I searched and didn't find any reference to this, so I thought I'd share.

I dyno'ed our shop Evo VIII on TurboXS' AWD Dynapack dyno and we had a very difficult time of it. The problem was that at certain RPM (3500 or so in fourth gear), the rear diff would start oscillating, and banging up and down against the car so hard that the rear valance looked like it was ready to shake off the car. It sounded like a jackhammer.

I'm assuming the resonance happens because the Evo, unlike most cars, has very little compliance in the subframe and rear suspension mountings, so driveline vibrations don't get soaked up like in most cars, but build up and transmit into the rear axle.

I haven't found anyone who knows about this problem, or who knows a solution for it. We jacked the rear diff up and down, and raised and lowered the front of the car, to try to change the weight on the rear, but it did it on every run to some degree. Even Dynapack corporate didn't seem to have a good answer for it.

Have you dyno'ed your Evo on a Dynapack and not experienced this problem? If you have, did you solve it?

I'll be using a Dynojet until I can find a solution.

--Dan
Mach V Motorsports
MachEVO.com

 

I am not positive but I think that Altered was doing the pulls in 3d gear on the Evos this past weekend.

 

I'm not entirely sure you're answering this thread, but that is true, those runs at Altered were made in third gear.

I'm talking about a DYNAPACK dyno that has this resonance problem, not a Dynojet like Altered Atmosphere has. I know I can dyno the car on a Dynojet. It's the Dynapack I'm asking about.

I experienced the resonance with the Dynapack in every gear -- it just moved up the rpm range if we switch to third gear.

--Dan
Mach V

 

It sounds more like an alignment issue.. Is it possible that the rear portion of the dynapack isn't seating square against the wheel hubs completely? Honestly, I have my reservations about running without wheels, since they would act as "Flywheels" and absorb and dissipate some vibrations inherant in the drivetrain anyway.

I'm just speculating, I've never actually worked with a Dynapack..

 

Understand that you were interested in Dynapack information but I saw you had used 4th gear and thought that 3rd might have made a difference. That is why I referenced the procedure used by Altered on their Dynojet. Had you indicated in you original post that it occured in every gear on the Dynapack or that you had already done it on a Dynojet, I would not have posted.

Just trying to suggest possible variables to your situation as I understood it from your initial post.

 

Just put jack under the rear dif and raise it up a tad. I've had no problems dynoing EVO's at all.

 

That was suggested, and worked on another car. We did it on mine, and jacked the diff to several different heights. Nothing worked.

--Dan
Mach V
MachEVO.com

 

I woud guess your diff is locking up and releasing due to differential wheel speeds. Which can cause an oscillation such as you described. It happens on some FD RX-7's with clutch plate diffs that are set very tight. You need to figure out what's causing the two wheels to spin at different rates. It happened on my FD when I switched to a KAAZ diff. Then again all systems have an inherent mechanical resonance. You may have just hit the right engine speed, angle of inclination, etc.. to get the system going. I remember seeing a jet engine test at Boeing once where they hit the test bed's resonant frequency. Wow! That was frightening.

 

I came to the same conclusion this Thursday when I had my EVO on a dynapack AWD dyno. If we "pre-loaded" the diff just a bit to get it to lock up completely before going WOT we didn't get the oscillation problems at all. If we did NOT pre-load the diff it was horrible!

Keith


PS: Found this thread while searching for differences in dyno results between dynapack and dynojet. Which reads higher numbers for the same exact car? Anyone know?

Thanks.

 

Not to bring this thread back from the dead, but here's what I was told by Hill over at AgileAuto.

It has to do with the fact that the frequency at which the dynapack motors work is the same as the internal frequency that is used on the Evos (STI's too). What ends up happening is that the driveshaft (and diffs) will start rattling really badly.

Hill said that they were sent a new circuitboard that changed the frequency in order for it to work correctly with the Evos. I got mine dynoed just today on a patched dynapack, and everything came out fine.

 

Yep, they said it was a software problem...

 

I recently ran the Evo again on the TurboXS Dynapack, which had the latest Dynapack software revision. No resonance problems this time.

--Dan
Mach V

 

I do occasionally run into this problem, usually once the dif starts getting warm (after many many runs). I have solved it simply by switching the RPM start point or by feathering the throttle during preload. It sometimes also helps to run it through the gears (even though on a Dynapack you don't have to). I will be calling Dynapack about the software update.

 

You never know what problems you will run into when you take the only component in the system (the tires) that is capable of damping out such mechanical resonances/out-of-roundness/etc,.. Doing so can't be very good for internal drivetrain parts, can it?

Shiv

 

Are you saying it's a bad idea to use the dynapack? In theory it sounds like it's pretty effective... at least, more so than the dynojets.