RPM questions

Is it better to have high power at low RPM's so you can reach your maximum speed quicker. Or, would it be better to have a very high power engine with high RPM's to continue to build power throughout the RPM range? Say were racing two 2001 mustangs or w/e one is a blue mustang and has a 680 HP 5600 RPM 660 ft lb at 4300 RPM engine. The other one is black and has a 400 HP at 4600 RPM 420 ft lb at 2800 RPM engine.

In general, provided all other things are optimized, the one with more power will always win. Now, if the comparison were mustangs with 400 hp at 4000/ 600 tq at 2000 and the other one 400 hp at 7000, 400 tq at 4000 the first one would most likely win. Why? More torque. Torque is a good measure of ability to get moving, and HP is a good measure of acceleration. Although they sound the same, they aren't. Torque is arithmetic, meaning that if you add twice the torque, you add twice the force. HP is exponential. Since acceleration forces are exponential as well, the two play equally important roles.

The secret is setting up the car properly. The blue mustang will need lower rear gears and a higher stall converter (if its an automatic) to properly make use of that power curve.

I know that doesn't really answer your question, but in your example, the blue mustang appears to be the clear winner. More HP and torque. The first 10 feet might be a battle, but the rest of the way, blue will dominate.

And to add... in general, where the power peaks and how it performs is also a function of the car itself. The engine from the blue mustang would do well in a small light car that doesn't need a lot of torque to overcome the car's inertia. I have one particular car with a 400-hp V8 that peaks its torque at 2500 rpms. Its in a heavy car. If I replaced it with a high-strung 400-hp four cylinder, it wouldn't do well at all, since it would make less torque at a higher RPM.

power under the curve wins. :smokin:

Another excellent reply Curtis,

I think the most important thing is that the engine is built to its purpose. even if you could get the reliabilty factor right up, you still wouldn't want to stick a top fuel engine in an F1 car and vice versa.

The mean power developed by the engine during the time the car accelerates is what affects the performance of the car.

As for the example curtis73 mentioned the 400 hp car at 7000 rpm will win if everything is equal and both engines have a transmission that suits their respective needs. (I also assume that the torque is in lbf*ft as no unit was mentioned). So why then?

600 lbf*ft @ 2000 rpm = 231 hp
400 lbf*ft @ 4000 rpm = 309 hp

7000-4000 rpm = 3000 rpm
4000-2000 rpm = 2000 rpm

As the car will accelerate the usuable engine speed range is usually given by max torque speed and max power speed + 500 to 1000 rpm overspeed.
The high speed engine does not only have a wider rpm range but it also developes a higher mean power as the engine developes 309 to 400 hp versus 231 to 400 hp.

Torque can never get a car to perform better, only a higher power output at the used engine speeds can increase performance. I can infact create more torque by hand than what any engine can do on its crankshaft, just give me a lever that is long enough. I can't however outperform the engines as they can produce their torque while also spinning very fast.

Given the right gearing a 350 Nm/900 hp F1 engine can get a 60 tontryck moving much faster than a 3000 Nm/600 hp diesel engine.

Lots of good info here!
What all of this boils down too is end usage of the vehicle. If this is strictly a race car, with no drivibility issues to consider, build the motor for a higher RPM life. 6 & 7 grand (or higher) banzai launches are okay for a drag car but they get difficult to live with on the street. 12,00 RPM small blocks are fast for sure but when they don't idle under 5,000 RPM's and can't get the car moving on a launch less then 10,000 they don't make for a fun daily driver.
Build a street car to make lowend torque. Torque does the work. Torque gets the car moving. As was stated earlier HP is a derivative of torque and should be secondary to the torque numbers. A 600 HP motor that makes only 300lb-ft at 2000 RPM is not impressive for a street motor but, one that makes 600 lb-ft at the same RPM is...
As an example; What is your common RPM range on a street car? Be honest. Off idle to say 4500 RPM? (I think the 4500 may be pushing it a bit). Does it make sense to build a motor that doesn't start to work until it hits 6 grand?
As long as the car can put the power to the pavement a torque motor is easier on parts and is, in my opinion, much more fun to drive on the street.
Mark

When you're talking typical V8s and V6s, most are factory designed for a power band in the idle-5000. Some gas engines were as low as 4200. In recent years things have become more performance oriented and technology allows for higher revs without a loss in reliability or much of a loss of low end torque production. Many today are in the idle to 6000 range or about 2000-7000 range in the case of the hotter performance engines like the LS1 or Ford's Mod motor in the Cobra. Usually above that RPM, the things you have to do to make it breathe that fast make you start to lose low end torque and other things like vacuum, drivability, and fuel consumption go way off the charts in a bad way for a street engine.

Sounds like you have a pretty good grasp of things, Astroracer.