The Official GMN Exhaust Size Thread

[Freshmeat]

Buckle up 'cause I'm diving right in...

The surface area of a 2.00" pipe is effectively 3.14".
The surface area of a 2.25" pipe is effectively 3.98".
The surface area of a 2.5" pipe is effectively 4.91".
The surface area of a 2.75" pipe is effectively 5.94".
The surface area of a 3" pipe is effectively 7.07".
The surface area of a 3.25" pipe is effectively 8.30".
The surface area of a 3.5" pipe is effectively 9.62".

There are debates about what size exhaust to run for each application. I'm going to state that for most HO-converted Panthers, a dual 2" or a single 3" will be the best route. I'll tell you why...

My research shows that a dual 2.5" exhaust system can support roughly 500hp before it becomes a choke. If you look, 2.5" is a little under 5" of surface area. So thinking about it, if you take the total surface area of the pipe (9.82" in a dual 2.5" system) and divide it by 2, then multiply by 100, that is ROUGHLY the power the exhaust can support before it becomes a potential restriction.

9.82" / 2 x 100 = 491hp

Running on that theory...

Dual 2":
6.28" / 2 x 100 = 314hp

Single 3":
7.07" / 2 x 100 = 353.5hp

Personally, I prefer to slightly undershoot my exhaust size (after running dual 2.5" on a Panther wagon for two years) to enhance torque numbers. On our large cars torque is a must. Some will probably state that they went from dual 2.25" to dual 2.5" and noticed a huge difference. I want track results. On mildly built Panthers, it's best to keep it modest to retain the torque instead of overkilling and losing bottom end performance only to possibly gain 1-3 horses in the top end.

To obtain the optimum performance level of an exhaust system, take its peak number and divide it by two. That will give you the lowest rating at which it may provide results. If that is the lowest rating and the peak is the highest, halfway between will be the optimum performance level. For example:

Dual 2":
6.28" / 2 x 100 = 314hp
320 / 2 = 160
160 / 2 = 80
160 + 80 = 240
Optimum level = 240hp

Single 3":
7.07" / 2 x 100 = 353.5hp
350 / 2 = 175
175 / 2 = 87.5
175 + 87.5 = 262.5
Optimum level = 262.5hp

Dual 2.25":
7.96" / 2 x 100 = 398hp
400 / 2 = 200
200 / 2 = 100
200 + 100 = 300
Optimum level = 300hp

Dual 2.5":
9.82" / 2 x 100 = 491hp
500 / 2 = 250
250 / 2 = 125
250 + 125 = 375
Optimum range = 375hp

Now... there are other things that play a role in what the system can and can't support. For example, on a dual exhaust system, an X-pipe will typically scavenge better than an H-pipe, allowing for better flow. In any instance, a crossover is damn near a necessity to balance the pressure between the two tubes. A catalytic converter may or may not noticeably slow the air flow. Muffler selection will also play a part in flow velocity and expansion, as well as audible volume.

As far as the sound from mufflers... it really depends on what sound you like...
Straight-through mufflers will typically provide a more raspy sound, but usually last longest.
Baffled designs provide a more mellow tone but are known to rust due to the design (manufacturers are steadily improving that).
Packed mufflers (stock style) will provide the quietest exhaust but greatly depend on the manufacturer's quality to determine how long the packing material will last.

[Blaze86Vic]

The problem lies in that those are theoretical values. In motorsports the most trusted values are empirical, not theoretical. And though 2" dual is enough for an HO, 2-1/2" dual flows more, and more flow can result in more HP. Do all the research you want, but that's the wrong way to bark up this tree. Because this is a heavily real world tested topic. And 2-1/2" dual exhaust results in notable performance increase (dyno, track, and road tested and proven) over 2" dual exhaust on a bone stock 5.0L HO FACT.

There is a big difference between being a bottle neck and being a restriction. Bottle neck means that an item is THE limiting factor in peak power. A restriction is something that marginally holds back potential. 2" daul, is a restriction.

