How to Determine Necessary Exhaust Pipe Size

[cudabob496]

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

[72cudamaan]

So according to the report that you trust to be true, I see no reason that a street driven motor needs 3" exhaust.
Unless of course your engine produces 679 rear wheel horsepower.

[Chryco Psycho]

I have always leaned towards the smaller side with increased velocity through the system , I figure around 400 RWHP you need to look at 3" exhaust , I would think free flowing muffler design in a 2.5" system is more important than jumping into 3 " pipes 

[cudabob496]

So according to the report that you trust to be true, I see no reason that a street driven motor needs 3" exhaust.
Unless of course your engine produces 679 rear wheel horsepower.

The 679 is the max hp a 3 inch system can handle, before you need larger pipe.
The max a 2.5 inch system can handle is about 460 hp.
For me to have a 600 hp, 500 cu in motor, the chart says I need at least a 3 inch system.
The chart says a 2.75 inch dual exhaust would limit my engine power to 566 hp.

[cudabob496]

of course, even though you have a 3 inch exhaust, or whatever size, all the other components
in your system have to flow equally as well, like the air filter, carb, intake, heads, mufflers.
Looks like most of the straight through mufflers flow around 1000 cfm or more, so they should not
be a problem.  As Chryco says, the whole system is a big air pump!

For most street applications, things may only start to get limiting when you are going above 5000 rpm.

[dodj]

Thanks for posting. Think I'll bookmark that one. 

[PlumCraZRT]

So according to the report that you trust to be true, I see no reason that a street driven motor needs 3" exhaust.
Unless of course your engine produces 679 rear wheel horsepower.

The article is referring to crankshaft hp, not RWHP.  But I generally agree, just about nobody on the street should need exhaust larger than 3".

I have always leaned towards the smaller side with increased velocity through the system...

I agree with this as well, since the increased velocity will provide better low-end torque.  I would much rather have every-day usable torque than a couple hp at 6,500 rpm.

The 679 is the max hp a 3 inch system can handle, before you need larger pipe.
The max a 2.5 inch system can handle is about 460 hp.
For me to have a 600 hp, 500 cu in motor, the chart says I need at least a 3 inch system.
The chart says a 2.75 inch dual exhaust would limit my engine power to 566 hp.

I also wanted to point out that these calculations are essentially to make sure the exhaust is not the "bottleneck" of the system.  Of course you could make more than 566 hp with 2.75" exhaust, but it's going to be more difficult (I think the F.A.S.T. guys are making 600hp with 2.25" exhaust).  The trade off, as I mentioned earlier, is that you lose low end torque as the pipe diameter grows, so it's probably safest to pick the smallest size that won't kill your power potential.

BTW I will be putting 2.5" exhaust on my 500hp stroker.

[cudabob496]

The article is referring to crankshaft hp, not RWHP.  But I generally agree, just about nobody on the street should need exhaust larger than 3".

I agree with this as well, since the increased velocity will provide better low-end torque.  I would much rather have every-day usable torque than a couple hp at 6,500 rpm.


I also wanted to point out that these calculations are essentially to make sure the exhaust is not the "bottleneck" of the system.  Of course you could make more than 566 hp with 2.75" exhaust, but it's going to be more difficult (I think the F.A.S.T. guys are making 600hp with 2.25" exhaust).  The trade off, as I mentioned earlier, is that you lose low end torque as the pipe diameter grows, so it's probably safest to pick the smallest size that won't kill your power potential.



BTW I will be putting 2.5" exhaust on my 500hp stroker.

Assuming the chart is correct, then going from a 2.5 inch dual exhaust to a 3 inch dual exhaust allows your engine
to generate over an additional 200 hp. This is not just a few hp at 6500 rpm? Also, my Cuda makes about 625hp. If I switch from
my 3 inch system to a 2.5 inch system, the chart says I will lose 200 hp?

Also, if the chart says the most horsepower you can make on a 2.75 inch dual exhaust is 566, how can you make more than 566, unless
maybe you go to a supercharger or turbos.

Lastly, if I have, say, a 500 cu in motor that makes 600 hp on a 3 inch dual exhaust, am I going to see a significant torque increase by going to a 2.5 inch exhaust.  Right now my engine makes max torque at 5400, and max hp at 6000 rpm.

One additional consideration. I have a 3500 stall converter. That is supposed to double the engine torque to the rear wheels in the low rpm range. So, I can get max higher rpm power, and lots of low rpm power. But even with a lower stall converter, I don't think the torque gain from a smaller diameter exhaust is going to be much, especially if your torque peak is in the higher rpms. If someone is serious about street torque, then the cam profile would be a major consideration.

[cudabob496]

The article is referring to crankshaft hp, not RWHP.  But I generally agree, just about nobody on the street should need exhaust larger than 3".

I agree with this as well, since the increased velocity will provide better low-end torque.  I would much rather have every-day usable torque than a couple hp at 6,500 rpm.


