steven975

[QUOTE=no_really,Feb 24 2005, 06:12 AM] ^ not exactly true.

skittzomaniac

"wheel hp" is the amount of hp at the rear wheels. there's a 15%-20% drivetrain loss. thats why our cars are rated at 240 hp, but only measure 190-200 whp.

the dyno measures the amount of torque. it ignores wheel rpm, as that would change with different diameter wheels. (wheels can effect hp/trq, but its because of the weight, not the diameter.) it then calculates hp with the above noted equation: hp = (trq*rpm)/5252 that equation leaves gear multiplication out of the picture so you can compare various engines on a level playing field. everything else you put in you post was right, but when he said wheel rpm is meaningless, he was right. a bulldozer is a perfect example. tons of trq, not tons of speed. why? the gearing. and low rpms.

rpms matter because, if you measure the same amount of trq and 2000 rpms and at 8000 rpms, the 8000 rpm hp calculation will be a whole lot higher. ex.: 2k w/150 ft-lbs = 57 hp compared to 8k w/150 ft-lbs. = 228 hp. thats why its important to stay at your pk trq for a long time. the hp will climb and climb. but if you keep the hp the same, and make 57 hp @ 8k, your trq would have to plummet all the way to 37 ft-lbs. keep the hp climbing by hitting your trq peak and holding it there for as long as possible.

and we need to dispense with this "power" and "force" thing. its either trq, which is measured, or hp, which is calculated. there is no "power" or "force."

skittzomaniac

this is a perfect dyno. trq curve is as wide and flat as kansas. hp climbs all the way to redline. it shows why holding the trq curves matters alot more than peak #s and why the hp curve shouldn't peak at all.

PathNull

so could you argue that Torque is a level of power at one point, while HP is the integral of that torque over a range?

Warren J. Dew

Er, no, I'm afraid.

Power is torque times rpm. Higher torque at a given rpm means higher power at a given rpm. The same torque at a higher rpm means higher power.

Since there's a direct relationship, you don't really need to know both if you have the entire curve - the power curve would be plenty. Unfortunately, if you only have one peak power number, you don't have any idea what the shape of the curve is. That's where the torque numbers come in - they give you some idea of what the shape of the power curve is, even if you don't have the entire dynamometer plot.

ttb

yea, i think that's right....power = work/time

dp = dw/dt

dp dt = dw

w = integral dp dt

EmptyPockets

So how does this explanation explain when cars have lots more torque then hp....for instance I have 222.8 wtrq and 180 whp? I'm just a little confused here.....

skittzomaniac

^^If you're talking pk numbers.... normally cars with more trq than hp have the trq curve that peak real low then tanks as the rpms climb. the hp increases to a point, but sooner or later the trq falls faster than the rpms can climb, so it results in a low pk number. less than ideal set up. if the engine would hold that trq to redline, the hp wouldn't peak until the instant before shift.

no_really

actually, many engines have more torque than horsepower. Hondas typically don't, and most engines that depend on high revs to make power don't. Many of the fastest cars have more torque than hp. Situations where you make the most torque right before redline are actually less than ideal, because you could make more power if rpms could continue to climb. You actually want to shift at a point where the next gear starts where your torque numbers are peaking. The rpm band between peak torque and peak horsepower is your "powerband," and ideally, your rpms would stay in this area. Your acceleration would steadily increase until you shift.

ttb

many of the fastest cars have more torque than hp? which ones?