Hello Chaps,
I thought that in this second instalment I would cover an erratum as well as answering the questions posed by the first posting. So here goes…
1. In the description of the engine, I forgot to mention one other advanced feature – and this is the arrangement of the balance shafts. I am sure that you are aware that an in-line four, unlike an in-line six (which has perfect primary and secondary balance) the four has a second-order vertical imbalance. One balance shaft will shift this to a lateral imbalance, but two contra-rotating shafts will cancel all imbalance forces. And that is what the B46/8 has – and the result is a very smooth engine indeed. The only forces that are left are the firing impulses, ‘which you can feel in the gear lever at low w rpm. These shafts are pretty common nowadays, but the B46/8 seems especially competent.
2. The question by Jon M. about whether I tried an auto before buying my manual car is interesting in a “I should have done that” sort of way. But no, I did not. I have a friend in the USA wha has recently bought an auto Z4 with the 2 litre engine, and he likes it a lot – and he habitually drives it in the Sport mode and manually shifts using the paddles. Perhaps I did not want to be convinced since I enjoy the experience of changing gear. And there is the question of throttle response lag, too. I realize that it can be minimised, and that it is probably not a factor in general driving, but a torque converter relies on the difference between the input and output shaft rpm to generate torque. So, in any steady-state condition, it you want to increase output torque (speed, acceleration), the engine must first spool-up in rpm to increase the output. The larger the delta torque demand, the greater the spool-up time and lag, obviously. The BMW auto box has the reputation of fast shifts and seems to be an excellent pice of engineering, and appears otherwise outstanding. The 0-60 times are not affected, of course, since they are conducted at WOT. The manual box has very wide ratios, and this would be a downside if the engine did not have such an extraordinary flat torque curve. It is interesting to compare the RPM/MPH figures of the two boxes, and I posted them in the Morgan Photos forum – I write this on an iPad and the right click function does not show the image links, so I cannot include them in this thread. As for the high gearing, BMW had a different set of requirements to a British sports car (mainly based on survival on the autobahns, emissions and economy I would imagine) when they were designing the transmissions and fifth and six in the manual box will certainly give relaxed high-speed cruising. It would be interesting to know which gear Morgan used when they achieved the 149 mph top speed – my guess is probably fourth.
3. Richard mentions the lack of data about twin compressors on a common shaft, and here I must confess that I have not yet seen a cutaway of the turbo design. In the training manual the key technical features of the engine are listed as:
Map-controlled oil pump
Electric arc wire-sprayed cylinder barrels
Twin-scroll exhaust turbocharger
Electrically adjustable wastegate valve
Direct rail
Double VANOS
Valvetronic
So I assumed that there were two, since the “scroll” word can only be applied to compressors. And I would imagine that the inertia of two small compressors is less than that of one large one. But if anyone has any more information I would be very interested. I should also say that it is not the compressed intake air which is dumped when the waste gate opens, but the exhaust which is routed around the turbines.
4. Marcher mentioned the LSD as an option. My “before and after” experience is limited to my 2002 Plus 8 to which I fitted a 4.5 litre Stage III engine that made around 285 BHP. It originally had the Dana LSD, but I did not like it – at slow speed it made cornering a bit “notchey” as it engaged. When one side tube of the axle cracked I rebuilt it with a lower (numerical) ratio and a Quaife ATB, which was very smooth, did not clonk, and was much nicer. This car, however, had a live rear axle and the torque effect of the engine always caused the right wheel to spin, so LSDs are very important with this design, which is why they were so common in the USA. I am not sure of the scenario when one would be of great advantage on a road car with an independent rear end except in the ice or snow. Racing would be a different matter, though, where you could get much greater weight transfer during cornering, and my fully independent Caterham CSR had one. A side effect would be that if you were to spin the wheels when you take off then both would spin, and there would be a dramatic decrease in directional stability so you would need to be pretty sharp to keep it pointing straight. However, if you do decide to get one fitted please let us know the results.
5. I have also been thinking about the Nitron damper settings – these thoughts were triggered this morning when I had to drive over a zillion comically configured speed bumps, some of which had a flat top. The Plus 4 did not care, and clambered over them like a mountain goat. But the damping ratio was clearly 0.6 or perhaps even a little higher – when you reach 0.7 it is effectively dead beat, and there is no overshoot. But it felt very competent. SSL recommended around 0.5 for the trads when they supplied Spax for my R100. This damping ratio gives closer to two overshoots. So I think that it may be possible to soften the Nitrons a little – it would be interesting to try a few more clicks out from full hard.
If you have been following the plot here, you will probably have figured out that if you soften the front dampers more than the rears, the car will be slightly more aggressive during dynamic turn in, as the damper-induced weight transfer due to the change in roll angle affects the tyre slip angles. The stiffer dampers at the rear will cause greater instantaneous weight transfer to the outer tyre and thus a larger slip angle and a reduction in dynamic directional stability. Steady-state characteristics will not be affected, of course.
On a final note to the moderators, perhaps you might consider starting a forum for “Commercial and Experts”. This would separate any marketing implications but allow access to the many experts, who, by their very need to keep employed, are otherwise excluded.
