Any ideas if the fan clutch significantly reduces this high speed loss? I'm
rebuilding a spitfire with a weber side draft, new headers etc., and wonder
if i should take the antipurist approach another step with an electric fan.
Has anyone out there converted to electric on a 1500 motor?
Thanks, Nick
79' Spitfire
>I spoke today with a fellow who is both car and aerodynamic-savvy. He
>explained that a crankshaft driven fan suffers from inefficiencies which
>become significantly worse than you might expect when operated at high
>speeds ( ie full throttle.) At lower speeds, the air moves over the fan
>blades in a fairly orderly and predictable manner. But as the RPMs increase
>the air is increasingly unable to follow the fan's contours and begins to
>show disorganized turbulence, which results in a great deal of energy being
>expended to overcome the resulting drag. The fan doesn't push more air, it
>merely stirs it around like crazy. Engineers have overcome this, to a
>degree by driving the fan via a belt at a speed significantly less than
>crankshaft RPM. They can increase the fan blade pitch for satisfactory low
>speed air moving performance, and make the blades somewhat flexible so the
>pitch 'flattens out' at higher speeds. I was told that 10-15 horsepower is
>a very realistic figure for the power to drive a crankshaft mounted fan at
>very high speeds.
> Having learned that, I apologize to anyone who may have been offended by
>my skepticism, shown below....
>Bob W.
>-----Original Message-----
>From: R. John Lye [mailto:rjl6n@server1.mail.virginia.edu]
>Sent: Wednesday, November 03, 1999 6:46 AM
>To: bwesterdale@edax.com; triumphs@autox.team.net
>Subject: RE: horsepower
>
>
>Bob W. asks:
>
>>Not to be too much of a skeptic, but 18% seems rather high-- if you had a
>>100bhp motor to start with, would it really take around 18 HP to run that
>>tiny little fan?
>
><snip>
>
>>Your thoughts?
>
>18% may be a touch high - however, I can relate J.K. Jackson's story and
>numbers here. He had a TR-6 on a chassis dyno and ran it with the fan,
>then he removed the fan and remeasured. Simply removing the fan gained
>him 13% more power on a TR-6 engine. The TR-6 fan is plastic rather
>than metal like the TR-3/4 fan, but it has more blades so those probably
>balance out a bit. Therefore, I'd guess that a TR-3/4 should see a
>similar performance gain as the TR-6 did.
>
>I hope that helps,
>
>John Lye
>
>'59 TR-3A, '62 TR-4, '70 GT-6+
>email: rjl6n@virginia.edu
>homepage: http://avery.med.virginia.edu/~rjl6n/homepage.htm
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