interpreting torque and power

[Craig C]

After doing a bit of reading on this I am still a little confused.

At the dyno day on the weekend I had an interesting change from last year on the GTV2.0.  My torque went up (130Nm@4600rpm to 137Nm@4000rpm) but the power went down (84hp@5200rpm to 80hp@4600rpm).

Overall the power figure appears low for a 2.0l nord engine but the torque appears healthy when compared to similar vehicles.

I'm not sure whether this is good or bad for a track car.  That is I assume that I wouldn't want to make changes in the pursuit of more power that resulted in less torque.

Any opinions on how to interpret this?

[Steve S]

Is the torque figure measured at the wheels? It's totally irrelevent to engine torque. You can only measure engine torque on an engine dyno. If the figures are indeed engine torque but measured on a chassis dyno then the figures are effectively nothing more than a guess.

[Darryl]

The revs are different, its not the power (or torque) numbers by themselves that tell you much.

I assume this was where the peaks were, which suggest the engine has lost some top end somehow.

Compensatiing for revs assuming a constant volumetric efficiency
80000/4600 = 17w/rpm

vs 84000/5200 = 16w/rpm

so yes, the torque has gone up but not far off what one would guess the power curve would look like (ie last year it probably produced 80kw at 4600 on its way to a peak at 5200, so that hasn't changed much) - but what is telling is that it must have tapered off instead of rising - maybe ignition/plugs who knows...

[Craig C]

I found the problem.  The butterflies in the carby weren't opening fully.  The throttle cable must have stretched over time because the throttle pedal stop had been screwed down as far as it would go.  When I removed the the pedal stop altogether I had full power back but then it started sticking at fully open.  Looks like I've got a bit of work to do at the weekend with a new cable and some adjusting.

Thanks for the feedback, it makes sense and gave me some direction as to what to look at.

Craig