[Shotimes] Re: Rotating mass and dyno tests

[Ron Porter]

As I understand it, running on a Dynojet is not a measure of constant rpm,
but rather a change over time.

Granted, if you let the car settle into a constant rpm & speed, things like
heavier wheels/tires/rotors/flywheels/etc won't matter as much, but this is
not a real-world example.

IMHO, this is one of the huge benefits of having chassis dynos, as it really
isn't very relevant as to what the engine puts out at the crank, but what
actually makes it to the tire contact patch.

Ron Porter

-----Original Message-----
From: shotimes-admin@autox.team.net [mailto:shotimes-admin@autox.team.net]
On Behalf Of MonsieurBoo@aol.com
Sent: Sunday, February 06, 2005 3:06 PM
To: shotimes@autox.team.net
Subject: [Shotimes] Re: Rotating mass and dyno tests

"What they determined is that the larger brakes caused the net 7 whp  loss, 
as these were the only changes."
 
I must be missing something here.  The logic is that there's a "force  
multiplier" effect with larger diameter rims/rotors/tires:  Even if the
total 
weight of the larger-diameter combo is less that the pieces it replaces,
more HP 
will be required to rotate the larger-diameter assembly, because the  weight
is 
located farther from the center of rotation.
 
That certainly holds true for ET, provided we assume the car is
accelerating 
all the way to the traps.  Taken to an extreme it would make  the car bog
off 
the line and when shifting.
 
But for HP at a constant RPM I can't figure where it would make the least  
bit difference, even if you are measuring it at the wheels.  Since the
engine 
isn't accelerating the spinning weight, no HP is being lost.   (Although, if

you had rims with "turbine fan" spokes, the parasitic drag  would kill ya.
But 
that's not a weight issue.)
 
Maybe it's as simple as when they increased the rolling diameter, they took

the engine out of the sweet spot of its power curve by changing the actual,

final drive ratio.  It was running flat out but at a different RPM and just

not on the sweet spot anymore.
 
Cheers
Mark LaBarre
94 atx 130k
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