So i read on the interwebs that one click of the parking brake disengages the front differential for some AWD 911's. I couldn't find a definitive answer if this is, in fact, true for the 997 Turbo. So, is this true for our cars? I tried that trick while driving a short distance while lightly driving and the car seems much peppier and responsive. Maybe i'm feeling the placebo? If this really does work, is it safe to drive in 2WD mode for long distances? Do i need to engage 2WD while stationary or can i do it while in motion? For instance doing a hard pull from standstill in 1st with AWD, then one click of the hard brake in 2nd to disengage AWD for better top end. I'm thinking theoretical and not really planning to do it. Just intrigued by the idea.

 

I've also come across this statement a while back on here I believe. Interesting to see if has any truth to it. Perhaps, anyone who has done a swap of the handbrake lever to the alcantara versions can confirm if there are any unusual sensors back there apart from the one that lights up the "!" on the cluster.

 

Why would disengaging the AWD system produce a better top end, even if it was possible? All the weight of the AWD system is still there. Just curious.

 

Reduction of drive line drag will result is a small increase of whp..

 

pwd-
The drivetrain is still being rotated!

 

Got it. Doesn't seem like it would matter too much, but maybe so. I could see it making a bigger difference on a car with relatively low HP.

 

I would not stress the system when your tricking the AWD linkage. Seems like it would put extra stress designed to go to all 4 wheels to 2. I am a big believer that the designers of the Porsche designed it to work the way its set up and if you change that your asking for trouble. My 2 cents from a Stock TTS pilot that spent a lot money modding other cars with reliable tuners and my results were stellar performance and shortly after that pushing my car more than driving it. I am adverse to messing with the cars now and just buy them how fast I want them.

 

I would think the opposite. Drivetrain loss is not a static number.

 

Interesting. I didn't think about it as a static number, but more as of a percentage of the overall power. Again; I don't know if this is true, I'm just thinking it through.

 

Front drive on the 997 is by an electronically actuated wet clutch pack just behind the diff. The porsche traction management (ptm) computer decides when to apply the clutches, and add front drive in - like launching, or oversteer etc. This clutch has only around a 200Nm capacity, and probably is always slipping a bit.

Our cars are very RWD, with just a bit of front drive added in. As power increased with mods, the handling of our car feels more and more RWD. It will spin the back tires, spin out etc with ease. It does not seem to have the awd traction of one of the GTRs we race with. Figure with 600ft/lbs at the wheels, around 400 of that is to the rear in best case.

I wish we had a display on the dash like the 991's showing the torque distribution while driving. I have seen it in a friends macan, and it is very actively adding and reducing front drive as you stop/start, turn, cruise.



You can just unplug the connector on the front clutch to try 2wd mode, but I'm sure all the electronic systems, ptm, pasm, psm will be pissed off.

 

I don't believe that the handbrake tricks the car to go into rear drive only. However, since it is an electronically controlled front diff, what I have in my car is a switch that opens the diff and thus making it RWD and I can attest that it definitely becomes RWD. To indicate that it is in RWD the program shows the handbrake on lights in the dash. The red HandBrake light is on in the speedometer and the center digital screen goes red and displays a message "Handbrake On" or something like that. This does not hurt the car or the drivetrain.

Caution 1: Since the stability programs, etc. are all built for AWD just disconnecting the diff is not sufficient, you need to account for that and in this case when I disengage the front diff, the car automatically goes into sport mode without showing the "Sport Mode" light. Hope this makes sense. As such, when in RWD mode, the car drives like a RWD Porsche in Sport mode.

Caution 2: Since the handling characteristics of the AWD cars are so different than those of the RWD especially in corner entry and exit (somewhat in mid-corner) you need to be extra careful as to what mode you are in vs. what mode the car is set-up for. E.g., when in AWD coming out of the turn you go hard into the accelerator and you let the car pull you out of the corner and you have a certain alignment for the front wheels that take power into consideration (i.e., you toe out the front more). In RWD mode coming out of turns you yaw the rear with the accelerator and when you are fairly straight you go hard into the gas and if you have a lot of toe out in the front the steering will not be as precise.

Bottom line for me is that I am going to take the drive shafts, diff etc. out of my turbo and make is a full RWD drive car as that to me is what a 911 should feel like.

As a separate note, weight of the car has nothing to do with top speed. Top speed is a function of hp and square of frontal area (aerodynamics). Getting rid of the drivetrain loss to the front wheels will increase wheel horsepower and thus top speed but not by much. Let's assume car has 600 hp, 15% drivetrain loss in AWD and 10% drivetrain loss in RWD - that amounts to 30 hp difference.

 

top speed is a function of tq at the wheels and all forces counteracting that. includes aero and rolling resistance.

you are still spinning all the parts in the AWD system, it would make virtually no difference.

 

Whoa!

Been following this thread for a few days.

Mr. Angry, have you unplugged the differential sensor and had success?

Webspoke, pointed out the electronical actuator engagement. You may not want to disengage your setup but, if you could, what would be your approach to do so?

Vid997, went through alot of headache with this 997 conversion. Reprogramming the tranny and changing his setup.

Pureporsche, changed out the dash and found that this (yet to be confirmed,) overrode the PTM etc.

Looking forward to more knowledge regrading this override.



Others have noted in the past that there is a safety option that can disengage the "ill" AWD system. Thought is, If your throttle is stuck, this will only allow you to control the wheels in action.

 

I'm not a physicist or an engineer, but this seems odd to me. Are you saying that all things being equal (HP, drivetrain loss, aerodynamics) except for weight, that two cars will have the same top speed? So if one weighs 2,000 lbs. more than the other, but everything else is equal, the top speed will be the same?

Interesting.

 

Within reason. Drag and friction outweigh vehicle weight at high speeds. This is why sport bikes accelerate ultra quick but have bad top end (relatively speaking).