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

Very interesting reading Derek, but.... im not sure the torque multiplication of the converter would be beneficial enough to compensate for the slow start you get with an auto to beat the 2.5 off the line, with a manual box coupled to the ES9J4 Engine i think (although not on paper) the V6 would be off the line faster than the derv and would then become uncatchable.

D

Hi Derek

The 2.5 box is a MG5TB not the later ML5T. Cannot lay my hands on a book to check that the ratios are correct for a TD. My experience is that there are times particularly when overtaking to avoid the gear change penalty I keep a 2.5 in third until the upper 70's. You might also try seeing how a 1st/3rd/5th only plot works out. Each generation of Citroen manual boxes seems to have been produced in a range of options each with its own set of gear ratios. Early Mk2 V6 manual's got the MG5T box while the ML5T was the manual box for later Mk2 V6's with the ES9J4/L3 engine. Sad that the UK market did not justify Citroen making any RHD manual V6 Mk2 XM's.
There are also a fair number of 2.5's with non standard ECU's and one of them has a real edge over a standard one. So a 2.5 plot with an added extra 20 hp might be interesting too.

John

What do you mean by "non standard" ecu's John? Not factory fitted? Or were Citroen experimenting with them?

Hi John, scanned this from the Revue Technique and citroen spec manual.

Hi Demag

In talking to one of the local indys it seems that a few of the 2.5's he had in for routine maintenance were chipped for the owners by specialists. Other than the improved torque it does not seem to have had an adverse effect on maintenance or life of the engines. Chipped on purchase by second owner at 80k, still running well on original clutch at 210k was an example. Typically when driving it 1800 rpm gave the same speeds in each gear that you would expect from a standard 2.5 at 2000 rpm.

John

Thanks chaps, for your input.

Hi Dean, in the same way that you have to slip the clutch to get off the line with a manual box, when you hit the loud pedal with the 4HP20 the revs immediately rise. This means torque converter slip and that in turn means torque multiplication.

Hi John, a slip of the pen with gearbox type. I took the info from Section 5 , 330-00/3 page 5 item 4 of the manual. According to that it's type MG5T. The ratios are correct as shown and I've altered the error. I've also corrected the torque outputs to coincide with the heading - NM, the figures I'd used were dNm!

I think we should stay with the standard engines, otherwise someone might suggest a rechipped V6, or even - horror of horrors - a turbo'd one! There is such a beast in a different car.

Derek

I've corrected another elementary error on my original posting, there are of course three gearchanges required on the 2.5, the last one at 66mph. That's a 2.75 second disadvantage and will probably decide the issue.

Watch this space.

Derek

I'm sure I change gear in significantly less than a second when I'm in a rush. Not when I am not in a rush though. But I would expect the V6 to have the edge.

I've been driving one of citroens EGS gearboxed cars (the electronic gearchange) which has quoted change times of 1.2sec in auto mode, 0.8sec in manual mode (still electronic but manual intervention with the flappy paddle).

It feels SOOOOO slow at changing gear compared a normal manual box, so by comparison i would estimate a std manual box gearchange taking 0.5sec and probably quicker if actually racing away. Certainly a 1 second change is pessimistic. Even a sluggish gearchange on a maual box is quicker than anything the EGS can do.

Its certainly an interesting calculation you have carried out though.

I have tried to be as neutral as possible in my investigation. As we no longer have a manual car in our household I judged the gearchange time by sitting at my computer and pressing an imaginary clutch pedal, moving an imaginary gearlever and releasing the pedal. One second seemed pretty quick. All my later manual cars had baulk ring synchromesh which prevented gear engagement from taking place until the engine revs had dropped to engagement speed, does the MG5T gearbox not have this? Because if it does, you have to factor in the time taken for a lumpy engine and its massive(ish) flywheel to slow down.

A quick experiment at a full-throttle start from standstill in my wife's ZX (don't laugh!) showed a jump to 2000rpm and the tacho staying ahead of the speed at a reducing rate up to the gearchange, showing that torque multiplication was taking place. If I am successful in producing these graphs I intend to factor this in as maximum multiplication (2.45x) at 2000rpm reducing to zero at the gearchange point. Second gear also allows torque multiplication but at a reduced rate - it's all there in the Citroen gearbox manual but takes some understanding!

I will then reduce all figures by 10% to account for losses, but I will also reduce the 2.5TD figures by 5% for losses in the manual box - I was told many years ago that each pair of meshing gears loses 5% of the input.

It is interesting that the auto changes at full throttle take place well below the maximum power point, and this is with a nearly flat torque curve. So this is a real performance killer. Of course you (I) can do manual selections and hang on to the gears up to peak revs, but the object is to compare standard performance.

Dieselman, I had intended to include the 2.1TD and the 2.0TCT but I haven't been able to find any info on the 2.1 power and torque figures, nor for the early manual box nor the 4HP18 autobox. Presumably they were included in the S! manual which I don't have. A Google search has been fruitless as well.

It is gratifying that so many are finding this interesting. Why don't some of you mathematically minded members come and join me?

Derek

Hi Derek

It's not the sort of info that you can easily get anymore but I think that the 2.5TD 0-62 was around 10.5secs and although I haven't seen a published figure for ages the 24v was I think around 8.5secs. That sort of seems to indicate that the 24v would be quite a bit quicker to 70mph. Although I never thrash my 24v I was behind the Missus (she hadn't noticed me) at a local set of traffic lights when she got cut up by a Saxo. The lads in the Saxo were revving the hell out of it but the 24v left them for dead off the line!

Subjectively I find the XM as poky in gear as my 2.2td Accord which has 350nm torque (although to be chipped to 410nm tomorrow!)

Cheers

Tony

Don't want to hijack you post with another way off topic discussion but i have been looking for accurate performance figures for the xm for a while but all i can find is waffle like this,

http://carinfo.autold.com/car-citroen.html

looking forward to the results Derek.

D