XJ6 & XJ12 Series I, II & III 1968-1992

V12 specific questions

 
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Old 02-15-2019, 03:16 PM
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Hello all,

I have a Jaguar XJ12 project that is slowly going somewhere. Nearly have the axles back under the car and am trying to figure out the driveline. The car isn't originally a V12 car. I have two V12 engines and one transmission: 1 carbed V12 from a XJ12 S2 and one Injected V12 from a Double Six ~1986. The carbed engine has 9:1 compression ratio and compression & oil pressure looks good. The Injected V12 has 11,5:1 compression ratio, only 150.000km on it but like every single V12 engine I've had it's starting to lose compression. It's not extreme on this engine yet and if I remember correctly only one cilinder was a bit lower but I'd like to drive the car a long time before I have to take the engine out again. So my first question is what your opinion is on this losing compression? Where does it come from? Is it serious? Is your experience with these engines similar? Is the issue easy to resolve now with the engine out?
Btw: Both these engine I bought from scrap cars that had been sitting for over a decade probably, never saw them run.

Secondly, I wanted to fit the V12 that is originally carbed because I trust the lower compression ratio more. I have a GM400 transmission that came with the newer engine with lower mileage. I've done a lot of reading and read the torque convertor would not fit older V12's that originally came manually or with a BW12. The crankshaft end both look different too. So, is there a way around this? I feel I should use the newest transmission possible due to reliability.

Third, if I use the newer engine with higher compression ratio would the carburetors be a straight and simple swap? How would this run in comparison to an engine with 9:1 compression ratio?

If anyone has advice/any knowledge it would be greatly appreciated. Have been reading so much here. Still a noob at this. Will probably add questions that I can't think of right now.
 
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Old 02-15-2019, 10:01 PM
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Oh boy, fun in the workshop.

1) Compression loss in generally LACK OF USE. As in garage queens have more issues than daily drivers. Ours are 12.5:1 on that HE spec engine, and some with lower than liked comps have responded extremely well to good hard run, exactly what the engine was designed for.

2) Depending what it fitted to. Carbies are easier for some. BUT, tha maintaining is not easy for others. The EFI of the HE will require the Electrickey, fuel pump etc etc to operate. (SEE #3 BELOW)
You are correct with the different crankshaft ends. I have done ONE only, where we had the convertor snout from a BW12 fitted to the TH400 convertor, it worked, but not as well as I would have liked.

3) NO, repeat NO. I have done it, but never again, and I was always fiddling with it to keep it on song, that Hi Comp engine is EFI only in MY opinion.
 
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Old 02-16-2019, 02:45 AM
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Originally Posted by Grant Francis View Post
3) NO, repeat NO. I have done it, but never again, and I was always fiddling with it to keep it on song, that Hi Comp engine is EFI only in MY opinion.
The car will be running mostly on LPG, very common here. Could I still hold petrol as a spare for crossing countries without LPG? The space in the boot of an XJ for LPG-tanks is minimal, holding 100l (~550km?) is a real struggle.
 

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Old 02-16-2019, 03:09 AM
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My experience with LPG is limited to cooking only.

I have been around a few of LPG cars, none of them mine, and the EFI LPG engines seem to be less trouble than the older carby engines, BUT, that was a very long time ago, and I do know LPG technology has come a long way.

I reckon the 9:1 would be happier with LPG, but only my observation.

One PreHE EFI car here had LPG, and eventually it was removed and reverted to the standard EFI system, it could not be set right, and many people that knew what they were doing were involved with it.
 
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Old 02-16-2019, 08:33 AM
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Originally Posted by XJeej View Post

I feel I should use the newest transmission possible due to reliability.
If they've come from the same scrap cars as the engines, you really can't know what condition the transmissions are in. Personally, I would assume they're both in need of overhaul.

The BW12 is a very good transmission. Robust, reliable. The 400 is a bit more refined and smooth and has the advantage of part-throttle-kickdown...which is why Jaguar decided to use it. But, if using the 400 is too problematic there's nothing 'wrong' with using the BW12.

Third, if I use the newer engine with higher compression ratio would the carburetors be a straight and simple swap? How would this run in comparison to an engine with 9:1 compression ratio?


Probably a bolt-on swap in terms of installing the manifolds and carbs onto the engine. If you have the desire and ability to dial-in four carburetors...including the possibility of re-jetting ..... it would probably run great. And it would be an impressive sight. And, in terms of total labor hours, probably less involved than switching over to fuel injection

Personally, given a choice, I'd go with the fuel injection, despite the initial effort involved. Carbs are a retro-grade step, IMO

Cheers
DD


 
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Old 02-16-2019, 09:43 AM
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Originally Posted by Doug View Post
If they've come from the same scrap cars as the engines, you really can't know what condition the transmissions are in. Personally, I would assume they're both in need of overhaul.
Well, this GM400 transmission has a black torque convertor with white marker writing on it. The transmission itself has liquid gasket on the rear piece and pump. I don't know what to make of this, haven't checked for metal flakes in the oil due to no drain plug. The white marker has me hopeful though.
 
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Old 02-16-2019, 12:41 PM
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back 15/20 yrs , i had 3 complete Jag V12 engines , had been collecting them for a few yrs,(thinking maybe someday worth something).

well on a quick change of lifes plans, we up and moved to TEXAS , tried selling the engines NO takers , well piled them up and trucked them to scrape yard, BIG nothing they gave me scrape price by pound,
got $75. US dollars fro the lot , $25.dollars each!!

how that for value ??
 
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Old 02-16-2019, 01:14 PM
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Same story here (netherlands), I tried selling the low mileage one for a really fair price, no takers. It seems im the only one buying.
 
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Old 02-16-2019, 07:24 PM
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I agree with Doug on the trans, the BW12 is STRONG, and HEAVY by comparison.

I noted, late, that you mention a 1969 XJ6 > 12, and assume this is the beast in question.

If that is the case, you will need to have fuel tanks WITH a return spigot for the EFI engine system, and memory?????, has me thinking the V12 carby cars also had return fuel lines, but I may be wrong?, whereas the 6cyl carby cars did not have return fuel lines.

Just another item on the "to do list".
 
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Old 02-17-2019, 11:42 AM
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A newcomer to this part of the Forum, but I believe trying to run an HE engine on carbs will be very hard to get right. The HE combustion chamber is designed to ignite a very lean charge, and fuel injection gives far better atomisation than carbs ever can. So I feel it might be very uneconomical to get the HE going properly on carbs, possibly giving even worse MPG than the V12 carburetted engines had, as to get the charge to burn reliably in the HE combustion chamber you may need an uneconomically rich mixture out of the carbs.
Plus the higher compression needs a more powerful spark, so that might mean using the HE ignition - or an equivalently powerful aftermarket system - so now you are well over halfway to the whole injection hog. As long as you have the ECU and the ECU to engine loom and engine looms from the donor it is pretty straightforward to fit it all.
 
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