Roger K Posted December 2, 2020 Report Share Posted December 2, 2020 Quick question (I hope) - will a Spitfire 1500 3.63 diff fit straight into a '73 non-roto MkIII GT6 with overdrive? The original diff is a 3.27 - are the mounts the same, and the three shaft flanges? Thanks Roger Link to comment Share on other sites More sharing options...
Colin Lindsay Posted December 2, 2020 Report Share Posted December 2, 2020 Yes it will fit straightaway but you'll notice distinct differences in performance. Link to comment Share on other sites More sharing options...
Roger K Posted December 2, 2020 Author Report Share Posted December 2, 2020 Thanks Colin. Given that until the very last few cars, GT6s with overdrive had a 3.89 ratio I'm hoping one half way between 3.89 and 3.27 won't rev too highly, especially with the overdrive. Plus, I'm told the 3.63 is stronger... Link to comment Share on other sites More sharing options...
Pete Lewis Posted December 2, 2020 Report Share Posted December 2, 2020 3.63 :1 its the best of the bunch remember the higher the ratio the more torque you have to transmit for the same performance as a lower ratio just part of the reason 3,27 dont last .............always on the loud pedal in general bigger diameter pinion needs a thinner crown wheel or diff case to accommodate the off set from the pinion Pete Link to comment Share on other sites More sharing options...
Nigel Clark Posted December 2, 2020 Report Share Posted December 2, 2020 Don't forget the original speedo will over read with the lower final drive ratio. About 11% difference, so you may want to have it recalibrated. Nigel Link to comment Share on other sites More sharing options...
Gully Posted December 2, 2020 Report Share Posted December 2, 2020 When I had my 3.27 ratio diff replaced with a 3.63 I got Speedy Cables to recalibrate my speedo to 1075 TPM, which appears to be spot on against the Sat Nav. Gully Link to comment Share on other sites More sharing options...
Roger K Posted December 2, 2020 Author Report Share Posted December 2, 2020 Pity it's not the Mustang speedo as I just change the plastic gear in the gearbox to recalibrate! Oh well, hello Speedycables... Thanks for the advice, all Link to comment Share on other sites More sharing options...
Clive Posted December 2, 2020 Report Share Posted December 2, 2020 6 hours ago, Roger K said: Pity it's not the Mustang speedo as I just change the plastic gear in the gearbox to recalibrate! Oh well, hello Speedycables... Thanks for the advice, all Not sure if there are different gears available for the d type. There certainly are for the J type. Yes, 3.63 is strong. I broke 3 x 3.27 diffs in as many years in my 2.5 vitesse, but the 3.63 ran for 10 years, then 2 years in a 150bhp spitfire, and after a reseal now resides in a Belgian MK3 Spitfire. Link to comment Share on other sites More sharing options...
Pete Lewis Posted December 2, 2020 Report Share Posted December 2, 2020 see https://www.overdrive-repairs.co.uk/buy-spares.php?cat=D Type Spare Parts List they only show 15t Triumph 16t MGB then if more teeth you may need to change the bearing mount ,as bigger diameter will change its mesh with the worm ( easy on our old truck gearboxes the drive housing was an eccentric you just turn it to suit the pinion size) to get the TPM my days you mark tyre with a chalk line on the side wall you add a card needle to the cable inner drive under the dash, mark the road 52,8ft align tyre mark and push /drive the 52ft carefully and count the turns of the inner cable needle multiply by 100 and thats the TPM for the speedo bear in mind it must read fast it must not read slow the old spec was 10% + 4mph so a speedo could read at 60 true + 10% +4 read 70 in the car Pete Link to comment Share on other sites More sharing options...
Roger K Posted December 2, 2020 Author Report Share Posted December 2, 2020 That, Pete, is pretty much how Speedycables ask you to check what calibration you want. Not quite as complex, though, they just ask for a chalk mark on the tyre and a card pointer and disc on the cable, and count how many turns of the pointer for a set number of rotations of the tyre. They do the maths, otherwise just as you say. Link to comment Share on other sites More sharing options...
