Series 2 V12 XJC - upgrade running gear or rebuild original?

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Series 2 V12 XJC - upgrade running gear or rebuild original?

Postby [email protected] » Mon Apr 15, 2019 2:43 pm


I'm brand new to this forum so thanks for having me and thanks in advance for your replies.

I'm a big Jaguar fan. I'm just coming to the end of the complete restoration of a 1967 series 1 E-Type roadster in opalescent gold on light tan but it's my new project which I'd appreciate your advice on.

I have a '78 series 2 Daimler Double Six coupe. It's essentially the same as the series 2 Jaguar XJ12 couple bar some chrome and badge differences. It's in need of a full restoration and I have the option of rebuilding the original v12 fuel injection engine and BW 3-speed gearbox OR fitting the car with a later V12 HE engine and four speed gearbox from an XJ-S (which was built on the same platform as the XJ coupe).

Do I stick to the original, less efficient and less reliable engine and gear box or upgrade? Bear in mind that sticking with the original engine and gearbox is the more expensive option (since I have access to decent XJ-S running gear already). Originality and unreliability over improved performance and cost saving? What do you think?

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Re: Series 2 V12 XJC - upgrade running gear or rebuild original?

Postby mike020150 » Wed Apr 17, 2019 2:36 pm

Hmm, an interesting conundrum. If the car is a true 1978 (production actually ceased in late '77) 12 cylinder coupe, it will have been supplied the better GM 3 speed gearbox, not the earlier BW unit. To drive, a pre-HE engine as originally fitted to coupes, is a different kettle of fish compared with the later HE version. Pre HEs are more responsive and revvy whereas the HE cars rely on stronger torque delivery coupled with higher differential gearing for a lazier power delivery. If your HE V12 engine is a 5.3 litre, it would have been fitted with the same 3 speed GM gearbox as the coupe but with slightly altered calibrations. Only the last 6 litre V12s came as standard with a 4 speed auto, this time an electronic version of GM's autobox. Fitting this electronically controlled box to a 5.3 litre engine isn't practical for various reasons including the need to supply it with electronic info from an engine which doesn't have the necessary sensors and information feeds. If you'd like a 4 speed auto to fit a 5.3 litre engine, realistically there are only two choices, either fit a ZF conversion (from Graham Whitehouse) or utilise one of the largely US conversion kits utilising the 700R4 transmission. The former has, IMHO, a much better set of ratios. If you plan to use the car a lot, a four speed auto with the appropriate matching rear axle ratio (typically 3.54:1) retains the car's essential character and makes it a better long distance cruiser. If annual mileage is likely to be low, a car with the 3 speed GM box is a lot better to drive than one with the BW version. The former is smoother and more sporting in operation but 2750 revs at 70mph (with the std 3.07:1 diff) is high by today's standards. As ever, you pay your money... Mike.
2014 XK Dynamic R convertible; 1977 Daimler Double Six Coupe; XE S

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Re: Series 2 V12 XJC - upgrade running gear or rebuild original?

Postby davidr » Wed Apr 17, 2019 10:44 pm

I've a feeling that the later XJS bodies had a larger transmission tunnel to accommodate the 4 speed GM box so you might find that it won't fit an earlier car without some tin-bashing.
1996 XJS Celebration Conv. (shown on the JEC stand at NEC Nov 2015)
2001 XKR Convertible

Previously enjoyed!
1989 XJ-S V12 Coupe. 2006 XJ TDVi Sovereign. 1995 XJ6 3.2. 2013 XF 2.2d. 2009 X Type 2.0d Sovereign estate

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