Mayf; I am a subscriber to the column of air theory because I heard it from
the Hofman/Markley group who seem to know what they are doing. That said the
resulting opening seems way to small for the speeds I run. So I double the
opening size to allow for the acceleration segment of my pass. Then I made
it a little bigger because it looked right. Maybe I'm not ready for rocket
science. Rich Fox
----- Original Message -----
From: "DrMayf" <drmayf@teknett.com>
To: <land-speed@autox.team.net>
Sent: Thursday, January 24, 2002 03:28 PM
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.
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