At what point is 3 inch exhaust required?

[1968Maro]

The thread title pretty much says it all, but in your opinion at what point is three inch exhaust neccesary? (as opposed to the 2 inch exaust I currently have).

I am sure there is a switch over point when it becomes neccesary when your engine goes over a certian amount of displacement or when you are running power adders (Turbo).

[64malibu]

yea mostly depends on power and if it needs to breathe 3in is reccomended for performance engines

[ProdigyCustoms]

3" is usually recomended past 500HP

[77thor]

3" is usually recomended past 500HP

I agree. I have read tha same thing.

[Nine Ball]

Based on the numerous cars I've had, we saw no gains going from dual 2.5" to dual 3.0" until the engine combo was making at least 490-500 rwhp. That is in the 550-600 crank HP range. Of course, that also depends on a few things.

1. Assuming a mandrel-bent system, not Billybob's Muffler World with the crinkle-bent crap.
2. Assuming you already have high-flow mufflers on both systems.

[TnBlkC230WZ]

Based on the numerous cars I've had, we saw no gains going from dual 2.5" to dual 3.0" until the engine combo was making at least 490-500 rwhp. That is in the 550-600 crank HP range. Of course, that also depends on a few things.

1. Assuming a mandrel-bent system, not Billybob's Muffler World with the crinkle-bent crap.
2. Assuming you already have high-flow mufflers on both systems.

This is interesting 9 Ball. I'm a firm believer in keeping the exhaust as small as possible. Do you have any specific examples where you tested this? It would be good for referrence as the bigger-is-better debate always comes up.

[Jim Nilsen]

Also a good point to be made is that reducing down from 3" to 2.5" after the mufflers has been proven to keep velocity up and increase torque on the bottom to midrange. Or 2.5" to 2" which ever is the case in your stystem. most factory exhaust systems emulate this.

[avewhtboy]

when we ran lt1 in imsa racing we had one 3 inch mandrel bent pipe no muffler, were were told by GM engineers thats good for up to 450 horsepower.

[parsonsj]

It also matters where you're measuring. Gases leaving the combustion chamber at 1400*F require more volume than when they are 450*F at the tail pipe.

jp

[wmhjr]

Yup. I used 3" oval X-pipes through 3" spintech mufflers through 3" round tail pipes, but considered reducing to 2 1/2" tailpipes because the exhaust gasses cool toward the rear. I elected to stick with 3" all the way because I could and frankly because I could actually use a little bottom end tq loss and free breathing upper end with my 462ci.

[parsonsj]

I won't bore folks with the math (which is fairly trivial) and the ideal gas law, but my calculations tell me that at constant pressure and liberal rounding:

a gas at 1400*F (1000K) in a 3" tube, will only require a 2.25" tube at 450*F (500K).

So use a 3" tube to the muffler, and then a 2.5 or 2.25 tube after it, and it will have little or no effect on horsepower.

jp

[1968Maro]

Okay, I will bite.

I was sure there was a formula to figure out flow rates out of the exaust but I have never seen one before.

What is it?

People call it back pressure all the time but its actually the ventri effect you are looking for in an exaust. You don't want to have your engine flow to freely or you will not remove as much exaust from the combustion chamber as you would with a well tuned exhaust.

[wmhjr]

Well, a couple things. The gas in question is not at constant pressure. There are harmonics at play here as well. As such, I'm not sure there is a single "formula" that applies to all. Therein all the black magic and voodoo. Too many variables in my mind to simplify to a single formula. Pulse amplitude and frequency, pressure variation, etc.

That being said, from a realistic perspective it probably makes very little difference after the mufflers IMHO. We're talking fractions that in a street car probably are barely measurable if at all but that all add up to minute advantages on a track.

[wmhjr]

People call it back pressure all the time but its actually the ventri effect you are looking for in an exaust. You don't want to have your engine flow to freely or you will not remove as much exaust from the combustion chamber as you would with a well tuned exhaust.

