It's a Question of Ratios.............?

Haven’t achieved much over the last few days apart from getting some of the front suspension parts into degreaser to soak. I have however been considering gearboxes and have realised that much more thought was required than I envisaged. The gearbox of choice for Cobra replicas is currently the Tremec TKO as shown below:

However the TKO’s come in three flavours:
So which one to choose?.... there are a few things to consider:

The first is the Starting Line Ratio (SLR) also known as the Launch Ratio and is particularly important when selecting a gearbox and/or differential.

Starting Line Ratio = 1st gear ratio X differential ratio.

The optimum SLR range is 9-11 with 10 being the ideal. Less than 8 will result in excessive clutch slippage to get the car moving and greater than 12 will provide lots of noise but not much movement. These figures are based on an overall wheel height of 28” (This info comes from our friends across the pond) and therefore the SLR needs to be factored to take account of actual wheel size:

SLR = 1st gear ratio X diff ratio X ( 28 / Wheel Height in Inches )

So now I need two bits of info – diff ratio and wheel height. My Diff is 46/13 or 3.54:1 there were various ratios fitted to the XJ6 dependant upon engine size and intended market (2.88, 3.07, 3.31, 3.54, 3.57, 3.77, 4.09, 4.27, 4.55) the 3.54:1 is fairly common. A number of these diffs were available either as Limited Slip Diffs (LSD or Powr-Lok) as well as Standard (or Open) Diffs. There are many opinions regarding the relative merits of LSD’s and open diffs and it seems to be an eternal quandary. However, having read many arguments I am sticking with my open diff as:

1) In normal or spirited road use it should prove more than adequate and should also provide improved turn-in of the car as opposed to an LSD.

2) LSD’s can produce some unintended “end swapping action” especially when accelerating out of corners, as a result of wheelspin with the diff locked – both wheels spin = no lateral grip. With an open diff only one wheel should spin therefore the other should still provide some grip.

I reserve the right to change my mind after driving the thing!! - and should the open diff not be satisfactory then the Grand Plan would be to fit Quaife ATB internals !!!

Getting back on track – overall wheel height is needed next. Many people fit 18” wheels with low profile tyres. I will however be going for the “period look” with 16” wheels and taller tyres. Now I don’t know the tyre size for the 16” wheels but I know that the Mark IV GD can be fitted with 18” wheels and 285x40 tyres (to fill the larger rear arches). This gives an overall wheel height of 27” (26.97).

So – for my car the SLR will be 1st gear ratio X 3.54 X (28/27)

Or 1st gear ratio X 3.67.

The TKO 500 has a first gear of 3.27 giving an SLR of 12.00

The TKO 600 has a first gear of 2.87 giving an SLR of 10.53

So this appears to be a “no-brainer” – TKO 600 it is.



I suppose you should argue that with around 400 hp and 400 lb ft torque you should be looking at the 8-9 range as a more powerful engine will be able to cope with the taller gearing. Certainly if you look at the original specs for 427 Cobras (420hp & 480 lb ft) its SLR is closer to 8 than 10. The 289 Cobras (271 hp & 314 lb ft) SLR’s are around 9.3.

There are two TKO 600’s the 600 and the 600RR. The RR is the Road Race version without an overdrive 5th gear – this would result in higher rpm (and increased fuel consumption) at cruising speeds – so a standard TKO it is.

So – a TKO 600 will give me the following:

Obviously aerodynamics will restrict 5th gear maximum speed!! One last thing to calculate is the distance to 2nd gear. Using the Calculator provided by http://www.5speedtransmissions.com/ I have calculated the distance to second gear to be:

1) For a normal gear change at 2500 rpm and less than 50% torque = 19 Feet.

2) For a full throttle gear change at 6000 rpm and full torque = 55 Feet.

Now all the numbers seem fine to me, but I still have a nagging feeling that the overall gearing should be a little taller (i.e. numerically smaller diff ratio). I’ll have a rummage on some of the forums and see what comes up?