Disappointed? With a steel flywheel in a LWB XJ12 5-speed conversion from 3-speed slush box?
I would ask you on what measurable evidence you base such a laughable comment, but I wouldn’t bother to rebut it so there’s no point asking.
Was wondering the same thing, and equally chary about asking why
Now, I’m no expert in all things that involve tranny swapping, but to my mind, going from a slush box to a standard, irrespective of any flywheel one chose, would not likely be “disappointed.”
I could be wrong: it would not be the first time this year.
Kirby is known for iconoclastic posts. He’s bright and an engineer so he knows hid stuff but he doesn’t always let reason get in the way of a strongly-held opinion…like many of us!
From having been there, done that.
Sorry, I don’t understand.
To make any sense or a meaningful ‘been there/was disappointed’ statement you must have converted your XJS auto to manual with an alloy flywheel. Then swapped to a steel flywheel on the same car and been disappointed.
Leaving subjective feelings aside, what shortcoming did you find that was measurable and what was your comparator? On what basis would you then extrapolate those feelings to be relevant to someone else’s bigger, heavier car?
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BTDT but on a bog standard ‘66 3.8 ‘S’ and I was not disappointed, even with the 3.07 diff that I had to put in at the time due to blowing up my 3.54
At the time this was the only C/w and p that I could source here in NZ (late 80’s) this was replacing the DG box, chalk and cheese 
Why? I converted to a manual with a steel E-type flywheel and lived to regret it. That flywheel is soooo heavy that the car would accelerate from standstill faster in 2nd gear than in 1st. I wouldn’t recommend the steel flywheel to anybody, for any application.
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OK, well ten out of ten for honesty.
I’m guessing your Nobel prize will be in the mail for rewriting the laws of physics. Car accelerates faster with less torque applied to the driving wheels. Let me get back to you on that one.
FWIW the 250 hp 5.3L V12 flywheel has effectively the same overall mass and peripheral mass distribution as the 3.4, 3.8 and 4.2.sixes. I doubt you could tell them apart with your eyes closed. The bigger clutch is a little heavier.
Cheers 
Pete
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Yes. You can convert to a 5 speed without removing the engine. I put my 85 XJS up on chunky blocks of wood under the wheels…about 500/600mm clearance on the sills and packed blocks of wood under the rear of the engine to support it when the gearbox was disconnected and removed. The TH400 is heavy!! I used a Tremec TKO600 5 speed. Rolling around underneath on a crawler is fine. A tricky part is cutting the trans tunnel for the gear level quite close to the steel fuel line …but slow and steady works.
By the way…I used a Fidanza Aluminium flywheel as well. I don’t have any problems getting the car off the line with the original 2.88 dif…it is a V12 so you are never far from a power stroke from one of the pistons.!!! The crankshaft on the V12 is a CHUNKY piece of steel and will have a lot of rotational inertia onboard. I would say nothing to lose using Aluminium flywheel…
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I removed a BW12 from a S2 XJ12 with 3.31 and placed the Toyota Supra W57,
no hoist, engine didnt come out, cant remember whether I loosened the mounts.
My flywheel centre was custom machined steel and I used the auto ring gear
Doubt you will be disappointed, although weight means it will never be a supercar
Mine has a very heavy clutch and 1st gear is a bit too short
I can explain the physics to you if you’d like.
Really? You guys are that clueless? It’s simple enough: the massive steel flywheel has sooo much inertia that the engine’s power is consumed bringing the flywheel up to speed rather than propelling the car. In fact, the car in first gear went from idle to redline only slightly slower than it did in neutral.
A flywheel serves two purposes. 1) it carries the rotation from one power stroke to the next; and 2) it serves as a backing plate for the clutch. In this latter role it has to absorb heat. For the rotation issue, any engine of 6 or more cylinders doesn’t need a flywheel at all, the power strokes overlap. The only reason to choose steel over aluminum is if you’re planning to abuse the clutch – and with a V12 you’d be seriously abusing it, there’s little reason to use the clutch at all.
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OK, couple of quickies. I didn’t express a strong preference for either metal -alloy cheaper, steel option more supportable in a bigger heavier car than Jag ever fitted a flywheel to. Either would be fine. It was you who said the OP would be “disappointed” going from slushbox to a five speed if he opted for steel. That’s the bone of contention.
Going from a three speed auto to a bog standard Jag setup with 4 gears and steel flywheel totally transforms any XK or V12 and the difference between opting for alloy or steel is minor by comparison. If you were disappointed, that’s your prerogative, but you’re out on a limb with that view, especially as you never tried the alternative, so even your butt dyno has no reference point. You have no idea how much quicker the engine would rev in neutral with an alloy flywheel, let alone how much quicker the car would accelerate. The difference in responsiveness between the two is insignificant compared to the massive difference between Tony switching from auto to T5.
I know how heavy a V12 crank is because there’s one leaning against the sink unit that I have to move every time I want to open the cupboard. I also have four steel flywheels and one alloy that I have to shuffle when I pull stuff out from under the bench. The flywheel mass for the V12 is not that different from a 3.4 so cannot be described as if it were stupid heavy and to be avoided or risk disappointment (IMO). Not least for precisely the reasons cited and that as a proportion of overall rotating mass a V-12 15kg is “lighter” than a 3.4 15 kg.
The physics you cite is fine, but doesn’t speak to your claim that the car accelerated quicker in second than first. I know a short first means you need to change up sooner, but you can’t claim that a car accelerates harder with less torque at the wheels in a higher gear.
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I hadn’t thought of pumping losses but I agree that’s likely a big contributor. -and one reason why turbo/supercharging is so effective.
[quote=“PeterCrespin, post:28, topic:384663, full:true”]
Never let the facts get in the way of a good story.
Actual Flywheel weights are as follows
3.8 E - 19.8lb.
4,2 E./ Sedans - 26.5lb.
V12 E - 22lb.
But flywheel weight bears no relationship to the inertia (flywheel effect), which depends where the weight is positioned.
I have fitted many V12 Manual Conversions with a Button Flywheel (14.3lb. ) that is used in conjunction with the Auto drive plate. Never had anyone complain of difficult take offs because you also have a massive V12 Crankshaft that weighs 75lb.
50% more than a 350 Chev.
For my 6.0L race engine I use a 3 plate Tilton clutch with a button flywheel that ways 8lb.
can drive it on the street with no problems
I stepped up to the pump for the same set-up, for my 1200 Datsun racer: needless to say, there was not any lugging done! I also did it for safety: with 9200 to 9900 rpm shifts, the thought of even a lightened steel flywheel coming apart frightened the willies out of me.
Different creature than a big- honking’ Jag engine, though.
Any opinions about using a jaguar 4 speed+ overdrive in an xj12?
If one has the funds, it’s probably worth looking at the newest box in the Tremec’s line; the TKX.
It should theoretically work in a Tremec kit, (TKO etc), so put the shifter in the right location, AND if you’re going for a high revving V12, this is a better box. It’s also built to withstand a lot of torque (600 ft/lbs) despite being a slimmer box. You’ve got some ratio choices as well
https://www.tremec.com/menu.php?m=184
~Paul K.
I ran that in my series 1 SWB for about 20 years, about 20 years ago.
It was a transformation from the Borg Warner auto. Shifting was never quick and ratios might not have been ideal but it was still a blast.
The overdrive unit never ever gave any trouble other than a pump cam failure at one time. I had the OD solenoid switched through the original capstan kickdown switch to mimick kickdown so it was tested at times.