Exhaust CFM engine calculation?

[rougeben83]

Just wondering, has anyone ever tried to calculate the max exhaust cfm that an ej205 is capable of putting out? I know its directly related to what turbo youre running and how much boost is being put into the intake side, but please humor me...

Reason for asking is, I know the general rule with turbo motors is bigger/free-er flowing exhaust the better, but just how much does a stock motor really need? I guess I'm saying I just want to find a muffler that can flow just enough to let the motor breath without being so large that it just becomes one long megaphone for the engine (sorry, I don't like the exhaust to be as loud any more what it needs to be).

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[CKxx]

I don't know the math, but there are general ideas what what is needed. Most setups are fine with a 2.5" or 3".

What turbo are you running?

[rougeben83]

vf23, currently tuned at 19psi.

I've heard of the 2.5" exhaust being capable of xxx amount of hp and 3" etc, but those assumptions had to have been based on something even more basic (like actual flow rates)...at least I hope. The thing is, even mufflers of the same pipe diameter can have a wide range of flow rates...from what I've read some of the older-style turbo mufflers can flow in between 400-500cfm for the 3" models, to up to 1100+cfm for the "straight thru" mufflers of the same diameter.

[eg33GC]

cid x .5 x Ev(.xx)x rpm
----------------------
1728(for converting to cfm

[Audiosavvy]

Loudness and flow do NOT go hand in hand. You could get an exhaust that will flow more than you'll ever need it to flow that's also whisper-quiet and subtle in appearance.

[rougeben83]

Originally Posted by Audiosavvy

Loudness and flow do NOT go hand in hand. You could get an exhaust that will flow more than you'll ever need it to flow that's also whisper-quiet and subtle in appearance.

that's what I'm saying, but the general consensus from the peanut gallery seems to be loud = power!!!.

For some of us that are building their own exhaust system (e.g. no one bothers to make an 00-04 legacy exhaust that uses a wrx downpipe ) the info may be useful when we're choosing mufflers and resonators and the like.

[Unabomber]

CFM has zero to do with exhaust selection. To determine what exhaust sounds best for your car in terms of loudness, you will have to use trial and error. Size and shape have all or little to do with quieting depending on the quieting methods used in the construction. I've heard about 8700 exhausts and trust me when I say, the #1, #2, #3, and #4 things to quiet down an exhaust are the muffler. #5-#99 are the muffler too. As well as #100-#678. Following them are pipe size, resonators, resonated tips, and all the other things, but they are so far down the line they are not worth mentioning.

[rougeben83]

Originally Posted by Unabomber

CFM has zero to do with exhaust selection. To determine what exhaust sounds best for your car in terms of loudness, you will have to use trial and error. Size and shape have all or little to do with quieting depending on the quieting methods used in the construction. I've heard about 8700 exhausts and trust me when I say, the #1, #2, #3, and #4 things to quiet down an exhaust are the muffler. #5-#99 are the muffler too. As well as #100-#678. Following them are pipe size, resonators, resonated tips, and all the other things, but they are so far down the line they are not worth mentioning.

ok, you guys are getting hung up on the volume factor. Maybe I shouldn't have put that in there in the first place.

I'm just saying can anyone care to make a rough estimate as to what the max cfm the exhaust side of a stock ej205 is capable off, so at the very least one can select a muffler/resonator/other exhaust components that can at least match that rate? So one can say "well why do I need this 1500cfm muffler when this 500cfm muffler will be just as good". Or one can then say "well I don't need to go any larger than xxx diameter piping to begin with since the engine won't be taking advantage of the extra flow capability". OR "well I understand that this exhaust system will only flow xxx cfm, but given other driving characteristics I want to focus on, I'm willing to make that compromise".

It's a simple math question about head flow of a stock engine. I'm not even asking about pnp'd components, different cams, built heads etc or anything that would change the breathing characteristics. Just a stock ej205, stock head/valvetrain .

[UkNuck]

Corky Bell covers this briefly in 'Maximum Boost' - basically he just refers back to the section on intake sizing and does a pretty crude adjustment for the expansion factor. His intake sizing rule is based on keeping the velocity below a threshold of increased drag (he uses mach 0.4 - I don't know if that's a widely accepted number). For the exhaust he uses a lower velocity value to kludge the result for the hot gases (unnecessarily confusing imo - it would be better to use the same velocity and just apply a factor based on the ratio of absolute temps).

Since you can also crudely relate intake cfm to bhp (i.e. cfm ~= 1.5 x bhp), this allows him to give a rough chart of pipe dia versus bhp. Try a google image search for 'corky bell exhaust diameter chart'.

A real engineer would probably use intake mass flow not cfm and include fuel mass in that figure - and then use gas laws to turn the mass flow to a cfm at whatever post-turbo absolute temp (just scaling the intake cfm by the abs temp ratio ignores the contribution of fuel to the amount of combustion gas - probably not a big deal for a rule of thumb given the inaccuracies in using ideal gas laws and likely not really knowing the temp ratio).

[rexblake]

This is Corky Bell's exhaust size chart.

[CKxx]

Originally Posted by Audiosavvy

Loudness and flow do NOT go hand in hand. You could get an exhaust that will flow more than you'll ever need it to flow that's also whisper-quiet and subtle in appearance.

What is a good free-flowing muffler for around 450whp? Or does nothing exist that can be quiet at that level w/o effecting power? Right now I have a BPM catback on a 30r. If I start the car in the garage, things fall down in the house.

I'm tempted to put a QTP-style plate in the exhaust back towards the muffler. Not as a cut-out, but simply as a sort of TB in the exhaust that I can control. I only need to quiet it down when I'm trying to get home late. Right now I just shut the motor off and coast to my house...

[Uncle Scotty]

Originally Posted by rougeben83

that's what I'm saying, but the general consensus from the peanut gallery seems to be loud = power!!!.

the peanut gallery are a bunch of ****ing idiots that dont know **** about ****

[Jubert69]

Originally Posted by Uncle Scotty

the peanut gallery are a bunch of ****ing idiots that dont know **** about ****

I don't expect anymore/anyless from you Scotty, but I agree with you 100%

[Chi_San]

Originally Posted by Jubert69

I don't expect anymore/anyless from you Scotty, but I agree with you 100%

rofl He does have a way of putting things... Sometimes, it's like Reality just decided to smack the crap out of you.

A quality turboback is what this thread needs. Cobb, BPM, Blitz, whatever. Then, get tuned. End of story.