A good question concerning RPM's

[FinallyGotOne]

3 questions:

I was watching a race the other day and the cars RPM guage started at 6000, and went to 19,000, it was an open wheel car. I was amazed at that RPM. My question is this?
Why are our 911's (79 sc) limited to 6250 RPM. My paperwork and stuff all shows that after about 6250 RPM 's the car does not gain anymore power?

Why are we limited to a lower RPM?

Somebody also mentioned a rev limiter. I think I finally noticed this. the other day iwas entering the freeway in 2nd gear as fast as i could (just for fun) and it sputtered right before i shifted toi 3rd. The RPM was in the red past 6250 I'd say. So is the rev limiter that device in the car that cuts power after a certaoin RPM is reached, and at what RPM is that.

Lastly, is there any way to make 1st gear a longer gear so it does not tack out so quick, but keep th eother gears as they are so the top end is still pretty good?

Thanks all!!

[bpoteat]

most (all?) production cars have rev-limiters to keep you from killing your engine. if your car isn't making anymore power past a certain rpm, why would you want to go that far into it anyway?

[herr_oberst]

F1 engines have pneumatic valves, ultra-balanced reciprocating mass and NASA quality electronics to help them to live at the upper stratosphere of the RPM range.

911' engines are semi mass produced with a balance between cost and quality. (Not to mention valves that are closed by wire springs) and 6250 is good RPM for an engine like this. I've always felt that the redline on the tach was put there by someone who knows more about these expensive parts than I do, and pretty much abide by it.

[DaddyGlenn]

Originally Posted by FinallyGotOne

{snip}...Lastly, is there any way to make 1st gear a longer gear so it does not tack out so quick, but keep th eother gears as they are so the top end is still pretty good? ...{snip}

Theoretically, you could have your transmission rebuilt with a taller first gear but you would end up with slower acceleration in 1st (along with greater wear on the clutch, etc.) and the 2-3 shift would come sooner (time-wise) because you would enter 2nd at a higher rpm to start with.

Not really worth the effort or expense. 1st gear is really just to get you going. 2nd through 5th are close ratios so that is where you should be doing your spirited driving.

[TechnoViking]

This may help explain.....

http://www.fastmachines.com/archives/f1/001603.php

2003 F1 teams...
Engine budgets $931,000,000
$185,000,000 Williams
$175,000,000 Ferrari
$150,000,000 Toyota
$140,000,000 McLaren
$110,000,000 Renault
$105,000,000 BAR
$ 24,000,000 Sauber
$ 18,000,000 Jordan
$ 15,000,000 Minardi
$ 9,000,000 Jaguar

[FinallyGotOne]

Daddy Glen, that is the funniest quote i think I've ever seen. (Slinkies)

I think I misunderstand something, i thought the power dropped after 6250 because of the limiter, but it is because of the design of the engine then.

let me see, if the crankshaft is turning at 6,250 rev per minute or 104 revs per second, and it is giving you a certain amount of pwer i now assume valves exploding, with the combustion of fuel end exhaust etc... that the fuel system cannot deliver anything really beyond that then. Would i be correct in assuming the the crankshaft turns once for every combustion and once for every exhaust? so really the crankshaft turns twice for each cycle of fuel/- (or is it that the camshaft turns twice for each crankshaft rotation? I seem to be forgetting something here...
So really the crankshaft could definitely handle turning faster, assuming bearing and all could take it, but the fuel delivery and such wuld not hold up. This sound right?

[Quicksilver]

For a good explanation of the basic workings of the standard 4 stroke motor look at Wikipedia's write up on the Otto Cycle.

[David E. Clark]

The “red line” on your tachometer refers to the maximum engine speed at which the motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. Thus, unless you are like Scrooge McDuck and bath in money, you don’t want to run your car above the red line for an extended period of time. The “rev limiter” is a factory installed device designed to prevent damage to the engine by interrupting the power that is distributed to the spark plugs. This prevents the engine from operating above a pre-determined RPM level (normally slightly above the red line); in essence a rev limiter is a factory attempt to save the motor from the owner!

For a really good primer on the relationship between horsepower, torque and RPMs, I suggest Horsepower Vs Torque, What’s The Difference, Why It Matters, And How To Get It By John Baechtel on the Carcraft Magazine web site.

[berettafan]

Valve springs can only close the valves so quickly. Too high a rpm can cause 'valve float'.

[old man neri]

Essentially you start getting something called 'valve float'. The valve springs cannot shut the valves fast enough.

[DaddyGlenn]

There are many possible limiting factors that could vary from engine to engine. Not being an engineer, I won't even pretend to know all of them. But all the metal bits can only take so much stress before they start to deform and there are the aerodynamics of moving the gasses in and out of the cylinders and managing the heat that is created, etc., etc.

