exhasut flow question?

[FlowSpecialist]

Here's an interesting little piece I've just found in Google which nicely supports my figures.

Auto Exhaust Science

Written by some unknown has-been apparently but his figures seem to be correct.

You have to go right down to the Muffler Flow Basics section in which you'll find this.

"A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (1.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe."

I've corrected the typo where it actually says 10.5 inches of mercury instead of 1.5 inches. Now the deal about 1.5" of mercury is it's an old standard for testing carbs and as mercury has a specific gravity of 13.55 it equates to 20.3" of water. Now there's a number that's rings a recent bell.

Above I've already said that even perfect flow per sq inch at 20.3" would only be 124 CFM so 115 CFM per sq inch for a real piece of pipe of muffler length is bang on the money. 560 CFM being the maximum for a real piece of 2.5" pipe tells you without a shadow of a doubt that the advertised numbers of over 1000 CFM the OP referred to are blatant lies.

Dave

[95LT1Z]

Originally Posted by FlowSpecialist

You want me to do even more calculations for you when I've been kind enough to give you the equations to use? Ho hum, ok one more.

Flow at 25" would be sq rt (25 / 5) = 2.23 times as much as at 5" so 175 CFM at 5" would be 391 CFM at 25".

Clearly that's abundantly possible from a 3" pipe and also drastically restrictive for your engine. Barely good enough for 175 bhp.

Why are they testing at 5"? I haven't the faintest idea. Are their figures even correct? Same answer.

Dave

Edit: They're probably testing at 5" because they don't have a big enough flow bench to pull a higher pressure drop across things as big as mufflers.

wow that thing is a big cork in my exhaust system. i think im gonna be cutting it off and doing something different. if they are gonna make/sell mufflers then they should have the equipment to flow them and post correct readings.

Originally Posted by FlowSpecialist

Here's an interesting little piece I've just found in Google which nicely supports my figures.

Auto Exhaust Science

Written by some unknown has-been apparently but his figures seem to be correct.

You have to go right down to the Muffler Flow Basics section in which you'll find this.

"A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (1.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe."

I've corrected the typo where it actually says 10.5 inches of mercury instead of 1.5 inches. Now the deal about 1.5" of mercury is it's an old standard for testing carbs and as mercury has a specific gravity of 13.55 it equates to 20.3" of water. Now there's a number that's rings a recent bell.

Above I've already said that even perfect flow per sq inch at 20.3" would only be 124 CFM so 115 CFM per sq inch for a real piece of pipe of muffler length is bang on the money. 560 CFM being the maximum for a real piece of 2.5" pipe tells you without a shadow of a doubt that the advertised numbers of over 1000 CFM the OP referred to are blatant lies.

Dave

thanks alot for the equations you posted for me! helped me out ALOT!

[1989GTA]

Thanks Dave. By the way I have seen David V and others use pipe OD. So when they say 2.5" they mean the OD. Larry Meaux even states his figures are using tubing with a .0625" wall with his PipeMax program. Allen

[FlowSpecialist]

Originally Posted by 1989GTA

Thanks Dave. By the way I have seen David V and others use pipe OD. So when they say 2.5" they mean the OD. Larry Meaux even states his figures are using tubing with a .0625" wall with his PipeMax program. Allen

Ok so reworking the numbers a 2.5" pipe actually has an I/D of 2.375" and an area of 4.43 sq inches. Flow at 20.3" should be 4.43 x 124 = 549 CFM.

That's plenty close enough to the actually tested 560 CFM for me. Any flowbench that's within 2% correct is doing very well.

Dave

[FlowSpecialist]

I've had yet another thought. A muffler (hopefully at least) is always going to fit over the system piping it's designed for rather than inside it. Doing that would impose a pointless restriction. So a 2.5" muffler should be designed to fit over 2.5" O/D pipe and will therefore have an I/D of 2.5" itself.

In that case my first set of calulations were correct to start with. Don't even these simple matters get complicated when things aren't precisely specified?

Dave

[Lasse]

How about x-pipes in dual exhaust systems? X-pipe will give simultaneous access to both mufflers for exhaust pulses, so flow doubles in theory. This will help low rpm of engine, but what happens at higher rpm, when exhaust pulses are closer each other at timeline? Will 2.2cfm/hp-rule need some correction with good designed x-pipe or is x-pipe useless at peak hp rpm?

[FlowSpecialist]

Pass. Not much call for crossover pipes on the 4 pot motors I deal with. You'll have to ask the boss about that. His article in the link I posted has some relevant info for you though.

Dave

[MilesTugoh]

I just found this ;

Darin's 116cfm per square is measured using what criteria? - Speed Talk

The third reply is Darin Morgan's ..

[MadBill]

....The maximum that a straight piece of tube with a nicely tapered inlet and outlet (a venturi tube) can flow at 25" pressure drop is 138 CFM per square inch of cross sectional area. That's not open to debate it's part of the basic flow equations. Flow follows a square root law so at 20.3" pressure drop that maximum will be 138 x sq rt (20.3 / 25) = 124 CFM per square inch.

...You say, and I've looked myself even though I can't quite read the small print it's so tiny, that they are claiming over 1000 CFM from a 2.5" muffler. That has to be the biggest load of bullshit I've ever seen on a website because it's physically impossible by a factor of nearly 2.

What about the length factor? A 12 foot length of 2 -1/2" pipe will have far less flow at a given delta than will a 12 inch one.
It follows then if say a 36" long muffler had short well-radiused 2.5" inlet and outlet tubes but the ~30" long main body was a 6" diameter empty can, it might well flow better than the same length of straight pipe...

[FlowSpecialist]

Originally Posted by MadBill

What about the length factor? A 12 foot length of 2 -1/2" pipe will have far less flow at a given delta than will a 12 inch one.
It follows then if say a 36" long muffler had short well-radiused 2.5" inlet and outlet tubes but the ~30" long main body was a 6" diameter empty can, it might well flow better than the same length of straight pipe...

Indeed it might and that's a common strategy for advanced muffler design. However it can never flow more than the size of the inlet and outlet pipe allow it to.

For a given pipe size the maximum flow potential is based on a very short pipe length. The straight section of a venturi tube is only about 1xD or 2xD. Above that we start to get wall friction effects coming in to play which gradually reduce flow as pipe length increases.

It's that maximum flow potential, even for a short pipe length, that prove the flow claims the OP referred to were out by a factor of two.

Dave