DCraig
CURRENTLY BANNED
My first post here. I’ve worked on Porsches since 1968 but I still love the project cars. Perhaps I can help by adding my two cents worth on the ZF boxes used in Porsches.
Unless there are some specific questions concerning the 901 boxes, I will not include them since they don’t get along well with higher horsepower inputs. Their normal mode of failure is to twist the rear of the main input shaft off. All Porsche ZF boxes use helical cut gears.
The 915 box is probably the all time favorite transaxle to use in mid engine cars because they are reasonably plentiful; you can flip the ring gear for correct rotation; parts are still available (but getting pricy); limited slip as well as Quaife differentials are available; and unless you abuse them, they will last a long time and give trouble free service. More on abuse later. The aluminum cases are a bit stronger than the magnesium cases but case failures, except for pushing out the differential side cover, are only found when internal parts get broken and fed through the gear train. The factory found that it was cheaper for them to produce the aluminum cases, the strength factor was a bonus. The early 915 boxes had skinny little ears on the guide sleeve that rides inside the slider. They were prone to failure. These were increased in size in about 1976 and are used in all later boxes. They are upgradeable to the early boxes.
The 930 boxes have gears that are 30% wider than the 915 gears. The G-50 gears are also 30% wider than the 915 gears. This was done because under racing conditions and big time horsepower, the gears would fail occasionally. I have never seen a gear failure on a 930/935 box that wasn’t caused by debris from another failed part.
The 934/935 boxes have internal oil pumps (915 race car boxes did also); a spray bar directing oil against the ring gear were it meshes with the pinion gear; input and pinion shafts have drilled oil slots; and the input and pinion shafts are nitrided. To my knowledge these are the strongest boxes made.
Now lets talk about abuse. We use friction synchros up until the G-50 box which uses GM style synchros. Friction synchros DON’T react well to speed shifting or short ratio shift towers. It overpowers the synchro and lets the slider teeth bang into the gear engagement teeth. That’s the grinding noise you hear since the gears on the input shaft and pinion shaft stay in constant mesh. If anti-friction gear oil additives are used the synchros stop working.
The number one killer of transaxles is when the driver downshifts, especially into first gear, and the expanders over speed and the anchor block breaks. This spreads the synchro ring and feeds it through the internals. Not pretty. If the driver shifted into first over 20MPH on a 928, even if the clutch was never engaged, the anchor block said bye-bye. The number two killer is starting the engine without oil in the transaxle. The needle bearing last less than 60 seconds. Number three killer is using regular gear oil in a limited slip. Limited slip gear oil STINKS badly. I have used Swepco oils for many years.
Under big torque loads open differentials break their spider gears; LSDs can warp where the ring gear meets the carrier; any of them can push the cover plate into deformation, which opens the ring and pinion clearance, and can then clean all the teeth off the ring gear; I’ve never seen a titanium spool hub fail.
I believe we are dealing with hard driven street cars here instead of all out race cars that take full power abuse for long periods of time.
If I can figure out how to do it, I’ll post pictures.
Unless there are some specific questions concerning the 901 boxes, I will not include them since they don’t get along well with higher horsepower inputs. Their normal mode of failure is to twist the rear of the main input shaft off. All Porsche ZF boxes use helical cut gears.
The 915 box is probably the all time favorite transaxle to use in mid engine cars because they are reasonably plentiful; you can flip the ring gear for correct rotation; parts are still available (but getting pricy); limited slip as well as Quaife differentials are available; and unless you abuse them, they will last a long time and give trouble free service. More on abuse later. The aluminum cases are a bit stronger than the magnesium cases but case failures, except for pushing out the differential side cover, are only found when internal parts get broken and fed through the gear train. The factory found that it was cheaper for them to produce the aluminum cases, the strength factor was a bonus. The early 915 boxes had skinny little ears on the guide sleeve that rides inside the slider. They were prone to failure. These were increased in size in about 1976 and are used in all later boxes. They are upgradeable to the early boxes.
The 930 boxes have gears that are 30% wider than the 915 gears. The G-50 gears are also 30% wider than the 915 gears. This was done because under racing conditions and big time horsepower, the gears would fail occasionally. I have never seen a gear failure on a 930/935 box that wasn’t caused by debris from another failed part.
The 934/935 boxes have internal oil pumps (915 race car boxes did also); a spray bar directing oil against the ring gear were it meshes with the pinion gear; input and pinion shafts have drilled oil slots; and the input and pinion shafts are nitrided. To my knowledge these are the strongest boxes made.
Now lets talk about abuse. We use friction synchros up until the G-50 box which uses GM style synchros. Friction synchros DON’T react well to speed shifting or short ratio shift towers. It overpowers the synchro and lets the slider teeth bang into the gear engagement teeth. That’s the grinding noise you hear since the gears on the input shaft and pinion shaft stay in constant mesh. If anti-friction gear oil additives are used the synchros stop working.
The number one killer of transaxles is when the driver downshifts, especially into first gear, and the expanders over speed and the anchor block breaks. This spreads the synchro ring and feeds it through the internals. Not pretty. If the driver shifted into first over 20MPH on a 928, even if the clutch was never engaged, the anchor block said bye-bye. The number two killer is starting the engine without oil in the transaxle. The needle bearing last less than 60 seconds. Number three killer is using regular gear oil in a limited slip. Limited slip gear oil STINKS badly. I have used Swepco oils for many years.
Under big torque loads open differentials break their spider gears; LSDs can warp where the ring gear meets the carrier; any of them can push the cover plate into deformation, which opens the ring and pinion clearance, and can then clean all the teeth off the ring gear; I’ve never seen a titanium spool hub fail.
I believe we are dealing with hard driven street cars here instead of all out race cars that take full power abuse for long periods of time.
If I can figure out how to do it, I’ll post pictures.
