Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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Moving on from another thread which might get derailed (as if - this is RR after all!)..
I’ve never found maximum torque ratings for overdrive units, specifically the Laycock LH unit as found in the earlier Scimitars, MGBs and the suchlike.
The only googling success was finding that it may be related to pump pressure to a point?
In short, the idea is to use the one in a Scimitar (3.0 V6, so approaching 190lb/ft peak torque, normally only allowed in 3rd and 4th), but also in 2nd gear.
I recall the works Tr4’s using it in more than 3rd and 4th (don’t think it extended to 1st gear, but certainly 2nd) but they’re still only a 2.2 4-pot. Was the LH also used in Jags?
In short, with a 2nd gear at around 2.2:1 (so around 500lb/ft effective in fag packet maths land), am I likely to shorten its life slightly, significantly, or just kill it first time it gets driven in anger?
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I'm not sure on overdrive units but if you're considering modifications have you considered a modern 5 or 6 speed box with an adaptor plate?
RX8's have 5 and 6 speed boxes that are cheap and there's BMW 330D boxes that are capable of taking plenty of torque too.
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1966 Ford Cortina GT 2018 Ford Fiesta ST
Full time engineer, part time waffler on Youtube - see Jim_Builds
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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Officially you are supposed to retain the original gearbox...and despite prescribe I still can’t change gear quicker than an overdrive (plus this would give us a 7 speed gearbox).
An RX8 box is also in stock, but so is the overdrive box and we know that fits 100%.
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Overdrives..peteh1969
@peteh1969
Club Retro Rides Member 107
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Having owned a car with overdrive I can say they can transform a classic car and how it drives into something that give better than a modern car can ever give regardless of engine or g/box it has fitted.
I would love to get a second 3 rail box for the Corsair and an over drive from a Transit and get them married up it would be so nice to drive.
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Last Edit: Aug 6, 2019 15:24:09 GMT by peteh1969
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Hi, I think it's a combination of low pump pressure, because it's driven by lower input shaft speed, and the higher torque multiplication of the lower gears overcoming the internal clutch.
Colin
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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Having owned a car with overdrive I can say they can transform a classic car and how it drives into something that give better than a modern car can ever give regardless of engine or g/box it has fitted. I would love to ge a second 3 rail box for the Corsair and an over drive from a Transit and get them married up it would be so nice to drive. Here’s one that should go in the “I never knew that” thread - Reliant cast the adaptors that go between the main casing and the overdrive for Ford for the Transits (with the side-change box as also used on the 2.0 Capri); a sort of reciprocal arrangement given they were having the engine and boxes from them.
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Overdrives..peteh1969
@peteh1969
Club Retro Rides Member 107
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The side change is the problem there Phil otherwise it would be a semi bolt on job.
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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That’s the later box - the one with the LH is the Zodiac box with the two external linkages to go round the O/D.
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scimjim
Club Retro Rides Member
Posts: 1,503
Club RR Member Number: 8
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Overdrives..scimjim
@scimjim
Club Retro Rides Member 8
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That’s the later box - the one with the LH is the Zodiac box with the two external linkages to go round the O/D. Which I imagine would bolt straight up to a V4. Available with cable or hydraulic clutches, they’re just one of many versions of the type 5 box with an LH on the back.
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I've learned a lot about this, I've never thought about overdrive as I've never had it.
I'll he on Ebay if you need me...
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1966 Ford Cortina GT 2018 Ford Fiesta ST
Full time engineer, part time waffler on Youtube - see Jim_Builds
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1966 Ford Cortina GT 2018 Ford Fiesta ST
Full time engineer, part time waffler on Youtube - see Jim_Builds
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scimjim
Club Retro Rides Member
Posts: 1,503
Club RR Member Number: 8
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Overdrives..scimjim
@scimjim
Club Retro Rides Member 8
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We have plenty of overdrives thanks - just looking at modifying one to be a seven speed with electronic engage/auto disengage and wondering if it’ll take the torque of a 200bhp Essex.
The event specific regs require the original gearbox.
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Last Edit: Aug 6, 2019 19:00:53 GMT by scimjim
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Overdrives..Deleted
@Deleted
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I think the weakest link would be gearbox to overdrive,unless it’s splined...
Years ago I made a controller for a Mitsubishi Tredia turbo. 4g63 engine. 280 hp. That fwd box was basically 4 speed with vacuum overdrive fifth gear. Ran that as a splitter box, 8 speed. Never broke the overdrive. Broke everything else 😂😂
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Last Edit: Aug 6, 2019 19:15:54 GMT by Deleted
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scimjim
Club Retro Rides Member
Posts: 1,503
Club RR Member Number: 8
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Overdrives..scimjim
@scimjim
Club Retro Rides Member 8
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Definitely splined. Here’s a description of the later J type - the LH is very similar.
