Six litre V12 double six

V12-Engine
21

FWIW my '88 XJS coupe with the 5.3 typically gave 12-13 mpg in city driving and 16-19 on the highway. If driven very conservatively…like 60 mph with cruise control, no long hills, no passing, just plodding along…20-22 was doable. Quite boring, though.

55-70 mph was the sweet range on that car. Over 70 and economy dropped quite a bit.

Cheers
DD

22

It doesn’t use 1st to start? I didn’t know that! What a waste! IMHO, it would have been a better idea to leave the 2.88:1 final drive in there, use 1st gear to start, and use the overdrive on the freeway.

23

That works Kirbert.
My 4L80E is controlled by way of aftermarket TCU and does use first gear and does retain 2.88 diff. Take off is fine and cruising a breeze at little more than fast idle.

24

If you don’t mind my asking who makes the TCU

25

TCI ez-tcu by fast. I am very happy with it, the only limitation I see is it does not support 12 cylinders. This is not an issue provided your car is either 6 cylinders or Marelli ignition taking speed signal from one coil.

26

Slightly off-topic, but mentioning cruising reminds me that yesterday I drove a couple of hundred miles in a Jag that did 70 mph at about 1700 rpm - close to your ‘fast idle’ and possibly the tallest geared car I recall driving. I averaged about 35 mpg (US).

Each cylinder was 500 mL Was it a 6L V12? Was it a 4L V8? Nope, it was a 2L I-4 turbo diesel SUV - probably the ssme performance as an XJS up to about 80 mph.

IMG_9526
IMG_95263264×2448 1.71 MB

27

It’s interesting the Marelli failure comes up a lot on forums, this must be a USA issue as I have never heard of Marelli failures here in Aus, My car (1989 Marelli) has 160,000miles on it and never had a failure.

I also have a 4L60e and 3.58 diff, as I live in a rural community I do very little city driving but I get about 26mpg from my car on the open road at 100km/h.

I have a spare 6.0L engine and the differences are minimal especially to the later 5.3L

  • Rear main seal is a 1 piece rubber unit same as later 5.3L
  • Transmission bellhouse is different but 6.0L block still has the pads for the TH400 mount they are not drilled.
  • Injectors are O ringed into the fuel rail
  • Throttle capstan is smaller and mounted to the intake manifolds
  • My 6.0L has dual ribbed belt
  • AC compressor is MUCH smaller.
  • Chambers are slightly larger to keep CR at 11:1 mod was to the exhaust side.

This is 6.0L chamber.

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P_20160905_124540.jpg4096×2304 2.3 MB

1 Like
28

Here is another difference I found today while doing valve clearance.

The valve shim diameter is 0.611 on the 6.0L and 0.680 on the 5.3L and the intake is 0.020" shorter on the 6.0L.

I need to find 0.611 valve shims.