For those who don’t know, a dynanometer is a device used to measure the torque of an engine.
There are 2 kinds: one where the engine is bolted directly to the machine,
The other is the one where you roll the car’s drive wheels onto a pair of drums in the floor, and the car is secured in place. The drive wheels spin the drums, thus measuring the car’s output at the wheels.
BTW…here in New Jersey this is an integral part of our
Emissions testing for Inspection.
Question is…how do they dyno a vehicle with full time all wheel drive: a car whose four wheels are all powered (and one axle cannot be disengaged). Cars like the Audi Quattro,
the Mitsubishi 3000/Dodge Stealth VR4, and Subaru?
For emisions testing purpose(I.e you don’t care about possible lost horse power) They just have two sets of drums, one for each axle. It’s kind of cool with a Subaru, because you get to jump out of the normal long line, and jump right up to the station in the empty AWD line, hehe.
Just a WAG because I have never seen it done, But I would guess that it is much the same for a power test. I would assume the set of drums would be locked together to prevent traction systems from causing power transfers between the axles.
In a normal 2WD car, the 2 drive wheels are sitting on the drums, and the drums spin the dyno…
The non-drive wheels are sitting motionless on the ground
The emergency brake is engaged and/or chains are secured to the car frame to lock it down, preventing the car from flying off the dyno drums.
Although it seems possible to create a dual-drum set up
(A drum set for both front and back axles) I’ve never seen one, nor do I think such a full-time AWD car work on a single drum dyno, with the Limited-Slip center Differential
supplying torque to the wheels which are being immobilized
I kinda wondered about dyno/emissions testing of full-time 4WD vehicles too. Here in Illinois, they put your car on a dyno and they have to “drive” it through a testing routine, to measure the emissions under normal accelleration. The problem is, many (particularly older-pre-1980) 4WD trucks have full-time 4WD (it’s on all the time, you cannot turn it off), with mechanical gears (not a viscous clutch) connecting the two axles. You can put two wheels on the dyno and block the other two, but then the speedo will read twice as fast - and for many of these early trucks, it is very bad for the center diff.
-Also the three local testing facilities I have seen didn’t have 4WD setups at all, and 4WD vehicles are very commonly different lengths, so they’d need a dyno that had an adjustable length. My guess is, in IL, they pass you with just an inspection (it’s too late for me to go and ask today). - MC
To restate what I said earlier, This is a true dynomometer reading, because they have to know that your car is running under a normal driving engine load, to make sure the emisions are okay under load. However I’m not sure if that type of system would be acurate enough for a spec test with the possible extra friction from reading two powered dynos.
P.S. also on the web site it lists a number of AWD vehicles that can’t be tested with the Colorado system. It appears to have a problem with 17 inch+ wheels.
If they dyno a AWD/fulltime 4wd on a 2wd dyno, it’ll kill the differential in the transfer case. Some states even exempt AWD vehicles from the dyno testing because the dual-roller dynos are really dangerous to work with.