Your right on it Jim, effectively there is no increase(if not an actual
decrease) in the mass of air pulled through the radiator(due to the fan and
excluding dynamic air pressure) at a certain point. The absolute best analogy I
can think of that visually represents this is an egg beater(electric variety).
You can put the beater on low and see the fluid flow(of whatever your beating)
inside and outside of the beaters. Put the setting on puree and watch the flow
separate to the point that you can see the bottom of the bowl inside the
beaters. Sure there is still an extremely low pressure zone that still 'sucks'
like a hurricane but without increasing the flow around it(i.e. the flow rate
curve goes flat). Boat props have a similar flow seperation problem where
cavitation(air gets beat out of the water) sets a limit to waterflow rate
around the prop for a given fixed pitch.
Since my latest custom exhaust installation, my (otherwise stock) TR6 now has a
little more spunk under throttle(especially 3Krpm to 4Krpm). I've been thinking
about getting rid of the stock fan for one of those 1)flex blade fans or 2)some
custom clutched fan 3)or an electic puller. As with most HP equations,$=HP and
1 through 3 increase in HP ...and cost.
Since #3 is going to get the most spunk(HP), we can simulate it by taking the
stock fan off(temporarily) and take a run down the road to see the benefit in
spunkiness(HP) on a cool day. If the spunk/$ ratio is worth it then I'll
probably go with #3. Then, you have to consider the highest cubic foot flow fan
per lowest amperage fan you can get(at lowest cost of course).
I've not tried the 'fanless' run down the road but I will soon. I live right
next to a highway so I can jump out and run 55-60mph and then hop off and park
without sitting anywhere and overheating the engine. I'm guessing the benefit
might be 5-10HP(maybe optimistic here) from 3000rpm to 4200rpm(which is where I
keep the engine when moving through the gears). Not having the benefit of one
of those dash mount dyno meters, I'll relay the results in spunk and is it
worth it(when I do) but I'm sure someone on the list has done this on a stock
TR6 and can tell you right now if its worth it.
I've seen where VB has an electic puller for $190 but I may opt the junkyard
Nissan/Honda/whatever route if their cheap enough and come with shrouds.
Good luck!
--
On Wed, 3 Nov 1999 09:28:58 Jim Barbuscia wrote:
>
> I've been following this thread with some interest so I thought I'd add my
> experience to it. I went to an electric fan setup on my TR7 and noticed a
> real increase in the power output when I removed the standard fan.
> What I attribute it to is the fact now the car rev's higher and thus more
> RPM = more HP. So an 18% increase in this case would be easy to understand.
>
> -jimb
> 80 TR7 F/I spider
> 74-1/2 TR6
>
>>
>>
>> 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.
>
>## Jim Barbuscia SE Sun Microsystems Kirkland, WA. ##
>## http://www.ptinet.net/~jimbar ##
>## Standard Disclaimers apply (Jim.Barbuscia@Sun.COM) ##
>
>
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