riofrio

Ok....

We are used to talk about horsepower to describe one aspect of performance. But as far as I am concerned, it is actually acceleration what governs the rate of increase of velocity. But there's also torque, which will create an important feeling, too.
Can somebody state and explain what each of these three is and how they are related to each other (mathematically if possible)? Hint: mass plays a crucial role. I know I always get them a bit mixed up. How come acceleration is not a spec?

giddyup

TexasGunRunner

...because torque translates into acceleration. If you have low torque, you will not accelerate very fast. If you have high torque, you might accelerate faster, all else being equal than somebody with lower torque.

Remembering back to old physics classes...torque is the force applied toward rotating an object. It has two components, distance from the axis where force is applied, and the amount of force.

If you think of breaker bar and socket on a bolt...if you apply force 10 inches from the axis of the bolt, and you apply 10 lbs of force, that is 100 inch-pounds. Now apply 100lbs of force one inch from the axis, that is also a 100 inch-pounds.

This why cheater bars work...a small force further from the axis creates more torque.

This is also why rotary engines produce relatively less torque than piston engines. The rotary 'arms' are very short. The piston engine has much longer arms at peak points in the rotating process (depending on all angles), basically the distance from the center of the camshaft to the center of the outer lobe on the camshaft.

Torque is what gets your car moving (transmissioin rotating, drive shaft rotating, diff rotating, wheels rotating).

mar3

iTrader: (8)

Theoritical equations and relationships are all nice and such, but have to be seriously corrected or framed to have any relevance to street or race cars. Nothing you see in a college physics book will have important relevance to street cars, though much of the math is necessary to get in the ballpark for a race car.

Proof? Here's the two ideal situations...

Huge torque at 2500 rpm through 225/50s will be a lot of smoke.

Huge HP at 7600 RPM will usually mean not that much torque at 2500 RPM, so no smoked tires and an easy win...

Torque rules the streets if you never go past 60 mph, but HP wins races anywhere...

Wankelguy

Diesel truck motors have lots of torque, but they don't accelerate very quickly. There's more to acceleration than merely torque and horsepower figures. This is the real key to rotary performance, that being the free-revving nature of the rotary engine. Think about it, if you install a lighter flywheel, the motor will accelerate faster, even though the torque and horsepower is the same.

Forseti

With most cars I look at the power/weight ratio as a rough estimate of how quick the car is. Obviously many other factors play in including the gear ratios, driveline efficiency, traction, drag coefficient, etc etc.

250 HP isn't much if you've got to pull a 5000lb car. Which is why a lot of users here are so keen on weight reduction.

F-

genrex

The generally-accepted equation that you're looking for is:

1/4 mile time = (6) (the cube root of weight/horsepower)

for example, if the cube root of wgt/hp = 2.5,
your 1/4 mile time will be about 15.0 seconds.

this is accurate to within 0.1 or 0.2 seconds, which is quite good.
the factor which accounts for the inaccuracy is actually the "6" -
which varies depending on how fast the car can be shifted - so if
you have a quick-shift kit or do speed-shifting, the "6" would be
something like a 5.9 or 5.85 for your calculation purposes.

the weight is the weight of the car with you in it.

another handy equation is: 234 / (the cube root of wgt/hp)
this calculates your trap speed at the end of the 1/4 mile.

these are the accepted equations used by the drag-racing crowd.
used for NA ENGINES.. they're not so accurate for turbo'd engines.

oni-dori

but that is only with ideal grip surfaces, right?

riofrio

Yeah, I'm sure these mathematical models assume lots of 'ideal' behaviors.

Forseti

Well even science tells us that life is full of estimates. Just take a course in quantum mechanics...you're dealing in probabilities a good part of the time.

If you wanted to really model the behaviour you wouldn't end up with a usable equation like genrex gave you.

Unfortunately sometimes you have to assume the ideal to get anywhere.

Pedestrian X

Paul Yaw has a good article on HP vs. Tourque anyways its very technical and long but heres the link.

http://www.yawpower.com/tqvshp.html

85rotarypower

I found this really neat program for testing multiple things. Its called cartest. It seems to be very accurate. It takes into account almost 50 variables, including losses from transmissions to driveshaft and tire weight. You can also input your own horsepower map. It comes with over 900 preloaded cars. Here is the link for anyone interested.

http://www.cartestsoftware.com/

It is the DOS based cartest. Cartest 2000 is not free, while the DOS one is free. It's called CarTest (Version 4.5).

peejay

Go to www.theoldone.com and do reading. There is some good info there

When they build race engines they don't care about HP, they only care about two things - how well the engine accelerates the car for its given load (weight, gearing, aerodynamic drag, etc., ALL taken into account) and how well the engine recovers after a shift (just imagien YOU are an engine happily accelerating at 8800rpm then WHAM you're down to 6500 again). They don't care about HP numbers and when they measure static HP they often find they are down 20% or more on the competition, never mind that in practice they run circles around the competition