of the GM engineers (the 'F'-car guys), who seemed to have gotten everything they 'knew' about exhaust systems from Hot Rod Magazine!

Best Regards,
R

PS: As I noted on a separate thread, the stock exhaust backpressure on an L98 around '89 - '90 reduced the power output by 30 - 35 HP. So there are big gains to be had with an "offroad" system, more like 25 HP than 30, but it could be 30 HP gained if you'd done several other mods to your car - big cam, orted intake and heads, etc.
And of course, the "off-road" system saves a lot of weight by storing the cats in your basement!

 

[Red73BB]

Adding on.....

On this topic...I'm planning on a new exhaust system this year. I have a '73 454. When I bought the car the motor was already rebuilt for 425hp (never dyno'd) what should I go with 2.5" or 3" what mufflers would you recommend. Should I have a H pipe installed, if so where is the best location
Thanks in advance.
Gary

 

[Redbob]

I hope you don't think I meant 3" duals.

Well, it sorta looked that way...:eyerole

 

[Redbob]

Re: Adding on.....

On this topic...I'm planning on a new exhaust system this year. I have a '73 454. When I bought the car the motor was already rebuilt for 425hp (never dyno'd) what should I go with 2.5" or 3" what mufflers would you recommend. Should I have a H pipe installed, if so where is the best location
Thanks in advance.
Gary

Gary:
I'm not familiar with the underbody of the '73, so I may recommend something that simply can't be done, but here's what I'd try:
I'd plan on a 2-1/2" system with mufflers that have 'true' 2-1/2" inlets: by that I mean, check and see if they don't just immediately inside the inlet neck down to say 2" - in that case I'd run probably 2-1/4" pipes.
I've seen 2-1/2" mufflers listed for your '73 in the Mid America catalog. Or maybe the 3-chamber Flowmasters, but some folks say these are awfully noisy, although a rear 'H'-pipe reduces this.
Try to find 409 "stainless" tubing, or better yet, aluminized 409, which is proof against salt corrosion for 10 years or more, but at the VERY least, use aluminized carbon steel.

IF there's room, fit a pair of straight-through resonators about 2/3 of the distance along the pipes from the engine to the mufflers. This breaks up any acoustic resonances and reduces low-RPM "boom" in the cabin.
Put a cross-over or 'H'-pipe towards the rear, just ahead of the axle if possible. Use about a 2" pipe here. Aluminized or stainless! (Don't use anything like 304 -309 stainless anywhere you'll be welding to it - it'll just break off down the road a ways)

A 2-1/4" to 2-1/2" 'H'-pipe up front ahead of the resonators might give a slight performance improvement.

Regards,
R

 

[Red73BB]

Thanks, Redbob I appreciate your input. When I go to a exhaust shop I will bring a copy of this post.