Since both Clive Wood and I had trouble with failed diffs at Donington I have been looking for possible improvements to reduce overheating in particular.
The general recommendations are to get maximum airflow over the diff (hence why some run without a floor) and to use plenty (at least 1.5 litres in a Mallock axle) of a good quality oil. Richard Mallock and Mark Charteris swear by Castrol 351, however, Kenneth Arvidsson put me onto a more recent development that has been used to good affect in Sweden - Omega 690.
Omega 690 isn't cheap but does appear to possess some excellent qualities. Take a look at the link below;
At the recent Snetterton meeting I ran Omega 690 in both my diff and gearbox - Omega 690 - (75W140) Super EP Gear Oil in the diff and (75W90) in the gearbox (a dog box - I don't think it would be compatible with a synchro box)).
At Donington my diff was showing a temperature of 145 degC on a temp tab, whereas on a hotter day at Snetterton on the Omega oil it was just 120 degC. Interestingly Richard Mallock told me that Alan Cook's diff (on Castrol 351) was showing 135 degC at Snetterton.
I think the 'clinging' quality of Omega is probably particularly useful on a live axle car where the oil tends to migrate to one end of the axle under long cornering which leads to overheating due to inadequate lubrication.
I will continue to run on Omega despite the cost and I have negotiated a discounted price to Clubmans Register members as long as you're ok to run with a small sticker on your car. I know it's pricey, but then so are diffs!!
Omega 690 Super EP Gear Oil 75W90 – 1 x 5 Litre @ £29.26 per Litre - £146.30
Omega 690 Super EP Gear Oil 75W140 – 1 x 5 Litre @ £39.98 per Litre - £199.90
Omega 690 Super EP Gear Oil 75W90 – 1 Litre @ £32.26 per Litre
Omega 690 Super EP Gear Oil 75W140 – 1 Litre @ £42.98 per Litre
Contact Paul Chesters at firstname.lastname@example.org quoting Clubmans Register.
This winter i built a new axle with strengthening in the lower center part of the axle wich created a bigger oil capacity of 2,1 liter. The bigger capacity gives more durability and cooler oil.
I use Eurol 75W140
After some trobble with the previous mk20 axle and Castrol B373 oil i choose to go for a modern oil (75W140) and now for a bigger capacity diff. Did 3 years with the previous diff on the 75W140 oil (1.2 liter diff) Last year the old mk27 diff had a leak and blew the diff due to lack of oil.
The new axle also gives the rearaxle 1 degr. camber for the radials.
Owh yes it has double oilseals on both ends.
Interesting, my car developed a mega vibration at Snetterton late on in race 1 which I suspected was either tyres (the balance weights came off the fronts), the prop shaft (which was ok) or something else transmission related. Temperatures were very high during race 1 and shot up alarmingly on returning to the paddock, water temp gauge needle went beyond maximum up to the stop pin. For the later races Mark rigged up a means of holding the rear deck open to assist air flowthrough (this kept racing temperatures to the high end of reasonable) and the wheels were balanced which reduced vibration from ridiculous to just teeth rattling. Inspection yesterday revealed a tooth was largely missing from the differential’s pinion and that a locking bolt was missing from the axle cage, the latter was likely to have been shaken out by the subsequent vibration.
I put the likely root cause of the above down to insufficient airflow through the car resulting in unacceptable running temperature for the diff compounded by the unusually hot conditions. If you are in doubt as to your car’s underbody and diff temperature under racing conditions, the air temp is in the 30's and there is no cloud cover to reduce the direct radiation from the sun I’d recommend introducing something to improve flowthrough. Any effect on driveability can ultimately be resolved.