Fundamental (i.e. dumb) question about engines

[round_midnight]

I see in many posts people wanting to have high-revving engines for track/race use. Why are high revs important? It seems to me that if an engine makes, say, 500 hp, it doesn't matter at what rpm (within reason) it makes that 500 hp. Gear ratios can be used to get the desired speed in any case.

The only difference I see is that a high-revving engine will produce less torque for the same power, so engine components don't have to be as strong, and can hence be lighter. But I'm sure it's more fundamental than that, or the F1 guys wouldn't be spending millions making 20+ krpm engines.

Thoughts?

[Bob In Ct]

The HP numbers you see posted everywhere are the "peak HP" numbers. It usually occurs at high RPM because HP = Torque * RPM / 5252. Torque numbers are also "peak" numbers but occur at lower RPMs. The max torque usually occurs at the RPM at which the engine breaths its best.

For racing purposes it's all about HP.

Bob

[patrickt]

Originally Posted by round_midnight

I see in many posts people wanting to have high-revving engines for track/race use. Why are high revs important?

This is a good article that helps you get your arms around torque, hp, revs and gearing: http://www.vettenet.org/torquehp.html But remember, race cars and street cars are two different animals, and what's fun on the track can be abysmal on the street.

[ZOERA-SC7XX]

Originally Posted by patrickt

This is a good article that helps you get your arms around torque, hp, revs and gearing: http://www.vettenet.org/torquehp.html But remember, race cars and street cars are two different animals, and what's fun on the track can be abysmal on the street.

Agreed! Choose your engine carefully, as you are the one who'll be driving it regularly on the streets & highways.

[bobcowan]

It's all about top speed. My car is a street and track car. I want it to be fun to drive on teh street. But I also want a higher top speed on the front stretch. If I want to raise the top speed in 4th gear, I have two choices. 1. raise the rear gear ratio to something like 3.00. But then I lose that off the line torque that's so much fun. or 2. Increase the rpm potential of the engine. Then I get both.

[GreenSnakeDoc]

A high reving engine, provided torque does not fall off too much, offers the ability to have a higher maximum speed with matched gearing, or more acceleration with a higher numerical ratio rear end that multiplys torque to the driving wheels. It's the amount of engine torque applied to the rear axles that actually moves the car.

[RICK LAKE]

round_midnight Speaking for a couple of us that play in a different sand box, we build large torque band motors that are under squared. With the extra torque you can then make the right gearing in both the trans and rearend for the car. I don't built a 7,000 rpm motor in an FE. Reasons small rod bearings, heavy rotating mass. The heads with even the best porter are a major limiting factor. If you get 420 cfm from the BT you have magic. 360-380 cfm with alot of work moving ports, sparkplug holes, off set rockers, sleeving pushrod holes, raising ports.150" and .300" is about the norm. Torque motor is an endurance motor. They live twice as long as high revving motors. Depending on what you are doing, autocrossing, or roadracing without 1/2 mile straights, the torque motor will stay with the HP motor until it's out of it's range and do this with less rpms. If you have unlimited funds, you can build a tricked out SB or BB and not worry. Budget racers don't have this option. I am building a 498 motor to replace the 482 in the car. The 482 makes 500hp and 540 ft. of torque in street trim and street pipes with mufflers from ERA. I am leaving 20+ ft. of torque and about 30 HP in the pipes. The 482 is a hand full. I may advance the cam timing to take some of the bottomend out and move the HP and torque ranges up a little higher in the rpm scale. I run 3.31 gears at this time with either a G-Force 5 spd or Richmond 6 spd with overdrive. The fun is coming with Diesels for racing, Look up Galebanks. 5,800 rpm diesel in a truck that will run down most cobras. Rick L. Midnight biuld a motor that fits your needs, don't worry about the other stuff. Higher the RPM the more maintainance and money. Also the shorter life span. The GT-40's that won lemans had a 6,200 rpm chip in them for both fuel milage and to make sure the motor stayed in 1 piece for 24 hours. Also a Holley carb had less moving parts than a set of webers. The webers made more power and used more gas. Funny trade off. Some times the turtle beats the rabbit.