What determines max safe RPM?

[Guest Roaces]

I tried searching, but couldn't find much good info... I have been wondering for a while, what stops you from using your engines at really high rpm ranges? I have given it a bit of thought and here is all I have come up with.

 

Valvetrain Spring Rate

Feul Injector/Carb flow rate

Strength of Con Rods, Piston and Crank

Natural Frequency of Crank

Oil Pressure (10psi/1000rpm)

Ignition System

 

Aside from that, I sure would like to know what usually limits power at higher RPM and what kind of advantages does safely running at a higher RPM give you?

[COZY Z COLE]

You might find this interesting on when to shift......

 

http://www.datsuns.com/Tech/whentoshift.htm

 

LARRY

[WickedWild]

piston speed

[grumpyvette]

"what usually limits power at higher RPM "

 

do a search on ...REDLINE..... IM sure youll get several posts on the subject like these

It basically comes down to several factors, valve float rpm, rod strength rpm and the increasingly shorter time available for the cylinders to fill as the RPMS increase, Im sure youve noticed that power increases with rpms due to the greater number of power strokes per second at higher rpms , but a some point the voluumetric effiecincy falls off due to the cylinders not filling completely due to time.(the valves just don,t have the open flow time necessary),( usually by 4500rpm-5500rpm, DEPENDS ON THE COMBO)voluumetric efficiency starts to fall off rapidly

 

theres a whole bunch of factors involved , like the cam, type of lifters, valve springs,etc. but if the engines a stock 350-383 youll be reasonably safe keeping it at about 6000rpm

above that your likely to be getting into stress that could dammage your engine that figure is found by keeting the engines piston speed under 4000fpm max and below valve float rpm levels

 

 

 

Maximum Average Piston Speed (redline) is the rpm level where stress starts to damage parts,both the engines lower end strength and the engines valve float limits will have a huge effect on the engine ability to safely operate in the upper rpm ranges, most stock hydrolic lifter engines tend to float the valves by about 6000rpm due to weak stock springs and lifter pump-up so even if the bottom end can take the strain the piston smacking a valve will destroy the engine, keep that in mind!

 

 

Crank stroke x rpm divided by 6 = Piston speed in feet per minute.

Conservative speed is 4000fpm, but lightweight aftermarket parts can hit 5000fpm.

 

 

http://users.erols.com/srweiss/calcrpm.htm

 

 

 

4000 feet per minute (FPM) of piston speed is generally considered max useing basically stock parts in a chevy v8. now no one in their right mind will tell you you can,t exceed that speed slightly but STRESS IS CUMULATIVE and if you EXCEED that speed regularly ,sooner or later IT WILL eventually FAIL!

4000fpm=48000 inches per minute

your crank stroke X 2= distance travelled bye the piston in ONE REVOLUTION

while its very true that its unlikely to fail the first or even the 100th time you exceed redline you are stressing the parts and the strain on those parts due to rpm levels climbs MUCH FASTER AS THE RPMS INCREASE than the RPMS,example the strain on parts spining 6000rpm is not double the strain imparted by 3000rpm, its more like 4-8 times higher (rod angles , rod stroke ratios, total stroke length, piston weight, crank counter weights, internal verus external ballance, etc. vary the results)

examples

327=7384 rpm

350=6857 rpm

383=6400 rpm

454=6000 rpm (454 big blocks with the 3/8 rods should not push that limit too hard, the aftermarket 7/16 rods and lighter weight pistons, can easily handle 7000rpm in a CORRECTLY set up engine)

 

 

 

 

I would have to say most blown engines that are brought to me are caused by (no particular order)

(1) lube problems

(2) overheating

(3) rod bolt failure(streched or not torqued correctly)

(4) broken pistons due to detonation or valve float

(5) broken rods due to spun bearings or trying to compress busted valves

rod bolts normally stretch and break on the exhaust stroke when they are not compressing anything to slow their inertia as they play crack the whip approching tdc

