pwarnery

Hello All,

I recently found out my car was run on a mustang dyno, maybe 5 or 6 years ago... A little while ago, a well know tuner recommended against having my car dynoed on a 4 wheel drive set up, because the front differential would eventually fail as a result of it...How can I figure out if my front dif, is or isn't working?Any opinion is welcome.

raineycd

If they don't have the rollers linked, then it may cause issues. Otherwise, should be ok like driving on a road... You will never know if they did or not from that long ago. Do you have any symptoms?

Kevinmacd

If the dyno has capabilty of 4 wheel drive with the front receivers that rotate, then there is no issue. Where the problem comes in, is when you dyno on a 2 wheel drive setup, and the front viscous clutch is slipping at 100% during the test.

Macster

The front diff is working to some level if you can drive the car and make a turn and the inside front wheel doesn't spin as the faster turning outside front wheel transfers torque to the inside and less loaded wheel.

Furthermore, if the diff showing signs of any fluid leakage? Is the cardan shaft flopping about? What do the front CV boots look like?

The front diff is driven through a viscous fluid coupling filled with special (silicon based IIRC) fluid that gets more viscous as it heats up and if this coupling was damaged during the dyno test I think there'd be clear signs of that damage.

Can't imagine how this car could have been dyno'd on a single roller setup anyhow. There is no way that I know of the viscous coupling would not try to transfer at least 5% (the min IIRC) of the engine's torque as the rear tires spun up on the rollers.

Absence any visible signs of leaks around the diff housing or at any place on the front drive hardware, a very loose/wobbly cardan shaft, or front CV boots torn or damaged, my WAG is the front diff and the assorted hardware is ok.

But how to test front diff is working, I do not know. It is an interesting question.

Sincerely,

Macster.

pwarnery

Everything looks good on the front differential. No leaks, nothing loose, no weird noises. The car will snap sideways exiting a hairpin turn in second gear when I step on it on dry pavement. The same thing will happen in the wet in 3rd gear while just breathing the throttle open. I would have thought the front differential would keep this from happening if it worked right? It might well be that low rpm torque overwhelms the traction available at both the rear and front wheels. Another reason for asking, is because of comments I've red on line which seemed to imply that some front differential may not be working and the owners don't even know it. Can't one stand the car on jacks and see if one front wheel turning leads to the other one spinning as well?

Brenton

I have about 525hp through my tip 04 tt
I do not get any wheel spin or powerslides in 3rd (I would have to in the wet really really force the issue and at those speeds I dont want to)
2nd yes I can light that baby up but third no.
What else has been done to the car?
I would also expect the car to do this if the front diff had an issue and was not working.

Macster

The diff is responsible for allowing the inner wheel/tire to turn slower and the outer wheel/tire to turn faster to allow a car to make a turn and especially a sharp turn with none of the drama that would be present in a car with a locked or solid axle.

(Watch the rearmost axle of a big rig trailer as the rig makes a sharp turn. The inner tire will spin, break traction. It is one thing for this to happen at slow speeds at the end of a trailer it is another thing entirely if it happens at the front wheels of one of these cars.)

I have been pretty careful with my car and have not explored its limits and behavior at the limits. But I have experimented some.

Under very hard straight line acceleration in 2nd gear I have felt the car's tires break loose. The thing has some get up and go!

Based on my experience (limited) it would not surprise me that even a stock Turbo if one dropped the hammer in 2nd exiting a sharp (hairpin sharp) turn that the car would behave as you describe.

Under these conditions the weight transfer is of course to the rear of the car, which already carries around 60% of the car's weight.

The front of the car, the front tires, are very lightly loaded. If the rear tires break loose the front tires may not have enough grip to keep the car going the right way. I mean there are the laws of physics and limits. If one goes over those limits...

Also, the front axle only gets a fraction (30% max IIRC) of the engine's torque (the coupling is a fluid coupling and there's an upper limit to how much torque this can transfer) and this may not be enough torque to pull the car out of an extreme situation.

