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loosecannon Reader
1/1/12 12:29 p.m.

I'll just add that the Jaguar V12s, while heavier than a V8, are still pretty cool with their ribbed aluminum blocks, narrow v-angle and smooth, smooth running. The stock 5.3 cam runs out of steam above 5500 but the engines will hold together up to almost 8000 rpm, so some stiffer springs and better cams would do wonders. I picked up 30 hp at the wheels on my engine just by straightening out the intake manifolds.

Ian F
Ian F SuperDork
1/1/12 5:04 p.m.
mguar wrote: The stock V12 has 3 basic versions.. 1971-1980 was the flat head era, 1981-1992 was the HE era (still 5.3) and from 1992 through 1996 Jaguar made 6.0 HE's.. 366 cubic inches instead of 326 in the 5.3 that added 40 cubic inches gives another 25 horsepower.. The first roughly year and 1/2 of the 6.0 production the crankshafts were forged hardened steel. Later cranks were made from sintered Iron.. Stock the fuel injection will flow1200 CFM more than enough to support 450hp. The earlier carbs used prior to 1975 will only flow 800 CFM but that is mostly wasted due to how it's arranged.. That is it enters down below the valve cover and is heated prior to it wandering it's way into the engine.. The MGBGT autocrosser guy cut off that goose neck and picked up a lot of power. There is even greater potential if you cut open the Fuel injection manifold and weld a plate across it to take 3 Strombergs per side. That's 1200 CFM.. (That's for us luddites who don't like the complexity of Fuel rejection).. Webers while sexy and a whole lot of eye candy, require dyno work and complete rejetting to run right.. Assume about $1000-$1200 for parts.. Power gain is right there with the 6 Stromberg mods.. Due to the too short intake stack to fit under the stock hood. Whereas almost any other mod requires a computer upgrade.. Headers: are wasted on the V12 (except all out designed to match the camshafts equal lengthheaders) Unlike a V8 the exhaust pulses are even and not in conflict.. Cam shafts: the stock cam is remarkably good.. For all out racing it can be beaten but for street & autocrossing etc.. it's great right as it is.. Regrinds by ISKY and Crower will add power if combined with higher compression ratios. a gain of 100 horsepower is possible. Port work will add to those numbers.. The stock Valve springs: were reved by the factory to 7800 rpm before they bounced and since the power peak is at 5750 rpm not a restriction. If tired ISKY sells good valve springs cheaper than I've seen Jaguar valve springs priced at.. The pistons are typically 7.8-1 compression through 1980 (a handfull are 9.0-1 in the earliest XK-Es) Starting in 1981 compression jumped to 11.5-1 However those pistons cannot be used in the earlier flatheads.. (1971-1980) What can be done to the later 6.0's is use the earlier 7.8-1 compression pistons suitably relieved for valve clearance on a 6.0 and you'll be right around 13.0-1 compression.. Stock the V12 has a 3.543 bore (both 5.3 & 6.0) max safe overbore is 3.681. which combined with the 6.0 crankshaft yields up 6.47 litres. All other things being equal overbore will yield an additional 25 hp. (plus whatever power improvement using higher compression ratio) In general it's safe to say V12's start near 300 hp. and the 6.0 makes about 25 more.. it's pretty safe to assume that with any mod the 6.0 will yield up another 25 hp over the 5.3 there is roughly 15 horsepower more from 9.0-1 compression to 7.8-1 compression another 10hp. at 10.0 & 10 more at each added one point of compression.. (those are factory numbers) The cheapest power gain is using a stone stock 5.3 with a pair of turbo's off junkyard salvaged Volvo's/ Saabs. (little T2's)

Mmm... paragraphs...

Ian F
Ian F MegaDork
1/1/12 5:22 p.m.
carguy123 wrote: Yes, and punctuation and capital letters. It's amazing how much more communication happens that way.

In his defense, he did use paragraphs, but only returned once. You need to do it twice when posting on this forum or the software will ignore it and create a one big paragraph.

Good info, thanks for posting.

singleslammer New Reader
1/1/12 6:13 p.m.

Terrific! I am still looking for a decent early or late XJS. Non HE or 6.0l would be a great starting place.

noddaz Reader
1/1/12 7:02 p.m.

Stop it! Stop it! I am NOT listening! La, la, la, la, la, la, la... (Jeez this sounds like so much fun to build......) Stop IT!!!!!!!!!! (12 cylinders of wonderfulness.......) Argghh!!!

unevolved SuperDork
1/1/12 7:03 p.m.

Things like this make me wish there was a GRM Wiki to consolidate all the weird knowledge that exists on this forum.

