What are the benifits of increased engine RPM?

[bob cook]

I'm curious to know the benifits of increased RPM in a Drag Race engine. I've recently lightened my valve train by 1/3 of a pound and noticed and increased capability to rev the motor higher. How would I take advantage of it? Would the cam need to be advanced or retarded to move the power band up? Do the heads need increased flow? Does the rear gearing need to be changed? I'm running a C4 with an 8.8" rear end. I'm stuck running the oem hydraulic lifter per class rules. The motor is a Vortech blown 309ci sbf.

[speedtalk]

Benefit?

This formula says it all:
Horsepower = (Torque X RPM) / 5252

Raise Torque or RPM, make more power

Nothing will move the operating range of your engine more than better cylinder heads.

[OldSStroker]

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

[Joe Mendelis]

You can not multiply hp by RPM, only torque. If this was the case a Formula one car would accelerate considerably faster than they do. Imagine 920 hp X 20,000 RPM. In reality it is 230ft lb times 20000. what is better, a 20,000 rpm motor with 230 ftlb, or a 230rpm motor with 20,000 ft lb? It's the same power, but one has almost 1000 times the gear!

600 Hp @ 6000 rpm. Say you have a 4.0 gear 2100ft/lb (525x4)
600Hp @ 9000 rpm. 1 1/2 times the rpm, so 1 1/2 times the gear. You have increased RPM by 50 per cent, so "wheel torque" is 50 % higher? 600 hp at 9000 rpm is only 325 ft lb... times 6.0 gear comes out to 2100, same as above. Now you have more friction in the driveline as well. It is 100% proportianal. I'm not saying more gear won't help in several different cases.
We have 8 cylinders firing times the net PV diagram area (torque) times the cycles per unit time. RPM only gives you more power if the torque doesn't fall off proportionately faster than RPM rises.

one horspower is 33,000 ft lb per minute. The standard unit of force being one ft lb. That comes from having one pound of force one foot from the axis. So with that you can see where 5252 comes from.
If the radius is one foot, the diameter is two feet, then the circumference is two pi. Divide 33000 pounds of force by a linear distance of 6.283 feet and you get 5252. If the force is going less than 5252 rpm, it is not going to make one hp. If it goes faster it will make more. It is exactly proportional. If you go to 10504 RPM you will have two hp, 2626 will be 1/2 hp.

The gears in the rearend multiply torque, not power so when you make the same power higher you just crutch it with more gear and multiply your proportionately less torque more to still end up with the same thing.

The cup engines will only make about 1.48-1.52 ft lb per inch. They want more RPM because they have heads good enough to raise the rpm where torque is highest, and that makes power. The valvetrain is so hard to deal with is why they don't make more power.

Will a 600 hp 6000 rpm engine really go the same as a 9000 rpm engine making 400 hp because the rpm times power is the same?

Sorry if that came out as being offensive.I just wrote that trying to put it in perspective (at least my perspective). I know there are things like inertia in the rotating assembly etc. to think about as well.

I had a coversation with a guy the other day telling me rpm made you go faster all by itself, but he couldn't explain it to me and got all mad and got off the phone. I listened and gave him a chance to explain, but he couldn't. I have heard other people say this before that were good engine/car people, but they never explained it to me in a way that made sense. Everybody just says "you can put more gear in the car so you go faster". There must be something I'm missing and hopefully someone can explain it to me in a way I understand. When the car is leaving the line/coming out of the turn it is the most vulnerable to a loss in average speed, which would mean higher et's or lap times. It makes sense that if an engine can get in to its power band imediately it can get toward the end of the straight/ 1/4 mile where the speed is much less critical in affecting final time. Cup and Pro Stock ARE making more power than a few years ago. One very prominent shop is up 8 % from just 2 years ago.

[Joe Mendelis]

In rambling I missed the question to begin with. Bene of increased RPM? The one and only benefit is the ability to make more power(And it sounds good)
I hope people with experience in very competitive racing will voice their opinion here.

[cboggs]

HP, CFM, .. all a measurment of time, ..

Once a cylinder head has reached the cfm demand of a given
bore / stroke, .. the way to extract more power from a better cylinder head
is more rpm.

Did that come out right ??? ( it's 11:30 at night and it's been a long day )

Helping the cylinder head & flow is rpm, .. cylinder head ports
are ( or should be ) designed for a bore / stroke combination at a particular
rpm and a primary design goal is air speed through the port.

Higher rpm means a higher air speed, so the cross section of the port
can be larger to help increase flow.

Big air pump, .. high rpm means pumping more air in a given time, ..
means more power.

Example,

300 ci v8, .. 4.125" bore, .. 2.8" stroke

at 7000 rpm the minimum port cross sectional area should be 1.92"

at 9500 rpm it's 2.6"

Curtis

[Racer7088]

Like Joe said we turn more rpm to make more power or to make more average power like an engine that holds onto it's power a long time.

