08-06-2017, 12:17 PM
(08-05-2017, 09:10 PM)The ferret_imp Wrote: The only flaw I see in Randys explanation is " horsepower will always rise as rpm increases" and that is not true. Most often the hp decreases in the far reaches of the rpm band having peaked earlier, often well before redline. The horsepower graph often looks like a lopsided mountain a long rise, a peak and a short drop off after the peak. If peak hp is at 10,000 and drops off after that, and redline is at 13,000, there is little need to rev to redline, since at that point, you are actually losing power. That's why they invented shift lights, to tell riders when is the optimum time to shift to take advantage of maximum horsepower.
Taken with the rest of the line that I originally typed it made sense. Yes horsepower will eventually drop off as torque drops off faster than rpm increases.
(08-06-2017, 12:39 AM)postoak_imp Wrote: Well, that's true in the practical world, but I don't always live there.
If I understand correctly, if you have low enough horsepower, you can still have reasonable torque via gearing but might get like 1 revolution per hour. You have to have enough horsepower to get the kind of torque you want in the rpm range you want.
I really think you don't understand torque and horsepower. This statement is not to belittle you in any way, just an observation.
Torque is not a product of horsepower, but the thing that horsepower is calculated from.
To have high horsepower you need to have high torque at a higher RPM. At any RPM under 5252 horsepower will be lower than torque and any RPM above it will be higher...no matter what. It is a mathematical fact that can't be changed because of the formula used to calculate horsepower.
Force x Time / 5252 = work done
Torque x PRM / 5252 = horsepower
With this formula you can essentially turn off the horsepower graph on any dyno and just look at the torque numbers and figure it out yourself very easily. But because the dyno does the calculations for you, you don't have to.
When you see the horsepower "roll over" at the top of a run it's usually due to either the valve train not being able to keep up (valve float), running out of available fuel, or hitting the limit of airflow in the intake system. Changing any of those can drastically change how and where an engine makes power.
For race engines high RPM is where they live and so they sacrifice low RPM power for high RPM power because that is where they live most of the time.
On the street you want that low RPM power (grunt) to get moving more easily without needing to rev to 4,000 rpm just to get moving from a dead stop.
Ever listen to an Indy car take off from the pits? They need to rev it pretty high just so they don't stall it because they make no power downstairs and drivability at low speeds is terrible.
Well, now that I have written a small book, I hope this helps clarify how this all works.
