RPM, HP, Torque & Top Speed

[**DONOTDELETE**]

In terms of top end speed, how much of a factor is horsepower and torque? If the rotating assemblies are spinning at a redline of 6K in top gear, does it matter or not if 2 spark plug wires come off? I realize that torque and horsepower are important factors for how quickly you reach top speed, but in the end, isn’t it just a factor of RPM, gearing, tire diameter, and wind resistance? Then again, aren’t HP & torque needed to overcome wind resistance and other drag factors? It just doesn’t sound right that you could replace the engine in a lakester with one from a Geo, run it at the same RPM and expect the same top speed as with the higher powered motor. What are your thoughts?

[Crafty]

You are right hp does have effect on top speed, I guess there is a calc somewhere that shows you, but I don't know it.. what I do know is that at 5252rpm torque and hp are equal..

[Darwin]

Go here; http://www.offroaders.com/info/tech-corner/reading/horsepower-torque.htm

[Machinos]

I think it's mostly a matter of gearing and wind resistance. If you had a REALLY powerful motor, but no overdrive and a steep rear end, and really bad aerodynamics, you wouldn't go very fast. The really high-end RVs with huge diesel motors and tons of torque can't have a very numerically low final drive ratio (as in, not much overdrive at all) because they're pushing so much air even at 70mph it wouldn't do any good. Like, a new Corvette or Viper has a really tall 6th gear, .50 or something, which it can actually make use of because it's EXTREMELY aerodynamic. In an old car like we've got you could drop the same drivetrain in and equalize the weight, but you'd still need a lot more power to get it up to the same speed.

[Drewfus]

I'm into 'Salt Flat Racing' and the following info comes from that basis. Ultimately, most of it can be argued as there are a few variables, but the point will be the last sentence.

'Here is a formula to find out how much power you need for a certain speed force = total drag + total rolling resistance + transmission losses X MPH divided by 375 = HP

But you need to know what drag your vehicle has(the aerodynamic part, this is a big ???), here is a formula to work out drag......

Drag = Af X Cd X 0.00256 X speed squared

Af = frontal area in square inches
Cd = coefficient of drag NB non dimential number only found in a wind tunnel as an aproximation use 0.035

0.00256 = air density
drag will be in pounds
also to get a rough approx of rolling resistance use this formula

Rolling resistance = vehicle mass X 0.013
I use 15% as a rough estimate for transmission losses

as you can see your drag is a squared function of speed, and alas power is a cubed function of speed.......

Cheers, and let the arguments begin,

Drewfus. http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[autocol]

[ QUOTE ]
It just doesn't sound right that you could replace the engine in a lakester with one from a Geo, run it at the same RPM and expect the same top speed as with the higher powered motor.

[/ QUOTE ]

it doesn't sound right, because it isn't.

top speed is - as drewfus has shown - a function of a number of variables, but for now, let's reduce them to drag and power. the greater the power output of a motor, the more air it can force the car through in a given period of time, and the faster it can go. when the drag on the car matches the power output of the motor (ie, the force propelling the car is exactly equal to the force braking the car) then the car will have reached a steady state... it's top speed.

it is true that if a less powerful engine were to be turning the drivetrain at the same rpm's, the car would have the same top-speed, but the truth is that it would never get there. with less power, the steady state between the driving force of the engine and the braking force of drag will be reached at a lower speed, so the car would never reach the top speed (or rpm's through the drivetrain) it was capable of with the more powerful engine.

[**DONOTDELETE**]

That's the type of info I was looking for. Perhaps the crowd I hang with is a little slow (could be taken more than one way I supppose), but no one I asked could provide a good answer to my question.

[ QUOTE ]
the car would never reach the top speed (or rpm's through the drivetrain) it was capable of with the more powerful engine.

[/ QUOTE ]

OK, I must admit that my experience behind the wheel has only occasionally gone past 100 mph. On those rare times that I've gotten my '66 289/C4 above 100 mph, I had more RPM's available to me. Drag obviously becomes a critial factor at high speed, and a Mustang is certainly not very aerodynamic. So the force of drag could keep me from redlining in top gear. Hmmm... Pretty cool stuff to think about. Makes me want to do some testing, but where can I go 'round here for some high speed runs?

Thanks to all for responding!

[**DONOTDELETE**]

Here's an interesting website I found related to the topic:

MPH Range (http://www.smokemup.com/auto_math/mph_range2.phb/)

Now the above sheet does not address drag and HP, but it does calculate speed based on gearing. Interesting reading. Oh, you have to register to use the website.

[**DONOTDELETE**]

Damn typo! Try this -

MPH Range (http://www.smokemup.com/auto-Math/mph_range2.php/) or paste http://www.smokemup.com/auto_math/mph_range2.php/