how much horsepower will a given size exhaust pipe support?

[Chris_Hamilton]

Apologies if this has been covered. I searched and didn't really find an exact answer to my question. What I'd like to know is if there are general rules of thumb as to how much HP a given exhaust pipe size will support. Example a single 2" pipe. a single 2 1/2" pipe, etc. etc.
Thank you Gents.

[RevTheory]

I've been wondering the same thing and didn't have any luck searching either. Dual 3 inch into single 4 inch tail pipe in my case. I'll be following the thread for sure.

[Tom68]

Apologies if this has been covered. I searched and didn't really find an exact answer to my question. What I'd like to know is if there are general rules of thumb as to how much HP a given exhaust pipe size will support. Example a single 2" pipe. a single 2 1/2" pipe, etc. etc.
Thank you Gents.

Not really for many reasons.
Exhaust is nowhere near as important as cylinder fill, intake.
Exhaust isn't constant flow, i.e, number of cylinders need to be specified.
Exhaust systems have various volumes depending on application.
So primary pipe size and length will be more important than tail pipe size as long as we're using rational numbers.

I've assumed you're referring to tail pipe sizing.

My first car was good for 140 mph (open road) with a single 2'' exhaust and rams horn manifolds on a fuellie headed 327 in a 3200lb sedan. But back then everybody was using at least dual 2'' pipes.

A mate took an MG RV8 to speed trials with a single 2.5'' outlet and was faster than many cars with flower pot sized exhaust outlets.

Point being most people run more tail pipe diameter than they need because they can.

[Orr89rocz]

Vizzard had a formula, for zero loss system. Something about you need 2.2 cfm per hp worth of flow and exhaust pipe can flow 115 cfm per square inch

So you back calculate. Min size for no loss. If you exceed that, it can make power but you are supposedly leaving alot on the table and suffering more losses in the system from pressure etc

[skinny z]

Vizzard had a formula, for zero loss system. Something about you need 2.2 cfm per hp worth of flow and exhaust pipe can flow 115 cfm per square inch

So you back calculate. Min size for no loss. If you exceed that, it can make power but you are supposedly leaving alot on the table and suffering more losses in the system from pressure etc

It is a scale of diminishing returns though. With the last bit only worth a few percent.

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Don't forget the muffler(s).
If I understand his approach correctly, this has a lot to do with providing the optimum platform for exhaust scavenging and the resulting exhaust induced induction cycle. (Provided the requisite overlap is there to begin with).

[pcnsd]

Vizzard had a formula, for zero loss system. Something about you need 2.2 cfm per hp worth of flow and exhaust pipe can flow 115 cfm per square inch

So you back calculate. Min size for no loss. If you exceed that, it can make power but you are supposedly leaving alot on the table and suffering more losses in the system from pressure etc

I have the below in my notes on mufflers... and the same graphic posted by skinny z.

Muffler flow rate required to prevent HP lost by David Vizzard:
Flow minimum is 2.2 cfm per open tube horsepower. Flow maximum is 2.6 cfm per open tube horsepower.
Flow rates are measured at 1.5" HG pressure. Higher flow rates than maximum do not improve performance but can impact sound levels.
1.5" HG = 20.39" water column.

[skinny z]

It's SO much easier with open headers!

[gruntguru]

Also depends on how far down the exhaust you are. You can usually reduce the tailpipe to one size smaller than the main exhaust and muffler because the gas is cooler - reducing the actual cfm. Also there is less pulsation after the muffler. Pulsation means the peak cfm is higher than the average cfm.

[digger]

IMO this is probably the wrong question as it cant be definitively answered because it depends. The more correct question is what size(s) exhaust should i use for my application.

[juuhanaa]

Hello Chris. Sorry i dont have answer for your question, but here is just something i stumbled across in this forum. Maybe it is something what skinny z was also thinking when he was writing about open headers..

if your cam timing is restricted to help protect bottom end power, maybe a light switch will go on...

Anyway the point im trying to say, it is a question of bigger area to make more torq in upper revs, but its a different story how to make horsepower.



-juhana

[modok]

Before or after the termination box? Muffler or no?
As said above, if flow is smoothed by a large volume or muffler or some other way, then it's easy to calculate the flow loss, and how big is big enough is a matter of diminishing returns, and can be measured by back pressure at the termination box. I'd assume David V's numbers are right, or right enough. But....that's basically only for tailpipes.
Your vehicle may not be long enough to need any tailpipe
If flow is not smooth then size needed will depend on the smoothness of flow and pressure waves.
I'm sure we can find some fairly good guidelines but it's going to be specific to engine type and header type.
So come back when you get an engine.