In the wet... I'm in even more careful -- given what I have felt the car do on dry pavement -- and have not experiemented any.

However, I've driven my Turbo in the snow (with summer high performance tires (new though...) and even at low rpms in a high gear I could feel all 4 tires break loose with no problem if I just breathed on the gas pedal.

Oh, regarding the comments you mention about others referring to suspected diff problems you may have come across posts by owners who are commenting on their cars' LSD (limited slip diff). In some cases the limited slip feature can stop working and if it does the fact it is not working is not obvious.

Being that I can't observe the car, experience the behavior directly (as a passenger) I can't say for sure the behavior is normal for a Turbo with a properly working fluid coupling and front diff. You are of course free to enlist the help of someone at the next track event to follow you and your car and observe the car's behavior and comment on the possible reasons for the behavior.

Anyhow, I meant to ask about this today at the local Porsche dealer but got distracted on something else and forgot. I will be back at the dealer again, maybe Thursday (picking up one car after having it serviced and dropping off the Turbo for some needed/due service) and I'll try to remember to ask the techs how to know the AWD system especially the front portion of it, diff, fluid coupling, etc., is working ok.

Sincerely,

Macster.

cannon1000

Worst case scenario if it is fried - convert it to a 2WD (GT2) setup.

pwarnery

The car has a 520HP Ruf engine kit. The numbers on the dyno read 505HP SAE and 567lbs of torque. That is a significant amount more than the stock #s. It may well be that with the outer tires loaded out of a left hand turn hairpin the front differential can only handles so much torque 30% and the excess has to go somewhere, as in spinning the rear tires. It is kind of fun even though a little surprising at first. Interested in finding out how the Porsche techs check to see if it operates properly.

Macster

Well, I asked both senior techs how to check out a Turbo's AWD system and both said 'drive the car'.

Sorry, but there's no secret test.

They both said the system is very reliable and seldom acts up. A few front (and rear) diffs have been replaced due to noise but the units are remarkably robust.

Now of course we're talking about stock cars.

They continued on telling me the fluid coupling is very simple and with no leaks and the cardan shaft tight there's not much more one can do.

To inspect all the components/areas of the AWD system some panels have to come off to expose the drivetrain so one can look for leaks, grab the half shaft and cardan shaft and give them a good push/pull test looking for any signs of play, excessive play.

Not sure what a bad one (or a good one feels like) since I've never tried this with any Turbo, with a healthy or sick AWD system.

I think in your car's case with its big bump in hp/torque the tires and the AWD system is being overwhelmed. That neither diff has LSD probably contributes to this, lets it happen sooner and with more violence.

Sincerely,

Macster.

pwarnery

Macster thanks for the info. What I find interesting is that a well known tuner who posts on this board, discouraged me from having my car "dynoed" in 4 wheel drive mode a year ago or so. Doing so would eventually lead to front differential failure. So I didn't dyno the car and stumbled on the previous owner's data. What is even more interesting, is that this well respected tuner is refusing to provide me with durametric logs of his customer's cars running on 91 octane with a specific kit he sells and I was considering... Back to the front differential. Are they bullet proof or not? A wilderness of mirrors...

ca993twin

I think that there is some confusion about the front diff and the viscous coupler. The front diff will not be harmed by any sort of dyno, screwed up or not. It is quite robust. The viscous coupler (VC), on the other hand is very easily damaged by the wrong sort of dyno. I've even heard reports of the VC being damaged by a spin at the track, where the car suddenly finds itself reversing.

I'm pretty sure that many of us are driving around with broken VCs, and don't know it... there are no symptoms other than no power gets transmitted to the front diff. There MUST be some way to jack up one wheel or another and figure it out, but I don't know what that procedure is.

Macster

An earlier post of mine my have been in error. I don't have the time to go back and re-read them, so let me say this just in case.

