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Been a while since I've posted any real content, however, I thought some of this info might be interesting...
I've raced my car for many years now I can say that from a transmission standpoint the weak point is the bell and the components within it (that is after you replace at minimum 3/4 shift fork, shift plate shaft, and steel synchro's on 3,4,5). Below are some of the components that gave into the beating of racing (ring'ed off 993 LSD housing, missing teeth on 8:32 r&p, busted stock side cover). My car has a tilton 3plate 5.5" clutch so it's an on-off switch. Works great for fast shifts, very little mass for the synchro's to fight against, however mis-shifts and things of that sort hit the trans hard. Will follow-up with info of how this all gets beefed up.
Below is a comparison of different bells in the view where the shift plate mounts. Primary forces from the pinion on bell in this view are shown in first pic with arrows. R&P trying to separate from each other due to helical gearing shown with blue arrow, acceleration force shown with green arrow and deceleration/engine braking/down shifting forces shown red arrow. Resultant force on acceleration would be arrow pointing toward around 10:30 and deceleration around 7:30.
Pics below show 964 5-spd, 993 6-spd, and 993 6-spd turbo bells with how the areas circled have been beefed up (mainly in the red circled deceleration force area). Note the turbo bell also has a steel cast in "8" for the input and pinion shafts. The 5 speed bell has been modified to be used in 6-spd trans by the addition of a 4th shift rod bearing and the upper shift rod bearings are removed on the 6-spd NA bell.
The 993 turbo also has a bigger ring gear. It's the same size as the GT3.
Part of your ring and pinion problems are cover flex. When it flexes the pinion gear rides up on the tips of the ring teeth. Replace that cover with CMS billet one and those problems go away.
Speaking of R&P, here's comparison of the motorsports 8:32 ring and pinions, Standard to Turbo/GT2/GT3. Most dim's are approx 10% larger on the GT3. Biggest difference being diameter of ring gear (190mm to 215mm), and as Bill noted the bell is 1" longer to accommodate the larger ring gear (which means longer input shaft is needed too). The GT3 (even though from I've found this part number first appeared in the 993 GT2 PET), have more oiling holes down its length compared to the single hole (not shown in pic) in the larger splined area which goes under the hub of the 1/2 operating sleeve in standard R&P. To use the add'l oil holes new idler gear bearings are needed with holes in the race, and a funnel that feeds oil down the pinion (I'll post pics later). Ring gear is also much heavier, appx 6 lbs Standard, 9 lbs on GT3.
Many times even with a base Carrera without a limited-slip diff the ring gear is found to have chips missing from it. I suspect the gear gets chipped when a tire breaks loose during aggressive acceleration and when the tire reestablishes grip it jackhammers the ring gear. It is interesting that even with the rather large chips showing up at the drain plug folks continue to drive their cars without symptoms for many miles sometimes until worn synchros prompt a transmission servicing where the chipped gear is first discovered.
Andy
One other downside to the 8/32 ring and pinion is it lacks a 'hunting tooth' feature - 8 divides into 32 evenly which means the same set of teeth mesh each revolution and does not 'hunt' around the ring gear evenly displacing wear and such.
Many times even with a base Carrera without a limited-slip diff the ring gear is found to have chips missing from it. I suspect the gear gets chipped when a tire breaks loose during aggressive acceleration and when the tire reestablishes grip it jackhammers the ring gear. It is interesting that even with the rather large chips showing up at the drain plug folks continue to drive their cars without symptoms for many miles sometimes until worn synchros prompt a transmission servicing where the chipped gear is first discovered.
Andy
Here’s a comparison of input shafts. The top one is the standard 993 street version with machined on 1st and 2nd fixed gear, below that is the 993 RS/RSR one with interchangeable 2nd, and bottom one is the two piece 997 Cup which is longer. 996 Turbo is pic below, with larger 1st and fixed 2nd, Cup looks 996TT except with changeable 2nd. It should be noted that the 997Cup one shown here is a Guard version with slightly different 1st ratio.
The 997 Cup also has center oiling from nose and wider 1st (gear pic is 993 and 997Cup 1st idler gear). Though the gear is thicker, the thickness of the inner hub is the same. Regarding the center oiling, if you look inside the 993 nose cover there is cast in nipple that points right to the center of the input shaft that drilled through with fitting on the outside could feed the input shaft.
I do find it very interesting in all these versions is the dimensions throughout the gear housing and the nose cover are the same.
@JoeMag thanks for sharing your sleuthing, guess for air to water cooled, the trans was a known working item that "ain't broke" don't mess with a good design. Evolve as needed.
Yes, that appears to be the case regarding the tranny staying the same through-out gen changes.
In the quest for a better diff, I ended up using a 997Cup diff case. I'm not sure what the exact differences are between the 996Cup and 997Cup diff cases. I needed some new clutch plates and belleville washers and CMS assembled all my old diff parts into the 997Cup housing. I do now between the 993 and 996Cup diff housing the radius between the body and the flange to reduce stress riser.
Also, with the larger Turbo/Cup ring gears a thrust bold can be added to the trans side plate that the ring gear can "push" against in heavy loads. I was told to adjust the thrust bolt to lock ring gear and then back of 1/2 a flat which makes for about a 0.005" gap. I have not removed the thrust bolt yet since I started running it mid summer to see if there is any evidence of wear.
11-25-2021, 04:05 PM
Building a HEAVY duty Trans...
