I've been wanting to ask this question for some time now, but didn't know if it's been brought up before on this site. When you look at a dyno graph, the horsepower and torque curves are supposed to cross at 5,252 rpm's due to the equation: Horsepower = Torque * RPM / 5252

Then how come some dyno graphs come out with the curves (or lines, depending on how good you tune ) crossing at another point in the rpm range?

I noticed this last week when we dyno'd our hatch, the lines cross at something like 6340 rpm's...anyone care to comment?

 

if the hp and torque curves are on the same scale say 400 hp and 400 ft lbs it will cross at 5252 but say the hp is higher 600 hp and 400 ft lbs they will cross at a point other than 5252 just because the scales are different

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by TurboLude97 &raquo;</TD></TR><TR><TD CLASS="quote">if the hp and torque curves are on the same scale say 400 hp and 400 ft lbs it will cross at 5252 but say the hp is higher 600 hp and 400 ft lbs they will cross at a point other than 5252 just because the scales are different </TD></TR></TABLE>

Exactly. Make sure youre looking at the graph correctly. It is IMPOSSIBLE for tq and hp to cross somewhere other than 5252 if theyre on the same scale because HP is only a mathematical derivative of torque.

 

One thing that has always eluded me is the HP/Torque difference…now I understand that torque is nothing but rotational force, and it’s measured in ft/lbs…so what’s hp? I know 1 hp is the amount of force it takes to move 550lbs a certain distance, but it somehow doesn’t make any sense why that measurement is important. Is the only way you change the torque of an engine without changing horsepower is to change the length of the piston rods coming off the crankshaft?

 

Now youre getting into controversial territory. There are some people that believe if you take 2 engines of exact displacement, give one a small bore and a long stroke, and the other a big bore and a short stroke, that the long stroke engine will make more torque down low, while the short stroke will have top end power as its greatest strength.

If you start talking about the ratio between the length of the crankshaft stroke vs the length of the connecting rod, well, im in the belief that that plays a big role in the longevity of the engine, but not so much in the power potential.

Short answer: Stroke is only ONE of the many factors that affect TQ. HP is important because it is how quickly the tq is put to WORK, if that makes any sense to you. The more you study dyno graphs, things will start falling into place and making sense.

 

basically, you need a hell of a lot of hp to maintain high tq at high rpms, correct?

 

Yes and no. As revolutions increase, TQ becomes less important (to some degree) because you have more firing pulses per given amount of time.