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Whats the biggest limiter to high RPM's in an engine

S

Shockwave

Banned
Whats the one biggest limiter to having a high RPM engine? For example, a Detroit V8 usually (stock trim) redlines around 5-6k. European V12's can touch 8 or 9K. So obviously number of pistons isnt it. So, what 1 upgrade could you do to an engine to allow it to rev higher without damage? Stronger pistons? Valve upgrades? Stronger crank?
 
For one thing you need to make sure that the combustion cycles can occur quickly enough, so you need fast fuel injectors.
 
Presumably its "valve float". This is when the valve spring/mass system hits resonance, and no longer operates in conjunction with the cam. "Big Detroit V8s", as you call them, are usually pushrod systems, which means there is more mass, hence the lower resonant frequency (which is usually the determined redline). The "European V12s" usually are DOHC, which have the cam touching the valve (or a hydraulic lifter) directly--so less mass and a higher redline.

There's no easy "swap", I don't think. You MIGHT could put stiffer valve springs in... but I'm willing to bet this has been tried by the manufacturer. It also places higher loads on the valvetrain as a whole.

Just FYI, Formula 1 went to electro-pneumatic systems to get past this point--a little diaphragm is moved with compressed air as opposed to a traditional cam. Of course, CART is routinely hitting 14K rpm with conventional DOHC engines.... but with pretty sophisticated interference-type nested valve springs (and super-light valves).
 
Originally posted by: HokieESM
Presumably its "valve float". This is when the valve spring/mass system hits resonance, and no longer operates in conjunction with the cam. "Big Detroit V8s", as you call them, are usually pushrod systems, which means there is more mass, hence the lower resonant frequency (which is usually the determined redline). The "European V12s" usually are DOHC, which have the cam touching the valve (or a hydraulic lifter) directly--so less mass and a higher redline.

There's no easy "swap", I don't think. You MIGHT could put stiffer valve springs in... but I'm willing to bet this has been tried by the manufacturer. It also places higher loads on the valvetrain as a whole.

Just FYI, Formula 1 went to electro-pneumatic systems to get past this point--a little diaphragm is moved with compressed air as opposed to a traditional cam. Of course, CART is routinely hitting 14K rpm with conventional DOHC engines.... but with pretty sophisticated interference-type nested valve springs (and super-light valves).
Click to expand...
What special does Honda do to hit 9kish in the S2000?

 
Don't forget the extreme forces that pistons are subjected to at high RPM's. Mass isn't meant to change directions that rapidly.

Viper GTS
 
Simply put, rotating mass. Reduce the mass in any component that moves/rotates and you increase the available redline. I'd say mass is the 1st order affect. Harmonics, stresses, heat, those are all 2nd order affects which increase/decrease with mass.
 
Originally posted by: Viper GTS
Don't forget the extreme forces that pistons are subjected to at high RPM's. Mass isn't meant to change directions that rapidly.

Viper GTS
Click to expand...

1 giant advantage of the rotary engine, but thats outside the scope of this thread. I assume that that force is actually hardest on the crank or piston connecting arm though right?
 
Originally posted by: Skoorb
Originally posted by: HokieESM
Presumably its "valve float". This is when the valve spring/mass system hits resonance, and no longer operates in conjunction with the cam. "Big Detroit V8s", as you call them, are usually pushrod systems, which means there is more mass, hence the lower resonant frequency (which is usually the determined redline). The "European V12s" usually are DOHC, which have the cam touching the valve (or a hydraulic lifter) directly--so less mass and a higher redline.

There's no easy "swap", I don't think. You MIGHT could put stiffer valve springs in... but I'm willing to bet this has been tried by the manufacturer. It also places higher loads on the valvetrain as a whole.

Just FYI, Formula 1 went to electro-pneumatic systems to get past this point--a little diaphragm is moved with compressed air as opposed to a traditional cam. Of course, CART is routinely hitting 14K rpm with conventional DOHC engines.... but with pretty sophisticated interference-type nested valve springs (and super-light valves).
Click to expand...
What special does Honda do to hit 9kish in the S2000?
Click to expand...

a short stroke. shorter stroke means the piston doesn't reach high velocities, given a certain RPM, therefore it isn't decelerating as much when it has to change directions at the top and bottom of its stroke.

it also means that you don't make as much torque. 🙂

Here's a quick mathematical explanation. V = rpm*radius. When the piston is at the middle of its stroke, this equation holds true. A short stroke motor has a shorter radius thus lower piston velocities.
 
