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      04-22-2007, 01:37 PM   #28
Drives: 96 Geo Metro 1.0L
Join Date: Dec 2006
Location: Los Angeles

Posts: 81
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Originally Posted by CokerRat View Post
Horsepower is what ultimately makes a car go fast -- it is a measure of power output. Torque is a measure of how much twist the engine will make... but torque is important in how the curve looks over the whole rpm range. Torque made at higher rpm's allows you to take advantage of gearing so that you get the same acceleration force at the wheels as cars making more torque but at lower rpms.
True, but a piston motor is not designed for torque at high rpms. Torque is simply put, the power of the explosion in the cylinders. It is directly proportional to displacement.

Originally Posted by leftcoastman View Post
While I agree that torque in and of itself isn't enough to determine how fast a car is, I wouldn't say it's so simple to say that torque multiplication is what makes low gear acceleration so rapid.

At low gearing, you are travelling at low speed. Air resistance increases as a square of speed. At higher gears, you're probably going at a high speed, therefore the power necessary to overcome that air resistance (which has been growing as a square) is much higher.
Absolutely true. But air resistance does not come into play at speeds less than 100kph at a coefficient of drag of around .3 which I believe our 3 series has. Below that speed, its the inertial resistance where torque is very important.

Originally Posted by bnj View Post
From engine's part, it is the integral of the hp curve that determines the acceleration. Torque slows you down since the transmission parts need to be heavier. Weight is performance car's worst enemy. The more curves and less straight line, the more the weight is an issue.

Originally Posted by bnj View Post
When it comes to how engine affects acceleration, power during the acceleration defines how fast the car accelerates.

Power is defined as how fast the work is done. The work can be e.g. getting car to 100 mph. How fast this work is done (ceteris paribus) depends on the average power during the acceleration.

You can't avoid having high torque if you have high power as it is closely related to power. However, average power is the measure affecting the acceleration. Power means performance. Torque is luxory and stress to transmission components.
Well said.

Torque has nothing to do with speed. At high rpms, its the motor's volumetric efficiency; how fast it can fill the cylinders with air and then expell it; that is power. Power is the only thing that will yield the fastest speed. With that said, the speeds at which most people will drive their cars, torque will still dominate. Around 250kph and higher, its all power.

AMG motors are much better suited for a street car and the speeds at which a street car will see. BMW motors really shine at high load, high rpms, and high speeds. Its not a matter of respect or power for money; AMG's and BMW M's are just different cars. Unless BMW changes its philosphy, M cars will never beat a Powerful Merc on the streets.

Last edited by homerunball; 04-22-2007 at 01:58 PM.
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