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Old 09-25-2011, 10:33 AM
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C.F.M. question

Hi all
Bit of a newbie question, here goes, when quoteing gas flow figures for throttle bodies, ports, mufflers etc. how many inches of H2O are they measured at ?
Thanks in advance
Clive W

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Old 09-28-2011, 09:47 PM
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2-bbl carbs are measured at 3" mercury, 4-bbls @ 1.5" mercury and cylinder head ports @ 28" water. EFI Throttle bodies - if it's a four barrel type performance throttle body, most likely 1.5" Hg but singles could be otherwise.
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Old 09-29-2011, 09:06 AM
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Quote:
Originally Posted by klx650sm2002
Hi all
Bit of a newbie question, here goes, when quoting gas flow figures for throttle bodies, ports, mufflers etc. how many inches of H2O are they measured at ?
Thanks in advance
Clive W
Generally speaking, 28 in/H2O- BUT you need to be certain of what the depression actually is because sometimes figures derived from a different depression will be used in advertising (for example) to "inflate" the figures, or flow at a higher depression will be compared to another manufacturer's flow taken at some lower depression- again to inflate the claims.

There are formulas to correct different depressions to each other or to a 28 in/H2O 'standard', as you may already know. Like to convert 25 inches of water to 28 inches of water: multiply by 1.06.

Some info on flow benches and how to fabricate them- http://www.flowperformance.com/
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Old 09-29-2011, 10:51 AM
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Thanks guys that's really helpful, especially the bit about how to correct for different test pressures, which would of been my next question.
So if I've got it correct 28/25=1.12 and 1.12^0.5=1.058
Thanks
Clive W.
The flow bench I have access to only goes up to 18 or 20, thats why I ask.

Last edited by klx650sm2002; 09-29-2011 at 10:57 AM.
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Old 09-29-2011, 11:41 AM
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We must be careful when comparing cylinder heads for purchase because we don't have a standard testing procedure. We don't know, in most cases, the size of the fixture the manufacturer is using, the distance from the cylinder block to the valve seats, the number of angles they used on the valves and seats, whether or not they used undercut valve stems, dry flow or wet flow, what equipment was used to do the flow tests, what the ambient conditions were at the time of test and on and on. RHS shows some good figures of their heads, and they should show good because they use a 4.200" barrel to test small block Chevy heads. If you compare their flow with another mfg who uses a 4.030" barrel, the larger barrel will flow more CFM every time and that's what sells heads to Ricky Racer.

Here's a nice little article that I found after a little surfing....
http://www.flowspeed.com/cfm-numbers.htm
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Old 09-29-2011, 11:45 AM
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Quote:
Originally Posted by klx650sm2002
So if I've got it correct 28/25=1.12 and 1.12^0.5=1.058
Thanks
Clive W.
The flow bench I have access to only goes up to 18 or 20, thats why I ask.
That is correct, sir!
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Old 09-29-2011, 01:35 PM
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Thanks guys you have been very helpful, much appreciated.
Clive W.
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