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You strike me as someone who can intelligently talk about driving into depths well deeper than I am capable of understanding, so respectfully I propose this question to you and others in hopes of expanding my knowledge:
Can you use Left Foot Braking with an open diff in RWD to come close to the performance of a LSD?
Maybe Senna or Schumacher could do such a thing, but for us mere mortals (who also don't have true sequential transmissions), left-foot braking is really only practical for turns before which a downshift is not required; otherwise, the left foot is required to enable downshifting before a turn. Even if it were possible to left-foot brake before every turn, a car with a proper, clutch-type LSD will always be more stable while braking, especially during hard trail-braking until the apex, due to the partial locking of the rear wheels during deceleration. No way around that---and this function is NOT provided by a Torsen or Wavetrac diff!
This is not to say that a LSD is absolutely required for the above, but since you are turbocharging your 944 engine, the corner-exit traction advantages of a LSD would also be quite desirable! Rebuilding my worn-out factory 951 M220 LSD with Guard Transmission parts was one of the best single improvements that I made to my car.
Besides cost, the main barrier to adding a LSD to a 944 trans is the installation procedure----while it is not rocket science, it is an iterative and time-consuming process. The factory manual calls for a collection of special tools to measure pinion gear depth and ring and pinion gear backlash, each of which is set by using specific shims. Ironically, the special tools were designed to ensure repeatable results from Porsche dealership technicians in a shop environment; most differential and transaxle specialists will actually "blue in" the ring and pinion gear contact pattern to optimize results, which eliminates the need for specialized tools, but requires specialized knowledge (and proper shims!). In either case, the design of the 944 transaxle requires pulling the gear cluster to allow removal of the differential----this can get tedious after 10+ repetitions!
If you are not up to the above, the cost of buying a known-good used LSD trans suddenly makes sense . . . . .
Besides cost, the main barrier to adding a LSD to a 944 trans is the installation procedure----while it is not rocket science, it is an iterative and time-consuming process. The factory manual calls for a collection of special tools to measure pinion gear depth and ring and pinion gear backlash, each of which is set by using specific shims. Ironically, the special tools were designed to ensure repeatable results from Porsche dealership technicians in a shop environment; most differential and transaxle specialists will actually "blue in" the ring and pinion gear contact pattern to optimize results, which eliminates the need for specialized tools, but requires specialized knowledge (and proper shims!). In either case, the design of the 944 transaxle requires pulling the gear cluster to allow removal of the differential----this can get tedious after 10+ repetitions!
This has been discussed before. Transmission specialists will use the factory tools because we know that they are the only way to get the pinion depth correct. Use of Prussian Blue on a static transmission doesn't work because the contact center moves under load, ie, when the engine is pushing the transmission one direction and road forces are pushing it the other. I've tried using Prussian Blue, it doesn't work, you cannot get an accurate assessment of the set-up. If you cannot afford the factory tools, or your boss is too cheap to buy them, you can try your best with bluing and there are some great posts on this topic in the 911 forums.
You do not need to reset the pinion depth when installing an LSD. You do need to reestablish differential carrier bearing preload and ring-to-pinion freeplay (also called backlash). The preload can be measured with a beam-type torque wrench, or a twist-dial type (as Pete Zimmerman includes in his excellent 915 tutorial). Freeplay can be measured using a homemade tool, like the following.
PS: Droops, you're right that it is an iterative, and at times frustrating process. It has a lot to do with small differences in the true thickness of the shims: the nominal 0.30 mm shim you try might be spot on, or it could be 0.27, or anywhere in between. And those small differences do affect the measurements. In describing the process for setting pinion depth, it's interesting to decipher the workshop manual where what it's truly saying is "you get get it dead nuts because our shims vary in true depth from shim to shim"!
This has been discussed before. Transmission specialists will use the factory tools because we know that they are the only way to get the pinion depth correct. Use of Prussian Blue on a static transmission doesn't work because the contact center moves under load, ie, when the engine is pushing the transmission one direction and road forces are pushing it the other. I've tried using Prussian Blue, it doesn't work, you cannot get an accurate assessment of the set-up. If you cannot afford the factory tools, or your boss is too cheap to buy them, you can try your best with bluing and there are some great posts on this topic in the 911 forums.
You do not need to reset the pinion depth when installing an LSD. You do need to reestablish differential carrier bearing preload and ring-to-pinion freeplay (also called backlash). The preload can be measured with a beam-type torque wrench, or a twist-dial type (as Pete Zimmerman includes in his excellent 915 tutorial). Freeplay can be measured using a homemade tool, like the following.
I use the factory tools, though, as a rule.
Hi Kevin,
I have all of the factory tools at my shop, and we always use them. My main point is that you either need these tools or need a good amount of experience with setting up ring and pinion gears (good point about the true R&P contact point being determined under load). A friend is an engineer at Weddle Industries here in town, and they always determine the actual contact R&P point----though it must be said that they design the stuff and know what they are doing!
I like the homemade backlash tool! I guess that between this and a good beam-type torque wrench, you should be able to set bearing pre-load and R&P back-lash according to the manual, which is good enough if you are only changing the diff and/or its bearings.
I like the homemade backlash tool! I guess that between this and a good beam-type torque wrench, you should be able to set bearing pre-load and R&P back-lash according to the manual, which is good enough if you are only changing the diff and/or its bearings.
Hey Chris, yes, I agree. The beam (or twist) type torque wrench needs to have a pretty low scale given the number you're shooting for. But yes, this is all that's needed. Cheers.
07-22-2018, 06:40 PM
