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      02-26-2013, 12:12 PM   #227
Pete_vB's Avatar

Drives: '69 GT3, GT4, 1M, 912
Join Date: Nov 2010
Location: SF Bay Area, Shenzhen, Oman

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Originally Posted by swamp2 View Post
Thus at any given speed the vehicle that can make the higher power to weight ratio WILL out accelerate the other. Period, no if and's or but's. Basic physics, no torque, no gear ratios, no nothing.
Correct. At any given speed the vehicle that can deploy more hp to the ground for its weight will accelerate harder (ignoring aero and rotational inertia). Again, I'm using the dynos above and I have adjusted weights so that the peak power to weight ratio is equal. This is just an example- it could be any two cars.

So, we now have the formula. Gearing we have. So let's look at how much HP the examples are putting down, both peak and at different speeds in 4th, and compare it to your statement above:

At 105 mph, per the formula above, the M3 is turning 6350 RPM. We go to the blue line on the M3 dyno above and see it's making about 255 tq, or 308 hp as you'd prefer. 4150 lbs / 308 hp = 13.5 lbs per hp.
The 1M turns 5150, makes 335 tq/ 328 hp per the graph, and again I'm using a heavy 3700 lbs to make peak power to weight equal. So 3700 / 328= 11.3:1.

Per your statement above, the best power to weight at that speed will accelerate harder, so the 1M is ahead at that speed.

Now look at 110 mph and so on.

105 MPH: M3 13.5, 1M 11.3
110 MPH: M3 12.9, 1M 11.2
115 MPH: M3 12.3, 1M 11.2
120 MPH: M3 11.9, 1M 11.4 (11.3 in 5th)
125 MPH: M3 11.6, 1M 11.5 (11.2 in 5th)
130 MPH: M3 11.4, 1M 11.6 (11.2 in 5th)
135 MPH: M3 11.2, 1M 11.7 (11.2 in 5th)

Now even without shifting, the 1M is averaging 11.4 lbs per hp over this speed range with these example numbers. The M3 matches its peak of 11.2, but it averages 12.1. So it's averaging 6% lower power to weight. 8% less power to weight if the 1M shifts. The M3 can't shift to a better gear, 3rd ends before 105.

So per your statement, two cars with equal peak power to weight ratios, but the one with more torque is accelerating harder almost everywhere. Again, shifted perfectly, no lazy driver. 8%- that's the equivalent of going from 415 to 448 hp.

This is the advantage the M4 is going to have in acceleration due to area under the power curve, and why you should care how much torque the car has. Even if you know how to shift. Or drive a car that shifts for you.
1M, GT4, 1969 Porsche 911 w/ 997 GT3 Cup Motor (435hp & 2,100 lbs)

Last edited by Pete_vB; 02-27-2013 at 12:43 AM.