CFM intake charge vs exh

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cuisinartvette
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CFM intake charge vs exh

Post by cuisinartvette » Sat Aug 12, 2017 10:09 am

So was reading the bathroom bible :mrgreen: saw the old "this muffler flowz xxx" stuff and wondered how much is really needed?

If an intake port takes in say 250 cfm, when burned it takes up more space...til it cools, no? Then as it travels through the exh and cools needs less area?

Kind of a waste thinking about it but tossing it out there must be some rule of thumb?


Say a dynomax super turbo moved 600 cfm and an Ultraflow 1000 when does it really come into play as the cyls are pulsesnot steady flow

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Re: CFM intake charge vs exh

Post by mag2555 » Sat Aug 12, 2017 1:32 pm

It may take up more space if you want to look at it that way , as the cylinder is powered by expanding hot gases, but that hot gas has far greater pressure also , do you know what that means?

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Re: CFM intake charge vs exh

Post by Stan Weiss » Sat Aug 12, 2017 3:04 pm

cuisinartvette wrote:So was reading the bathroom bible :mrgreen: saw the old "this muffler flowz xxx" stuff and wondered how much is really needed?

If an intake port takes in say 250 cfm, when burned it takes up more space...til it cools, no? Then as it travels through the exh and cools needs less area?

Kind of a waste thinking about it but tossing it out there must be some rule of thumb?


Say a dynomax super turbo moved 600 cfm and an Ultraflow 1000 when does it really come into play as the cyls are pulsesnot steady flow
Unless you have a one cylinder engine why are you looking at what the intake port flows?

The carb / throttle body CFM is a good indicator of what the engine is taking in @ WOT.

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Re: CFM intake charge vs exh

Post by RDY4WAR » Sat Aug 12, 2017 3:52 pm

The style of muffler has to be considered as well. If you want the muffler to act as a cut-off for harmonic resonance in the exhaust, then larger mufflers and sometimes chambered mufflers work better. If you want to keep up exhaust velocity past the muffler, straight-thru style is needed with the internal piping the same or only slightly larger than the inlet and outlet piping.

I use Larry Meaux's PIpemax software to determine the size of the header and exhaust needed.

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Re: CFM intake charge vs exh

Post by panic » Sat Aug 12, 2017 4:33 pm

The longer and larger the exhaust system is, and the more cylinders in common, before the muffler -the smaller it needs to be.

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Re: CFM intake charge vs exh

Post by pdq67 » Sun Aug 13, 2017 1:09 am

^^^^^^^^^^^^^^^^^^^^^^

Panic,

Please elaborate more on this??

Are you talking about installing smaller dia. tailpipes because the exhaust gasses are cooling, so therefore their volume get's smaller?? So a smaller tailpipe will still keep the exhaust gasses velocity up?

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Re: CFM intake charge vs exh

Post by mag2555 » Sun Aug 13, 2017 6:21 am

The stated goal is that the Exh should flow 75% of what the Intake does in the lift range that the Cam provides, but depending on many other combined factors I have seen ratios as low as 68% produce good results.

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Re: CFM intake charge vs exh

Post by Rick360 » Sun Aug 13, 2017 11:24 am

mag2555 wrote:The stated goal is that the Exh should flow 75% of what the Intake does in the lift range that the Cam provides
This is definitely NOT one of my goals.

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Re: CFM intake charge vs exh

Post by Frankshaft » Sun Aug 13, 2017 12:23 pm

mag2555 wrote:The stated goal is that the Exh should flow 75% of what the Intake does in the lift range that the Cam provides, but depending on many other combined factors I have seen ratios as low as 68% produce good results.
You ever work on 4v clevland stuff? Intake flow rates over 350, ex, about 175 if your lucky. Still make 720hp on a 408 "bracket" style build, single carb. I personally think 4v clevland stuff is cool as hell. Even thought about building a 408 4v Cleveland with factory cast iron heads for my 73 Camaro. Guys put sbc, ls and bbc gm engines in fox body cars all the time. Why not the other way. I never would really, I just like those engines.

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Re: CFM intake charge vs exh

Post by peejay » Sun Aug 13, 2017 12:51 pm

CFM ratings are at a certain pressure drop.

The pressure drop between atmosphere and the intake stroke, and the pressure drop between the exhaust stroke and atmosphere, are FAR different.

In other words, how much does that exhaust port flow at, say, 50psi?

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Re: CFM intake charge vs exh

Post by Steve.k » Sun Aug 13, 2017 11:53 pm

Frankshaft wrote:
mag2555 wrote:The stated goal is that the Exh should flow 75% of what the Intake does in the lift range that the Cam provides, but depending on many other combined factors I have seen ratios as low as 68% produce good results.
You ever work on 4v clevland stuff? Intake flow rates over 350, ex, about 175 if your lucky. Still make 720hp on a 408 "bracket" style build, single carb. I personally think 4v clevland stuff is cool as hell. Even thought about building a 408 4v Cleveland with factory cast iron heads for my 73 Camaro. Guys put sbc, ls and bbc gm engines in fox body cars all the time. Why not the other way. I never would really, I just like those engines.
Cleveland heads you say? 368.6@.600 and 211.5@.600.

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Re: CFM intake charge vs exh

Post by mag2555 » Mon Aug 14, 2017 6:41 am

I did not state that it was MY goal for a 75% Exh to Intake ratio!
I guess I should have detailed my post more as in stating that most times what a novist will read in Mag articles is the the goal is to get a 75% ratio.

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Re: CFM intake charge vs exh

Post by Warp Speed » Mon Aug 14, 2017 8:36 am

cuisinartvette wrote:So was reading the bathroom bible :mrgreen: saw the old "this muffler flowz xxx" stuff and wondered how much is really needed?

If an intake port takes in say 250 cfm, when burned it takes up more space...til it cools, no? Then as it travels through the exh and cools needs less area?

Kind of a waste thinking about it but tossing it out there must be some rule of thumb?


Say a dynomax super turbo moved 600 cfm and an Ultraflow 1000 when does it really come into play as the cyls are pulsesnot steady flow
Hot exhaust gasses prefer an ever increasing area throughout the system past the headers.

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Re: CFM intake charge vs exh

Post by mag2555 » Mon Aug 14, 2017 8:47 am

That's not what I have found!
On any car of 600 hp or less with high flowing Mufflers under the rear seat area I can drop back to 2.5" Madrel bent pipes from 3" inlets with no HP drop off!

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Re: CFM intake charge vs exh

Post by Warp Speed » Mon Aug 14, 2017 3:56 pm

mag2555 wrote:That's not what I have found!
On any car of 600 hp or less with high flowing Mufflers under the rear seat area I can drop back to 2.5" Madrel bent pipes from 3" inlets with no HP drop off!
I've done extensive exhaust system testing from 400hp to 875+hp. No mufflers, but about 82-84" system length past collector's. This testing, plus general exhaust gas physics seem to point otherwise?
That's what I've found! :P
Because you didn't record a loss, does that make my statement untrue?!?

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