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Mopower
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Joined: 07 Apr 2002
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 Posted: Wed Dec 12, 2012 6:13 am Post subject: Calculating Approximate CFM Requirements |
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A few guys use a pretty good estimation method for figuring CFM requirements for their puller engine's induction requirements. There's been some pretty good dyno results out there after shoring up a carb, manifold, and head. What general method for calculation do some of you YT posters use?
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MLPANKEY
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| Posted: Wed Dec 12, 2012 7:50 am Post subject: Re: Calculating Approximate CFM Requirements |
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| Mopower wrote: | | (quoted from post at 07:13:36 12/12/12) A few guys use a pretty good estimation method for figuring CFM requirements for their puller engine's induction requirements. There's been some pretty good dyno results out there after shoring up a carb, manifold, and head. What general method for calculation do some of you YT posters use? |
What ever the choke points minimum cross sectional area cfm is usually makes 1 hp for 1 cfm conservately usually 180 cfm per cylinder will make 200 hp around 2000 rpm with piston speed being equal to or higher in fpm than engine rpm.
Last edited by MLPANKEY on Wed Dec 12, 2012 7:56 am; edited 5 times in total |
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the haulin harris
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| Posted: Wed Dec 12, 2012 12:53 pm Post subject: Re: Calculating Approximate CFM Requirements |
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cubic inches X rpm divided by 3456
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A.P.
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| Posted: Wed Dec 12, 2012 1:56 pm Post subject: Re: Calculating Approximate CFM Requirements |
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There are a number of on line tools available for calculating the CFM needs of an engine.
Here is just one of the many
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north puller
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| Posted: Wed Dec 12, 2012 3:29 pm Post subject: Re: Calculating Approximate CFM Requirements |
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3456 is assuming that your engine is 100% efficient. However, most N/A engines are lucky to be 80% efficient (only moves 80% of the air it acually displaces) and therefore would only require 80% of the CFM. A little more accurate formula would be inches X RPM / 4320.
Last edited by north puller on Wed Dec 12, 2012 3:33 pm; edited 1 time in total |
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Superih
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Location: Flushing MI
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| Posted: Thu Dec 13, 2012 7:58 pm Post subject: Re: Calculating Approximate CFM Requirements |
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| north puller wrote: | | (quoted from post at 16:29:22 12/12/12) 3456 is assuming that your engine is 100% efficient. However, most N/A engines are lucky to be 80% efficient (only moves 80% of the air it acually displaces) and therefore would only require 80% of the CFM. A little more accurate formula would be inches X RPM / 4320. |
I think you would be surprised at the amount of air required to run these pulling engines.
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Mopower
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| Posted: Fri Dec 14, 2012 7:10 am Post subject: Re: Calculating Approximate CFM Requirements |
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Your explanation is the closest estimate to my understanding. I was curious how some folks figured it, because it sometimes seems to be an afterthought for pullers, as it was on my MM. I'm hoping for some better power from appropriate CFM and 1/8" more bore next year.
436cu.in. X 1716 / 3456 X 80% = 173cfm
I've probably been running 130cfm at the very most.
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