MG Bryan
MG Bryan HalfDork
1/1/12 11:00 p.m.

Since we now have this wonderful thread, what's the cheapest way to bolt a 5 or 6 speed manual to one?

ronholm Reader
1/2/12 12:27 a.m.

Somebody buy this one... Before I get in trouble.. I Hate you guys...


loosecannon Reader
1/2/12 1:13 a.m.
MG Bryan wrote: Since we now have this wonderful thread, what's the cheapest way to bolt a 5 or 6 speed manual to one?

I have the bellhousing, Fidanza aluminum flywheel and pressure plate to bolt a T5 to one-total cost was $1200 but I'll sell all of it for $750. You will have to put a spacer between the tranny and bellhousing to avoid the input shaft problem I had, though.

Ranger50 Dork
1/2/12 10:48 a.m.
noddaz wrote: Stop it! Stop it! I am NOT listening! La, la, la, la, la, la, la... (Jeez this sounds like so much fun to build......) Stop IT!!!!!!!!!! (12 cylinders of wonderfulness.......) Argghh!!!


unevolved SuperDork
1/2/12 4:03 p.m.

I think the big question everyone's wondering is what's the smallest, nimblest chassis these can be easily swapped into?

chaparral Reader
1/2/12 6:33 p.m.


e30 would be my first guess, but there's already a cheap and upgradeable V12 to fit to those (the 750i engine).

How about an old Celica Supra?

mikeatrpi Reader
1/2/12 6:37 p.m.

Datsun! You could build a Zerrari

HappyAndy UberDork
1/2/12 6:41 p.m.
mikeatrpi wrote: Datsun! You could build a Zerrari

Don't you mean Zaguar?

unevolved Dork
1/2/12 7:36 p.m.
chaparral wrote: but there's already a cheap and upgradeable V12 to fit to those (the 750i engine).

I didn't realize how cheap those engines were...

Next you're going to tell me there's a cheap way to bolt up a 5MT to one, right?

stroker HalfDork
1/2/12 7:52 p.m.

dunno what an uncorked Jag V12 sounds like at RPM but it seems a shame to put turbos on it and muffle that exhaust note...

Conquest351 HalfDork
1/2/12 8:09 p.m.

I think a whistling uncorked turbocharged V12 would sound insane. Please do. PLEASE DO!!

Timeormoney Reader
1/2/12 8:42 p.m.

So...The grm formula: POWER
1981-1992 V12 Jaaaag
Junkyard T2's, two of them
Junkyard Intercoolers, two of them
Small 45 Amp Alternator
Vortex fuel tricky thingy
Remote Tank power steering pump

What are the gotcha's?
Ie the self immolation fuel lines?
Known weakpoints?
Does this era have some form of GM tranny that can be easily handled or is it a ZF nightmare?

Just a quick summary with some follow up questions to perfect my craigslist search. I have actually worked a v-12 jag of some late model vintage. The fuel lines self immolated the car, and my buddy decided he would rewire it and get it running. He did, although I was brought in to change the starter. The jag manual called for pulling 83% of the car apart, I created an "access port". For all I know the car still runs today.

mikeatrpi New Reader
1/2/12 9:56 p.m.
HappyAndy wrote: Don't you mean Zaguar?

I was thinking of a 250 GTO kit, but Zaguar has a nice ring to it!!

calvindoesntknow New Reader
1/2/12 10:14 p.m.

okay guys so i just read this, and i am building a jaguar v12, its already in the car so no head machining can be done atm. what are your best suggestions for performance? i wanna hit 400 hp but i also want some horsepower in the normal driving range because honestly i wanna beat anything in my highschool parking lot. cus yeah im still 17 haha

Conquest351 UltraDork
1/2/12 10:55 p.m.

Anything will take a 125 shot of nitrous. LOL

Timeormoney Reader
1/2/12 11:14 p.m.

I think my post above probably summed it up. Do you happen to have the 1981-1992 model and access to a junk yard?

Tom Heath
Tom Heath Web Manager
1/3/12 9:30 a.m.

The problem with page 2 has been fixed; there was some wonky formatting in one of the last 4 posts that was quoted, so we lost exactly 4 messages in the process.

Now, on with the V-12s....

HappyAndy HalfDork
1/3/12 10:20 a.m.

In reply to mguar:

Do you have any pictures of this junkyard twin turbo system that you described, and what pressure was it running?

Timeormoney Reader
1/3/12 3:29 p.m.

I am now confused, which happens often.

Do we have a formula of parts for the best bang for the buck? Is there a model year that is the "best" or do we need to grab parts from different years and meld them together?

Any help would be deeply appreciated. My convertible v-12 fantasies need a road map.

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