Randy pobst to drive 550hp evo this weekend at Laguna Seca
11-13-2006, 06:25 PM
#31
Rennlist Member
Thread Starter
Hey Seth!
Sure. The two charts show two cars that happen to have almost identical HP curves. not surprising that the 1987 porsche S4 which i beat on, and a newish BMW M3 which both are rated at near 320hp (315 for the 928 and 333 for the M3), would have near identical curves. Contrary to popular view and that goes right up to most of the racing anouncers like Dorsy Schrader, who still get caugth up with the "Torque" numbers of race and street cars, it is HP that determines a vehicles acceleration at any speed. what this means is its the HP that determines the torque as seen at the rear wheels, as muliplied through the gear box. So, yes, its torque that everyone gets so excited about, but what they dont understand, is that it is the torque as mulplied at the gear ratio. when was the last time someone asked what your "torque" was at 100mph cresting the start finish hill at Laguna seca? Since you are a skilled racer, you know that shifting at near redline keeps the average HP applied to the tires at a maximum.
so, what do the comparisons of the two charts mean? it means we have two vehicles, both with almost identical HP curves by value and shape, but have huge differences in torque values. Does anyone care? does it make one car faster in any area than another?? No. it means that at any speed of the vehicle, both the BMW M3 and Porsche 928 as represented by their HP curves will accelerate exactly the same (barring all other factors such as aero, etc). the differences of these two cars acceleration, will not be due to the engine performance or "engine torque " values. Why, because hp will determine torque as multplied through the gears at the rear wheels at any speed.
example:
(assume same spaced gears )
both cars going up the main straight at laguna just passing the start finish line and at 95% of the max RPM. both cars ar at near 285rwhp. because the bmw is at 95% of 7800rpm (ie 7400rpm) and the porsche is at 95% of 6000rpm (ie 5700rpm) both are putting down their engine torque at that engine speed x the gear ratio to give that vehicle speed. . since the rpms are skewed 1.3x for the bmw due to its engine design, its torque value will be multiplied through 1.3 x more gearing. in the end, both cars are putting down approx. 1,200ft-lbs of torque to the rear tires at 100mph, even though their engine torque varies by 30%. (calculated by the 928 having 265ft-lbs in 3rd gear at 100mph. so thats 265 x 4.5:1 3rd gear ratio vs the BMW at 200ft-lbs x its 3rd gear ratio of 5.9:1)
both cars produce close to the same rear wheel torque, at 100mph
The force to accelerate is the 1,200ft-lbs that we care about, not the engine torque which is meaningless unless we tie a rpm or a speed to it.
as a race car driver, all we care about is maximizing our time at the max hp possible over a lap. Thats why you are faster than Richmond. he short shifts!
Mk
EDIT: Here are some revised curves. the first curves above used metric measurements for torque on the 928 curves.
Sure. The two charts show two cars that happen to have almost identical HP curves. not surprising that the 1987 porsche S4 which i beat on, and a newish BMW M3 which both are rated at near 320hp (315 for the 928 and 333 for the M3), would have near identical curves. Contrary to popular view and that goes right up to most of the racing anouncers like Dorsy Schrader, who still get caugth up with the "Torque" numbers of race and street cars, it is HP that determines a vehicles acceleration at any speed. what this means is its the HP that determines the torque as seen at the rear wheels, as muliplied through the gear box. So, yes, its torque that everyone gets so excited about, but what they dont understand, is that it is the torque as mulplied at the gear ratio. when was the last time someone asked what your "torque" was at 100mph cresting the start finish hill at Laguna seca? Since you are a skilled racer, you know that shifting at near redline keeps the average HP applied to the tires at a maximum.
so, what do the comparisons of the two charts mean? it means we have two vehicles, both with almost identical HP curves by value and shape, but have huge differences in torque values. Does anyone care? does it make one car faster in any area than another?? No. it means that at any speed of the vehicle, both the BMW M3 and Porsche 928 as represented by their HP curves will accelerate exactly the same (barring all other factors such as aero, etc). the differences of these two cars acceleration, will not be due to the engine performance or "engine torque " values. Why, because hp will determine torque as multplied through the gears at the rear wheels at any speed.
example:
(assume same spaced gears )
both cars going up the main straight at laguna just passing the start finish line and at 95% of the max RPM. both cars ar at near 285rwhp. because the bmw is at 95% of 7800rpm (ie 7400rpm) and the porsche is at 95% of 6000rpm (ie 5700rpm) both are putting down their engine torque at that engine speed x the gear ratio to give that vehicle speed. . since the rpms are skewed 1.3x for the bmw due to its engine design, its torque value will be multiplied through 1.3 x more gearing. in the end, both cars are putting down approx. 1,200ft-lbs of torque to the rear tires at 100mph, even though their engine torque varies by 30%. (calculated by the 928 having 265ft-lbs in 3rd gear at 100mph. so thats 265 x 4.5:1 3rd gear ratio vs the BMW at 200ft-lbs x its 3rd gear ratio of 5.9:1)
both cars produce close to the same rear wheel torque, at 100mph
The force to accelerate is the 1,200ft-lbs that we care about, not the engine torque which is meaningless unless we tie a rpm or a speed to it.
as a race car driver, all we care about is maximizing our time at the max hp possible over a lap. Thats why you are faster than Richmond. he short shifts!
Mk
EDIT: Here are some revised curves. the first curves above used metric measurements for torque on the 928 curves.
Originally Posted by Seth Thomas
Mark, I don't fully understand what you are trying to say in in the thread above. The two charts and the correlation you are trying to draw from there isn't laid out well. Can you explain this a little bit better for me.
Last edited by mark kibort; 07-15-2009 at 06:17 PM.
