Tuesday 29 September 2020

Part 2: Commissioning and Performance of the Marshall Supercharger on the MG PA

The first attempt at starting the car proved to be an anti-climax and something of a damp squib. Although I had never slackened off the distributor clamp during the fitting of the supercharger, or at any other time, my buddy noticed that the distributor appeared to be loose as I attempted to start the car. Indeed it was …so, time to check and reset the timing.

With the distributor top off we found that the car had been fitted with electronic ignition as there was a sensor and a 2-wire pickup rather than a set of points. Luckily the manufacturers name was on the side and so, after getting the wiring data for setting up the timing with a multimeter and using the TDC mark on the flywheel (observed through the inspection cover on the bell housing), the distributor position was reset to the equivalent of “points just opening” (multimeter voltage falling to zero), to give a static (fully retarded) timing at TDC. The electronic ignition takes care of the advance curve.

Next try at starting: Nothing. Not even a misfire, a backfire or anything. Nothing. So, plugs out and checked for sparks. There was a spark, although not as strong as the fat blue spark that I would have expected from an old-school points system, but as the car was running perfectly before taking off the twin SUs I assumed this was just a characteristic of the electronic ignition.

The next check was: “is fuel getting into the cylinders?”. With soaking wet plugs the answer to that was obviously “yes”. Now, in my experience and also that of my buddy (who has also spent his entire working life in the automotive trade) is that if you have fuel and sparks you will get something to happen. It may not be the right thing but there will be an ignition event of some kind - it may be when the exhaust valve is open, when the inlet valve is open or, if the engine has been timed on the wrong stroke, then at “phut” during TDC valve overlap. But, here, there was absolutely nothing.

Now, I have mixed feelings about electronic ignition. I am a mechanical engineer by profession and pretty well all my non-professional experience is with magnetos on vintage motorcycles, which I understand and have got to trust over the years. We have good people that will re-magnetize the magnet, rewind the armature and take care of the few mechanical bits and pieces of points, bearings and the advance/retard. They are also straightforward to set up with a timing disk on the crankshaft. Advance/retard is manually controlled with a lever on the handlebars connected to the magneto with a bowden cable.

But with electronic ignition you never quite know what is going on. Out of my two dalliances with electronic systems I have had one good and one bad experience. The good experience was to replace an original Lucas magneto (because it needed rebuilding) with a BTH self-generating magneto on my Vincent Black Shadow. It came with clear instructions and worked “out of the box” and, many years later, I have never ever had a problem with it. It starts, hot or cold, with just a lazy push on the kickstart.

The bad experience was with a (name removed) electronic system fitted on a Velocette Thruxton.  This was similar in concept to that on the MG in the respect that it consisted of a sensor that replaced the usual magneto and required a coil and battery. This was not my bike but that of a good friend of mine who was, at the time, a very competent restorer of Velocettes in Chicago.The ignition system and pretty well everything else in the engine was brand new. We kicked and checked all morning – there was fuel and a good spark at the plug (at least externally) but all we could get out of the engine was an occasional “phut”. In spite of the fact that there appeared to be plenty of power in the battery – a good spark, lights worked etc. we decided to connect in another battery. The bike then started on the next kick.

The lesson learned from this experience was that the battery must not only be good (it was already a brand new battery) but also absolutely fully charged to ensure that the electronic ignition system would function correctly. It transpired that the battery had been charged a few weeks earlier but had lost sufficient power in the intervening period to preclude a good in-cylinder spark. The corollary to this is that electronic systems can appear to be functioning OK, judging by a strong spark outside of the engine, but are failing to produce a spark under compression pressure to ignite the charge.

So, back to the MG. I reasoned that the additional power required to turn over the engine with the supercharger could reduce the voltage sufficiently that there would be no in-cylinder spark and decided to replace the entire ignition system with a good old-fashioned points-based distributor. A new distributor, leads, plugs and Lucas coil were ordered and fitted. Now, I know that advocates of electronic systems will decry this move as a retrograde step (and me as a dinosaur) but I personally have more confidence in these “old fashioned”  systems and ….they have been powering engines successfully for an awful long time.

With the new distributor and coil fitted, time for the next attempt at starting. Nothing. Absolutely nothing. Each starting attempt was getting to be quite an anti-climax.

Back to the fuel system. After another phone call to Oliver Richardson it transpired that I had an incorrect jet and needle in the carburettor. The diameter of my jet was 0.125” whereas it should have been 0.1”. It might not sound much but it’s a 56% increase in area. A new jet and RA needle were ordered from Burlen Fuel Systems. However, the needle is fitted into a short sleeve at the bottom of the dashpot and a new sleeve was also required and so this was ordered too.

