I think I am probably wrong, but I do remember read something like acceleration determine largely by torque, top speed determine largely by horsepower...

 

Correct ... you are wrong
Sorry, couldn't resist. Yes, this is conventional wisdom. As is "hp sells cars, torque wins races" (or somesuch). But like so many pithy lines, it is at best half correct.

See the many discussions here and in every car forum. If you know physics, you know that the two are intertwined, but hp really tells the story. Key is, it is the hp you are at, not the spec peak hp that matters.

Also, torque specs will give you a good idea as to how about the engines go about making their power.

Should this be moved to Car Talk?

 

torque is what matters.

 

I didn't answer to the poll because I believe both aspect of an engine are very important for acceleration and top speed. It's true that you can't just take a simple equation to figure out what type or force matters more because of all the variables. At the same time while the variables can be solved for (not easily) they are going to be different for virtually ever car.

 

If you are just going to hit the race track all day, horsepower is more important.

For the street, a broad torque range allows an engine to be more flexible and therefore you can roll on the gas and go at a wide range of rpm. People mistake this flexibility as high torque. And while it is good for the street, it doesn't necessarily make the car fast when you are going all out.

Gordon Murray (designer of the McLaren F1) when designing a car sets a torque to weight ratio that he strives to achieve, not a horsepower to torque ratio. What he is really saying is that the ultimate street car needs a wide, torquey powerband so that at any rpm, in any gear, the car just moves out. And that is not a bad philosophy. I would expect no less if I were buying an F1. Wouldn't you?

But horsepower IS fast. Why? Because hp = torque x rpm. Engines that make their hp peak at a high rpm will be faster than engines that make the same hp at lower rpm. Here's an experiment to try. Ride your bike up a small hill in a medium gear. Try to apply a constant force to the pedals, (your torque). Now, take a runup at the hill, same gear, with a much higher pedal rpm, and apply the same force (your torque) to the pedals as you did in the first run. It'll be much easier to make it up the hill this time. Why? Because you made more horsepower the second time around. Hence the reason that the S2000 is as fast as it is. It makes its horsepower at a really high rpm.

Not my best explanation, but I hope this helps somewhat.

 

 

I'll challenge that. Well, it depends on what you mean by "what type of force matters". True, the feel of the car, and the way it goes about its business are more complex. BUT...

Tell me the peak hp of the car and its weight and I'll tell you its 0-60 within, oh about 0.5 sec, with a fair degree of certainty (within a std deviation). And within 1.0 sec most all the time. Tell me only its torque and weight and I can be way off on my guess.

Example:
240 hp, 2800 lbs. 0-60 will be about 5.5 sec. That holds for S2000, Boxster S, old Z3 M. Simple eqns get you close.

Now, 150 lb-ft and 2800 lbs. 0-60 in in 5.5 again (S2000) right? Or was I talking about the 8+ sec Cavalier??

To be sure, they go about their power in different ways, but hp and weight get you real close on accel. hp alone gets you close on top speed (also need Cd*A to get real close).

 

The way I explain it to non-mathematical types is making a reference to a Peterbuilt. They have gobs of torque, but not much HP. And they have trouble climbing steep hills. That gets them thinking. Then I mention a street bike like the R1. 180 or so HP, but no torque. Why can they run 11's? Then if they still don't get it, I start to make hammering in a nail analogy. Imagine if I had enough force (torque) to ram a nail all the way in with one shot. That would take me about 1 second for the windup, then a small time to hit the nail. At the same time, some little guy hits the nail 5 times and slams in in before me. Who wins then? Yeah, I am stronger, but he can make more smaller hits that combined over time creates a greater effect (that is horsepower). Of course, the little guy probably can't carry an 80lb bag of cement, but he is faster. And in cars, faster is what matters, not stronger.

The S2K was not meant for towing boats. But you could do it with a Camaro!

 

[QUOTE]Originally posted by S2kRob
[B]Gordon Murray (designer of the McLaren F1) when designing a car sets a torque to weight ratio that he strives to achieve, not a horsepower to torque ratio.