Turning rpm to turn rpm though is what people do when they also have exhaust tips that are 6 inches in diameter even though they are making 150 hp.

[Trev]

Does this also apply if your engine starts dropping of in the hp after a given rpm

Trev

[BRENT FAY]

I agree with with David Reher's explanation to why more RPM is better. Go to www.rehermorrison.com and click on tech talk and go to article #47 "Raising the Redline - Why RPM matters". Food for thought reading. Brent

[bob cook]

I figured there was not going to be a straight forward answer due to all the variables involved. I'm just trying to plan my next move to try and get something out of this increased rpm capability. My car dynos at 786 peak rwhp around 7000rpm. It starts dropping off after that. I probably should evaluate it as an average hp based on an rpm range and use the range with the highest average hp at the track.

[panic]

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...???

[OldSStroker]

I probably should evaluate it as an average hp based on an rpm range and use the range with the highest average hp at the track.

That sounds like a good plan to me.

I'm stunned.

Reher has "explained" how 1,000 HP @ 8,000 RPM (657 ft/lbs.) X 4.88 gear (= 3204 ft/lbs.) or 5.14 (= 3374 ft/lbs.) is better than 1,000 HP @ 7,000 RPM (750 ft/lbs.) X 4.56 gear (= 3421 ft/lbs.).

He also "explained" (same page) that an " the engine speed should ideally drop 1,000 to 1,300 rpm after a gear change".
Perhaps he could identify this extremely rare transmission with 16% RPM drop on the shift... any shift. Perhaps one of those 9 speeds??
He then goes on the completely confuse dropping below the converter's stall speed (shifting early) with ratio changes (inherent mechanical component, beyond the driver's control).

But he must be right because he's famous....

Don't panic, panic....

If Reher had suggested a 5.21 screw instead of the 5.14 vs. the 4.56, the wheel torque would be the same, right? Maybe he was suggesting the 5.14 because the car would be a couple mph faster (about 3 mph @200) with the higher revving/more gear package. Anyhoo, that's how I read it. The principle still works. FWIW, Reher isn't the only one on this band wagon, but he's one who is talking about it.

If you want to pick on Reher's message, take issue with "multiply the power ", when he meant multiply the torque which he explained earlier.

Not apologizing for Reher, but physically and practically he's correct.

If you get a chance look at some of the ratios in the intermediate gears in a P/S 5-speed. Even NASCAR outlawed ratios closer than 1.3 on "gear rule" tracks. Some of those Cup 4-speeds had some ratios probably closer than 1.1:1. If I recall, at Pocono shifting dropped the revs from 9500 to maybe 88-8900.

I doubt that a 6700 stall converter EVER gets close to zero slippage under high hp loading, so shifting it at 8000 is still going to give quite a few hundred rpm slippage after the shift and tighten up the engine's rpm band more than the mechanical gear ratios would suggest.

My $.02

[dbusch]

Bob, considering how NMRA 310" pure street engines with Systemax EFI manifolds routinely turn past 8000 rpm, there is no reason why you cant go AT LEAST that much rpm. They make peak hp at about 7300-7500rpm and carry it out far enough to justify shifting at over 8000. With boost that should equate to even more rpm. If your power is dropping off after 7000 rpm it could be one or a combination of several problems:

1-the blower is being turned too fast
2-out of fuel
3-out of manifold/cyl. head cross section/flow
4-exhaust system too small
5-not enough valve spring pressure
6-pushrods too small/weak
7-cam duration too small/ centerline too advanced


believe it or not, turning the blower too fast is the most likely problem. You are achieving peak efficiency too soon and the air is getting superheated above 7000 rpm. turn the blower less rpm and you will need more converter and more rear gear; however, your power above 7000 should come up as long as you have the rest of the elements in place. and, as long as you power is up, more rpm will definetly have helped your average hp...

I would like to see an 8500 rpm shift point and a converter that goes 7000-7300 rpm. Maybe even 8800 through the lights if the valvetrain can take it. With titanium valves, the right cam lobe, and some killer pushrods i dont see that being a problem...

[bob cook]

This is the type of info I'm looking for and others can learn from. Hopefully, not the competition . I'm turning the YSI at 10:1 or 80,000 when I shift at 8,000. Probably a bit too much and way above the sweet spot in the YSI efficency map. More is better isn't it? Boost yes(30+lbs above 6,500), heat no(300degrees F crossing the line). Running 90 to 95 percent duty cycle on the 83lb injectors w/Weldon 2035 pump. Intake and Head flow should be good but, not sure. 1-7/8" into 3" collector. Springs - 150lb seat/400lbs open(Comp Cams 977). 5/16" diameter .08 wall hardened Smith bros. pushrods. Duration may be a bit too small 242/256@.05. Not sure about my converter but, I'll contact the experts on the details. Valve Train, I'd bet it's the lightest in the class.

[Jay Allen]

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.......