Question about max rpms

From an engineering/technical standpoint, what determines an engine's max rpms. It's just something I've been wondering and haven't been able to find an answer. Any help would be great, Thanks.

An engine needs 4 things to run:

Spark
Fuel
Air
Structural integrity.


Reach the limit of any of those and you reach your max RPMs.

Running out of fuel will cause a lean condition and destroy a motor if kept there too long, and of course reaching the limits of Structural integrity is also fatal to a motor.

Running out of the other two will just stop the engine revving any further.

Yup. There are kinda two redlines; the one where you stop making usable power and the one where pistons start disintigrating. The redline on most tachs that you see is the first kind.

if you wanna get detailed, the max RPM of a port injection vehicle is the rpm where it takes too long for the flame front to dissipate before the piston starts coming back up. but before this point is reached, your valves will be floating and hitting pistons, your connecting rods/wrist pins/pistons will break, and other bad things happen. direct injection helps by removing the pockets of fuel that occur slowing down combustion. a long rod/short stroke engine with direct injection, DIS, and a desmodronic valvetrain will be able to rev pretty much until things start melting.

if you wanna get detailed, the max RPM of a port injection vehicle is the rpm where it takes too long for the flame front to dissipate before the piston starts coming back up. but before this point is reached, your valves will be floating and hitting pistons, your connecting rods/wrist pins/pistons will break, and other bad things happen. direct injection helps by removing the pockets of fuel that occur slowing down combustion. a long rod/short stroke engine with direct injection, DIS, and a desmodronic valvetrain will be able to rev pretty much until things start melting.

Huh?
That statement is about as full of it as some of the statements in your little signature animation.

Huh?
That statement is about as full of it as some of the statements in your little signature animation.

to be fair, he had a couple half-truths in there :D

So basically it's about the design of the engine. Do I6 and V12 have higher redlines since they are better balanced? What about OHC versus a cam and rockers, or a cam and pushrods?

Edit: Oh and UncleBob, I love your sig picture. I feel the same way...most of the time ^_^;;

To an extent, the engine's layout can affect max RPMs, but for 90% of the engineering you'll see, they can all be tuned for whatever RPM you want. There are some I4s that won't rev past 3000 and there are some V8s that frequently see 8000. Almost all of the street engines you'll ever see could easily be engineered for many more RPMs than would ever be necessary in any form but racing.

If you're comparing the same displacement (let's say 5.0L) in different cylinder configurations, it does stand to reason that a V12 would be able to rev higher than an I4. The main reason is that the rotating mass (although greater) is divided among more cylinders. Since inertial forces and acceleration are multiplied by the square of its speed, 12 small pistons typically represent less inertia than 4 large ones. The additional pistons add lots more friction, but the extra revs you can achieve are a HP trade off that exotics are willing to make.

Another factor that can't be overlooked is bore/stroke ratio. A 5.0L that is made with a short stroke and big bore can rev higher than the opposite. Although the pistons would weigh more, they are moving slower. Overcoming inertia (which is exponential) is a bigger factor than the weight of the pistons (which is arithmetic)

Its all a dance of physics.

Huh?
That statement is about as full of it as some of the statements in your little signature animation.

^^^good refutation...i was simplifying but its all true. why cant you point out any mistakes you see instead of trashing my sig? its meant to be funny not critical.

^^^good refutation...i was simplifying but its all true. why cant you point out any mistakes you see instead of trashing my sig? its meant to be funny not critical.

if you wanna get detailed, (1) the max RPM of a port injection vehicle is the rpm (2) where it takes too long for the flame front to dissipate before the piston starts coming back up. but (3)before this point is reached, your valves will be floating and hitting pistons, (4) your connecting rods/wrist pins/pistons will break, and other bad things happen. (5)direct injection helps by removing the pockets of fuel that occur slowing down combustion. (6)a long rod/short stroke engine with direct injection, DIS, and a desmodronic valvetrain will be able to rev pretty much until things start melting.

1) port injection doesn't really have much to do with max RPM's.

2) I assume you are refering to burn speed. In a gas engine, this is by far the last thing to limit the RPM. Not until 25K RPM's or so anyway

3) Valve floating is caused by valve train overspeeding, not detonation or burn speed

4) rod/bearings getting damaged would be from detonation, not burn speed

5) direct injection has several advantages, although I'm not so sure a more even distribution of fuel is one of them.

6) Those are component designs that can help overcome inherent limitations in those areas, yes. "things start melting"....well, you can melt any engine with enough stupidity. Ask for pics, I'll send them.

car I'm building rev's to 11.5K, it has CARBS! *gasp*

^^^good refutation...i was simplifying but its all true. why cant you point out any mistakes you see instead of trashing my sig? its meant to be funny not critical.



