English Diff Ratio's

[Adrian Southgate]

So I've got to a point where I need to make a decision on the diff but on checking them today I seem to have a bewildering range of ratio's. I assume that the numbers stamped into the crown wheel are part numbers and that a segment is the number of teeth on the pinion and another the crown wheel.

This being the case I have the following... 

That one is definitely 9-35 and I have a second one to match which needs new planet gears... Both have 69 part numbers so made in 1969?

That one is 11-39 and has a 71 part number and a large flange...

This is the one I have just taken out of the axle, looks much newer than the others but its a 10-39 and has a part number with no conspicuous date.

Now working from these numbers I would hazard a guess that the 10-39 is actually a 3.9:1 (3.89:1?)

That makes the 9-35 a 3.88:1 and the 11-39 a 3.54:1.

Bearing in mind i will be using a type 3 box and a 1630 xflow should I fit the 3.54:1?

[James Doulton]

Patrick, I think the 180km/h was in 4th gear. 5th would be more like 220km/hr. It would get to its top speed in 4th and 4th would be the best gear for motorway driving. 5th might be okay at 100km/h on a level road but it would be struggling below about 70km/h and, with the top speed in 5th being less than 150km/h (4000rpm), then it will never feel like it has got into its power band.

[Patrick Buchfink]

Just looked it up again. It is about 220 km/hr @ 6000RPM in 5th gear, you're right. That is over the top.

Just did a comparison of 4 speed and 5 speed with 3.54 and 205/60/13. Shifting up is set to 6000RPM in the tables. Speed is given in km/hr.

What about reducing engine speed with the 5th gear compared to the 4th gear? Somewhat about 500 RPM on 100 km/hr. Is this remarkable on the crossflow?

RPM	1000	2000	3000	4000	4500	5000	5250	5500	5750	6000	Loss on shifting gear up
1.gear	8.1	16.2	24.2	32.3	36.4	40.4	42.4	44.4	46.5	48.5	2762 RPM
2.gear	15	29.9	44.9	59.9	67.4	74.9	78.6	82.3	86.1	89.8	1827 RPM
3.gear	21.5	43.1	64.6	86.1	96.9	107.6	113	118.4	123.8	129.2	1620 RPM
4.gear	29.5	59	88.5	118	132.7	147.5	154.8	162.2	169.6	176.9	1080 RPM
5.gear	36	71.9	107.9	143.9	161.8	179.8	188.8	197.8	206.8	215.8

RPM	1000	2000	3000	4000	4500	5000	5250	5500	5750	6000	Loss on shifting gear up
1.gear	8.2	16.5	24.7	33	37.1	41.2	43.2	45.3	47.4	49.4	2631 RPM
2.gear	14.7	29.3	44	58.7	66	73.4	77	80.7	84.4	88	1821 RPM
3.gear	21.1	42.1	63.2	84.3	94.8	105.3	110.6	115.9	121.1	126.4	1714 RPM
4.gear	29.5	59	88.5	118	132.7	147.5	154.8	162.2	169.6	176.9

Source

[James Doulton]

With 205/60x13's and a 3.89:1 diff, then 70mph is 4200 rpm and, assuming a 1600 GT spec engine (86bhp), there should be about 72bhp available from the engine, of which 25 is needed to maintain that speed. This is enough to allow it to climb a 13.6% gradient.

With a change to 3.54:1 diff, then the revs drop to 3800 rpm at which the engine can develop 67 bhp which is enough for a 12% incline. 6000 rpm in third would be 81 mph.

In acceleration off the line, the lower gearing of the 3.89 takes about 0.2 seconds off the 0-60 time (9.3 seconds) and 0.3 seconds off the standing 1/4 mile (17.3 seconds/80mph terminal). In the first 5 seconds then it would cover 55.5m as opposed to 53.5m, so half a body length.

I reckon that top speed is 100mph with the 3.89 if you limit it to 6000rpm but you could get to 107mph @5800rpm with the 3.54.

