Crown wheel and pinion ratios plus LSD questions. SD1 axle

paulmanta · Post by **paulmanta** » Sat Jan 12, 2008 6:50 pm

I have a Rover V8 (3.6, offy 360, edelbrock500) and an LT77 manual box.

I have removed the diff casing out of an SD1 axle by cutting and removing the axle tubes and have fitted the axle tubes from an Opel Manta so i have the correct spring hangers and halfshafts to allow me to run manta wheels.

The crown wheel and pinion i have worked out as a 3.9 ratio.

I want an LSD for this and also to know if other ratios are available for this diff, or indeed if i need a different ratio, i am wanting good cruising speed without it revving its nuts off, acceleration would be nice but i know its a trade off.

what are others with similar set ups running ? I will not be doing 1/4 miles but may do a bit of distance to shows and the like.

Paul V8 · Post by **Paul V8** » Sat Jan 12, 2008 7:07 pm

I have a triumph tr7 which uses basiclly the same axle, my standard ratio was 3.9 which made 1st gear pointless. I now have a 3.08 ratio from a SD1 vittesse. I'm doing 80mph at 3000rpm ish. seems to work well. Depends what size wheels you have. I know other ratios are avalible from Rimmer Brothers, 3.45 or 2.84. depends what you want to do.

paulmanta · Post by **paulmanta** » Sat Jan 12, 2008 11:17 pm

To Be honest a mate has offered me a 2.84 and it is seeming like a very attractive offer, i dont want balls out acceleration from standstill but a good lick of speed for lower revs when actually moving would be nice.

Post by **katanaman** » Sat Jan 12, 2008 11:31 pm

paulmanta wrote:To Be honest a mate has offered me a 2.84 and it is seeming like a very attractive offer, i dont want balls out acceleration from standstill but a good lick of speed for lower revs when actually moving would be nice.

I would do some calculations before using a 2.84. Its meant for a 3 speed auto and yes it will drop the revs but if you don't have the torque to back it up it could make for a sluggish drive. It could be ideal for you as well, just make sure before you go through all the work.

CastleMGBV8 · Post by **CastleMGBV8** » Sun Jan 13, 2008 11:07 am

Paul

If you tell me what size tyres including width, aspect ration, and diameter you are going to use, then I can calculate the gearing for any final drive ratio using an Excel program.

A 2.84 is really too high unless you are using smallish wheels.

Kevin.

paulmanta · Post by **paulmanta** » Sun Jan 13, 2008 1:36 pm

The wheels fitted to the Manta are Borbet A. they are 16inch with 215/40/16 tyres.

the engine is a 3.6 Rover V8 with an unknown LT77 5 speed manual box.

At the moment the axle is standard manta but the SD1/manta hybrid is almost ready to go in.

I will look at the box tomorrow and get the numbers off the oval machined section to tell us what the box was in originally.

CastleMGBV8 · Post by **CastleMGBV8** » Sun Jan 13, 2008 10:47 pm

TIRE SIZE - 215 40 16
TIRE DIAMETER - 22.77 NUMBERS IN BLUE ARE INPUT PARAMETERS,
TIRE WIDTH - 8.46 OTHER NUMBERS ARE FIXED OR CALCULATED,
REAR END RATIO 2.84 AND CANNOT BE CHANGED

GEAR GEAR RATIO RPM/10MPH RPM AT MPH
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
1 3.32 1392 1392 2784 4175 5567 6959 8351 9743
2 2.09 876 876 1752 2628 3505 4381 5257 6133 7009 7885 8762 9638
3 1.40 587 587 1174 1761 2348 2935 3521 4108 4695 5282 5869 6456 7043 7630 8217 8804
4 1.00 419 419 838 1258 1677 2096 2515 2935 3354 3773 4192 4611 5031 5450 5869 6288
5 0.82 344 344 688 1031 1375 1719 2063 2406 2750 3094 3438 3781 4125 4469 4813 5156

GEAR GEAR RATIO MPH/1000RPM MPH AT RPM
1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500
1 3.32 7.18 11 14 18 22 25 29 32 36 40 43 47 50 54 57 61
2 2.09 11.41 17 23 29 34 40 46 51 57 63 68 74 80 86 91 97
3 1.40 17.04 26 34 43 51 60 68 77 85 94 102 111 119 128 136 145
4 1.00 23.85 36 48 60 72 83 95 107 119 131 143 155 167 179 191 203
5 0.82 29.09 44 58 73 87 102 116 131 145 160 175 189 204 218 233 247

Not sure whether this will work.

Kevin

CastleMGBV8 · Post by **CastleMGBV8** » Sun Jan 13, 2008 11:07 pm

Obviously not, I'll have to do it the hard way a bit later.

Kevin.

CastleMGBV8 · Post by **CastleMGBV8** » Sun Jan 13, 2008 11:31 pm

Right the simple way.

MPH per 1000RPM with your wheel and tyre combination. 2.84/1 diff.

1st = 7.18
2nd = 11.41
3rd = 17.05
4th = 23.05
5th = 29.09 @ 0.82 if later box 0.79

To calculate max speed in gears just multiply by 5.5 if 5500RPM is red line.
With you low profile tyres it appears that the 2.84/1 ratio will work.

Kevin

5000SE · Post by **5000SE** » Sun Jan 13, 2008 11:43 pm

With that wheel and tyre combination the 2.84 will give the Manta almost identical overall gearing to a 3.08 diff on a Vitesse or other manual SD1 with standard wheels - not bad at all unless you're wanting to drag race.
I think it's a very good ratio for the road - good long motorway legs but you'll still have to take care not to smoke the tyres in first.

minorv8 · Post by **minorv8** » Mon Jan 14, 2008 6:47 am

I have a 3,08:1 ratio in my Moggy and it is not that bad on 1/4 mile. Being a relatively light car (just under 1000 kg) there are some traction issues to deal with but it pulls quite nice (PB ET13,039 s). OK, 5the gear is not needed, I don´t even reach the rev limiter on 4th gear. Fitting a 3,45 or 3,9 ratio (which I also have) would ruin the car, even less traction and a total disaster on highway. I have 185/60x14 tyres.

Provided you have a lot of grunt you should be OK with 2,84 ratio. Manta is not that heavy. But if you intend to drive a lot then it is 2,84 or 3,08.

paulmanta · Post by **paulmanta** » Mon Jan 14, 2008 7:24 pm

Thanks you very much for all of that, You are stars !!

That has given me all the info i could ever want and has sorted a lot out in my head.

By the way my gearbox says 19A on it.

cheers,
Paul