Does such a thing as an RPM Factor exist? When we compare two engines, we typically settle for peak HP and peak Torque. Does the velocity of RPM play an impact? If an engine can rip through the RPMs quicker in one engine than another - is that a factor worth measuring and considering for any combo?

If you are talking about racing engines, you have to remember that they are tuned to operate in a very narrow RPM band - maybe 2,000 RPM. Thus, I don't see how the veolicty of RPM would be a meaningful measurement.

Yes, it does matter. That's why they build big bore / small stroke
engines and use light weight parts - because they accelerate faster
in rpm. This is important in drag racing. It will get the rpm up quicker
and have better throttle response. Thats what the 302 and 327 are
known for.

------------------
67RS CAMARO - STREET CAR, 502 PUMPGAS, HYD. ROLLER, TH350 w/ATI 10" 12-BOLT w/373 ~ ON MOTOR, ET STREETS WITH MUFFLERS.
BEST 1/4: 60FT= 1.63, 6.71@106.75, 10.39@131.66
BEST 1/8: 60FT= 1.54, 6.59@106.93, ???
-----------
65 BISCAYNE - NO MOTOR OR TRANS - WILL BE 454 SLEEPER
---------
DAILY DRIVER: 91 B4C CAMARO, 305TPI, TREMEC 5-SPD, 342 GEARS, K&N, CRANK PULLEY, EDELBROCK HEADERS, 4" SINGLE EXH., ET=14.1@98 60FT=2.06
SOON TO BE 383 W/ SUPERRAM

Yes, it is important. The faster an engine revs, the faster it will accelerate a car. This is why dirt track cars use automatic flywheels with mini clutches. It makes the engine rev very fast. I read somwhere that if you reduce the weight of the rotating assembly by a few pounds, it is equivalent to reducing the car's weight by a few hundred pounds. It depends on how far the weight is from the center of rotation also. The down side to a very light rotating assembly is that you don't have all of that energy stored in a heavy spinning flywheel so the engine is easier to bog down when accelerating from zero. This is fine for a dirt track or road course car because they don't acclerate from zero, but not good for a drag car. A heavy flywheel helps you get off the line better, but will hurt you after you get moving because the engine can't rev as fast. You have to find a happy medium somewhere.

I'm not sure I buy that. I've got a little 12Volt motor that revs up pretty fast, but it sucks in the 1/4 mile. Torque is what applies force to the pavement. Internal inertia is responsible for RPM accelleration. That being said, it's worth considering that in a manual driveline, wrapping the engine up and dumping the clutch might give an off-the-line benefit due to the rotating inertia of a slow reving motor. I suspect the benefit is small in either case. All a fast reving motor means is that it has enough tourque to overcome it's own internal inertia which is small in comparison to the torque required to launch and accellerate the car. If I could have the same overall torque in either a low or a high inertia motor, I suppose I'd take the low inertia one. But torque will win in every case. IMHO

-dnult


[This message has been edited by dnult (edited 07-25-2002).]

I am hearing that torque may help the engine rev faster thus move the car quicker.

If you have a car that has a 4" stroke and makes peak torque say 400 lbs at 4000 RPM and car with 3.75" stroke and makes a peak torque of 450 lbs at 5000 RPM but gets to 5000 quicker than the other one does at 4000 - can that speed be somehow considered?

We all know the 302 is perhaps the best winding engine Chevrolet built within the small block family. When you compare it to a 350, it has 48 less cubic inches or is 86% less of an engine by size. If all things were equal you could say the 350 is 14% more engine, however if there is such a thing as RPM factor how much would the 14% be discounted.

Does this make sense?

Dano:

I'm not expert, I should say, and stated my opinions based purely on physics. Theoretically, a longer stroke will give you more torque, but I don't know enough about the differences in various motors as far as stroke, bore and horsepower. I look for a motor with a torque curve that kicks in resonably soon and stays high. Peak torque doesn't really tell the who story.

Did you mean to say that the 4" stroke gives 400lbs at 4000rpm and the 3.75" stroke gives 450lbs at 5000rpm? I ask because I would think that the 4" stroke would give higher torque, but then again we are talking about a difference of 1000rpm. However I would choose your 3.75" motor based on the extra 50ft-lbs of torque and the rev factor would be icing on the cake. The only thing that might make me think differently is if the 450ft-lbs motor's torque profile was weak until the high end of the rpm band making the 400ft-lbs motor a better choice. But that would be a difficult choice to make since 50ft-lbs is significant.

There was an interesting discussion in super chevy a couple of years ago talking about stroking and how you can stroke too much. They stated that 396 was about the biggest CID you could afford to go with because of piston speed and the possibility of raking the skirts and rod streatch. Rotating mass was a lesser concern. My 2 cents.

-dnult

Peak torque isn't what makes a fast car. If the 350 in your theoretical case made 400 Ft lbs from 1500 RPM to 4000 RPM and the 302 made 150 ft lbs at 1500 RPM with torque increasing as RPM rises, the 350 would win every time.

There is a book I was browsing the other day at the bookstore on dyno simulation software. I think it was written by Motion Software. It had VERY good and detailed discussions the various dynamics of IC engines. Next time I go, I'll see if RPM velocity is discussed.