No need to refute, I have an uncle that knows everything :evillol:

As for your sig?
As a joke its old and done to death, and in your case done badly. :nono:

1) port injection doesn't really have much to do with max RPM's.

2) I assume you are refering to burn speed. In a gas engine, this is by far the last thing to limit the RPM. Not until 25K RPM's or so anyway

3) Valve floating is caused by valve train overspeeding, not detonation or burn speed

4) rod/bearings getting damaged would be from detonation, not burn speed

5) direct injection has several advantages, although I'm not so sure a more even distribution of fuel is one of them.

6) Those are component designs that can help overcome inherent limitations in those areas, yes. "things start melting"....well, you can melt any engine with enough stupidity. Ask for pics, I'll send them.

car I'm building rev's to 11.5K, it has CARBS! *gasp*


1.) port injection is under much lower pressure than direct injection. this creates localized rich spots in the air fuel mixture in the combustion chamber. think of a garden sprayer. on low pressure it will spit out big drops of water, and if you turn the pressure up it will mist finer. the smaller the drops of gas in the mixture, the less hot spots in the combustion event. this makes the mixture burn more evenly and faster. thus allowing higher rpm without predetonation.

2.) see above. i know its not one of the things that limits rpm in most situations but it still theoretically is a factor.

3.) i know. thats why i listed valve float as an rpm limiting factor, not a result of detonation.

4.) again, this list was things that limit rpm, not things that are affected by burn speed.

5.) yes it is. see above. if that doesnt convince you, read up on it more.

6.) that was my point. things that theoretically limit max rpm in an engine. isnt that what this thread is about?



p.s.
moppie i dont know your uncle, but it seems like either he doesnt know everything or he doesnt tell you anything, since you cant seem to put any facts on the table. and if my sig is overdone, i wouldnt know. i havent seen anyone else with it except the guy i stole it from, and i doubt you know him.

1.) port injection is under much lower pressure than direct injection. this creates localized rich spots in the air fuel mixture in the combustion chamber. think of a garden sprayer. on low pressure it will spit out big drops of water, and if you turn the pressure up it will mist finer. the smaller the drops of gas in the mixture, the less hot spots in the combustion event. this makes the mixture burn more evenly and faster. thus allowing higher rpm without predetonation.


The point is, direct injection doesn't change the piston speeds, which is by far the biggest factor that automotive engineers use to govern max RPM If you are at 5000+ ft/min, It doesn't really matter what injection setup you have, you're playing with mechanical failure (in the long term) and it won't be caused by detonation, fuel distribution, etc.

direct injection offers more uniform atomization of the fuel, helps keeps the piston temps down, and allows an added paramter to play with, fuel injection time to further enhance engine management, but all of these things don't counter mechanical limitations via piston speed. Otherwise you would find direct injection engines passing the 5000 ft/min barrier, and I bet you can't find any

The highest piston speed engine I know of is the Honda S2000 (going from memory) around 5200 ft/min, way higher than just about every car out there. Not direct injection. Most stock cars are less than 4200. Even those insanely high reving motorcycles (R6, 16.2K redline) dont break 5000 ft/min on piston speed. (no direct injection, btw)

Then there's the F1 cars, drag race cars, with way higher piston speeds (some as high as 8000+), no direct injection.....other than diesels

i know i know. im not dumb, i know that piston speed and mechanical friction/failure come into play much before anything else. im just saying that burn speed can be a limiting factor, theoretically. i wanted to address all rpm limiting factors.

i know i know. im not dumb, i know that piston speed and mechanical friction/failure come into play much before anything else. im just saying that burn speed can be a limiting factor, theoretically. i wanted to address all rpm limiting factors.

well, when you find any example where it was a factor in a gas engine, I'd like to hear about. There might be some out there, but I don't think those engineers are talking about it

i remember reading something about it somewhere. if i can find it ill post it up. stay tuned.

yes, yes, we all have opinions and some are wrong-er than others.

i would like to add to this that non-interference engines are usually engineered to INDUCE valve float as a rpm limiter of sorts.
:wink:

yes, yes, we all have opinions and some are wrong-er than others.

i would like to add to this that non-interference engines are usually engineered to INDUCE valve float as a rpm limiter of sorts.
:wink:

I don't know if I'd call it "engineered" for that. The spring rate and preload for the valve springs are always reduced as much as possible for the design parameters (redline) for top end longevity. Stiffer springs increase valve seat wear, valve lip wear, and possibly cam/rocker wear