Roger K Posted December 2, 2020 Author Report Share Posted December 2, 2020 7 hours ago, Gully said: When I had my 3.27 ratio diff replaced with a 3.63 I got Speedy Cables to recalibrate my speedo to 1075 TPM, which appears to be spot on against the Sat Nav. Gully Do you find the ratios OK for motorway driving as well as twisty bits with the 3.63 and o/d? What would you be able to do for a couple of hours on a French autoroute - 75-80? Link to comment Share on other sites More sharing options...
rogerguzzi Posted December 2, 2020 Report Share Posted December 2, 2020 Hello Roger A 3.63 in our Spitty with a 28% J type o/d shows 3000rpm at 70mph. So 70+ cruising is ok and you will have more power lower down. But then Spitty does run EFI with ITB's so does tend to be better Roger Link to comment Share on other sites More sharing options...
Pete Lewis Posted December 2, 2020 Report Share Posted December 2, 2020 i use 52.8ft as the maths is ....simpluze Ha easy with the rolling road for my tacho calibration set up in the factory to get the tyre rolling circumference and gearbox TPM you could always take the car to a Tachograph centre and ask if the have a car speedo drive adaptor that fits Pete Link to comment Share on other sites More sharing options...
NonMember Posted December 2, 2020 Report Share Posted December 2, 2020 1 hour ago, Roger K said: Do you find the ratios OK for motorway driving as well as twisty bits with the 3.63 and o/d? My GT6 had done four Round Britain Reliability Runs before I figured out it was running a 4.11 diff. It's now on a 3.63 and it's fine for motorway. Link to comment Share on other sites More sharing options...
Colin Lindsay Posted December 2, 2020 Report Share Posted December 2, 2020 29 minutes ago, NonMember said: My GT6 had done four Round Britain Reliability Runs before I figured out it was running a 4.11 diff. It's now on a 3.63 and it's fine for motorway. How did that perform? I have more of those than Spitty or GT6 diffs... Link to comment Share on other sites More sharing options...
NonMember Posted December 2, 2020 Report Share Posted December 2, 2020 Well, on one of those RBRRs we arrived at Bristol, after an enthusiastic drive through Wales with a few other cars (Stag, PI, Spitfire, and possibly a Sprint) and one of the other drivers described mine as "the very fast GT6". I do prefer the more relaxed cruising with the 3.63 but it's not quite as spirited on windy roads. Link to comment Share on other sites More sharing options...
Pete Lewis Posted December 3, 2020 Report Share Posted December 3, 2020 guess to sum it up with a higher ratio diff any spirit will need 3rd gear more if triumph got their gear ratios right 3rd OD could be a split shift but they didnt and 3rd OD and 4th are about the same redesign by moving a few teeth around might have given us 6 ratio's but no you get just 5 pete Link to comment Share on other sites More sharing options...
Gully Posted December 3, 2020 Report Share Posted December 3, 2020 22 hours ago, Roger K said: Do you find the ratios OK for motorway driving as well as twisty bits with the 3.63 and o/d? What would you be able to do for a couple of hours on a French autoroute - 75-80? It's great - I do quite a few motorway / A-road journeys in mine and it's fine. I found with the original 3.27 diff I was rarely using the overdrive unless I was cruising, but now I find flicking in and out of OD in 3rd is really useful on twisty roads! I've not driven a 3.89 ratio diff car (the standard UK market Mk3 with OD), but I'd guess it would be a bit revvy cruising on motorways. I see 3000RPM (or just under) at 70. Gully Link to comment Share on other sites More sharing options...
Gully Posted December 3, 2020 Report Share Posted December 3, 2020 22 hours ago, Roger K said: That, Pete, is pretty much how Speedycables ask you to check what calibration you want. Not quite as complex, though, they just ask for a chalk mark on the tyre and a card pointer and disc on the cable, and count how many turns of the pointer for a set number of rotations of the tyre. They do the maths, otherwise just as you say. Or if you know the TPM for the 3.89 ratio diff and also for the 3.27 diff (it's written on the different speedos) you can interpolate what you need for the 3.63 - which is what I did to get the 1075 TPM. I gave that info to Speedy Cables, they worked to that and it's correct. Gully Link to comment Share on other sites More sharing options...
Roger K Posted December 3, 2020 Author Report Share Posted December 3, 2020 8 minutes ago, Gully said: Or if you know the TPM for the 3.89 ratio diff and also for the 3.27 diff (it's written on the different speedos) you can interpolate what you need for the 3.63 - which is what I did to get the 1075 TPM. I gave that info to Speedy Cables, they worked to that and it's correct. Gully That's fine - I'll get Andy on it for me! Thanks Roger Link to comment Share on other sites More sharing options...
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