New engine--Another V-12 vs Chevy small block

Hi: I too, have done some research on this event and the pro’s I talked to said the Chevy Ls1 injected unit is the one to buy, in the long run it runs better and less future problems.
I hope this helps…

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I would stick with the original V12. Fix it, even Bore it out to 7 Liters
much more fun than changing to a boring V8!



Same, IMHO.

My LT1 can be boring or challenging. Hissy fits at times. Boring at others. Punch the gas and anything but boring…


…and about 6 armored trucks more of cash.

You left the TH400 off the V12 when you weighed it. The LT! / 4L60E package (aluminum heads, aluminum intake, lighter transmission) is quite close to 300 Lbs lighter than the V12 with TH400 - close to what the AJ16 weighs with the 4 speed ZF. Plus much better performance, way better fuel economy, reliability, and low maintenance. Lighter condensor and radiator to boot. The LS packages (aluminum heads & block, 6 speed stick or 6 speed auto) are maybe 100 1bs. lighter than the LT1 / 4L60E.

Take at the offerings at “jaguar

Good guy, well experienced I Jag swaps and related.

Immense help when I did my car…


I agree about ripping out the rear cage mounts, bin there done that. I converted mine to 5sp manual 9 yrs ago, installed Bywater’s ECU & put in a 3.54 diff. IMO my V12 with that combo is way more than the stock xj-s IRS & 10 bolt can handle regarding High torque.

I have owned two Xjs couples over the years and while the V12 is challenging most of the V8 swaps I have seen seem silly to be. It’s like putting a lawnmower engine in the space shuttle.
Also consider how much work goes into a swap especially a good one with fuel injection. You can spend a couple of weeks reading the forums and updating the V12 and then have a very solid reliable engine that is historic in its significance.

Just my opinion

And, surely respected…

But, suppose the V12 is badly damaged. Rebuild one??

A good lawn mower engine ain’t all that bad… Knowing HD mechanics well realize the two banger is as simple as a Briggs and Stratton. yet oh so desirable…

The LT1 is a highly developed version of Chevrolet’s famed SBC… alloy head or iron heads. Reverse flow cooling a major feature.

The LS engine, all alloy even better… Power to weight advantage.

And, hands down, a vastly superior engine management package.

Not to say that a healthy V12 would not be welcome in my drive…


I currently own 3 Jags (XF Portfolio, XJL Supercharged V8 and my first ever: 1986xjs V-12 Coupe). I can understand those who are purest but that has never been me. I bought these cars to enjoy and over the years I have made modifications to my XJS for technology (replaced the radio but still have the original) and enjoyment (I swapped out after many many years to a 5-speed manual transmission, got some different wheels and added front and rear end sway bars). My V-12 engine had its’ last leg and to be able to keep it I would have to do a complete rebuild (had it checked by experts) and that would cost between $15,000-$18,000. I could have found a replacement V-12 but I had done a lot of reading and research and I came to the conclusion that for me I would like to convert (one reason is because there are not that many mechanics these days that actually know how to take care of that V12). I now have a LS2 400hp going in and with that I have had to go with T6 transmission. This engine is quite a bit lighter and more mechanics will be available if needed. Due to the COVID-19 whatever, my mechanic has 2 of his top guys out and has put it on hold until he can get full staffed again. I will update on the car and be honest about whether I made the right decision (for me). Thanks for all the input and help.

Was it one of the experts who then offered a rebuild at $15,000-$18,000?

You think there might be a conflict that is unusual???


CosmoXL, It wasn’t. The mechanic who tried to get the engine running (he has been taking care of this car since 1988) had it for a long time and tried to get it unlocked (it was froze up). It was really my fault. I wasn’t driving it and my garage was full with the other 2 so I bought what I thought was a top notch car cover. It was left out in the weather too long and it got moisture in the engine (probably condensation from being covered and the heat and humidity here in the South). I talked to several shops who did rebuilds and told them all that my mechanic told me and it came to an average of that. It would have been a complete rebuild from scratch (it was gone due to my neglect).

Carl, Not sure if that question was for me or exactly what it means but there was no conflict. The engine could be rebuilt but at a great expense. I had already considered converting due to the age and problems I was having with the V-12. After researching and reading how well others ran and how much they enjoyed the ride after converting I decided to convert. No biggy. I could have replaced my old V-12 with another old V-12 but time for something new.

Not directly. It was an extension of Cosmo’s comment.

“Your engine needs a complete rebuild.I can do it, costs between 15 and 18 grand”. That is the conflict. Seen everywhre actually…


In the UK I’ve seen a common price stated about £8500 from specialists and even that sounds real high to me.

Though that probably doesn’t include removing and reinstalling.

Having owned an LT-1 Corvette, it’s a good engine but I think if I were to junk the V12 it would have to be for at least an LS-1. Preferably LS-3,

The LT is a 300hp motor.


My wife bought a beautiful '89 XJS with a sick V-12. I’ve had the local jag guy look at it and he says the engine is done. A Chevy conversion sounds like the way to go as I want her to be able to drive it worry free. Since you’ve been down this road before, could you give me some advice. What company’s conversion kit would you recommend using to stick in an LS? I’ve heard of Jag Specialties, John’s Cars, and Suncoast Conversions.

Thanks for you help,


Welcome, Craig!

Go to the “Lumps” forum, and you will find a HUGE trove of information, and helpful people.

All three are good options. Good guys head each.

Google up their we sites.