It is all fascinating stuff and if I had a life time I'd go back to school and learn them all and start experimenting.

Oh, and I can't take credit for the quote. It came from a friend of my wife. I don't know the original source but I thought it was hilarious, too.

[bpoteat]

Originally Posted by FinallyGotOne

I think I misunderstand something, i thought the power dropped after 6250 because of the limiter, but it is because of the design of the engine then.

That's right.

Take a look at a dyno-graph (a plot of an engine's HP and torque over an RPM range) and you'll see that each engine has a peak where more RPMs do not produce any more torque or power, and will actually start losing power. A simplified version of your goal (for acceleration anyway) is to make sure that, at any speed, the transmission is in a gear ratio where the engine's RPM is so where you are getting the maximum amount of HP and Torque - i.e. the 'peak' of the dyno-graph.

[svandamme]

more revs = less longevity, more complexity and an exponential increase in cost and usually less drivability as well

Porsche could have designed for higher revs
but it would have come at the expense of all those things, and then some

the valve opening closing part could have been solved for instance by using something like Desmodromic valve train, eg, camshaft controls opening the valve, aswell as closing the valve, that would complicate the valve train, as would working on it

then retuned high rev camshafts
then the intake would have to be retuned for a higher rev range, and higher intake velocity

so the fuel injectors have to be higher up in the stack,
doing this , causes lowdown loss of torque, which means lesser drivers will find it more difficult to keep the car in the right rev range...
or it means a second set of injectors, but that isn't an easy thing , especially not in the pre-efi era

then the exhaust needs a similar retune
gearboxe need to be retuned,
and as everything spins faster, the strain on the components from start to finish will be worse and they will have more warranty problems

and in the end, most drivers buying Porsches new, aren't capable of using that car anymore because it's a pita to sit in a t-jam with your engine idling at 5000 rpm and you can't hear your radio anymore...

more revs is only interesting if you cannot go up in displacement
like in Formula 1
most other situations, will benefit more from displacement or turbo, or supercharging

[livi]

Good thread!

Always a pleasure Stijn. Between us for'ners, I wish I could take advantage of the English language like you do. You seem to have annoyingly good knowledge about engines too (envy).

[svandamme]

Markus,i just got that knowledge from a couple of well known books(Bruce anderson, Wayne's,C.Probst),and this forum

and i don't know half the stuff some others here know, the 911 engine forum has a couple of hardcore folks in it.. BeepBeep , Henry from Supertec, Grady Clay and so on... i just try and "get" what they say, and recycle that stuff in the appropriate threads, like this one

[dd74]

Originally Posted by FinallyGotOne

I think I misunderstand something, i thought the power dropped after 6250 because of the limiter, but it is because of the design of the engine then.

Actually, as I recall from my 3.0 days, the engine stops making real power at 5,500 RPM; anything higher, and it's just spinning.

I remember reading an old Porsche Euro road test, where the magazine tested a 3.0 race car that was allowed only to be lightened. Everything else was required to be stock. When asked about the 204 hp 3.0 and how high it revved, the driver suggested that after 5,500 RPM, one may as well shift up if they can, because otherwise the engine isn't creating more power.

I think, however, if you really want to utilize that last 750 RPM before the rev limiter cuts in, you need to get a 7.31 ring and pinion for your transmission. That's what I have, and my 3.0 easily ran with power past 6,250 RPM. Of course it helped that I didn't have a rev limiter.

[FinallyGotOne]

DD74

7:31 Ring and Pinion change ... hmmm. would i get a little longer gears doing this? What year is that set up from?

does it run a bit slower from the start vs my stock 8: ring and pinion?

[RWebb]

what I do not see noted above is that a street car lives mostly in conditions near idle -- you don't want the motor to sit a 3k rpm when it is stopped. That means that a range from say 2 or 3k up to whatever they can engineer while making the motor work well at 2 or 3k is the most important rpm range for a street car. To extend that range you need vario-cam, vario-valve and etc.

Racing Rules often limit displacement, making high rpm the only way to make good power in a NA motor.

[Gogar]

Shifting at a certain RPM after peak HP might be desirable still if it puts you more firmly in the optimum power range of the NEXT gear. (I'm guessin.)

[arerrac]

A little off topic, but speaking of engines that rev to19,000 rpm (purposely limited, can actually do more than 21,000 rpm). F1 engineers are stating that soon they will be able to rev even higher. Their concern is that the air passing the valves would be traveling at supersonic speeds and their not sure what effect this will have on the engine. Freakin awesome stuff.