[Healeys] 3.5 Rearend (was Re: On the Road Home from Conclave (Day 4))

[Bob Spidell]

Hi John,


re:
"  Lower RPMs means lower 
> fuel consumption.  At $4.39 per gallon these days, it should pay off rather 
> quickly."


Don't expect miracles; i.e. a linear reduction in fuel usage due to the taller gearing.  My MPG didn't increase much when I installed the 3.54 rearend (maybe 1mpg highway at 65mph).  As it was explained to me--and this makes sense--you'll need the same amount of power to push a car at 60mph regardless of the gearing (you'll need more torque for the taller gears/lower RPM, hence horsepower required will be the same).  That is, you'll have the throttle open farther--and thus use more air and fuel--to do 60 with a 3.5 rearend and 28% O/D than with a 3.9 rearend and 22% O/D, but RPM will be reduced (the reduction in engine noise and vibration at highway speed is worth the price of admission, IMO).  Also, I think it's easier to hold a more constant speed with the taller gears, which might account for the small improvement I saw (sorta like a poor man's "cruise control").

This is consistent with aircraft engines with constant-speed propellers--there's literally an infinite combination of prop speeds and throttle settings (in manifold pressure expressed in inches of mercury) that can produce any particular power setting (e.g. 55, 65 or 75% power), although with turbosupercharged engines running "oversquare"--e.g. 50 inches of manifold pressure at 2,100RPM--can markedly increase range (as Charles Lindbergh proved to the AAF in the Pacific in WWII).  But, this is largely achieved with the volumetric efficiencies achieved with supercharging (effectively raising the compression ratio; which will increase "mileage").


bs

--
***************************************************************
Bob Spidell         San Jose, CA        bspidell at comcast.net
'67 Austin-Healey 3000             '56 Austin-Healey 100M
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 -------------- Original message ----------------------
From: John H <jhomonek at mindspring.com>
> Bob,
> 
> I just recently installed a newly rebuilt sideshift trans and 28% out of extra 
> parts I had laying around.  I haven't been on a long trip yet to log mileage 
> benefits but it really lowers the RPMs when kicked in.  The real test will come 
> at SE Classic in September....Atlanta to Lynchburg, TN.  Lower RPMs means lower 
> fuel consumption.  At $4.39 per gallon these days, it should pay off rather 
> quickly.
> 
> By the way, it is matched with the 3.54 rearend too.  First gear starts on 
> inclines only and second gear on level or downhill starts.
> 
> John Homonek
> Atlanta AHCA
> 1959 BN7 - 1960 AN5 - 1974 Jensen-Healey