There was a discussion a short while back on the relative performances of the 2.5TD and the ES9J4 V6. As it is highly unlikely that we will ever get one of each together on an empty dual carriageway, the chances of actually carrying out a test are probably nil.

So I decided to try to do a calculated comparison. The weights, frontal areas, drag coefficients and tyre rolling circumferences of the two cars are close enough to be considered identical. So we can make direct comparisons of the torque available at the driveshafts.

I hope it is accepted that gearing has two effects, to change the torque and the road speed at which it's delivered; both in direct proportion to the gear ratio. Similarly, that overall gearing is found by multiplyig the gearbox ratio by the final drive ratio.

I intended (intend?) to produce a spreadsheet of torque in each gear against road speed, then use the graph plotting function to produce comparison graphs. I was going to plot the "torque required" curve on the same graph so that the surplus torque would give a reading for acceleration, but as the torque required will be the same for each car it can be ignored.

Torque output at the crankshaft for each car, in Nm, is:

RPM . . . . . .1500 . . . 2000 . . . 2500 . . . 3000 . . . 3500 . . . 4000 . . . 4300 . . . 5000 . . . 5500 . . . 6000

Torque

2.5TD . . . . .230 . . . . 285 . . . .267.5 . . . .250 . . . . 230 . . . . 215 . . . . 200

V6 . . . . . . . 235 . . . . 235 . . . . 250 . . . . .250. . . . .255 . . . . 270 . . . . 265 . . . .255 . . . . 250 . . . . .230

Sorry about all the dots but it's the only way to get spacing.

Gearbox ratios. The main source for these is the Series 2 workshop manual.

There is an error in the published figures for the 4HP20 autobox; the figures for the gear ratios and final drive ratio are correct, but the author has missed the 59x68 step up gear between the main gear cluster and the final drive. The figures below are corrected to include that. The effect of the stepup gear is to reduce the torque multiplication and increase the mph/1000rpm figures. The ratios are calculated as gear ratio x stepup ratio x final drive ratio and are shown as the torque multiplication factor:

Gearchange speeds are taken at max revs for the ML5 box and the full throttle change speeds for the 4HP20 autobox.

MG5T box (2.5TD)(Does not have a step up gear)

1st gear torque multiplication factor = 13.88; mph/1000rpm = 5.29; change speed 1st-2nd = 23mph

2nd gear . . . . . . . . . . . . . . . . . . . . . = 7.57; . . . . . . . . . . . . .= 9.71; change speed 2nd-3rd = 41mph

3rd gear . . . . . . . . . . . . . . . . . . . . . = 4.78; . . . . . . . . . . . . . = 15.37; change speed 3rd-4th = 66mph

4th gear . . . . . . . . . . . . . . . . . . . . . = 3.5; . . . . . . . . . . . . . . = 20.98; change speed 4th-5th = 90mph

5th gear . . . . . . . . . . . . . . . . . . . . . = 2.6; . . . . . . . . . . . . . . = 28.25; max speed is 121mph

4HP20 autobox (V6)

1st gear torque multiplication factor = 10.81; mph/1000rpm = 7.24; change speed 1st-2nd = 35mph

2nd gear . . . . . . . . . . . . . . . . . . . . . = 5.89; . . . . . . . . . . . . = 13.2; change speed 2nd-3rd = 73mph

3rd gear . . . . . . . . . . . . . . . . . . . . . .= 3.98; . . . . . . . . . . . . = 19.67;change speed 3rd-4th = 107mph

4th gear . . . . . . . . . . . . . . . . . . . . . .= 2.9;. . . . . . . . . . . . . = 27.33; max speed is 140+mph

The 2.5 top gear mph/1000rpm is lower that I expected.

Looking at the figures and taking acceleration up to the legal limit of 70mph, the two cars (much to my surprise) seem to be closely matched with the 2.5 possiblly holdng the advantage. But the V6 does hold two hidden aces up its sleeve. The first and major one is that the torque converter is not just a fluid flywheel, it increases torque, particularly in first gear where you can more than double the torque at low rpm decreasing to zero increase at high rpm. The second advantage is that there is only one seamless gearchange on the autobox (time penalty not more than 0.25s) whereas you have three gearchanges in the 2.5 and I don't think anyone can declutch, move the gearlever and re-engage the clutch in less than a second, so that's a 2.75 second disadvantage in some 12-13 seconds.

If there's anyone still awake and interested, would you care to comment?

Derek