A few months ago there was a thread about flow testing in which I said the flow in a running engine isn't even close to what it is on a flow bench. Using the example of a 360 engine running at 6000 rpm the calculated airflow (100% volumetric efficiency) is 624 cfm. 624 / 8 = 78 cfm per cylinder, this was the basis for me saying what I said.
Well I was wrong, no one corrected me, and now I must correct myself.
Up until now I always stopped at the 78 cfm (624 for eight cylinder four stroke 360 @ 6000 rpm). My mistake was not realizing the 78cfm/624 cfm is average flow and that the actual the intake flow is crammed into a much shorter time span. The port has to flow about 312cfm during the intake stroke to flow the average 78cfm.
If I did the math right a 45 cubic inch four stroke cylinder @ 6000 rpm will have around 332 ft/sec average intake velocity (average over 180 degrees crankshaft rotation) through a 2.25 sq in cross section at 100% volumetric efficiency.
78cfm average flow @6000 rpm
312cfm @ 6000 rpm ((720 / 180) *78 )) (average flow during 180 degrees of intake stroke)
312 * 1728 (1728 cubic inches in a cubic foot)
=539136 (cubic inches per minute)
/ 60 (seconds per minute)
= 9885.6 (cubic inches per second)
/ 2.25 (intake port cross section area, or volume @ 1" length)
= 3993.6 (average inches per second velocity)
/12 (inches per foot)
= 332. ft./sec average velocity during 180 degrees crankshaft rotation.
There is more about this at speedtalk;
http://www.speedtalk.com/forum/viewtopic.php?p=4774#4774Quote:
maxracesoftware;
.50 Mach = 582.5 fps @ 105.5F = 77.4 Inches of Water
.60 Mach = 699.0 fps = 111.5 Inches of Water
700 Fps times .5 = 350 FPS , and 350 FPS = approx. 28" Inches of Water
Flow Testing at 28 Inches of Water is roughly "half" the Air Speed Velocity
in Live Engine conditions.
Note the velocity maxracesoftware is talking about easily believable since the calculations above show the average velocity in the 360 intake port to be 332ft/sec. The 332 ft.sec from above is an average velocity during 180 degrees intake stroke. The peak velocity is sure to be higher. I suppose someone somewhere has instrumented an intake port on a spintron or running engine and measured actual conditions.
Testing at high depressions never made sense to me before but it does now.
