Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss

A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system but I figured this would give a great foundation to build on.

Just a side note that he stated that I thought was neat... “Just as fish don’t feel the weight of water, we don’t readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons!”

Hopefully I’ll get some more understanding of flow in a closed space in my fluid dynamics class next semester.

 

Did the article mention of the effect of backpressure on low-end torque? Just curious.

 

From what I gathered; for these systems you can go over 2.2 cfm per hp and still make similar hp numbers, but once you go over the 2.2 cfm per hp you start hurting your low end torque.

Hope that helps.

 

I see. Thanks for the info!

 

You're welcome.

 

I spent some time with the gas law equations and trying to generate some
curves for loss vs. pipe diameter. It can get complicated estimating the gas
composition and temperature of exhaust with all the variables. When you
simplify it to CFM per square inch you may be overlooking edge losses. A good
estimate of the edge loss is necessary to compare a single pipe to duals. My
guess is that the HP numbers for the dual systems need to be reduced a bit.

Here is a sample. The total friction surface for a 3.5" single is 11" and
the same number for 2.5" duals is 15.7". That is, for an equivalent cross
sectional area, duals have a greater circumferential surface for edge losses,
or wall shear stress as it's called in a textbook. You have a boundary layer
and a core flow in pipes.

This is all very simplified, but I eventaully gave up because fluid mechanics
is not my forte and I was just digging a deeper hole. The place to look if you
are interested in digging is in any engineering fluid mechanics book and in the
chapter for flow of viscous fluids in ducts.

 

Good info thanks OldSchoolSS.

-Dustin-

 

truedualws6 - You're right there will be some losses due to edge losses. I don't know how much will exist but there will be some there. All the information above is "theoretical" and is just for a general baseline guide while doing an exhaust system. I guess I could email my Powertrain Systems professor and he could give me a simplified equation that would take edge losses into account. The guy's a genious he has his PHD from MIT and is one of the worlds experts on dynamometer design and hybrid vehicle technology.

 

You guys are right, there is a thin outer layer of air that is stagnant.

Find out oldschoolss if you can I'm curious.
-brandon

 

maybe coating the pipes will lower the friction coefficient of the pipe walls, The exhaust will stay hotter, flow will be better, but who'll spend the money like that for a few extra ponies. just do a cutout right after headers when racing. Good info though. I have a ti-89 titanium too.

 

OldSchoolSS - If you have a chance to get some basic equations for this
please share them. I have always wanted to do a spreadsheet and generate
some curves for different exhaust configurations.

Duals will also weigh more than a single, but if you are like me, I did it for the
sound mostly, and performance is just the frosting on the cake.

 

2.2 cfm, but at what head pressure? This is exactly what I tell everyone who says a high flow cat doesn't restrict HP. Look at this link and they quote the same 2.2 cfm number, and show that even a single high flow cat will choke spom HP in our engine. Now if you go dual high flow cats, then you are fine up to about 340 HP with no loss.

http://www.installuniversity.com/ins...n11.122001.htm

My big question is how much HP do you start to lose when your exhaust does not support 2.2 cfm?

Dan

 

another thing to conside is the market. Mufflers aren't like heads or carbs they don't give cfm #'s and if they did what would be the test pressue 20.4 in. of H20??? wouldn't that be nice. something to think about...

-b

 

2.2 cfm is at 20.4 in. of H20. I think some manufactures do have cfm info on their sites. Dynomax does for sure I think it's even printed on the muffler box itself. I'll email my prof and see if I can get some simplified equations.

 

i thought backpressure was a myth?