Talk Morgan Forums Morgan Sports Cars Morgan Plus Four, Supersport & Plus Six (CX & CXV Generation Vehicles) Plus 4 CX - Initial Thoughts

Hello Mogsters,

I have exchanged my R100 for a manual 2022 Plus 4 CX from Williams. It is in Sport Green (which looks identical to their old Connaught Green) and it has tan upholstery, just like my last three Morgans. It had 2500 miles when I bought it a couple of months ago. The wheels were painted and the grill was black, so I asked Williams to change the wheels and grill to SS, just like my last three cars. They also added a luggage rack, Nitron dampers and a Le Mans filler cap. I know – I am in something of a rut here.

There have been a few surprises after it got it, though, despite a test drive and pretty exhaustive research, and I thought that I would list them here as well as a couple of questions… but fast forward if you allergic to technical stuff.

1. I did not appreciate how sophisticated the engine is. It was obvious that it was very efficient from the posted mpg figures, but… BMW has a training web site, and you can get access for £30/day. So I signed on and downloaded two manuals – one on the B48 (D, I think), and the second on the B46/48 TU. The first is very comprehensive and covers the theory of many of the subsystems, but is earlier than the closed deck design of our engines. The second manual specifies the B46TU as the 255 BHP version, with the B48 as the 302 BHP version, but other sources are less clear. Otherwise the engines are identical and have the new lighter crankshaft with bigger journals, VANOS (variable cam timing) on both cams, Valvletronic (variable lift) on the inlet cam, two complex ECU controlled cooling systems (the result of the closed deck design), ECU controlled oil pressure and a remarkable 350 bar (5075 psi) fuel injection pressure. It also has IROX coating on the upper conrod bearing shells, and to quote BMW “The polyamide-imide ensures, in combination with the hard particles, that the bearing shell surface is so hard that material abrasion is no longer possible.” Wow.

2. BMW calls the turbocharger “twin power”, which actually means that it has two turbines and compressors on a single shaft. Each turbine is connected to a pair of cylinders – 1 and 4, 2 and 3 - to provide even pulse intervals. The twin compressors are of the scroll design, in which the compressors do not rotate but move in an eccentric motion that compresses the air in a scroll – like a circular labyrinth. This means that the only rotating parts are the small turbines and the common shaft, which must be responsible for the really fast spool-up times. The electrical controlled waste gate dumps the compressed air into the exhaust, and I presume that this I what you can here woofing when you blip the throttle when you change down, or take you foot off after a burst of acceleration.

3. The Nitron shocks are part of the dynamic handling package, which on the later cars comes with a rear anti-roll bar. If you want this fitted to a 2022 the rear sub-frame has to be dropped to drill holes for the ARB mounting. You are probably aware that ARBs increase spring rate in roll, without affecting the two-wheel bump rate. They also affect the lateral-direction static stability of the car – a front ARB increases directional stability while a rear ARB reduces it. So the rear one would make the car more agile in the twisty sections, but I did not think that this was a worthwhile trade for the reduction in straight line stability considering the cost involved in fitting it, but chacon a son gout. I asked them to set the dampers to the recommended road setting of 11 clicks out from full hard. The recommendation for the track is 7 clicks out, but I have not experimented with the settings or the ride height. Yet. But I would be interested if anyone else has done so or tried different tire pressures.

4. The ride with the Nitrons is firm, well damped, and not at all high-frequency-rattley, which the trads can experience. The bump damping (tested over speed bumps) is about 0.6 zeta which is firm – as a rule of thumb the damping ratio can be calculated by subtracting the number of overshoots from 7/10. Pretty nice, though I think that SSL likes them a little softer (around 0.5 zeta, I think) for the trade. Perhaps the stiffer CX chassis can take a little more suspension damping.

5. I also ran some directional stability tests using wheel raps on a deserted road at 60 mph. The directional frequency response seems around 0.8 Hz, but it might be faster since the power steering affects the measurement. The damping ratio is about 0.5 zeta with the Avon tires, so both the frequency and damping are pretty much ideal, I think. It would be interesting to talk to the chassis designers, but I am not holding my breath. It would also be interesting to hear what Peter Ballard of SSL might contribute… I copied this to him, but he cannot post a comment under TM commercial rules. Nice to hear from him again, though. You are also probably aware that you can change the direction stability with tire pressure delta (front/rear – softer fronts mean more stability) and the roll contribution to turn in stability with front/rear damper stiffness settings.

6. I played around with S+ mode, and now use it all the time. The car feels a little sharper, which I like, and there does not seem to be a downside.

So in summary I think that it is a great car. Probably one of the best sports cars I have driven, and a lot more friendly for passengers than my Caterham CSR, which was from a different universe. The six speed box is very nice – almost as good as the CSR which sets the standard. The Bluetooth system is excellent and useful up to motorway speeds, although I plan to add a left/right and front/rear balance. I would like a permanent nav mounting, and have a plan as I do for more storage space. I bought a scissor jack from Germany, and after I dumped the extra spanners there was room in the bag for the lead hammer.

Leaving for Spain on the ferry and a drive down the West Coast in a week or two.

Regards,
David Poole

Interesting info David,

Did you test an Auto before settling on the Manual ? I did a back to back test at Donington a couple of years back and didn't like the gear spacing on the manual

Many thanks David for insight. Regarding the TwinPower turbo, I didn't realise it has twin compressors as well on the common shaft as haven't seen it mentioned elsewhere.

Very useful information, thank you David for troubling to post it up. I had no idea you could do online tutorials with BMW but thanks to you doing so find myself rather reassured that we’re driving something quite technically advanced.

Like you I drive a ‘22 manual Plus Four (no auto for me thanks) and am also wondering about the Nitron upgrade - and perhaps for even more fun the BMW LSD that Morgan can retrofit. Any thoughts on that?

Hi David,
thank you very much for your contribution. This is very interesting information. Do you have a link to the BMW training website?

Thanks for the interesting and educational info, David. Half of it went over my head, but that means I now know what to seek out in gaining knowledge!

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.

This explains the twin scroll turbo clelrly.


Basically 2 feeds to one impeller. This is the same idea Lotus use on their AMG engine.
The full article is here

Hello John,

Thanks. Nice to see the inside…but I have not heard the turbines being called a scroll before. Live and learn…where did the image come from?

The article in the link has the diagram. The scroll is the chamber the gases go thorugh see picture below. So it isn't like a Rootes blower it is effectively a reversed centrifugal pump with two inlets.

Interestingly the Lotus one had 2 nozzles rather than what look like slots.