4-Speed Gear Ratios
My XK-140MC came from the factory with an overdrive transmission plus a close ratio gearbox (JLE36132CR). Does anybody have an idea about how rare the CR box is on the XK-140 and how many might have come with the CR/OD combination? I realize that factory records are nonexistant, but maybe someone has a "sense" based on experience.
Would an XK purchaser have special ordered a car from the factory with a CR gearbox, or would a certain percentage of exports have included these features? When I bought my Series III E-Type in 1972, the dealer told me that special orders from the factory were virtually impossible (the factory strike most of that year might have been part of the problem). - Mike Carpenter S812797DN
I saw a question a while back relating to the close ratio gear box. The question was, what is one and why do you have one. Alas no one replied and I am still uninformed. My XK has one of these fitted and I would like to know what is different about it and what does it do for me? I imagine it was an option, was it popular? Help! - Roger Puttman XK 150 FHC 1957
Standard Intermediate Close Prefix (all) GB (4 speed) or GBN (overdrive) Suffix: None or "O" JS CR or MS Ratios: 4th 1 1 1 3rd 1.367 1.238 1.210 2nd 1.982 1.860 1.750 1st&Rev 3.375 3.377 2.980
On a standard gearbox the ratios range from 3.375:1 to 1:1. On a close ratio gearbox they range from 2.98:1 to 1:1. The ratios of the gears are therefore "closer together", hence the name. From a performance standpoint, all gears except top are "higher", as are the top speeds in each gear. If you have an overdrive, the close ratio box partially compensates for the lower gearing of the differential. Either way, the 3.4 litre engine has sufficient low end torque to use the "higher" low gear of the CR box. My 3.4 MKI with overdrive has a CR, but my 3.8 MKII with overdrive has a JS. First gear on the 3.8 is pretty much superfluous. - Mike Eck '51 XK120 OTS '62 3.8 MKII
Mike gave you 99% of the answer. The main reason for using a close ratio gearbox is for racing, as the engine does not have to slow as much or speed up as much to go from one gear to another, allowing for faster shifts. Purists and realists in the olden days didn't rely on the transmission gear synchronizers - they 'double-clutched' all shifts, up and down, first putting the transmission into neutral, then speeding up or slowing down the engine to match the next gear, then putting in the clutch again and actually selecting the gear. This procedure is done very fast, and dramatically prolongs the transmission life (though it does add wear to the clutch and throw-out bearing, but those are relatively cheap!). Hope you now know all you ever wanted to know about close-ratio gearboxes!
It was optional, and popular in those cars purchased for racing; as it bacame a production run item, Jaguar probably fitted a number of them to export cars, just because they were on the shelf! I got the one I had from an otherwise-mundane 3.4 XK150 FHC. Sold it to Bill Welsh years ago.
By the way, if you get really good at speed synchronizing, you can upshifty and downshift without using the clutch at all, save starting off! Impress knowledgeable chicks, but don't ever blow a shift! - Larry Schear Twin Cam, Inc.
The idea of close ratio gears was to give gear ratios closer together between the gears (I am not trying to be funny here). The comparison I could find fastest was for the 120 boxes (available from April 1953) where the comparison was as follows:
Standard Close ratio 1. 3.375 2.98 2. 1.982 1.74 3. 1.367 1.21 4. Direct Direct
So the overall range from direct top 1:1 to first gear for the standard car was 2.375 and for the CR car was 1.98. I understood that this was often better on racing circuits where 1st gear was of little significance. Others will offer more sophisicated views about all this! - John Elmgreen
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