412 Diff Ratios/Gearing Upgrade

[peasantslife]

Hello all. I'll just introduce myself - a medium term owner of a 412 S2, since 2001. Bought as a running but rotten smoker...I ran it solidly through the first winter, then we gutted it from lockers to rear wheelarches and up into the roll hoop to put back metal. Of course with so much damp for many years all the trim was shot, so took a Rover 800 Coupe and 'installed' trim (not quite finished but useable). I now alternate use with other classics and neo classics Aston/Rovers/Triumphs. So the Bristol is asleep in France at the moment and I'm in the UK. My next projected enhancement is to improve driveability by diff ratio swapping...3000rpm not required for 70mph....So, looking in my Owners Handbook I see possibly 2.88 or 3.07 may be fitted. I'll take a stab it is the 3.07, but short of the ratio tag or actually checking by rotation is there any wisdom out there as to what was actually fitted? (I could happilly fit the 3.07 into the V8 Aston replacing the stock 3.54 - a recognised improvement on manual AMV8's), then sourcing a 2.88 from a V12 Jag (some XJS models only?). Or what about 4 or 6 speed box swaps. Any good knowledge of options? I' m not afraid of buying stateside and shipping and 'engineering' to make fit is no issue. Though probably would go Injected engine with associated box...circa 300-350bhp I dont need sprinting but want legs for better cruise refinement.

[GREG]

Martin Barnes at ACCS , London has done more of these types of mods to a V8 Bristol than anyone else and is probably best to advise - Jeff Marsh at Brabazon as well

[Kevin H]

A 3.07 ratio diff won't make enough difference

Assuming of course that it doesn't already have that.

I looked at retrofitting either an overdrive or a 4 speed box to my 411 (same chassis as the 412) and it was going to need modifications to the floor of the car plus a shorter prop shaft, so decided against it. I also embarked on modification and injection of the original engine, which I now regret.

With hindsight I would go with a modern injected Chrysler engine with associated box. I believe this is what Bristol has done this numerous times so you would be doing something that the factory has already done. Take a look at the 411 "S6".

[peasantslife]

Thank you both, I think next time home I'll grovel and see if there is a ratio tag...and in the meantime keep eyes and ears open for an XJS HE 2.88, (the simple option) and in parrallel search out more detail on 4/6/8 speed auto's and which engines drive them! Nothing like a challenge!

[peasantslife]

Today I had a very interesting discussion with a firm who import and factor from the US. Their suggestion is the Torqueflite 518 4 speed which is a straight bolt on to the LA engine. 3.5 inch longer, so shorten the prop and a bit of tunnel panelwork....brackets etc, use the Hydraulic control version (it went electric control mid '90's.) Ratios are same as the 3 speed +0.69 o/d in 4th which can be a manual switch or easily pedal/vac controlled. Delivered in gwo for under £500. So now I have two 'simple' starting options, Diff or gearbox? Maybe both, but that would take me to over 40mph/1000rpm! That could be too much for the stock engine, but they make a very persuasive argument for sticking with the carb 360 engine and with a simple 'modernisation' rebuild deliver oodles more torque and no (direct comparisons) economy loss.... Anyone with experience out there?

[GREG]

And this is how to do it properly

Dodge 727 Transmission Swap to 518 Transmission from PATC, 46RE 47RE 48RE


But will future owners want all these mod's or wish we had left our cars standard ?