Your question piqued my curiosity and I double checked my reference and found this about power testing these cars:

"Power test. Power tests can only be carried out on 4-roller type test stands!"

So if you can find one of these '4-roller type test stands' and someone who knows how to use it...

My info is based on what I've read about these cars before I bought one, and during my ownership and with talking this over with various techs is these cars are quite sturdy, well made and have reliable drivetrains.

At least when the engine is left stock.

If one starts shoving larger HP or torque numbers through the drive train then while the hardware might still hold up due to the fact that most of these car's systems are overengineered (thank Goodness!) the odds are more owners will report problems.

The extra hp/torque and often the type of usage this brings with it starts to find the cars with the weaker, marginal hardware.

As for the reluctance, refusal of this tuner to provide you with the info you seek I have no comment.

Just so this is clear: My interest in these cars, my layman's knowledge of these cars, such as it is, is based on these cars being stock. Tunes, and other mod's bring too many unknowns to the party and I can't keep them all straight even if I wanted to.

As I touched upon in an earlier post the techs I talked too reported encountering very few cars with bad front diffs, with any real major hardware drivetrain problems.

In almost every case I would guess the car came in for a 'noise' and the techs traced the problem to the diff or the transmission or the viscous fluid coupling. Fluid leaks might have been present too.

Even though I had my car in the snow for a while I really couldn't tell you if the AWD system was working. I believed I felt all 4 tires break loose when I goosed the gas pedal -- while driving in 5th gear at below 50mph following a big rig east on I-40 west of Santa Rosa last Nov. -- in a pretty heavy snow storm with at times several inches of snow on the pavement and it had not been plowed, sanded or salted.

But I never got a chance to take the car someplace with lots of room to really play in the snow.

Now, what I did do today was after leaving a parking lot I drove through some standing water (sprinkler water) and upon pulling out on a (dry) surface street I nailed the throttle with the transmission in 1st gear. After a tiny fraction of a second I felt the rear tires break loose (the feeling was not all that obvious) and I felt a bit of a tug from what felt like from the front of the car.

Quicker than I can type it, quicker than you can read it, the rpms were at redline I lifted off the throttle and proceeded to drive more sedately back to the office.

In the rear view mirror I could see tire marks from at least the rear tires. If the front tires broke loose (and I suspect they did not since at best they only get a fraction of the torque the rear tires get, but there was that water on the tires...) I couldn't tell from the marks on the pavement.

I did feel what I believe to be the front tires providing some of the tractive force but I didn't feel this right away. There was a count or two, must a fraction of a second or two, before I felt this.

It sort of makes sense: The viscous coupling fluid must get quite hot (over 300F) before it gets viscous enough to transfer torque and this takes time. The heat comes from the fluid friction created by the spinning of the discs driven by a shaft from the transmission, these discs spinning between the discs in the housing that in turn are connected to the front diff's input or drive shaft. The greater the difference in rotational speed the hotter the fluid gets and the quicker. But there's a limit to how quickly it can react. (The 997 uses the computer and hydrauilcs to control the degree the discs engage in the housing and the word is the reaction time is shorter and there's more precise control. I have no direct experience, just repeating what the reviewers who've driven both the 996 and 997 Turbos.)

While the car was hot when I parked it before this little test it sat for just a few minutes while I waited for my food. I only drove the car a few hundred feet a a walking pace before I pulled out of the parking lot and turned onto the surface street.

Maybe if the car had been driven longer and the fluid given a chance to get hotter the reaction might have been quicker.

I think the AWD system of the 996 Turbo is bullet proof as bullet proof as any AWD system in any other modern car and probably very close to if not at the top of the list in this regard.

But I have nothing more than feelings to base the above on.

Sincerely,

Macster.

Turb-OH Brad

THIS! Drop a couple hundred pounds and get rid of a bit of understeer!

sweanders

Macster, do you have experience of all the things you post about?