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A thought....have two pumps and two coolers. Even though they are shared fluids, have one sucking from the rear drain plug, going to a cooler then back into the diff. Then have a second cooling system off the front drain plug feeding back into the upper center of the case.
I think that one big cooler that sprays only on the gears would be more efficient than that, no? The communicating fluid temperature across alloy compartments is about the same, so the pickup point shouldn't matter that much.
Yea, it was just a random thought that popped in my head while typing that last message. However, keeping an LSD cool on the track is somewhat important, even if our differentials are not prone to failure. I have no ideas the longevity or what not with those clutch discs.
The cooler and pump ended up in a clean package with the hoses and brackets. There’s room for a small puller fan behind if that ends up being necessary.
Here's an idea, not to be implemented now but to be filed for future use:
Use the exhaust pipe gas velocity, slash cut tube, GM check valve, and a hose to pull vacuum into the transmission case. One could pull it from the top of the differential compartment where the GTS transmission breather is. Put a vacuum regulating valve into the stock G28.13 breather hole. Would also require a straight thru rear muffler or RMB. That vacuum in the transmission case would help with potential oil leaks and possibly release a couple of ponies.
Porsche wanted only GL-5 gear oil used. Technical Bulletin #8813. Book E, Page 147.
They did not want GL-6 or GL-7 used, because of the potential to damage the bronze syncros.
You don't want/need anything thicker than 75/90....especially if you control the transmission temperature at 210 degrees. I don't think you want the temperature to be lower than that. What most people don't understand is that the Borg Warner style synchro has the friction surface "hidden" from the gear oil, which makes it difficult to both circulate the gear oil and cool the gear oil at the friction surface of the synchro. I add spray systems to race transmissions to not only keep oil right at the engagement point of the gears, but mostly to allow gear oil to spray directly onto the synchros.
I won't bore everyone with the evolution of the different synchros and what applications they came from....it's very involved and not everything makes sense.
In the '85/'86 transmissions, Porsche used bronze synchros with a sprayed Moly friction surface in all 5 gears (and reverse). In 1987, they switched to a sintered iron synchro in 1st-3rd gears and kept the bronze synchros in 4th, 5th, and reverse. The sintered iron synchros had the same Moly surface....they were just stiffer and didn't distort as much as the bronze pieces. The Moly surface is identical to the surface on the clutches for the limited slips.
Current technology, in most all manual transmissions, replaces the Moly surface for a Carbon Fiber surface. The benefits are huge. Better friction. Larger friction surface because of the way they retain oil (no oil "slots" required, like the Moly synchros have.) The Carbon Fiber won't "smear" like the Moly surface when it gets hot. The Carbon Fiber surface is way more resistant to wear. The list goes on and on.
I have Billet Steel Carbon Fiber lined synchros custom made for the 928 transmissions I do, in house. Fantastic pieces. Half the price of a Porsche steel synchro. Virtually bulletproof.
Hi Greg,
English is not my native language so sorry I'm not sure I understood your last sentence in this post : Do you manufacture custom steel/carbon fiber synchros or do you use aftermarket ones for the gearbox you rebuild ? In addition, do you also change the bronze moly lined synchros with steel/carbon fiber or do you let the bronze synchros (4th and 5th on 87-95 gearbox) when you rebuild them?
Have a great day,
Raphaël
07-02-2019, 12:37 PM
