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Old 07-18-2000, 06:54 PM   #1
Mike Moffa
Scooby Newbie
Member#: 27
Join Date: Jun 1999
Location: Montreal, Quebec, Canada
9805 STi V3 -LegGT05
Rally Blue - Satin White

Post Dyno Runs.. AWD car on a 2WD dyno... WTF

Hi guys, here is a message that I have receive from a friend.. Her story seen to be really good, but like her, I never heard of anyone doing it..

Can someone help us on that ?

Here is the message :
Do you know Giroux/Raceshop 2000? Now located on De Lussac in Boucherville.

I stopped in there on Friday night before heading out to Napierville, and
spoke to Serge (proprio). I mentioned my car is far from its potential for
its mods, and he suggested to dyno it, "pas de probleme". I said something
like "ben wee, sest pa mal de travaiy poor fair shanjay kelkul shows dan la
tranny ... sofe see too ah un AWD dyno???" He laughed and repeated "pas de

What he said he does is put the rear wheels on a neutral "dolly" so they can
spin harmlessly against them while the front wheels spin the dyno cylinder.
Sort of like the contraption a non-flatbed tow truck has to put under our
rear wheels in order to tow an AWD.

He told me he's got a Porche club, 15 of 'em coming to dyno on Saturday
morning, and after that he's closed for the next 2 weeks (construction).
Implication being that some of these porches are AWD and that it doesn't
hurt them.

I can't help but wonder if this is so "pas de probleme" why nobody else has
thought of it/doing it? Shops that dyno AWD DSMs temporarily convert the AWD
to FWD by changing to a welded center diff and disconnecting the transfer
case in order to dyno.

Why doesn't EVERYBODY do this, which is clearly faster, cheaper, easier? Is
it possible that nobody thought of it before, or is there some good reason
why I haven't heard of anything like that before?

Any thoughts? I mean, the theory sounds good, but can it hurt anything in an
AWD drivetrain?
Translation of "Pas de problemes" = No troubles

Let me know what you think..
Mike M

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Old 07-18-2000, 07:07 PM   #2
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Join Date: Jul 1999
Chapter/Region: SCIC
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1999 Outlasting
the competition.


it will make the center diff go BOOM!!!
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Old 07-18-2000, 07:18 PM   #3
Mike Moffa
Scooby Newbie
Member#: 27
Join Date: Jun 1999
Location: Montreal, Quebec, Canada
9805 STi V3 -LegGT05
Rally Blue - Satin White


kartboy, thanks for the reply, but why it will make it go boom ? Is it because the rear will be too much free ? In comparaison to the front that will have some tension on them and not the rear ??

Mike M
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Old 07-18-2000, 07:23 PM   #4
Eric Gagnon
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Member#: 46
Join Date: Jun 1999
Location: Saratoga Springs, NY
2001 Jetta 1.8T



There is a way on DSM's to disconnect the center diff and you can dyno a awd DSM. (Rallispec dos this) But on the subie i don't know. I wouldn't do it, just wait and find a 4-wheel dyno.. It is safer and more reliable/accurate.
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Old 07-18-2000, 07:56 PM   #5
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Join Date: Sep 1999
Chapter/Region: MWSOC
Location: Carol Stream, IL USA
MY99 RS-RIP / MY95 MX-5


Spinning it like that will cause the center diff to start overheating, because it will go nuts trying to compensate for the zero resistance in the rear tires.

Overheat it long enough and then your warrenty won't cover damages (because it says not to do that in the manual). So before trying that procedure, make sure that the guy doing the dyno will guarentee you a new diff when he blows it.
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Old 07-18-2000, 09:02 PM   #6
Joel Gat, 1.8L
Scooby Specialist
Member#: 32
Join Date: Jun 1999


Well, the other problem is that dyno'ing that way is pretty silly. You have absolutely no way of knowing what percent of power is being transfered to the rear wheels due to driveline losses, VLSD efficiency, etc. And like others have said, the VSLD will heat up and make a mess.

Now, one good thing could come out of that sort of dyno'ing. I suppose if you did it like that and then with a locked center diff, you could map out the efficiency of the center diff over it's rotation rate... That could be interesting


Oh yeah, mah fran-says ess tre mal may ton copeen esse peer que mu-ah hee hee hee

Translate the following: Pas de liex Rhone que nous.
give up? Paddle your own canoe...
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Old 07-18-2000, 09:27 PM   #7
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Join Date: Apr 2000
2005 Legacy GT
Royal Blue Pearl


I was thinking (oh no ...). Why doesn't the dyno manufacture the front and rear rollers w/ a pulley? That way, if someone wanted to dyno a AWD car, just attach a belt between the front and rear rollers. Now, all the wheels would be connected to the dyno, and if both the pulleys are the same size, then the resistance would be the same for the front and the rear.
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Old 07-18-2000, 09:31 PM   #8
Patrick Olsen
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Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
1997 Legacy 2.5GT
1996 Impreza coupe


I've thought of doing that kind of thing myself, but I figured the center diff wouldn't like it. As the guys above pointed out, you'd smoke the diff due to the (most likely) huge speed difference between the front and rear ends.

Of course, a WOT dyno run only takes a matter of a few seconds, so maybe it wouldn't be too bad. So the other concern would be what Joel pointed out - how much power is going to spinning those free-wheeling wheels?

Pat Olsen
'99 Legacy 2.5GT 5speed sedan
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