Tom @ Fast Forward
12-31-2007, 02:47 PM
There has recently been a load of BS about the measurements that are made on a Dynojet chassis dyno. The answer is simple. All a chassis dyno does is measure torque at the wheels. There is a large drum of know diameter and mass. The Dynojet measures the rate of acceleration vs. time and calculates the torque required to make that happen. It also measures RPM from the engine so that it can make the simple calculation for HP. That is made by Tq x RPM/5252. Very simple math. That's all there is to it. No more, no less.
A chassis dyno has no idea what the drive train losses are for your vehicle or the one that was on before you or the one that will be bolted down next. It has no idea if you ahve a manual transmission or an automatic (much higher drive train losses).
Matt Harwood (Marketing director for Mustang Dyno) had the following to say on the subject:
From Car and Driver
"There's no really accurate way to get engine horsepower from a chassis dyno," says Matt Harwood, marketing coordinator for Mustang Dynamometer, a major supplier of chassis dynos in Twinsburg, Ohio. Some tuners use the so-called 15/20 rule, which assumes a 15-percent driveline loss for manual transmissions and 20 percent for automatics. But, says Harwood, "I've seen losses as high as 35 percent." So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car, any flywheel horsepower number quoted by a hot rodder under the shade tree is most likely just a calculated guess.”
And from Hotrod Magazine:
The Truth: Are Chassis Dynos Legit? What you need to know about getting your car tested
“There is really no way to measure, predict, or otherwise determine engine flywheel power from a chassis-dyno test with any repeatable certainly.”
I would like to add this:
A major misconception is that flywheel HP and flywheel torque can be measured from a chassis dyno. The ONLY thing a chassis dyno measures is wheel torque ........
In recent discussions, the topic was diverted to the Dynapak dyno. From their website "Dynapack™ chassis dynamometers are such a radical departure from the stereotypical roller dyno......". They do not strap the chassis down with the tires on an inertial roller. They remove the wheels all together and hook directly to the axles. The software in the Dynapack is completely at the whim of the operator to adjust to read anything he/she desires it to read. Keith (Flyin Miata) has stated that he wrote the software for the one they use. He supposedly has it programmed to emulate the Dynojet. In which case, it is displaying the HP and Tq at the wheels of the vehicle.
Other Dynapack operators have inserted offsets in the software to "GUESS" what the flywheel HP and Tq would be. In which case, the graph displays both estimated crank/flywheel HP and crank/flywheel Tq rather than actual wheel HP and Tq.
There is NO KNOWN DYNO in the world that displays wheel HP and crank/flywheel Tq. In all cases, it is measuring the troque at the wheel/axle and simply multiplying by RPM/5252 to calculate the HP and then, by operator guess factor if so programmed, estimating crank/flywheel HP and Tq.
A chassis dyno has no idea what the drive train losses are for your vehicle or the one that was on before you or the one that will be bolted down next. It has no idea if you ahve a manual transmission or an automatic (much higher drive train losses).
Matt Harwood (Marketing director for Mustang Dyno) had the following to say on the subject:
From Car and Driver
"There's no really accurate way to get engine horsepower from a chassis dyno," says Matt Harwood, marketing coordinator for Mustang Dynamometer, a major supplier of chassis dynos in Twinsburg, Ohio. Some tuners use the so-called 15/20 rule, which assumes a 15-percent driveline loss for manual transmissions and 20 percent for automatics. But, says Harwood, "I've seen losses as high as 35 percent." So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car, any flywheel horsepower number quoted by a hot rodder under the shade tree is most likely just a calculated guess.”
And from Hotrod Magazine:
The Truth: Are Chassis Dynos Legit? What you need to know about getting your car tested
“There is really no way to measure, predict, or otherwise determine engine flywheel power from a chassis-dyno test with any repeatable certainly.”
I would like to add this:
A major misconception is that flywheel HP and flywheel torque can be measured from a chassis dyno. The ONLY thing a chassis dyno measures is wheel torque ........
In recent discussions, the topic was diverted to the Dynapak dyno. From their website "Dynapack™ chassis dynamometers are such a radical departure from the stereotypical roller dyno......". They do not strap the chassis down with the tires on an inertial roller. They remove the wheels all together and hook directly to the axles. The software in the Dynapack is completely at the whim of the operator to adjust to read anything he/she desires it to read. Keith (Flyin Miata) has stated that he wrote the software for the one they use. He supposedly has it programmed to emulate the Dynojet. In which case, it is displaying the HP and Tq at the wheels of the vehicle.
Other Dynapack operators have inserted offsets in the software to "GUESS" what the flywheel HP and Tq would be. In which case, the graph displays both estimated crank/flywheel HP and crank/flywheel Tq rather than actual wheel HP and Tq.
There is NO KNOWN DYNO in the world that displays wheel HP and crank/flywheel Tq. In all cases, it is measuring the troque at the wheel/axle and simply multiplying by RPM/5252 to calculate the HP and then, by operator guess factor if so programmed, estimating crank/flywheel HP and Tq.