XK150 Reflecting: Was it worth it?
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For me, given the way I use and enjoy my XK150, installing the T5 in the car was worth the cost, time and effort. 

Have I compromised the value of the car? Possibly yes.

Given a week, and help from a friend, the T5 gearbox could be removed and the original gearbox and overdrive refitted.  No wiring was cut, the overdrive switch is still in the dashboard, the throttle switch is in place, and all of the original components which were removed are safely stored ready for re-use.

Cost: The T5 conversion project cost has been (AUD) $8,400.

What were or are the negatives?

There are two technical aspects of the performance of the T5 gearbox that have a negative connotation:

Oscillating whine

An oscillating whine occurs in fifth gear, largely but not completely silenced when all the normal XK150 cabin sound-deadening carpet and underlay is in place. It transpires that this whine is quite common in the T5 gearbox. In modern cars, maybe it's extensive sound-deadening techniques being used to prevent the sound entering the cabin.

The whine is possibly more severe due to the very tall 0.63:1 fifth gear ratio in my T5. The ratio of the fifth gear pair of gears is 51:25, or 1:0.49 .

I went to the trouble of removing the T5 from my car, and sending it to the most prominent Tremec service company in Australia to be checked. The check showed no fault in the fifth gear pair tooth engagement pattern nor in the pre-load setting. So the oscillating whine is there to stay, but it is not all that audible at 100 km/h.

Chatter in first and reverse gear

The chatter occurs when travelling at very low speed in first gear or in reverse, such as when on a level surface entering or backing out of a garage. The chatter is most noticeable when the car has been driven sufficiently to reach full operating temperature (15-20 km). The chatter can be more severe in reverse gear than in first gear. When the chatter occurs the whole drive chain and exhaust pipe assemblies can shake and rattle.

There is no chattering when accelerating, gently or hard, and not on over-run in normal driving. There was never any chattering under any condition with the Moss gearbox installed!!

[Postscript June 2016] The chattering in first gear and reverse gear appears have been eliminated by placing, on an experimental basis, a firm neoprene rubber steady block under the rearmost point of the T5 rear entension. The experimental steady block appears to satisfactorily perform the very same function as the firm rubber steady block fitted at the rear of a standard XK150 overdrive unit (part C8398).

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