Just because an engine has more torque than another doesn't mean that it will rev faster. Just look at a Diesel engine. They produce huge torque ratings but don't usually produce that much horsepower. Given engines that are identical except for the stroke, the engine with the longest stroke will produce more torque than the other because the pistons are pushing on a longer lever arm. Its just like using a cheater bar to break a bolt loose. Using the longer lever arm exerts more torque on the bolt. Torque is simply a way to measure rotating force. For example, applying 1 pound of foce on the very end of a 1 foot breaker bar will exert 1 foot-pound (or pound-foot) of torque on the bolt. With proper gearing, I could make a 20hp motor make 400 foot-pound of torque. It just wouldn't accelerate whatever the torque was exerted on very fast.

Horsepower (which is a unit of power)is equal to 746 Watts. Power is simply defined as the ability to do work over time. This means that with more power, you can do the same amount of work in less time than you would with less power. Now, if you had two identical cars with identical engines except that one had a rotating assembly that weighed 15lbs less than the other, the car with the engine that had the lighter rotating assembly would accelerate faster than the other car, if they started from a rolling start. Both engine make the same amount of horsepower so the one with lighter rotating assembly will rev faster. This also means that the horsepower that would have been wasted accelerating the rotating assembly can now be used to accelerate the car. You;re not really changing the torque or horsepower ratings of the engine, you're just redistributing the horsepower that you have. This is why round track cars use really light flywheels. I have never experienced it my self, but my roommate has raced dirt track cars for years. He once had a car with a stock flywheel and clutch. He said that the difference was very noticeable when he changed to an automatic flywheel with a mini-clutch. The much better acceleration that he noticed was due to the lower weight of the rotating assembly.

Starting from a dead stop introduced a whole other set of problems. When a flywheel is spinning, there is energy stored in it. If two flywheels are spinning at 1000RPM, the one with the higher weight will have more energy stored in it. Since it takes more energy to start moving a car from a dead stop to 60mph than it does to accelerate a car from 10mph to 70mph, engine manufacturers use the stored energy in a flywheel to get the car going. The heavier the flywheel, the more energy that will be transfered to the rear wheels during takeoff. Too heavy a flywheel will melt the tires while to light a flywheel will bog the motor. A heavy flywheel will get you off of the line good (if you don't fry the tires), but it will hurt you after the car gets going because of the higher rotating assembly weight. When drag racing, you have to find a happy medium somewhere. Round track cars use a lighter flywheel because they don't usually have to worry about starting from a dead stop.

Hp = t x rpm/5252.

Do a study using D2K on a set of cams holding the rest of the engine combination constant. Pick cams that are identical except for advertized duration!

I started at about the 240 advertized duration figure and increased it in five degree increments up to 305 if I recall!Do a set of hydraulics and a set of solids if you want to see how solids increase power production over hydraulics. It's quite interesting. And if it hasn't driven you bonkers, do a set of roller cams, too!

What you will find is that a given cam will produce a pair of corresponding power curves. DAH???

The little bitty cams will make power right off idle and drop off fast as rpm increases. Medium cams will move their curves over into the middle of the rpm range and finally the big cams power curves will move over to the right! Relatively speaking!!

I don't have an integrating program but I will bet that if you integrate the power curves of all the cams tested that the areas under the curves will be almost the same for a chosen rpm range!! AND not measured from the same rpm starting point.

(I.e., for little cams, off idle-2000 to 4500rpm, for medium cams, 2500 to 5000rpm, big cams, 3000 to 5500rpm and so on...).

This tells me that for a given engine combination that it will produce the same power regardless of cam used, but in the cam's appropriate rpm range! This is why you have to gear and tire a car accordingly to the engine combination, (and vehicle weight).

This will occur regardless until you have to factor in engine rpm efficiency drops like the lowering of the dynamic compression ratio caused by bleed-off of big cam overlap. And high rpm breathing capacity!

I did a study like this once and found this to hold true, (theoretically), which led me to the conclusion to build a medium rpm great big "tow-truck" engine and just use my stock drive train and a big/wide tire.

My big engine produces enough power in the street driving range, (that to me is from 2000 to about 55 or 5700rpm), that it can theoretically propel my car down the strip very fast and quick IF I can get it to HOOK!!

Hope I haven't bored you. pdq67

PS., 9.8 to 1 CR., 496BB. Theoretically---551hp at 5500rpm and 579t at 4500rpm! M-20, 3.31's and 26" to 26.5" hi-po tires. About 10.50sec. and 130mph!

The nice thing about big torque is you can really dig deep into the gears. If you get massive torque from 1000-6000 rips, then any gear change in this rpm band will be thunderously crisp, and the engine will have no trouble winding up into the new gear. Torque is the automotive equivalent of a power lifter who can move a tremendous amount of weight from standing still to jogging, but wouldn't win the 100 meter dash http://www.camaros.net/forum/wink.gif . I think high-RPM horsepower is best suited for a machine that has many quick-shifting shallow gears, where the engine continuously floats within a few thousand RPM, and all the gears are cycled through very quickly; ie: indy car.

I like big torque, steep gears, with a relatively low RPM band, like idle to 5000 or so. Personal preference, that's all it really boils down to.

[This message has been edited by BreathWeapon (edited 07-29-2002).]