Good exhaust info.....

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 :bow: 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. :D

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/install_university/installu_articles/cat_flow_test/cat_results_discussion11.122001.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?

i thought backpressure was a myth?

Well, backpressure does exist, so its not a myth. But what you are referring to is the myth that less backpressure will create less low end torque. And this is a myth. I have proven it on the dyno with doing back to back dyno runs, one with longtube headers going through the stock restrictive catback and then opening up the cutout and gaining nothing at all. They were identical graphs.

My understanding is that haveing less backpressure will only move your power up in the rpm range. But this is only a theory of mine, so please correct me if i'm wrong.

From what I understand it is a matter of volume (cfm), having a cutout closed is actually more restrictive than a stock exhaust due to the disruption in the exhaust flow.

Everyone has a rough idea of the boundry layer on the surface of a car but on the inside of an exhaust tube? I've never read a discussion or math equation yet. It would be different from just pumping air through due to the pulses of the engine so measuring it would be difficult at best.

I missed the artical somehow, is there a link to it we can read?


I apologize for my spelling

About backpressure,

What getting an aftermarket product, a high quality one, lower back pressure and increase power?

I'm asking cause I would like to know if I can reduce stress on the engine by adding er, lowering back pressure.

My theory is that the exhaust pipe has less pressure in it, the piston has to do less work to move that exhaust in the cylinder and in the pipes.

Exhaust is one of the few areas I have little understanding in, even a basic understanding of. People write more articles about nitrous than basic pewrfomance upgrades...>.>(off topic..)

To inform you more, I am really interested because the only thing I want to do to my mom's car when it becomes mine, is exhaust headers and possibly pipes, and intake. I want to reduce the stress on the engine from stock parts.

About backpressure,

What getting an aftermarket product, a high quality one, lower back pressure and increase power?

I'm asking cause I would like to know if I can reduce stress on the engine by adding er, lowering back pressure.

My theory is that the exhaust pipe has less pressure in it, the piston has to do less work to move that exhaust in the cylinder and in the pipes.

Exhaust is one of the few areas I have little understanding in, even a basic understanding of. People write more articles about nitrous than basic pewrfomance upgrades...>.>(off topic..)

To inform you more, I am really interested because the only thing I want to do to my mom's car when it becomes mine, is exhaust headers and possibly pipes, and intake. I want to reduce the stress on the engine from stock parts.

Exhaust systems become less of a concern the farther away from the engine. In other words the headers are more important than the tail pipes. If you want to get a deeper understanding read nateh's posts. I think he designs oem systems for a living.

Basicaly you want a system that's going to support your engine's cfm requirements, noise requirements and opperate most effeciently in the rpm range of your engine. Most of the setups on this site don't need to go that into it though.

-b

Thought you might find this useful:
http://www.broaderperformance.com/muffler_flow_tests.htm

I think I'll go with hooker mufflers from now on :shock:.

Also, a question, what are "dumps", "over axle", "under axle", etc and what are the sound and performance differences, if any?

This has been an interesting read for sure as I'm still figuring out what diameter of piping, 2 cats vs one, etc.

Just a few questions though:

Were the initial numbers posted HP figures at the motor?
Would one Large High Flowing Cat equate to 2 smaller ones on each side?

What diameter exhaust would you guys recommend for a 383 Stroked LT1 with about a 200hp shot of nitrous (rwhp)?

from my understanding the myth about wanting backpressure for low end tourque is a farse, it isnt wat is actually happening, in a system like the internal combustion engine u want no restrictions on your engine to make the most power, but the reason that you can go to big of an exhaust pipe and lose power is the gases will expand, cool and therefore slow down if you have to much 'free room' in your exhaust system, people mistake this as 'needing back pressure' when in fact going to big will cause back pressure due to the gases slowing down and piling up. So at slow rpm that gases are going to slow, but as rpm picks up the amount of gases being pushed through the exhaust pipe is increased therefore to flow more exhaust in the same diameter pipe you must speed up the gases, making a bigger exhaust pipe more desirable for upper rpm. Please take this information with a grain of salt and If any of this information is wrong please correct me im no expert, and i hope this helped anyone intersted.
steven

Do the figures take into account what type of headers you have hooked up the the exhaust pipe? Does it matter?

I think we may be exagerating the importance of removing the exhaust pressure on our cars. There is alot more to designing an exhaust than cfm and pressure. Take for example turbo cars. They make tons of power with extremely high exhaust pressure. A properly designed exhaust will draw a vacuum near the exhaust port due to the speed of flow and Bernoulli's principle. We are not dealing with a one cylinder car. Oversized pipes and True duals IMHO are not necessarily the answer. Hence the X pipe on Corvettes and Y pipe on F bodies....:)

Is there any rough estimate for losses if your pipe diameter is too large? It seems, according to this thread, that a 2.25" dual exhaust system is more than adaquate for my needs; however, that limits me to a summit house brand with poor mufflers and no balance pipe. I'd like to use the flowmast 2.5 system, how much could I expect a 380hp pontiac 400 to suffer?

Oversized pipes and True duals IMHO are not necessarily the answer. Hence the X pipe on Corvettes and Y pipe on F bodies....:)

X pipes are the be all end all, y pipes are merely a packaging issue ;) :D

Its all about sizing for your needs and the x pipe creates scavenging.

X pipes are the be all end all, y pipes are merely a packaging issue ;) :D

Its all about sizing for your needs and the x pipe creates scavenging.

The Ypipe creates a scavenging effect as well. Given the choice of a y pipe and true duals I would take the Y pipe. If you don't think so try removing one of your smog/air fittings and feel the vacuum near the exhaust port.:)

More than just diameter- there are the number of bends, for example- each twist and turn adds "friction" to the flow. Also, as posted before, the gas is both expanding and cooling, and a well designed system will allow for this to keep the flow speeds up. Part of that expansion, or lack of, has to do with exhaust temps, basically you don't want the gasses cooling too quickly as they will loose energy- hence the benefits of coated systems that keep the heat in the exhaust rather than allowing it to radiate under the hood.

Lastly, there is tuning, where the entire system can be made to resonate at specific frequencies, and increase exhaust scanvenging. It borders on a black art, lots of trial-and-error as well a formulae and theory. Most systems have certain 'sweet spots', RPM's that they add the most effect. Car designers frequently will tune exhaust to add some benefits at one RPM range, tune the intake to help at a different RPM range, and maybe adjust valve timing for something else, so that, taken all together the engine will produce good power and torque across a broad range of speeds. This makes a car more driveable, though maybe at the cost of absolute peak numbers.

Rather than arguing H-pipes and X-pipes, maybe we should just run tuned stacks....... That would be impressive sitting at a traffic light belching flames! ;^)

k
Rather than arguing H-pipes and X-pipes, maybe we should just run tuned stacks....... That would be impressive sitting at a traffic light belching flames! ;^)

I fully agree,Zoomies with no baffles are the only choice for traffic lights. I will argue with anyone H-pipes and x-pipes. X-pipes are better