[Freshmeat]

You're talking about strict horsepower. In a Mustang, that's all fine and dandy.
In a 4,000# sedan, torque needs to be a huge factor in exhaust system selection. If you want the most flow, just run a dual 3.5" system. It CAN result in better horsepower... but will it improve real world performance in a heavy sedan? Not likely.
That said, you're right that these are theoretical numbers... but it gives these guys something to go on. It's better than nothing.

[Blaze86Vic]

Even in a sedan that is under 4000#s 2-1/2" made an improvement overall. And even in a heavy car, HP is what matters when it comes to high speed. Because lets face it, you aren't going to be doing 100mph at 2200rpm.

[88Vic]

So a single 3" system with the same cat and same muffler as a dual 2" system will give better torque? Somehow that doesn't sound right. But putting the numbers aside, which system will be louder and deeper?

[Freshmeat]

So a single 3" system with the same cat and same muffler as a dual 2" system will give better torque? Somehow that doesn't sound right. But putting the numbers aside, which system will be louder and deeper?

If you're going for volume, you'll probably want a dual system, as the single exhaust system tends to blend the exhaust gas "pulses" together more and it comes out in more of a hum than a growl.

[mrltd]

Yeah, 2.5" is way too much... my 16 second lopo proves that...

The thing about theory is that it doesn't account for the diameter change in the pipes from bends and all that other neat stuff.

I remember dropping the exhaust after the cats at the track and picking up at least 1/10th or better in back to back runs with the lopo. That was with equal length shorties and the stock rear cats on each side. And that was with a stock dual exhaust system that had striaghtpipes instead of mufflers.

[Southern_Pride]

So your samurai should have a .22 barrel size exhaust then instead the whatever it comes with? Yeah. Ill keep my big pipes.

[Freshmeat]

Stop being jackasses. This is a general reference for the guys looking to modify their exhaust systems on daily driven cars, which do not see wide open fucking throttle through every god damned gear. It's about improving the daily performance through BALANCED building, otherwise we should all go out and buy straight 4" dual exhaust for every fucking car we own.

Have a nice day.

[Grand Marquis GT]

Alright, enough dicking with his post.

[Enigma]

and the stock exhaust valve size is what... 1.46" inches.???
think about it.

[gadget73]

Generally speaking, he's right. Big exhaust on a motor that doesn't move enough air to have good exhaust velocity is going to suck torque. The problem is, on our cars the over-axle pipe with all its twists is a restriction, so you need bigger pipe back there to keep it from becoming a restriction. Same reason 1 5/8" headers on a stock motor do not make as much power as a 1 1/2" header. The ports on the side of the head are not bigger than 1.5" and the size mismatch results in poor exhaust gas velocity and lousy scavenging.

[turbo2256b]

Generally speaking, he's right. Big exhaust on a motor that doesn't move enough air to have good exhaust velocity is going to suck torque. The problem is, on our cars the over-axle pipe with all its twists is a restriction, so you need bigger pipe back there to keep it from becoming a restriction. Same reason 1 5/8" headers on a stock motor do not make as much power as a 1 1/2" header. The ports on the side of the head are not bigger than 1.5" and the size mismatch results in poor exhaust gas velocity and lousy scavenging.

Wrinkles from pipe bending are not good either mandrel bent stuff is the best. Sound better too.. In the case of ported exhaust ports. The ones in your heads now flow more than the intake did stock.

Heat is also a factor as the exhaust loses heat the air speed slows exaust volume cotracts so smaller pipe at the rear and or a smaller muffler the closer it is to the rear can work out better.

[Blaze86Vic]

I didn't say bigger was always better for performance. 4" exhaust on a LOPO or HO would cause a complete reduction is power and torque for anything under 5500rpm. But I do think that your suggested exhaust sizes are slightly on the small size.

I would have to say that 2" to 2-1/4" duals would be the best thing to do for a LOPO, and 2-1/4" to 2-1/2" duals for the HO guys.

[Freshmeat]

But I do think that your suggested exhaust sizes are slightly on the small size.

What can I say? I've grown accustomed to using small pipe?

Nah- I'm big on torque- it's probably the offroad driver in me... but I'd rather sacrifice a little top end to have the torque down low.