What are the benifits of increased engine RPM?

[putztastics]

More rpm means more gear for the same speed, so torque to the wheels is multiplied. Torque at the wheels divided by loaded tire radius gives FORCE or thrust at the wheel contact patch, which is the "F" in F=Ma. That force accelerates the vehicle.

Let's say you increase rpm 5%, but maintain the same hp figure. You are actually producing less torque at the higher power peak, but all the way thru each gear you are multiplying the torque by 1.05 because you changed gears by 5% to maintain mph.

Example: 600 fwhp @6500 or about 485 lb-ft. 600 hp @(6500x 1.05) or 6825 gives torque of 462 lb-ft. Let's say final gear was 4.10. In 1:1 top gear that's 1989 lb-ft at the wheels at 6500, but with 5% more gear, 4.31, there is still the same wheel torque. Now drop back to torque peak, and with the same peak torque there is 5% more at the tires. a= F/M, so a is larger.

I think that's what we've seen in Nextel Cup and perhaps Pro Stock in the last few years. Even with the same peak hp, the car can accelerate faster in it's usable rpm range with more gear.

My $.02

Very, very, very good reading.

Everytime I try to say this on any other board, I get ridiculed. It is refreshing to hear and to see other very compitent people sharing great knowledge.

I am surprised you did not get piled on.......

What you have to remember is if the torque is falling fast at that increased gear and RPM, the torque at the tire patch might actually be less. If you expect to go faster with more gear and RPM you better be making torque at that increased RPM. I fixed up a spreadsheet into which you could input engine RPM and torque from a dyno test, rear tire diameter, transmission gear ratio, rear end gear ratio, etc. and it would show torque at the tire and lbs. force at the tire patch. I use it to estimate rear gear ratios for dirt track racing. There aren't too many people interested in the results though. Around here they want to turn higher RPM so the car is "easier to drive". They are well over their torque peak RPM so the car isn't as loose coming out of the turns. So I ask well can't you run less gear and feather the gas out of the turn and have some power in reserve? Well yes but... Some think you will automatically go faster with big RPM, kind of like driving faster when you are about to run out of gas. It's dumb but people still do it.

[OldSStroker]

I see you didn't bother to read my post, or his article:
"If Reher had suggested a 5.21 screw instead of the 5.14 vs. the 4.56, the wheel torque would be the same"
Yes, it would, and the advantage is therefore what?

"With an automatic transmission, the engine speed should ideally drop 1,000 to 1,300 rpm after a gear change"
Not "a P/S 5-speed"
Again, which automatic transmission is that...???

Interesting edit, panic. I saved your original post for you.

I DID read your original post and I studied DR's article, and even critiqued it. Did you miss that? Perhaps DR didn't get his message across clear enough: the extra gear multiplication works throughout the engine's operating range. In the 5.14 vs 4.56 comparison there is about 12.7% more multiplication at every point. If the higher rpm engine is only down say 6-7% at a given vehicle speed over the "low" rpm engine, there is a net gain in wheel torque.

Engines that breath well often have hp curves that peak and then hang on for a while without taking a nose dive. IOW, they aren't peaky. From what I've been able to learn, Cup, P/S and even F1 engines exhibit these characteristics. Why is another subject.

Anyhow, that means the torque curve is dropping only slightly faster than the rpm is rising, so the overall torque curve hasn't changed shape much in the engine that moves up to 10k from 9.5K. Here's where more overall gear creates more thrust throughout the usable rpm band.

As for automatics: If I remember correctly, a THM 400 has a 1.48 second and direct third. If there were no slippage ("Clutch-O-Matic"?), an 8000 rpm 2-3 shift would drop to about 5400, right? With a 6700 stall (citing earlier example), there's no way the engine is going to drop anywhere near 5400. It will be a LOT closer to the stall rpm, which is just about the 1300 cited. My guess is that an engine that peaks in the mid/high 7's will have a fat amount of torque in the mid 6's.

Even in production cars the torque converter smooths over the larger gear steps better than a manual clutch does. I suggest that in part-throttle, suburban traffic type driving, if you held the accelerator steady, the engine would stay maybe in a 2400-2800 band from launch to Interstate speeds. Shoot, a C5 with a 4L60E with terrible gear spreads does that now.

The newer trend to 6-speed autos, like the 6L80E in the STS-V, XLR-V and C6 (soon) allows the engine to work in a narrower rpm band, OR in the case of heavy SUVs where you'll see MANY more of them, it allows a wider total gear spread. You'll see a 6:1 or more between 1st and 6th vs. about 4.4:1 on a 4L60. The 6-sp vehicle will launch better, tow better, and with the intelligent PCM, keep the engine in the most economical/lowest pollution/fastest accelerating (choose one at a time) mode without having loose, inefficient converters. With clutch-to-clutch shifting, gear changes are almost invisible. It approaches a CVT.

BTW, I like your screen name, panic.