I also wanted to point out that these calculations are essentially to make sure the exhaust is not the "bottleneck" of the system.  Of course you could make more than 566 hp with 2.75" exhaust, but it's going to be more difficult (I think the F.A.S.T. guys are making 600hp with 2.25" exhaust).  The trade off, as I mentioned earlier, is that you lose low end torque as the pipe diameter grows, so it's probably safest to pick the smallest size that won't kill your power potential.

BTW I will be putting 2.5" exhaust on my 500hp stroker.

Assuming chart is correct, then a 2.5 inch exhaust will limit your engine power to 466 hp? You could never make the 500 hp at the crank?

[PlumCraZRT]

Think of it this way...

If you have a straw and you blow through it with light effort, it doesn't create any additional resistance but the air isn't moving very fast.  This is analogous to using a pipe diameter that is large enough (or too large): you will make as much power as possible since the flow is not resistricted, but the velocity is low so your torque suffers.

If you blow through it hard, it will create some resistance, but it will still move the air faster.  This is analogous to using a pipe diameter that is slightly too small: it's a bit restrictive so you will lose a little bit of power, but the velocity is high so you have fantastic torque.

Now take an air compressor and blow through it.  You will move more air through the straw than when you blew hard and by nature the air will move faster.  This is analogous to using a pipe diameter that is way too small: it's restrictive so you will lose a lot of power, but you are still going to make more power than a smaller motor pushing through the same diameter exhaust.  Now the velocity of the exhaust is REALLY high, so you will have superior bottom-end torque.

Does that make any sense?

The reality is, with your setup (automatic with a high stall converter), you're not too worried about low end torque.  If you had a wide-ratio 4-speed with a 2.76 rear gear, you might be more interested in torque at 2000 rpm.

[cudabob496]

there are some informative links below the chart.

[cudabob496]

so, when we get more power at lower rpms (ie torque) because of higher exhaust flow, what's happening is you are getting a more
efficient scavenging of exhaust gases from the combustion chamber?  Another reason why they don't do much port work on the exhaust ports,
to keep exhaust velocity up?

[PlumCraZRT]

so, when we get more power at lower rpms (ie torque) because of higher exhaust flow, what's happening is you are getting a more
efficient scavenging of exhaust gases from the combustion chamber?  Another reason why they don't do much port work on the exhaust ports,
to keep exhaust velocity up?

I'm not sure if you're actually asking questions, but the answer to both is generally 'yes'.  In racing applications (particularly F1), you will tune the runners to optimize power at a certain RPM (which effectively covers a narrow RPM band) since the close-ratio transmission keeps your RPM fairly consistent.  Obviously, removing "obstructions" from your ports should always be a priority, but many times keeping a smaller port cross-section (to promote velocity) is more important than removing said "obstruction".

[Aussie Challenger]

In a street driven car the largest exhaust to deliver max HP is not really needed, lets face it having 600+ HP on tap is good but how often are you going to use it in reality. Coming down slightly in size will restrict top end output but give more low down torque where street driven motors tend to spend most of their time.
Over the years the Hot Rod community has followed the modifications done to race cars wanting to have that something special and different to factory.
Smaller dia. pipes are easier to fit and tend not to bang up against axles and bodies. I am not saying that we shouldn't have high HP motors in street cars as this is a personal choice and as I mentioned can be a lot of fun too.
What many did back in the day was have the required large dia. head pipes with a cut out for when they went racing, best of both worlds.   

[cudabob496]

In a street driven car the largest exhaust to deliver max HP is not really needed, lets face it having 600+ HP on tap is good but how often are you going to use it in reality. Coming down slightly in size will restrict top end output but give more low down torque where street driven motors tend to spend most of their time.
Over the years the Hot Rod community has followed the modifications done to race cars wanting to have that something special and different to factory.
Smaller dia. pipes are easier to fit and tend not to bang up against axles and bodies. I am not saying that we shouldn't have high HP motors in street cars as this is a personal choice and as I mentioned can be a lot of fun too.
What many did back in the day was have the required large dia. head pipes with a cut out for when they went racing, best of both worlds.   

Cutout means cops pull you over. Actually, you can use the 600hp quite easily, as I just have to take the motor to 6000 rpm.  I do it quite regularly when I need a "fix". It also came in handy, a few months ago, when I toasted a 560 hp Mustang Shelby GT500.  It helps to have a lot of wide open spaces and nice uncongested highways.  If I lived in Portland or Seattle, out west here, I'd be lucky to get above 40 mph on the street.  I live in eastern Washington. I have fighter pilot friends. They'll take an F16 over a Boeing 737 any day. The most eye-opening fact from this chart, is going from a dual 3 inch system to a dual 2.5 inch system takes 200 hp away from an engines potential power output.  And anyone in this chat room who races, will tell you they will do anything reasonably possible to make as much power as possible. Everyone is different. Some just cruise, and some hate to lose on the street. Because when you drive a Mopar, most everyone will challenge you!