Originally posted by: Skoorb
Originally posted by: HokieESM
Presumably its "valve float". This is when the valve spring/mass system hits resonance, and no longer operates in conjunction with the cam. "Big Detroit V8s", as you call them, are usually pushrod systems, which means there is more mass, hence the lower resonant frequency (which is usually the determined redline). The "European V12s" usually are DOHC, which have the cam touching the valve (or a hydraulic lifter) directly--so less mass and a higher redline.

There's no easy "swap", I don't think. You MIGHT could put stiffer valve springs in... but I'm willing to bet this has been tried by the manufacturer. It also places higher loads on the valvetrain as a whole.

Just FYI, Formula 1 went to electro-pneumatic systems to get past this point--a little diaphragm is moved with compressed air as opposed to a traditional cam. Of course, CART is routinely hitting 14K rpm with conventional DOHC engines.... but with pretty sophisticated interference-type nested valve springs (and super-light valves).
Click to expand...
What special does Honda do to hit 9kish in the S2000?
Click to expand...

I don't think its necessarily anything "special"... in that they try to increase the ratio of valve spring stiffness to valve mass (if you don't know, resonance frequency = SQRT(stiffness of spring/mass)). They DO use titanium valves and pretty darn stiff valve springs (they probably put in nested valve springs, but I haven't actually opened up an F20C). Although, 9K isn't as "surprising" as one may think. I've seen DZ302s (small block Chevys from the late 60s) hit 8K regularly--with pushrods. The more impressive thing is that Honda is actually warranty-ing it. 🙂

ViperGTS has a good point--although, I think its a more easily overcome-able one (with the right amount of cash). Pistons, and even more often, connecting rods--are still frequently made of cast-iron (for self-lubricating reasons). Switching to aluminum alloys/titanium pistons/conrods could lessen that mass.

Keep in mind that more of this is reliability driven--some of the modern DOHC engines can turn much faster than their posted redline. But I doubt the manufacturers will warranty it. 🙂
 
Originally posted by: Triumph
Originally posted by: Skoorb
Originally posted by: HokieESM
Presumably its "valve float". This is when the valve spring/mass system hits resonance, and no longer operates in conjunction with the cam. "Big Detroit V8s", as you call them, are usually pushrod systems, which means there is more mass, hence the lower resonant frequency (which is usually the determined redline). The "European V12s" usually are DOHC, which have the cam touching the valve (or a hydraulic lifter) directly--so less mass and a higher redline.

There's no easy "swap", I don't think. You MIGHT could put stiffer valve springs in... but I'm willing to bet this has been tried by the manufacturer. It also places higher loads on the valvetrain as a whole.

Just FYI, Formula 1 went to electro-pneumatic systems to get past this point--a little diaphragm is moved with compressed air as opposed to a traditional cam. Of course, CART is routinely hitting 14K rpm with conventional DOHC engines.... but with pretty sophisticated interference-type nested valve springs (and super-light valves).
Click to expand...
What special does Honda do to hit 9kish in the S2000?
Click to expand...

a short stroke. shorter stroke means the piston doesn't move as quickly, therefore it isn't decelerating as much when it has to change directions at the top and bottom of its stroke.

it also means that you don't make as much torque. 🙂
Click to expand...
Yup - with an 84mm stroke, 9000 rpm is not really a big deal.

 
Originally posted by: sward666
Pistons, and even more often, connecting rods--are still frequently made of cast-iron (for self-lubricating reasons).
Click to expand...
Nobody makes pistons out of cast iron.
Click to expand...

A LOT of people make conrods out of cast-iron... i should have qualified. 🙂
 
Extremely light(expensive) reciprocating components, titanium rods & valves and unobtainium pistons 😉

Piston speed is the true limiting factor but that seems to be moving up, about 4500' per minuet.