OR

most comonly,when they have been over reved, exceeding the valvetrains rpm limits, then the valves float, the piston smacks the valves bending them , locking them in place, from then on each successive impact (hitting with 300-500ft lbs of energy at 50-70 times a second) further bends the valve,untill the valve breaks off, the piston , unable to compress the broken valve causes the rod to bend, rods comonly bend then break, now you have a bent rod slaming everything it can reach at 5000-7000rpm with 300-500ft lbs 50-70 times a second untill something breaks,it locks up when something jams, at that rpm, the result is normally a long list of destroyed parts

 

we have all heard it, " you need massive low rpm tq" "you need a screaming high rpm hp peak" well heres some info,

More in-depth description:

http://www.revsearch.com/dynamometer/torque_vs_horsepower.html

http://www.dynacam.com/Product/Torque_vs__Horsepower/torque_vs__horsepower.html

http://vettenet.org/torquehp.html

http://auto.howstuffworks.com/horsepower4.htm

http://homepage.mac.com/dgiessel/engine/hpvstq.html

 

first thing to keep in mind is that theres no such "thing" as horsepower, horsepower is a mathmatical formula for the RATE at which TORQUE can be applied the formula for hp is (tq x rpm/5252=hp

example

450 ft lbs of torque at 3000rpm=257hp

450 ft lbs of torque at 6000rpm=514hp

because the torque at the higher rpm useing gearing can be applied faster

here read this

http://www.69mustang.com/hp_torque.htm

 

http://www.ubermensch.org/Cars/Technical/hp-tq/

 

http://vette.ohioracing.com/hp.html

 

where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.

example

in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)

in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,

you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.

a 3.08 rear gear and 700r4 trans matches the first example well but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.

 

links youll need to figure out correct rear gear ratios

 

http://www.miata.net/garage/tirecalc.html

 

http://www.wallaceracing.com/reargear.htm

 

http://users.erols.com/srweiss/calcmph.htm

 

http://users.erols.com/srweiss/calcrpm.htm

 

http://users.erols.com/srweiss/calcrgr.htm

 

http://www.geocities.com/z28esser/speed.html

 

http://server3003.freeyellow.com/gparts/speedo.htm

 

http://www.pontiacracing.net/trannyratios.htm

 

http://www.tciauto.com/tech_info/gear_ratios.htm

 

torque is the force your engine produces, rpm is the rate at which it can be applied, the correct combo will almost always be the combo that supplies the best AVERAGE torque curve within the HIGHEST possiable RPM RANGE that the cars engine and GEARING can sustain so that the greatest advantage from gearing can be used effectively , using the above torque curve Id want to shift so the rpms fell aproximately back at 4400rpm and I shifted at aproximately 6200rpm if I could.

you might want to play with these

 

 

http://users.erols.com/srweiss/calcrpm.htm

[Guest Roaces]

Thanks Vette! For the thoughtful reply, I have followed up what you said about piston speed, and it would seem that as well as speed of the pistons, the Acceleration of the piston is a factor as well, I found an article on how to tweak your piston accelerative rates here http://e30m3performance.com/tech_articles/engine-tech/rod-ratio/kin2.htm

Though, Im not sure what gains are to be had, the idea seems like it should hold water.

 

I have looked a great deal into how horsepower and torque relate to acceleration though, and thought I should add somthing. You cannot re-gear to get more horsepower at the wheels.

 

I think there is a lot of potential wasted in many engines because people are afraid to go into higher RPM ranges though (Not without good reason though).

[Pop N Wood]

First off, excellent post. Always learn a lot from such posts. Good dissection on how an engine is destroyed.

 

BUT the statement

 

first thing to keep in mind is that [b'] theres no such "thing" as horsepower, horsepower is a mathmatical formula for the RATE at which TORQUE can be applied

 

is misleading to the point of being untrue. People constantly quote that and it is simply a wrong way to look at the problem.

 

This has been discussed many times. If anything power is the only "real" thing and torque is a mathematical contrivance. The second law of thermodynamics governs HP, it can neither be "created or destroyed". Torque is a number that varies as it goes through gearing. It is calculated by dividing power by the shaft RPM. Torque can be increased or decreased at will. Just use different gears. But the power in and out of a gear set (less losses) is constant.

