Neil,
As usual you have hit the nail squarely on the head. Now......how to vary
the inlet size AND the size of the diffuser. (Air Box as Joe has
suggested). If all that can be done in direct relationship to the speed of
the car/air density, then we have the perfect inlet!!! AND the best ram
recovery.
Skip
At 11:16 AM 1/25/02 -0600, Albaugh, Neil wrote:
>Maybe a variable- area inlet is what is needed. ...works on an SR-71; that
>goes pretty fast.
>
>Regards, Neil Tucson, AZ
>
>
>-----Original Message-----
>From: DrMayf [mailto:drmayf@teknett.com]
>Sent: Thursday, January 24, 2002 4:29 PM
>To: land-speed@autox.team.net
>Subject: Scoop Openings Revisited.....
>
>
>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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