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Re: 4.55 differential ratio

Subject: Re: 4.55 differential ratio
Date: Sun, 22 Apr 2001 13:26:37 EDT
In a message dated 22/04/01 9:40:14 AM Pacific Daylight Time, writes:

>   Tell us more about using the OD correctly in racing; how U use it &
> when not to.  Pretty please. ;^)

OK - this is also going by blind CC to the MG Vintage Racers group as it may 
be of some interest to them as well, though I wouldn't presume to tell them 
how to drive their cars ;-)

The early non-synchro MG transmission used on all B series engined cars prior 
to the 1969 MGB came with a Laycock OD unit built to take less torque than 
the larger units used on, for instance, Triumphs. You can use them for 
competition use, but MUST observe certain limitations. While it is possible 
to build a heavier duty unit using some springs and other bits from the units 
used on the all synch MGB GT V8 installations, these are hard to get and it 
is really unnecessary.

Rule 1 - for competition use, NEVER use synthetic oil in the transmission. 
You can get away with it on the street, but if you are using the car the way 
you should be and have any power at all in a racing context, you WILL fry the 
friction surfaces - the synthetic lubricant is just too slippery (if you 
don't believe me, ask my friend with the TVR race car and the brand new 
trashed OD unit). Keep the synthetic for the engine and diff.

Rule 2 - use the OD ONLY in 4th gear - this gives the least slip inducing 
torque, and if you have a close ratio gear set, shifting into 3rd OD is just 
busy work anyway - it won't give you any faster lap times.

Rule 3 - NEVER do full throttle upshifts into 4th OD - momentarily lift your 
right foot to allow it to engage with no load - again minimising slippage.

Rule 4 - NEVER shift down out of 4th OD under braking - again, you'd be 
making the friction surfaces do work that will wear them. Complete your 
braking, downshift into second gear in the turn, and then flick the switch so 
that your next 2-3 shift will engage 3rd direct. If you are in a fast corner 
that only requires you to drop to 3rd gear, you will have to be a tad quicker 
and flick the switch in between 4th and 3rd while downshifting. That may mean 
doing a slightly slower than normal downshift while under braking, but if you 
have the switch handy (I have always preferred on the dash a finger's length 
away rather than on the shifter itself, but to each his own) you won't have 
any problem, and besides, you are a racer, and therefor pure of heart and 
lightning quick in reflex, so you should be able to do all that and wave to 
the cute corner worker all at the same time.

Note on Rule 4 - in the bad old days when brakes were more marginal, people 
used to do very nice sounding double-shifted downshifts to help decelerate 
the car. This is no longer necessary, it adds unneeded wear to engine and 
gearbox, and in addition tends to unsettle the car under braking.  Think 
about it - if you are truly braking as hard as is possible without wheel 
lock-up, and then you drop a gear and add engine braking to the equation, 
what do you think will happen? If you DON'T get rear wheel lock-up, then you 
were wrong - you weren't really braking at 'the edge' (or you have a brake 
bias  adjustment to attend to).


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