Dear Sirs, please tell me why emissions dyno tests are bad for our cars.
 

It's been brought up in a recent thread that dynometer tests, upon which a 4WD/AWD vehicle is driven onto 4 rollers to have its emissions checked, are bad for an SVX's AWD system. Unfortunately and very surprisingly, the thread contained no evidence of how this is so. Since I didn't seem to have any luck asking about this in that very same thread, I was wondering if somebody would be so kind as to explain the reasoning behind this claim. Thanks so much. :rolleyes:

OK, next week I'm going to bop on down to the station & ask my new best friend, Kevin, just why are these rollers always "under repair". I can't remember whether when testing my FWD van, if the vehicle was placed in neutral, & the drums were rotated, or if the drums were free, & the vehicle was put in gear & reved to 45 mph, thus spinning the drums. Perhaps "spindrumophobia" is a hypochondria of AWD vehicle drivers, & has no basis in fact. And, if our fears are strictly an urban myth, I'll breathe easier (in two years, heh-heh).
Anyone else with an explanation of drum engineering, please chime in.

Ron (marching to a different drum).

Thanks for the response Mr. Mummert. I've been under the impression that the drums spin freely with the wheels. So it's as if the car is being driven normally. I could be mistaken about this though. I was just a bit concerned to hear it might be a detrimental thing, considering I've had this test done before and will be required to have it again soon. Perhaps the dyno systems vary from state to state.

the problem is in the fact that they usually don't have 4wd or awd dynos, and you deffinatly can't put it on a 2wd dyno. (i think)

Right, but the place I go to has a 4wd lane with 4 rollers. I'm sure this isn't true for all, however, and you are correct that you don't want to put the vehilce on only two rollers.

dyno
 

e.p.a. and d.o.t dont know about our fwd fuse to unlock the rear wheel drive so in california our svx are exsempt from dyno smog,
all awd are exsempy from dyno

I was thinking back to the Subaru tour we went on at the 10 anniversary meet. Towards the end of the line, they put their car on a dyno line (both front and back wheels) and they ran them at different speeds to test. They must have them like Davie indicated for awd cars. :)

I've never even seen one, and certainly have no knowledge of why they would be bad.

However it seems likely to me that the rollers would be poorly maintained in state test environments.

Given that, what would be the effect if they were off in the speed of rotation relative to one another because of maintenance issues or torque split (since there isn't the road to keep it all in step). Seems it would unduly stress the AWD mechanism.

:confused: :confused:

I think you've made my point, Lee. State maintenance by lowest bidder. Whether a 60 second trip on rollers NOT NECESSARILY turning at the same speed will queer the clutches, is anyone's guess, but I've yet to hear of an exhaust tip broken off by a sniffer.

Ron (rolling off to a high school friend's son's wedding today. God , I'm old).

This AWD dyno thing, goes right back to, why the US version is different to the European version.

When the SVX was designed, it was the European version that was the preferred one. Constant AWD through geared shafts.

Unfortunately the US technology wasn't up to the standard that the car was designed to use. AWD dynos didn't exist. This caused a major rethink and the US AWD version was built to allow the car to run on a two wheel drive dyno by inserting the FWD fuse to disconnect the rear drive.

This will allow the car to run only on a two wheel dyno. If the car is run on a 4WD dyno or uncoupled rollers, with the fuse in, and the front, rear rollers, are not coupled the near wheels will not be driven, this will cause the center transfer clutch to have maximum speed difference, the driven plates will run at front wheel speed, the driven plates are stationary along with the rear wheels.

If it is run on the same 4WD dyno without fitting the FWD fuse. The TCU will detect the speed difference and will constantly attempt to bring them to the same speed. The delayed action of transfer clutch accelerating the rear wheels up to speed, checking rotation and accelerating again produces a surging effect that wants to spit the car off the rollers.

If the 4WD dyno rollers are coupled but there is some play between the rollers, the same effect will happen due to the difference in the wheel speed.

The European version, would do the same thing on a loosely coupled dyno or drum rollers, but it has the DIFF LOCK fuse that locks front and rear wheels together to prevent surging.

The end result is the US version should only be run on a two wheel dyno or drum rollers with the FWD fuse in place. The rear wheels should not be driven or allowed to be driven on the test they should be stationary, and the test should only be for a minimum time.

The European version should only be run on a 4WD dyno with the DIFF LOCK fuse in place.

Harvey.;)

Oops. :D

Thanks for the concrete info Harvey; it's much appreciated as I'd have never known otherwise.

This is very interesting. So, the engines are the same, I know that. Do you have more info about the differences with the transmissions and the pros and cons for each? Now i'm very interested. Thanks for the info.

It's in the locker.
 

There are a few scans out of the shop manual in my locker that shows the European version.

Harvey. ;)