[Michael]

The first law of thermodynamics deals with work and energy. It's the principle of conservation of energy. It has nothing to do with horsepower.

 

The second law of thermodynamics deals with heat, work and entropy. It's the principle that heat can't be 100%converted into work, without some other change taking place. It has nothing to do with horsepower.

 

Mathematically, horsepower is the time-derivative of work. For a system in rotation, such as an automotive crankshaft, work is the integral with respect to angular displacement of the torque. So, alternatively, power is the dot-product of torque and angular velocity - hence our "layman's" formula relating power and torque.

 

Torque is the cross-product of the force vector is the position vector of said force, with respect to the point of rotation. In other words, a force times a moment arm. That's a real, physical quantity, isn't it?

 

Dynomometers measure torque, NOT horsepower. Horsepower is inferred by multiplying torque by rpm, times a proportionality constant. So, physically, torque is real, and horsepower is "mathematical" - Grumpy is exactly right.

[Guest bastaad525]

heheh I love watching this argument. I definately agree with the torque being more 'real' than HP idea.... at least in the case of rotating engines. It just makes more sense to me. But it's not really that's it's more 'real' at all, rather, it's just that hp is not what's directly measured on a dyno, and is not the force you feel accelerating your car down the road when you punch the throttle. Torque is the force, HP is just telling you at what rate your motor is applying that force.

 

 

Agreed this is a very informative post, thanks to all who provided the great info. This actually relates very closely to a question I just asked in the Turbo/Supercharger board, about how to extend a motors torque curve to get more HP.

[260DET]

Crankshaft harmonics, camshaft harmonics too?

 

Lots of things affect durability and reliability, although it usually though comes down to one crucial factor which ultimately limits RPM.

[Pop N Wood]

Should know better than to get back into this again. Look back through some of my old posts where I have either worn everybody down or convinced them. Don't know which.

 

But I would like to point out a few things in your statements. First off, I misspoke. Should have said the first law of thermodynamics. As for the statement

 

The first law of thermodynamics deals with work and energy. It's the principle of conservation of energy. It has nothing to do with horsepower.

Power is energy per unit time. Energy can neither be created nor destroyed. Hence power is conserved. Work is a form of energy. Power, work, energy, heat. They are all related. The idea the first law of thermodynamics has nothing to do with power, when power can be thought of in terms of energy per unit time, is conceptually wrong. Energy is conserved, hence power is conserved. Mechanical work or thermal energy, it is all the same, just "altered in form"

 

Mathematically, horsepower is the time-derivative of work.

Power is energy per unit time. Think light bulbs. No work, yet they have a power rating. How is that consistent with your definition?

 

Dynomometers measure torque, NOT horsepower.

Automotive engine dynos are made to be inexpensive. The cheapest and easiest way for them to determine power is by measuring torque and doing the math. Not all dynos work that way. Electrical generating plants measure the voltage and current out of a generator. This gives them power. They can then “do the math†to get torque. Does that mean with electrical plants power is real and torque is not? Maybe different laws of physics apply to electrical systems than mechanical?

 

I am finally starting to realize that this is the source of the "HP is mathmatical" statement. There are other forms of dynos out there. Auto dynos are built to take advantage of the mathmatical relationship between power and torque. There is nothing about the underlying physics that make them that way, so don't read too much into them.

 

Grumpy is exactly right.

Grumpy is the man. Grumpy is usually right. Unfortunately I disagree with him (and seemingly half the internet) on this one.

 

Shall we get into what moves a car, torque or power?

[Guest bastaad525]

Oh Oh I know this one!!!

 

 

Torque moves the car. But you can have torque without movement (applying 50 ft lbs of torque onto a bolt that's already been tightened to 60 ft lbs). Torque that results in movement is work. Work done over time is expressed in Horsepower. Horsepower is the measure of the rate at which the engine can produce the torque that moves the car. So horsepower determines how fast the car can be moved by the given amount of torque. But it's TORQUE, the force of the engine rotating, that actually moves the car.

