Interesting musings! I think that you are on to something, though. I had
started this thread a few weeks ago relating my problems with negative
pressures (-9.7 in H2O) in my scoop at above 6000 rpm. When the engine
was shut down at the end of the run the pressure went to zero or a bit
positive. At the Lakes, after cutting the nose off the scoop, it still
went a bit negative in the lower gears (as much as -1.4 in. H2O), but
through high gear it was positive, at the top rpm it was reading 0.3
positive and went to 0.7 positive immediately after the engine was shut
down. Now the scoop sizes were... 2.5 in. X 3.25 in. = 8.12 sq. in.
before the nose was cut off and 5.0 in. X 5.5 in. = 27.5 sq. in. after.
My calculation were 4.8 sq. in. needed for my 304 cu. in. engine at 8000
rpm with a 2.76 gear.. So much for theory.
I suspect that position and the design of the opening had something to
do with it, but I am still experimenting and looking for some improvement.
Tom, Redding CA - #216 D/GCC
DrMayf wrote:
>Scoop Openings.
>
>Well, we have all heard the hypothesis that the scoop opening should be
>sized to permit a column of air at such and such speed to enter the engine
>and fulfil it's needs. On the surface this seems like the right thing to do,
>but, I am not in my right mind today and so my thinking may be a bit faulty.
>Here's the deal.
>
>a) given an engine of 370 cubic inches
>b) which turns at 8000 rpm
>c) and has a carb with 4 butterflies of 1.75 inches diameter
>d) with a target speed of 240 mph
>
>Engine air needs at 8000 rpm are:
>
>= (Disp * volumetric efficiency * rpm) / (2 * 1728)
>
>Where: Disp = displacement = 370 cubic inches
> Volumetric Efficiency = 93% = 0.93
> RPM = rpm = 8000
> 2 because air is inhaled only every other revolution per
>cylinder
> 1728 to convert displacement cubic inches to cubic feet
>
>So: Air flow need is = 796.53 CFM at 8000 rpm.
>
>The conventional wisdom is that at speed of 240 mph (352 ft/sec or 21,120
>ft/min) the scoop opening needs only to be big enough to
>
>Then the scoop size is = 796.53 Ft^3/min / 21,120 ft/min = 0.0377 sq ft =
>5.43 square inches.
>
>If the 950 DP with the 1.75 inch diameter butterflies is used, it has a flow
>area of 9.62 square inches. So does this mean that the air flow through the
>carb is roughly 44% less in velocity (( 9.62 - 5.43)/9.62 * 100)? Does this
>make sense?
>
>Another oddity for me (and what's unusual about that, you ask..) is during
>run up to speed. At launch with our motor screaming to high heaven, well,
>maybe not that bad, but think about this. The engine requires 796 cfm at
>8000 rpm, but if we are going only part way to our target speed the scoop
>opening does not appear to be big enough! Ok, you asked.
>
>Say we are at 100 mph or 146.7 ft /second, then the scoop size should be
>around 13 square inches. But it is only 5.43 square inches. So what is
>happening during this situation? I think the engine is starving for air
>because it is having to "suck" through that small straw which only works at
>full speed. I further think that the carb velocity signal is weakened to the
>point that the fuel is leaned out (ie, metering signal is not as high as it
>needs to be to meter the correct fuel amount into the venturi). I think that
>this is the reason that we see damaged engines when using the scoop formula
>or at least the target speeds not being met.
>
>I would suggest that air scoops based on this methodology need additional
>air inlets to permit the required air flow at low speed. Just like the big
>air liner nacelles which have blow in air doors to let more air in at low
>speed.
>
>Am I crazy? Probably, but hey, this is fun.
>
>Mayf, the red necked, ignorant desert rat in Pahrump whose house and
>workshop is coming along fine the painters are inside today all interior and
>exterior doors are installed except for garage roll up doors and some
>concrete work still has to be finished as does some electrical connections
>and water well plumbing and one soil line from the workshop and the roof
>tiles need to be laid and the fence put in and and and slap slap slap..
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