Shape of the dyno power/torque curve?

[427dart]

Comparing an engine dyno to a DynoJet wheel dyno would the shape of the power and torque curves be the same even thought the readings are different?

[David Redszus]

Similar perhaps, but not the same.
Tire to drum slippage must be measured, not estimated.

Chassis dynos are best use for comparative evaluations, not absolute values.

[PRH]

With an auto trans, the curve rarely looks the same as if the motor were on an engine dyno.
And the looser the converter, the less similar the curves will look.

As an example, if your motor makes peak tq at 3500, but have a 4500 stall speed converter in the car, the highest tq reading from the chassis dyno will be around 4500, and there will be almost no (useable) data below that point.

Trying to extrapolate the chassis dyno numbers into a realistic crank hp result is an exercise in futility(imo).

I’ve run many engines on the dyno that got put into cars and retested on a chassis dyno.
The correlation between the two varies greatly depending on how the car is set up.

[ClassAct]

I think the hub dyno would have a more accurate curve compared to a wheel dyno verses the crank dyno. You still have the converter to deal with but no tire slippage.

[427dart]

This would be a manual trans in 4th gear of course.

[cjperformance]

This would be a manual trans in 4th gear of course.

In this case and assuming 1:1 ratio in 4th gear the biggest change in hub hp compared to fwhp will be the % of frictional loss thru the crownwheel and pinion.
Seeing that this loss is an observed percentage of hp in V hp out the graph starting rpm/ hp difference starts with X hp differential and as rpm/hp increases the rpm / hp differential increases but the graphs follow the same curve in relation to frictional loss %.
There is a factor of rpm rate rise V inertial loss (weight of all components) which i have little knowledge on.

[In-Tech]

Did they use a rpm signal on the dynojet?

[427dart]

Did they use a rpm signal on the dynojet?

As in a cable hooked to the coil? I don't remember that even though the graph does show RPM. That could have been figured from the wheel/drum speed?

[In-Tech]

Did they use a rpm signal on the dynojet?

As in a cable hooked to the coil? I don't remember that even though the graph does show RPM. That could have been figured from the wheel/drum speed?

Yes, and very common with many dyno operators to do that because it's fast and easy. Truly not a problem if you are making changes and just want to see what helps and what does not. It is a very complicated subject.

[digger]

The peak hp rpm point on chassis dyno goes down a bit

[MadBill]

Damn! When we put our 302 SBF road racer on a Dyno jet, we ended the pulls at 8,000 while it was still slightly climbing (530 HP). On the other hand, think of the stress reduction from shifting at 8200 instead of 8500+ ...