I am writing to gather some informed opinions on where I need to go next with my car.
This is a 1990 convertible, and I have fitted nearly all the AJ6 goodies: ECU, TT Exhaust, fitted bored throttles (no plus torque intake, however). I run Thrush turbo mufflers into glasspacks. I have removed the A/C and the air pump (all of it: all plumbing, all the bits), which has removed some weight as well as making things a bit more accessible under the bonnet. The suspension has been warmed over, too: I have fitted poly bushes to the front end, and fitted larger bars, front and rear. The ride height has been tuned as well; I run heights that mimic the XJSR. Part of that was accomplished through tire fitment: I run XK8 revolvers (17") with 245/50 in the rear and 225/45 in the front. Has that slightly nose-down attitude of the XJRS. The rest of the height was trimmed out through packing ring removal in the front and very minor amounts of spacers in the rear. The car is fitted with a Tremec wide ratio 5 speed, and a 3.54 diff.
All in all, it is a very taught, sharply behaving car (well, as much taughtness as can be reasonably obtained with a convertible), without completely forsaking that Jaguar-esque ride quality.
But, it could use an ounce or two more of encouragement, as it is a 4200 lb hulk.
Toward that end, I have acquired two engines:
The first is a 1979 pre-HE, pulled from an XJS in the early 1980’s (circa 1983, I am told). This was pulled at very low mileage due to the owner’s inability to ‘get it running right’. I have the race-track fuel rail, and it has been fitted with new, HE-model injectors. So this fellow went to great lengths to try to get the car to run properly before he pulled the engine. I would be surprised if this engine has more than 30K on it. Year and S/N suggest it is fitted with 7.9:1 pistons. I got it for the heads, but the 7.9 pistons make it a turbo/supercharging candidate.
The second is a 1994 HE 6L motor (yeah, the one with the forged crank). Mileage unknown, pulled from an XJ12, from what I can devine from the serial number.
Why have I acquired these? My first plan was to pull the flat heads and use these on the 6L block and crank, and fit new pistons and liners, taking this to 6.7L using Chevy 305 pistons, or to 6.8L using 96mm Rob Beere kit. The logic being that the larger displacements will need better breathing.
So far, I have been acquiring the various bits and pieces of tooling with which to do full rebuilds, and expect to begin opening and disassembling one of these starting this season (the sun will return sometime, here in Ohio).
I have been reviewing how I want to proceed, and this is where I am looking for input:
The courses of action that I can see in addition to the original plan are:
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open and inspect the 5.3, re-seal it if it looks good and put two smaller turbos on it (I hate the prospect of turbolag)
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rebuild the 6L and turbo it, as is (although I think those pistons will have to be trimmed down somewhat)
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find a longer stroke crank- maybe one of the TWR-spec 84 mm models. I suspect this will entail many other changes (like rods and pistons, and maybe oiling?).
I eyeballed the 92 mm crank that xjr5006 offered on another site a while ago, pretty seriously, but I could not convince myself to go this route (5 mm head plate and all the rest of it).
My kid brother is into boats and has a 38’ fitted with twin 540 cube big blocks that are supercharged using Prochargers. I like the clean nature of that setup, and figure that a Procharger could actually be accomplished under the bonnet of an XJS. So, supercharging may be the route, and I happen to like this ALOT more than turbocharging, from a tuning and drivability perspective.
Yes, most any of this will necessitate a new ECU. That’s a given in this overall picture.
And yes, the trans/diff gearing will be getting a fresh look, too, as the present overall ratios will not be well suited to a significantly more torquey and powerful motor. I may drop the 2.88 back in, or, I will go to a close ratio T56-based box that can work with a diff as numerically high as the 3.54.
Of course, the chief objective is getting there through a minimum path, in terms of cost (let’s be clear- what I am outlining will not be cheap, but I also don’t want to shell out more than reasonably necessary), but also in terms of long term livability: the final result has to be reliable enough to approach daily-driver quality. I don’t want a setup that requires constant nursing and care (which is one reason I have not gone the nitrous route).
The primary use is as a street vehicle- not a stop light drag racer, I don’t track the car, anything like that. Just looking for bags of torque to motivate the 4200lb of bulk to move along. So, basically looking for an engine that is optimized to 6000 RPM, no real need to go higher, with plenty of grunt from ~2000 on up.
Anecdotal information suggests the cams from the 6L will be better suited to turbo/supercharging, although I do not know for sure. The somewhat lighter valve train components of the 6L would be transferred, if possible, to the flat heads.
Any thoughts or opinions? Anything I am missing, here?
The easy way I guess is to focus on a clean rebuild of the 6L, and not worry about those flat heads. I suppose if I stopped at 6L, that’d be the way to go. But if the flat heads are fitted, pistons must be bought, and since I am in there doing this, might as well go for larger liners…and, we’re off to the races! See what I mean?
-M
