Hi,
Nice story - Just a month ago there was a report on an MGC being equipped -
tuned and tested with such an air duct.
I'll try to translate from Dutch to English the important findings this
weekend.
If my memory serves me right, it was rated as a practical low cost "turbo"
boost.
Cheers,
Hans
-----Original Message-----
From: Max Heim [mailto:mvheim@studiolimage.com]
Sent: donderdag 13 juli 2000 17:41
Cc: MG's
Subject: Re: Bonding fiberglas [to ram air or not to ram air]
OK, I see. I was not considering the indraught of the carburetor as a
significant source of negative pressure in the engine compartment (maybe
it is on a Camaro!). Neither was I considering the movement of air around
the engine compartment as a significant factor in engine cooling. I still
wouldn't consider carb inflow very significant compared to the effect of
1) the high pressure in front of the radiator; 2) the low pressure under
the vehicle extending upwards into the engine compartment; 3) the fan.
This can be demonstrated by electric fan cooled vehicles -- when
stationary, with the fan running, engine temperature is stable. If you
shut off the fan, engine temperature climbs. The airflow generated by the
carburetor suction at idle apparently has no effect on cooling (possibly
due to the fact that it doesn't draw air through the radiator, but from
the vast open spaces around the engine below the car). I doubt that it
has any greater proportional effect at speed (carb indraught would be
greater, but radiator air flow would be greater still).
While the RamAir scoop on, say, a 69 Mustang CobraJet was not
particularly effective, due to its position in a high speed/low pressure
area in the center of the hood, you may have noticed that the current
Firebird TransAm has a RamAir intake mounted far forward on the leading
edge of the hood, placing it in a high pressure zone. They actually
tested this in a wind tunnel, and claim it is effective. It struck me
that the proposed position of James's scoop is very similar, being in
front of the "down break" of the hood.
The width of the opening (the MGC hood bulge) would seem to make possible
a very wide, flat duct (1-2" deep?) that would not significantly intrude
into the engine compartment space, so I think that that problem can be
avoided. As for the air exiting the radiator not being "cool", well,
that's the whole idea, isn't it? That means it has already contributed
its cooling effect via the radiator. And the idea of the air scoop is to
avoid this warm air and use higher-density cold air for both a denser
charge and greater thermal efficiency of combustion. The "convection
cooling" effect of the warm air circulating in the engine bay would seem
to be negligible; and, at any rate, is only different in this case by the
amount of the carb draught, which I also suspect is negligible. As for
having "nowhere to go", a cursory glance at an MG engine bay would reveal
vast openings into the underchassis area that dwarf the radiator surface
area.
But methinks I protesteth too much, in that I don't suppose that James
chiefly had the performance benefits of cold air induction in mind when
he brainstormed this project. I would guess that he wanted the MGC hood
for carburetor clearance, but was trying to think of something that would
make it both more functional (since the length of the bulge is excessive
for a V8) and less obviously "MGC-ish". Maybe he should also consider a
front air dam to help ensure low presssure in the engine bay, and
therefore maximum airflow through the radiator.
Michael Singleton had this to say:
>My understanding of the situation was that the scoop would injest cool air
>from outside and convey it directly to the carb intake, with the duct
sealed
>from the engine bay. This being the case, the following remain possible:
>1: Whatever pressure drop is provided by the carb air intake will be lost
as
>the carb is getting air directly from outside.
>2: The ducting will take up considerable free room inside the engine
>compartment, and will make the hood effectively much closer to the engine.
>3: The air exiting from the radiator is not cool. Further, if it has
nowhere
>to go, the fan can try to pull (or push, as the case may be) all it wants
>to, but it will be much less effective.
>4: I'm speaking from some experience, not a theoretical excersize.
>
>Incidently, on a street car, at legal speeds, I kind of doubt there would
be
>all that much ram effect in any case. If the duct is not fairly tightly
>sealed to the carb, it won't make any difference. It doesn't make all that
>much difference at racing speeds either. With our ductwork in place on a
>Trans Am Camaro, it didn't amount to a second a lap at Riverside. Still, it
>could look cool.
>
>Mike Singleton
>
>> But not if it's sealed to the air cleaner! In this case the duct and
>> scoop are completely independent of the radiator airflow and ambient air,
>> and have no effect on them whatsoever. In other words, the addition of a
>> scoop and sealed duct makes no difference to anything else in the engine
>> compartment. Cooling airflow remains exactly as before. Ambient pressure
>> remains exactly as before. Carburetor intake air pressure presumably
>> increases, but that's neither here nor there.
>
>
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.
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