Power Band

[pwgus]

another thing about the 5250 rpm equilibrium theory is, it sounded to me (and I could have misinterpreted it) like they were talking about HP and torque at the flywheel, while I believe most dynoes use HP/torque at the drive wheels, which is after the transmission, so wouldnt that throw things?

[smokeshow]

I have not read this article on this, I am just speaking from experience and seeing things on a chasis dyno and their wheel numbers. That being said I think I got a little of topic from the original post/thread...

If I get the time, and the dyno is free'd up, I will see if I can throw my n'Fest-ation on and see where the hp starts to take a drop in each gear. But those are two huge if's..

[blkfordsedan]

Side note, I have been at the dyno when a few cars have dynoed and their numbers did not cross at 5250 rpms...

HP & Torque will ALWAYS be the same at 5250 rpm. It is a law of Physics!

If you follow the HP vs Torque equations on the website Jim gave the link to, you can see how it works out. From a formula standpoint, as RPM increases, torque decreases, IF the HP remains constant. This is to satisfy the equation and is evident in the case of electric motors and mechanical speed reduction (like gearing or belt drives). Internal combustion engine are a little unique because HP is variable and dependent on volumetric efficiency and as RPM increases, HP increases to a point then takes a nose dive as you pass the effective breathing range. (they just run out of breathing capacity to support the required flow)

As for the RPM of peak HP and torque, it depends on the design of the individual engine. Head flow, camshaft profile, intake design, compression, (you get the idea), all effect the HP & torque curves.
It is safe to say that MOST stock & good street engines will have peak HP around 4500 to 5500 RPM and preferably a semi-flat torque curve with peak torque around 2200 to 2700 RPM. When you add long duration cams, single plane intakes, big port/valve heads, etc., you trade the flat torque curve for a narrow torque curve with a higher peak torque. This is OK for a race engine where you can keep the engine within the narrow range, but impractical for everyday use. This is important to remember! You can do a mod (like an intake or cam swap) that may not add much PEAK HP or Torque, but if it adds 3 or 4 HP and 5Lbft of torque across the entire RPM range (like 2200 - 5500) then your car will be faster than a mod that adds 10HP @ 5500 but adds nothing (or costs you torque) below that.

[muscle_Car1]

^:thumbup:

[smokeshow]

I am not questioning the theory, I know that what is it supposed to be. If need be, I will post pics of the dyno sheets... I am just curious as to what causes the numbers not to be correct... Anyways.. I think we are still following the lil white rabbit.

[blkfordsedan]

I am not questioning the theory, I know that what is it supposed to be. If need be, I will post pics of the dyno sheets... I am just curious as to what causes the numbers not to be correct... Anyways.. I think we are still following the lil white rabbit.

Sorry Smokeshow, wasn't questioning your knowledge specifically. I was just trying to shed some light for some of the less experienced guys who were asking. Your statement was just the most direct to the subject.

As for why they don't cross on some of the Dyno sheets, I don't understand that one either. Has to be a difference in the RPM scale between the HP & Torque? If you find an old sheet, I'm sure you'll notice right away.

[smokeshow]

I didn't mean for it to sound demeaning/offensive/condisending... Just inquizitive, and I tend to type faster than I think... LOL...


Rock on!!!

[pwgus]

I didn't mean for it to sound demeaning/offensive/condisending... Just inquizitive, and I tend to type faster than I think

I have done this more times than I can think of in my life lol

HP & Torque will ALWAYS be the same at 5250 rpm. It is a law of Physics!

I contend this in that in a perfect physics law system you may be correct, but like I say, other real world infinite variables will always present themselves, throwing our wonderfull calculated numbers into chaos (enter quantum physics, but thats somewhere I don't want to go lol) I liked the lower paragraph, tho most interesting. I just doubt that in practical aplication your always going to end up getting HP & Torque to cross perfectly at 5250 rpm's.

[FB71]

Tq and Hp, as measured at the crankshaft, will ALWAYS cross at 5252rpm. No outside influence affects this constant. Now, as measured at the wheels, is a completely different story. Drivetrain losses must be accounted for.

[pwgus]

Tq and Hp, as measured at the crankshaft, will ALWAYS cross at 5252rpm. No outside influence affects this constant. Now, as measured at the wheels, is a completely different story. Drivetrain losses must be accounted for.

That I can agree with I think

[blkfordsedan]

Guess I never actually thought about it, but it seems to me that although drive train losses reduce efficiency, but both HP & Torque are reduced.

I may be wrong, but the dyno actually measures Torque (force) & RPM and calculates HP, right? The mathematical correlation between them doesn't change. In theory, you should be able to calculate HP at any given RPM if you know the Torque. What's the difference between the dyno taking readings at the crank vs. at the wheels (other than being less)? Obviously, you have a torque converter and tires than can slip and absorb varying amounts of torque, but that's the whole point of a Chassis Dyno. It measures the bottom line force that the wheels put to the pavement and takes all other factors out of the equation? I guess I can't visualize how a chassis dyno would have any effect on the 5250 point.....I would say that the only factor would be the accuracy of the dyno and procedure.

I know it's pointless, but to me this is a very interesting query and I am a self-admitted Geek!

[smokeshow]

I agree with your thoughts on the 5252 rpms... It was just my personal observation that 99% of the time on chasis dyno's they still cross at 5252.. ( I just put 5250, because on most dyno screens you can't visably see 2 rpm).. but yes, physics/math never lies!

[smokeshow]

Going completely outside this spectrum of engine, but still on same topic.. but on low torque vehicles ( i.e. Festiva ) weight reduction is usually the best/cheapest way to make a car quicker.. Because you are changing the power/torque to weight ratio.. The old rule of thumb being 1/10 off of the ¼ mile time for every 100 lbs you lose.. but this is different and actually somewhat increased on low torque vehicles, because you will bigger changes when you drop 100 lbs.. due to the lower torque ratings...

My 2¢

[pwgus]

well what do we have, rear seats, air bags on the aspire, air conditioning, passenger seat, that probably only amounts to 100-250 lbs, not worth it in my book, the ac maybe but the others are too much for me justify. if it was whole seconds that would be one thing, but completely stripping down the car for .1-.2, maybe .4 s at tops of an increase seems unjustifiable

[smokeshow]

odds on, you are probably right PWGUS... but I would be interested to see someone to post data to back up my statements...