FOR A SMALL BLOCK CHEVY!!! i don't know about your guys 12 cylinders, although they're incredibly smooth, but it's been a decent improvement on mine. i actually went with a roller cam 305 as opposed to the 350. thought it was more fitting in line with the European style small displacements, but the best thing about it, was not having to change the springs as the six and eight are about the same weight, and the 3:54 rear end gears on the six's. worked out great, british hot rod. NOW I'M GETTING AT LEAST 7 MILES TO THE GALLON! holy crap these carburetors suck, literally! i think i have it tuned correctly, too, might be rich, i can't tell, but I've asked enough experts. best go to the throttle body fuel injection system. any one have any questions about it let me know, it was a decently fun project, and easy, too.
i even asked my wife if she wanted a corvette motor for her xk8, she said absolutely not. she's right, i do love that jag v8. but given the fact that they made 120 thousand xjs's over a 21 year span, there are plenty of stock ones out there for histories sake, even though mines a facelift six, but i tell you, it's a brute now, and i kinda like it. i even have gotten used to the "pick-up truck" v8 sound it makes with just two 18" glasspacks.
Note the chevy orange peeking through, that was done intentionally. Nothing a better combination in my opinion than a good European chassis and Detroit iron.

 

oh one other thing...if it had been that last 3 year engine, the aj16 motor, i'd of not bothered, that was a pretty strong motor, but this "weak sister" aj6 was just too pathetic, not to mention, the fact is i was gonna wait a couple of years to do this, but i broke the german transmission. my Scottish jag mechanic said, we've not had a customer break one, so i said, then i'm gonna get something that can handle the load, 1990 700r4 with a shift kit i bought off some old hot rodder, which is actually annoying, the shift kit, as it doesn't take much to chirp the tires off the line shifting from first to second, good thing i got that heavy duty rear end!

 

I've got a 350 sbc in mine too. I agree that it's the best combo. I had a 383 stroker but it was so obnoxious I got rid of it. Also had very little vacuum and the turbo400 needs vacuum. So last summer I installed a 350 with moderate cam. I swapped the motor in a weekend by myself.

 

yeah, I was told to use a moderate cam in order to have vacuum for the brakes, then I go all the way to the point of hooking up the brakes and then realized it had electrically powered brakes! as it is the cam works with my stock torque converter so no great loss, but I found a supercharger kit that works with my thousand dollar gm front end accessory kit, but it's 3k, so I'm guessing upgrading to an $800 throttle body fuel injection might give me a bit more horsepower and much better fuel economy.

 

Blasphemy but heck, to each their own!

I drive a Jag V8 as a daily driver. I like V8s but the XJ-S is a V12 car. Like a BMW 8 Series. 850i or none Though a nice V8 bubble inbetween is a nice thing it doesn't quite match the elegance. It is like fitting a wankel engine in a Viper. Doesn't match

But looks well done

 

very true, my car lost it's refined whatever, now it's a brute, but, that six was way to boring, just like a series 3 xj6, talk about slow. i'll sacrifice a little sound and vibration for what's historically been a good proven combination. don't get me wrong, the v12's are nice and very smooth, but I can get as much or more out of this little 305 with less weight than a 5.3. never understood why they don't produce a lot more. I know the merlin started with just over 100 hp, but ended the war with over two thousand hp.

 

Well, the 5.3l can produce a lot more. I mean, even by removing the cats you can gain a good 30 hp (with cats around 265 hp, without 295 hp). As it was launched, it was up to date, but after that no more evolution apart from upping the displacement... And using some more modern ignition systems...

 

In Australian trim the 5.3L made 285hp and 310ft/lb of torque in the late 1980's compared to a C4 Vette that made 230hp 330ft/lb in 1989. The V12 is just not as tunable as a Chev engine, and costs more due to limited supply and there being 12 of everything. It's a BIG engine as heavy as a BBC.

I'm the owner of an 89 V12 and I have no issue with Chev swaps into Jags. Been there done that 30 years ago with a S1 XJ6 and a 350SBC.

 

I got around the low vacuum for the brakes with a 12v vacuum pump to operate the accessories. But as you know it doesn't help with the tranny. Now what to do with the stroker motor. I see a V8 Vega in my future. I built 1 in hi school and it was a crazy fast car

 

A milder cam will solve the low vacuum issue.

 

don't confuse "slow" with "quick". My 87 Series III was far from slow it just wasn't "quick" off the line. Then again it was never intended to be "quick" nor "exciting" it was a luxury saloon, smooth and refined. I had an 86 C10 with a 305/700r4 combo, talk about a gas guzzling dog, that thing was anything but fast/quick or exciting. Not intending to crap on your decision, just my observation based on personal experience. No idea of the trucks gearing, I do know the 350 in the same truck was better on gas. Terrible MPG, in mine, 15 tops on the highway with my foot out of it.