Originally Posted by MechEE

Absolutely. The question is how much of a change are we talking about. If you remove your spare change from your center console, you're also reducing the car's total mass and increasing your theoretical acceleration for the same engine power output.



I made that comment, and it is not off. When you dyno your car on an inertial dyno (which measures the rate of angular acceleration of a drum to calculate instantaneous power), any drivetrain mods that one claims increase acceleration on the street due to decreased inertia will show up (after all, the drivetrain is accelerating on such a dyno). The gains from such a mod depend completely on the acceleration rate of the drivetrain, which depends on the load (remaining vehicle mass) and power output. You should read the link I provided for a background on gains from inertia changes.



As discussed, such gains will show up on an inertial dyno. Such gains can be easily predicted based on the acceleration rate of the driveline and the change in inertia. Again, please see the link I provided that explains all of this in detail if we are still not on the same page. Here it is again (written a long time ago now): http://www.stanford.edu/~mpg/lighten...omponents.html



Lightened flywheels have over an order of magnitude greater change in inertia than that from lightened crank pullies. As such, gains in 1st gear can easily reach 10+% which is noticeable. Even so, gains decrease with the square of the gear ratios, so they quickly drop to negligible levels in the higher gears.

Originally Posted by ATL_Red_G35

Dang.....you must be physics major....

Anyhow, this is not KJR, but the concepts applies.

https://g35driver.com/forums/showthread.php?t=139333

9whp gain must be a fluke then, if according to your theories.

Originally Posted by ATL_Red_G35

9whp gain must be a fluke then, if according to your theories.

Originally Posted by MechEE

Absolutely. The question is how much of a change are we talking about. If you remove your spare change from your center console, you're also reducing the car's total mass and increasing your theoretical acceleration for the same engine power output.



I made that comment, and it is not off. When you dyno your car on an inertial dyno (which measures the rate of angular acceleration of a drum to calculate instantaneous power), any drivetrain mods that one claims increase acceleration on the street due to decreased inertia will show up (after all, the drivetrain is accelerating on such a dyno). The gains from such a mod depend completely on the acceleration rate of the drivetrain, which depends on the load (remaining vehicle mass) and power output. You should read the link I provided for a background on gains from inertia changes.



As discussed, such gains will show up on an inertial dyno. Such gains can be easily predicted based on the acceleration rate of the driveline and the change in inertia. Again, please see the link I provided that explains all of this in detail if we are still not on the same page. Here it is again (written a long time ago now): http://www.stanford.edu/~mpg/lighten...omponents.html



Lightened flywheels have over an order of magnitude greater change in inertia than that from lightened crank pullies. As such, gains in 1st gear can easily reach 10+% which is noticeable. Even so, gains decrease with the square of the gear ratios, so they quickly drop to negligible levels in the higher gears.

Originally Posted by sean_mcnair

Good info. I researched this years ago when I used to drive an eagle talon. I made a spreadsheet that took into account acceleration, gear ratios, power, moment of inertia of parts, etc. It calculates the maximum gains from different lighter rotating parts and backs up what you said about the effect of gears and the comparison between flywheel and pullies.

On a talon, 5 lbs reduced from a flywheel(12" diameter on a talon) was equal to 6hp in first but only 2hp in second gear, <1 hp after that. The same 5lbs on a pulley (5" diameter) was about 1.2hp (1st), 0.6hp (2nd).

The gains on a G35 will be less than on a talon because a talon's first gear is much lower than a G35's. Anyways, here is the old spreadsheet:

http://www.geocities.com/sean_mcnair/dsm.xls

Someday, I gotta update that file with G35 gear ratios, and torque curve because it also calculates where to shift for the fastest et.