 

Though, once there's any movement of the car, that is work, and it can be measured in increments of time, so then it took horsepower to move the car... just like the examples of the horse lifting the coal. So then horsepower moved the car.

 

This is very confusing... real chicken or the egg stuff...

 

 

You know what... it's a trick question... it takes BOTH torque AND horsepower to move a car!! it took torque to get the car moving, but torque that results in movement IS horsepower!!

[Guest Roaces]

I have a comment for the dyno measurment thing though. My father has worked with Model Airplane engines for some time, nobody manufactures dyno's for those, so you have 2 options, custom rig your own, or use what ammounts to a torqometer. You mount the base of the engine to what would ammount to a torque wrench. Start the engine with a prop and everything, Measure the RPM it is spinning at, record the torqe on the casing of the engine and math it back to horsepower. It doesnt matter what you measure, because horsepower and torque are direcly related. Horsepower is directly related to Acceleration. Torque, when coupled with RPM is directly related to acceleration.

Beyond all the math though... you need good torque over the entire RPM band to have good horsepower, Horsepower is just a different way of measuring torque. How comfortable would you be knowing only one, if someone tried to sell you a car with 800ftlbs of torque and 80hp. It would be slow, or if someone tired to sell you a 800hp car with 80ftlbs of torque, you would kill it at every stop sign.

Rather than argue about it, it might just be better to shoot for a flat torque curve over you entire powerband, so you will get bothe good torque and good horsepower.

[Michael]

“Pop n wood†–

 

Of course, heat, work and energy can be converted into mutually one another. This is, in fact, the first law of thermo. So of course, as in your example, one can say that a light bulb, which radiates a certain amount of electromagnetic energy per second, also absorbs a certain amount of energy per second – and therefore, power. However, we can not calculate how much power the bulb absorbs merely from measuring how much light it gives off, without knowing or guessing the overall system efficiency. Alternatively, we can not calculate how light will be given off, merely from measuring the voltage and current input to the light bulb.

 

RATE of doing work, of expending energy, or of generating/absorbing heat is power. But thermodynamics is not concerned with how to make this conversion process happen! You can differentiate the second law with respect to time, and obtain, if you like, an expression for what can be written as power. But the point is that not all of the terms in that equation will comprise the “useful†power, as in the rate of energy conversion to do useful work per unit time – again because some of the power is always wasted.

 

Torque is a very useful quantity because it’s easy to measure. Torque, moment of inertia and angular acceleration are for a rotational system the equivalent of the good old F=m*a for translational systems.

[Pop N Wood]

Spent a lot of time in nuclear power school computing energy transference and losses. Electrical, thermal, radiation, mechanical, potential energy even mass. They are all forms of power. Spend a few months working steam tables and you will see what I mean.

 

It is easy to compute how much power is dissipated by the light bulb because it is very easy to measure the electrical power being consumed by the bulb. What ever the bulb doesn't dissipate in light energy, it dissipates as heat. So put the bulb in an insulated box, measure the heat loss, then you now know how much power is beign radiated as light. So yes, the light output can be measured by observing the electrical consumption less the thermal losses.

 

An engine reved against a trans brake can be producing the same HP as one powering a moveing car, yet because the car is not moving some people would say no work is being done. I look at that and know that with the trans brake on, the power is being dissipated as heat in the transmission. Release the brake and the power that was formerly heating up the trans fluid is now accelerating the car. The same amount of energy is being disspated either way, just in different forms.

 

Pick up an engineering text on automotive design. In a good one you will find an equation relating the maximum acceleration potential of a car to engine power. The analysis should include the effect of gearing. What you will find is two things: the ability of a car to accelerate decreases with vehicle speed and the maximum acceleration is related to the engine's mechanical power. Not torque. Torque is actually somewhat nebuluous because it can assume practically any value at the rear wheels depending upon what gear ratios are used. However, there is a limit what you can do with gearing because at a given vehicle speed you can't shift too low or it will over rev the engine.