That is absolutely true. I also play around with Harleys. I laugh ever time I see some hacker with "drag pipes" or open pipes. Somehow there is still that impression that completely opening the exhaust makes the motor run better. In fact, a well designed 2-1 stepped header makes significantly more power. They're ridiculously loud and are always popping - while I pull right past them with my Bassani Road Rage 2-1.

[parsonsj]

The gas in question is not at constant pressure. There are harmonics at play here as well. As such, I'm not sure there is a single "formula" that applies to all. Therein all the black magic and voodoo. Too many variables in my mind to simplify to a single formula. Pulse amplitude and frequency, pressure variation, etc.

Of course that's true. Obviously, I made a whole bunch of simplifying assumptions in my calculations. The point is that the temperature of the gas makes a significant difference in the necessary diameter of the exhaust tube assuming constant pressure.

You dismiss my analysis, and offer this instead: from a realistic perspective it probably makes very little difference after the mufflers IMHO.

So... are you agreeing with me?

jp

[wmhjr]

I'm not dismissing your analysis. Just saying that from a purely theoretical and scientific perspective it's probably flawed but from a practical viewpoint for the general masses it's probably right. My comment was really directed at the post after yours seeking a "universal" formula.

[wmhjr]

BTW, no insult intended. It's fun conversation - even if it has been beat to death many times over.....

BTW: I had a heck of a time with my project. There aren't many "good" exhaust fabricators in my area. The one guy who is pretty darned meticulous is also the guy who believes anything bigger than 2.5" is a waste no matter what the engine, and that x-pipes are useless. Great fabricator. Not so great engineer. We're not talking tailpipe sizes. We're talking from the collectors back. At just under 600hp, I dare to disagree with him. I finally talked another guy who is a fantastic fabricator to work with me and the Ram Air Restoration oval/x-pipe stuff. I'm really glad I was able to do that. Excellent TIG welding, excellent fitment, beautiful job.

[parsonsj]

Sounds good. I've run into old school folks with the same idea that 2.5 is big enough for anything.

I did a 4" exhaust for a local fellow with an 800 hp single turbo Supra engine a couple of years ago. Boy was I wishing for a 3" exhaust. All my fab tools were just a bit too small. I wonder if guys in the 2.5" camp are the same way. 3" stuff is bigger then their tools.

jp

[wmhjr]

I think that's really it. I didn't expect him to have a mandrel bender that could handle 3" well - that's a seriously expensive piece of hardware. But the RAR materials really made it a process of fitting, cutting, welding rather than bending. The bigger the pipe the harder it is to fit. So I think they just try to discourage it. The guy in question kept talking about the Hemi Cudas he's done that only needed 2.5". My answer was that so long as I never went above 75% throttle (which equals max output of a Hemi Cuda in stock form) I just "might" be OK - but I'd rather use the entire gas pedal and not just part of it. Some of these guys don't realize that we're often making lots more power today that we could easily do 30 years ago. There are a whole lot of factory sedans built today that can smoke a Hemi Cuda in a 1/4 mile. My AWD MB can do it - with the nav turned on, heated seats, powered rear sun shade, listening to the stereo

[wiedemab]

Okay, I will bite.

I was sure there was a formula to figure out flow rates out of the exaust but I have never seen one before.

What is it?

It's called Fluid Dynamics and I have a couple of text books that I can drag out......................

Seriously though, I consider myself a fairly intelligent human being and Fluid Dynamics was the hardest I've worked in my life for a "B". To get close to real world results from calculations you get into some pretty serious differential equations and even at that you are making a bunch of assumptions. We would typically use the calculations as directional tools and do the actual testing on the actual system to get the real results. Computer simulations are probably getting pretty accurate, but even at that you still need to do the testing.

It's kind of like FEA - I'm not sure I would ever completely trust what a software package spits out. You have to validate with testing.

The nice thing about this issue is that the worst case scenario is some loss of power. Not catastrophic failure.

This reminds me why I wanted to be an engineer. Since school though I've pretty much gone a different route - hell I'm in purchasing now, so take my comments for what they are - from a rusty engineer from years past!!!