Dominic Sorresso

Cliff,

What would be your expectation simply taking 33lbs of dead weight off the car?

Dynomite

Now that is very simple as compared to lighter weight Rotating mass
See Item #2 for some initial Calculations Horsepower Losses from Angular Acceleration of Rotational Mass

That is simply Mass times Acceleration or (Lbs sec^2/ft)(ft/sec^2) or Lbs of force required to get a fixed acceleration.

If you reduce the mass (or weight by 33 lbs) and you have the same force (tractive effort at the wheels) you will get greater acceleration and as a result higher quarter mile speeds in less time. You are reducing the weight by 1% (99% of original weight) so you should increase your acceleration by 1% (101% or original acceleration) more or less

If you increase your acceleration by 1% lets say average top speed was 120 mph (176 ft per sec).....in 13 seconds or an average acceleraton of 13.5 ft per sec^2. Then you new acceleration would be say 13.63 ft per sec^2 for a top speed of say 177 ft per sec or 120.7 mph

This is not precise because the assumptions are made to give you a ball park and as you can imagine the accelerations very as you shift and in each gear as the gear ratios get higher the accelerations are reduced (tractive effort reduced) as the gear ratios are increased in the transmission.

Dominic Sorresso

120mph car is going to be high 11s/low 12s. How does that affect ur calc?
If u kept everything constant, what increase in power would yield .7 more trap speed?

Dynomite

The initial calculations were a quick look to see where we were in terms of horsepower requirements to rotationally accelerate rotors.
A second or two in the 1/4 mile run will not appreciably change the ball park estimates if those initial calculations are correct. I will go over the calculations and fine tune some assumptions a bit later with the biggest assumption being uniform acceleration which is not the case because of the gearing and wind resistance. Which assumption will still end up with a result showing the "Ball Park" possibilities (horsepower savings rotationally accelerating rotors) being good, so-so, or insignificant (we are currently at insignificant). I will put in the 11 second for you in the situation where you are around 120 mph. But again.....no matter if you have 500 hp or 400 hp the horsepower to rotate rotors up to speed seems very very small.....but I will check. If you kept everything constant, what increase in power would yield .7 more trap speed......I am not sure what the .7 is???

On another note....in talking with Marc (e-mail)....

Marc thinks the DynoJet 248 that he uses does a good simulation of the rate of acceleration of the car. Marc observes that the time to accelerate the car from 2000 to 7000 rpm in fourth gear on the street is very similar to the time it takes to make a DynoJet 248 pull in fourth gear from 2000 to 7000 rpm (This is most significant). Marc also thinks that the other DynoJet model, 224 has a much lighter drum so he recognizes the acceleration differences using different Drums such as on the 224.

Marc then observes that the 1/4 mile times with 500 hp engine having a lighter flywheel are in line with approximately 15 more hp available using a lighter flywheel such as the Fidanza flywheel.

That makes sense to me also and albeit it is difficult to determine the overall positive effects of lighter flywheels in the 1/4 mile because of the three shifts (three gears) and engine changes in rpm in each gear, I will give it a shot of some calculations to see if I can duplicate the 15 horsepower savings with a lighter flywheel which I think I can.

The TRICK is to break down the shifts, acceleration incremental times (1st, 2nd, 3rd gears) and then rationalize a uniform acceleration in the 1/4 mile that will give the same results (top speed and elapsed time). In other words.....there will be variable (greater) horsepower losses from using heavier rotating rotors in some gears and at some times as vehicle speed increases.

zrracer

I was told years ago that a heavy flywheel will get you down the track faster as rotating mass is your friend at the launch ?

Robin

Dominic Sorresso

Yeah and that rpms drop less because of inertia. But it takes more energy to get it to change direction from decel to accel. Besides the front rotors are not transmitting power. Just unsprung mass. Not sure u could say rear rotors are transmitting power either.

Dominic Sorresso

Found a very interesting C&D article that was alongside the main topic of C7 Corvette Z51 v Shelby GT350. Forgetting for the moment, the topic of the main article, C&D looked at an interesting aspect of the GT350.
The Carbon Fiber Wheels that are available. They did some testing to see what the effect of 58lbs lower unsprung weight had on performance.
I have attached a pic of a table outlining this. You can see it in the Feb. edition of C&D pg. 18. Significant performance enhancement that I thought were relevant to the discussion at hand.

zrracer

wow what a difference.
I heard they had carbon fiber rotors also. Could that reduction include the brakes and rims?

Dominic Sorresso

That's just the rims.

Dominic Sorresso

40d in Chicagoland today so took the opportunity to throw on new rotors. Offset is same as stock rotor so wheel fitment shouldn't change.