So not a lot of difference between the two, but 3800 rpm is better than 4200 rpm. And with a 4 speed, the 3.54 won't feel too high geared when you are in top.

[Big Vern]

James why is 3800rev.min better than 4200? surely since fuel economy would be the only reason for contemplating such 'tall' diff ratios then the optimum BSFC would be at peak torque since you would have the smallest throttle opening there for any given road speed. less throttle = less fuel used. Since 16GT/mex have peak torque 4000 - 4500 they would be the best for overall BSFC. Not sure on your overly optimistic 0-60 or 1/4 mile times lol

[James Doulton]

I was only thinking about the engine sound. It would feel like you wanted to change up all the time that you were cruising and, with plenty of response available, it might feel a bit 'on the edge'. My Melos had a 4 speed with a 3.89 and it seemed very low geared at 50mph (with a 2litre Pinto). It'd be fine on a track but tiresome on the motorway.

My Legerra has a 3.89 and a 5 speed box and that seems pretty good. 80mph is about 3900 rpm and that is where the engine feels really comfortable. I bought a 3.54 for it because that would reduce the wheelspin and improve the 0-60 time slightly (according to my spreadsheet) but it would ruin the cruising so I'm keeping that in the spares store for now.

[Adrian Southgate]

My Legerra seems happy at around 3k - 3.5k so that will stay as it is. 

4 out of 5 of my fellow Northern Duttoneers run four speeds so XSC will be able to keep the pace, CMV can leave all four of them with ease on the motorway, I just choose not to. Martin's Westie on the other hand.....

[Big Vern]

James you old duffer :-)))  why would you want to be changing up at such low engine speeds? maybe you should trade the Leggie for a Mundaino LOL 

[Adrian Southgate]

So, parameters set, decision made.

These are the figures I get from 'gearcalc' using the Capri 4 speed box. 

Gear Mph per 1000 RPM      Mph @5500 RPM      Mph @6000 RPM
--------------------------------------------------------
1                5.33                              29                            32
2                9.49                              52                            57
3               13.65                             75                            82
4                19.07                           105                          114

Mph RPM (in Gears)
----------------------------------------
        1       2       3       4
----------------------------------------
5     939    527    366     262
10  1877  1054    733     524
15  2816   1581  1099    787
20  3755   2108  1465   1049
25  4694   2635  1832    1311
30  5632   3162   2198   1573
35            3689   2564   1835
40            4216   2930   2098
45            4743   3297   2360
50            5270   3663   2622
55            5798   4029   2884
60                      4396   3147
65                      4762   3409
70                      5128   3671
75                      5495   3933
80                      5861   4195
85                                4458
90                                4720
95                                4982
100                               5244
105                               5506
110                               5769

Gear Change RPM drop (change @5500) RPM drop (change @6000)
-------------------------------------------------------------
1 -> 2 -2412 (to 3088) -2631 (to 3369)
2 -> 3 -1677 (to 3823) -1830 (to 4170)
3 -> 4 -1563 (to 3937) -1705 (to 4295)

Of course all of this is theoretical, as BV pointed out there is a certain amount of wind resistance to overcome. Going to be fun trying it out though.

Time to order some heavy duty gaskets then :)

ETA I have (somewhere in the garage) a microdynamics rev limiter which is set at 5850 RPM. Has anyone used one of these before?

[James Doulton]

What wheels and tyres are those figures for, Ade?

[Adrian Southgate]

205/60/13's

I put my old 205's on a pair of the MS Compomotives and I like the way they sit. I'll have to get some 205/60/13 R888's or something sticky for summer though.

I clean one up on Sunday and post pics...

[Steve Kerswell]

Got a 3.08 dolomite Sprint back end, just can't get the enthusiasm to fit it under the B+. I've had 118mph from the Sprint engine, timed by officials and recorded, bloody fast enough for me ! The old car was shaking a bit too :-)))

[Steve Kerswell]

Could be a goer Daryl, I know the hot rodders are now using freelander diffs and Sierra shafts.