DYNO RESULTS -- STOCK vs AFTERMARKET WHEELS

[16v Jason <jason@scirocco.org>]

(2nd attempt at posting this message)


Hey all,
	There was a local dyno day sponsored by the RX-7 club in my area.. $65 for 
4 runs-- so I talked to the organizers and asked if I could do 2 runs 
instead, but swap my 15" TSW Hockenheims for the 14" stock teardrops while 
I was on the dyno.  I figured it would finally put an end to the mystery of 
how much power the wheels robbed.

	I haven't weighed either of the wheel & tire combos, but the consensus 
among guys there was that the Hocks are slightly heavier than the 
teardrops.  In addition, because of the larger wheel size, the weight is 
distributed further from the axle, and therefore, the hypothesis was that 
running larger wheels would (significantly) reduce effective power output 
of my engine.

	From a physics point of view, the effect of greater wheel weight would be 
present the faster the wheel was accelerating.  Obviously, in 4th gear (the 
gear of choice for dyno runs because of traction), acceleration is not as 
brisk as it would be in, say, 1st gear.  Therefore, I was expecting the 
power loss to be less than what it might be in lower gears.  I expected 
there to be little difference at, say, 1200rpm, when the wheels are barely 
being accelerated, and more of a difference at the torque peak, when the 
wheels are being accelerated the most quickly.

	I had the dyno operator start the test at 1000rpm in 4th gear (usually 
they start between 2000 and 2500), and continue until the rev limiter 
kicked in, no matter what the tach said.  So, the results for both runs are 
from 1000rpm to 7300rpm.  I haven't yet scanned the results (my scanner's 
dead), but I will do so as soon as possible.

THE RESULTS:
	There was *no* appreciable power loss anywhere in the rev range using the 
heavier wheels.  At lower rpms (and therefore, speeds), the two curves were 
almost exactly the same.  As the revs increased, I expected the 15s to 
effectively lose power when compared to the 14s, but that wasn't the 
case.  The 15s actually put *more* power to the ground.  Why?

	Well, the dyno results show a vibration in the wheels caused by an 
unbalance in the tires.  Of course, those vibrations are speed-dependent, 
and lasted from 3000-4000rpm and then again from 5000-6000rpm.  Betwen 4000 
and 5000rpm, when the curve was smooth, the 14s put less power to the 
ground than did the 15s.  Why?  Again, most likely because of 
inbalance.  The difference, however, (between 1 and 2hp) is statistically 
insignificant, as back-to-back dyno runs can show differences of 2-3hp.

	Therefore, the conclusion is that for my tire & wheel combination, there 
should be no appreciable difference in power to the ground using either the 
14s or the 15s.  The biggest factor in this test was probably the balancing 
of wheels.  So, rather than worrying about losing power by using larger 
wheels, you'll gain more by having your wheels balanced.

	Therefore, the handling benefits of using larger wheels is not compromised 
by their weight (at least at the 15" level).  Of course, common sense 
dictates that 17" wheels would be even heavier than 15s, and may show a 
significant power loss...

	Now, if I could find my Physics books and get the formula for the force 
required to turn different weights, we could do a theoretical analysis... 
but for the time being, I'm satisfied in knowing that I'm not losing any 
power by running 15" wheels.

Jason



----------
1987 Scirocco 16v
1989 Mercedes 190E Sport Euro


http://members.aol.com/rocco16v  

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