Tom, many folks are focused on the
"opening" area, how much thought is
going into the "VOLUME" of
the said intake scoop, or as I like
to think of it, the "air box"???
Joe
Thomas E. Bryant wrote:
>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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