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Overdrives..Deleted
@Deleted
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Pump pressure at lower speeds might be problematic. No reason you couldn’t run an engine driven pump with a regulator and bypass the internal pump.
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dreadspeed
Part of things
1972 triumph 2.5 pi on carbs
Posts: 66
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The strongest overdrive box fitted to any triumph was the A type overdrive fitted to the 6 cylinder fuel injected triumph engines ie Tr5 Tr6, Mk1 and Mk2 2.5Pi saloons, the Jtype is a weaker overdrive unit not used in the triumph scene for raceing or uprated engines, ive added overdrive to second gear in my triumph saloon by wireing in an extra relay no problems, but it is 160ftb torque at 4000rpm in top gear, i don't no what Lh overdrive means all ive heard of is Atype Jtype or Dtype fitted to triumph. also correct me if I'm wrong but the scimitar gear box is a very heavy cast iron unit where as most others were alloy
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Overdrives..Deleted
@Deleted
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So the way that this thread came about was that I noted that samta22 had a MGB gearbox with a Laycock GKN LH overdrive on it - Sam was cleaning it up ready to sell it on and I put my name on it - now I don't have a MGB but I do have several of these to restore over the next few years Jowett Jupiter's great little cars but the 1500cc boxer engine is rattling it's beans at 65MPH - fit a overdrive conversion and it results in much calmer cruising speeds - having determined that the LH OD is the most suitable & durable unit - Sam then found another for me - I don't need the gearboxes but I do need the OD units This is the Jowett gearbox This is the Jowett gearbox with the tailshaft removed and a owner / engineer fabricated gearbox to OD unit adaptor housing on a lathe bed in order to test it I need the excellent skills of @johnnybravo to cut the keyway to the mainshaft on the gearbox And manufacture / machine the adaptor housings So back to the thread once I have more info / progress
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Last Edit: Aug 6, 2019 20:24:01 GMT by Deleted
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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We have plenty of overdrives thanks - just looking at modifying one to be a seven speed with electronic engage/auto disengage and wondering if it’ll take the torque of a 200bhp Essex. The event specific regs require the original gearbox. As Jim says - basically run it as: 1st Clutch change to get: 2nd - the gearbox we have has a small ratio gap between 1st and second, but a big ratio gap between 2nd and 3rd, hence: Flick a momentary switch to engage: 2nd overdrive Clutch change to get: 3rd Flick a momentary switch to engage: 3rd overdrive Clutch change to get: 4th Flick a momentary switch to engage: 4th overdrive All this makes a 7 speed close ratio gearbox from mid 60’s standard components as long as the OD holds out.. There’s a second switch to disengage the overdrive at will too. The logic wasn’t too bad to sort out - just as well given it’s one of my day jobs!
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Overdrives..Phil H
@philhoward
Club Retro Rides Member 133
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Info found from googling (no hard references as yet): www.mgb-stuff.org.uk/gearstext.htmInteresting bits copied here for discussion: Overdrive: D and LH Type Differences
Fault diagnosis There are both ratio and electrical differences between the early and later ODs fitted to the MGB. The earlier D type has an OD 3rd ratio that is closer to 4th than the LH type - 73% as opposed to 65%. Thus the LH type has closer to a '3rd and a half' ratio than the D type, but both offer a useful mid-way point between 3rd and 4th for spirited twisty bits or a long steepish climb. The LH also has a higher OD 4th ratio that the engine is well capable of pulling, giving even more relaxed touring.
The D type is not as strong as the LH and has vacuum switch and relay to prevent the driver from disengaging OD under certain conditions - high revs and no throttle - which could overstress the unit.
LH Solenoid Cover
Geoff Dunlop wrote to me from Australia asking if I could shed any light on a green label cover on his factory V8. They should be black on all chrome bumper 4-cylinder cars, and blue on 4-cylinder rubber bumper, the colour change representing the change in speedo drive gear ratio. V8s always had the same OD, regardless of chrome bumper or rubber. Chris Betson of Octarine Services seems to recall that the V8 originally had a red label (mine is black), and the MGC a green, these colour differences representing the higher operating pressures, as well as perhaps a different speedo drive gear ratio on the MGC. However Overdrive Repair Services say that if Laycocks ran out of the usual colour they used what they had to hand, but stamped it with the correct number!(Scimitar cover is red from memory)
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Last Edit: Aug 6, 2019 20:43:47 GMT by Phil H
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Overdrives..peteh1969
@peteh1969
Club Retro Rides Member 107
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The big problem is I would need a new tail shaft and housing as the gear shift is on the tail shaft and there is not enough room for the side selector box and I would be wanting to do the box change without cutting the floor. I would only want the overdrive in 3rd and 4th as any of the lower gears would put too much load on the over drive which is why over drive only engage in 3rd & 4th gears.
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Last Edit: Aug 6, 2019 20:45:57 GMT by peteh1969
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