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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