V-12's normally have very short strokes so they can generate high RPM's at relatively low piston speeds.

Getting rid of valve springs and using pneumatic valve control also helps, but that is for the future, F-1 uses it now.
 
Originally posted by: Triumph
a short stroke. shorter stroke means the piston doesn't reach high velocities, given a certain RPM, therefore it isn't decelerating as much when it has to change directions at the top and bottom of its stroke.

it also means that you don't make as much torque. 🙂

Here's a quick mathematical explanation. V = rpm*radius. When the piston is at the middle of its stroke, this equation holds true. A short stroke motor has a shorter radius thus lower piston velocities.
Click to expand...

Yep... this is why a lot of the high-revving V8s (even the pushrod types) are the short-stroke ones. The DZ302 in particular... but also the ones typically used in the NHRA
 
I remember reading somewhere that accessories can also limit the max rpm of an engine. Parts like water pumps, alternators a/c pumps can only spin so fast before it gets dangerous to them, even if the engine could spin faster.
 
Originally posted by: Triumph
Simply put, rotating mass. Reduce the mass in any component that moves/rotates and you increase the available redline. I'd say mass is the 1st order affect. Harmonics, stresses, heat, those are all 2nd order affects which increase/decrease with mass.
Click to expand...
Bingo, that's why your big block engines have redlines around 7-8k rpm and some small blocks can reach limits up 11-12k rpm.

I still have a 351C built to run 11.5K rpm on premium gasoline, too bad I don't have car to put it in. 🙁

 
Originally posted by: flamingelephant
I remember reading somewhere that accessories can also limit the max rpm of an engine. Parts like water pumps, alternators a/c pumps can only spin so fast before it gets dangerous to them, even if the engine could spin faster.
Click to expand...

Can water pumps cavitate?
 
Originally posted by: Shockwave
Originally posted by: flamingelephant
I remember reading somewhere that accessories can also limit the max rpm of an engine. Parts like water pumps, alternators a/c pumps can only spin so fast before it gets dangerous to them, even if the engine could spin faster.
Click to expand...

Can water pumps cavitate?
Click to expand...
Yes, but usually because of low coolant. Most high-performance engine run electric water pumps to reduce drag and prevent cavitations.
 
Quix, for quick rise and fall of RPM rotation mass must be reduced but for sustained high RPMs reciprocating mass is the limiting factor. Piston speed is the major consideration. Piston ring flutter comes into play also.

EDIT: One other thing. A fast burning combustion chamber is a must as the descending piston can out run the flame front
and pull away from the charge. Yes, that does happen and HP drops like a rock.


Shockwave, even fully open t-stats restrict coolant flow at high RPM, as a result pressures run upwards of 45psi
in the block. This helps to eliminate cavitation at the w/p. Never remove a t-stat for this will cause further overheating.
NASCAR runs a restrictor orifice in the coolant return of 7/8"-15/16" dia.
 
Originally posted by: flamingelephant
I remember reading somewhere that accessories can also limit the max rpm of an engine. Parts like water pumps, alternators a/c pumps can only spin so fast before it gets dangerous to them, even if the engine could spin faster.
Click to expand...

Certainly, but you can design that out relatively easily. Simply increasing the gear ratio into the alternator would slow it down.
 
Originally posted by: Quixfire
Originally posted by: Shockwave
Originally posted by: flamingelephant
I remember reading somewhere that accessories can also limit the max rpm of an engine. Parts like water pumps, alternators a/c pumps can only spin so fast before it gets dangerous to them, even if the engine could spin faster.
Click to expand...

Can water pumps cavitate?
Click to expand...
Yes, but usually because of low coolant. Most high-performance engine run electric water pumps to reduce drag and prevent cavitations.
Click to expand...

I remember cavitation being a bad thing in The Hunt for Red October...what does it mean in this context?
 
In my engine, the oil pump cavitates at a little bit above 6000RPM. A couple of tweakers managed to get the engine to 8k but they needed to run an external oil pump. The current cams and intake make over 5500 pointless, and the computer says "knock it off" at 6000 though
 
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