The Laycock Overdrive unit has an epicyclic gear train internally, that when not engaged simply allows 100% input revs (from output shaft of gearbox) to be transmitted 1:1 onto propeller shaft and thus into rear axle, so is a totally independent unit to rear axle and thus all the options available for a rear axle ratio. When the OVERDRIVE is engaged - an electrical solenoid engages a cone clutch, the epicyclic gear train now multiplies the input revs from the gearbox by its designed gear train, which in case of XK140 Overdrive is 28%. So again the increased revs out of the overdrive then drives the rear axle, again independently of whatever rear axle ratio is fitted.
This is why in Jaguar Literature the GEARBOX (and if fitted the OVERDRIVE, ratios are quoted, independently of any choice of rear axle ratioā¦
And as above - the Jaguar Engineers have worked out the overall best combinations of gearbox and raer axle ratios for desired customer drivingā¦,
Added OVERDRIVE was a preferred option for medium and high speed freeway cruising, where a constant speed could be maintained at lower engine revs, with benefit of improved fuel consumption.
But for spirited driving and indeed racing, where the acceleration capability of an XK engines mid-range torque in the intermediate gears was a priority, overdrive was not a preferred extra weight/cost option, as the torque of an XK engine (was too much for the overdrives cone-clutches, thus slippage and eventual failure). This is why both Jaguar and Laycock were quite explicit re use of an overdrive - something to only engage at steady state driving, to reduce the revs at the cruising speed; and not something to be used as say a fifth gear during accelerationā¦
These days it doesnāt matter, as few XKs are driven much these days especially with any vigor, and when they are its usually be a knowledgeable enthusiast/driver. With show cars and weekend coffee runs, its all a bit irrelevant what gears and rear axle ratio is fitted, with or without overdrive, except the cost and expertise in repairing an abused overdrive unit can be a major issueā¦
with regard to differential ratioā¦IFā¦you are on roads where most cars are doing 60-65mph, and IF you care whether you drive at 2400rpm or 2800rpm or 3100 rpm cruise speed to maintain 60, then it may matter to you. Play around with this rpm-speed calculator: Pick your rpm, vary it, pick your ratios and vary them. Most current 600-16 tires have approx 27.7 loaded diameterā¦so enter radius of 13.85in. If you believe you have a 28 diam loaded, use 14, but most will deflect about 1/2 inch under weight, a little is made back up at speed.
Copy and paste, or type in: https://wahiduddin.net/calc/calc_speed_rpm.htm
as to available ratios for a Salisbury 4HA, it is basically the same as a Dana 44ā¦so many Dana 44 ratios are availableā¦beyond what Jaguar chose as options.
Your description of the operation of the Laycock de Normanville Overdrive is fine but I think it is necessary (essential?) to add one aspect: hydraulic pressure.
You state that āan electrical solenoid engages a cone clutchā is of course true in the end, but I think it is essential to mention that the solenoid opens a valve that allows hydraulic pressure (developed by the built-in plunger pump) to engage the cone clutch using two hydraulic pistons.
There are Jaguar owners thinking itās the solenoid that moves the cone clutch but there is much more power required, hence the (cleverly developed) hydraulic system within the overdrive.
The break-point for the two Salisbury 4HA and Dana 44 differential units is 3.73 and down and 3.92 and up. That is, you can install a 3.54 ring and pinion set on a 3.73 carrier or a 4.09 set on a 3.92 carrier. If you have a 4.09 and you desire a 3.54 ratio you will also need the appropriate carrier. Once you know what to look for, the different carriers are easily identified.
Yes, I am well aware of how these Overdrive Units work; I was being - I thought - suitably simplistic, not wanting to complicate the explanation of the overdrive ratioā¦
But thanks for your clarificationā¦
Thatās one way of looking at it, but rarely quoted as suchā¦
Buts shows the ālong-legsā and thus fuel consumption benefit of having overdrive for highway cruisingā¦
Hmm I just bothered to work out what my old 3.8 āSā saloon had 3.07 āEā cwp and o/d something in the region of 2.2:1 now thats relaxed cruising
I took a photo of this when I was changing ratios on my 140. This illustration shows how I got away with changing the whole carrier as a unit when I did the Mk7 swaps. Later when I changed only the ring gear and pinion I had to consider this. IIRC I got Dana parts from East Coast Gear in or around Raleigh NC.