Unfortunately, the sleeve is a tight interference fit in a blind hole and the existing sleeve cannot be removed easily. I tried! It is also hardened steel and it’s not possible to bore it out in the lathe and so another trip to my tame spark-eroder to take out the inner of the existing sleeve leaving a ~0.010” wall thickness that could then be collapsed without damage to the dashpot sleeve hole.


Although the engine would not have run well being so much over-jetted it should still have run and so there was not really any optimism that this would be the solution.

I also found that there was a large pool of petrol at the bottom of the blower resulting from the many failed attempts at starting and this was soaked up with blue-roll and allowed to dry out. It would be more than unfortunate to get a backfire into the inlet manifold and set fire to the supercharger after all this effort….

What next? Having checked and rechecked everything there comes a time when you run of ideas as to what to look at next. By this time I had added another nearly brand new and fully charged battery off my Austin Healey in parallel with the MGs battery to ensure there was no shortage of power during cranking. I went back to the ignition system and, although everything was new, again checked the spark at the plug. I noticed that the spark on the plug of cylinder #1 was a bit more intermittent and anaemic than I would have expected. I took all 4 plugs out and checked their sparks with them resting on the blower. I was hoping to see 4 strong sparks and hear 4 electrical “cracks” during cranking …but that was not the case. There was an occasional weak spark from one or other of the plugs but nothing that would inspire confidence to start and run an engine.

I removed the HT lead from the coil to the distributor at the distributor end and, as hoped for, there was a massive, nearly 1” long continuous stream of blue sparks during cranking. There is only the (brand new) rotor arm and the carbon and brass pickups between the input and output of the HT leads at the distributor so the resistance of the rotor arm was immediately checked and it turned out to be an open circuit. It should have been a 5 kohm resistor to supress interference but that was not the case here. It would seem the problem had been found and another rotor arm was immediately procured (and checked!).

Next try. Nothing!

At this point I was running out of ideas and called Oliver. He came over a few days later and quickly went through the setup. It turned out that I had not set up the new needle and jet quite correctly (not centred properly - the dashpot should bottom with a satisfying “clunk”), the number of turns of the jet adjusting nut was wrong and I had screwed-up the timing.

Next try. It bust into life. What a relief! This was the first time that the engine had started with the supercharger fitted. We spent an hour or so checking various things until an oil leak from the top of the engine brought proceedings to a halt.

A few days later, with a new cam cover gasket and putting the remaining bits of the car (radiator tie rod, bonnet etc.) back together it was time for a first road test. Although the car went reasonably well, the performance was not markedly different from the original twin SU setup and was running significantly rougher. It had all the characteristics of being far too rich, which was confirmed by a check on the plugs. In fact, they were so black (as was the inside of the exhaust tailpipe) you could have mistaken them from being inside the chimney of Battersea power station for 10 years. It was time for someone that had a greater understanding of SU carburettors than I to set this up properly.

A slot was booked on a reasonably local rolling road and I duly drove the car over with the hope that the dynamic timing and especially the carburation would get sorted out.

It turned out that the fully advanced timing was at 24 deg BTDC so that was reset at 33 deg BTDC and then attention was turned to the carburation. The CO measurement of 12% @ 4000 rpm confirm just how rich the setup was. Another needle that should have been only very slightly weaker was tried (based on the flow areas of each needle/jet vs needle position in the jet - details here ) but the engine would barely run on this needle. By this time we had to call a halt to testing as, among other things, I now had a broken exhaust manifold that was leaking at the front cylinder. This was probably a delayed consequence of dropping the engine at the front to put on the new pulley (before I had decided to make the 2-piece pulley)


At this juncture I was learning that setting up SU carburation properly requires a lot of experience. This approach, ie selecting a needle from the literally 100s of needles with different profiles is quite different from setting up the carburation on vintage motorcycles where the jet size, rather than the needle, is the variable. I am quite accustomed to making my own jets for ancient Binks and AMAC carburettors using a set of drills in 0.1mm steps. In fact some of these early carburettors don’t even have needles  ….but I digress.

And so my first foray into rolling roads was not entirely successful. If I summarize the day, it took me 11 hours from when I left home to when I returned (I only made it 3 miles towards home from the rolling road before I had to call out the RAC to rescue me), I had a car that performed worse than when I started the day, now had a broken exhaust manifold and it had cost me over £300. Not a happy bunny!

Another call to Oliver along the lines of “help!”, as I was running out of ideas as to what to try next. He strongly recommended that I take the car to Sigma Engineering in Gillingham down in Dorset, who had the right expertise to set this up.

The first step was to sort out the damaged exhaust manifold. A quick call to Barry Walker soon had a replacement plus gaskets on the way and, after grit blasting and a couple of coats of Frosts heat-resistant paint, was fitted.

As the car was at least now running again I called Sigma Engineering and made an appointment for a slot on their rolling road. It’s about 100 miles from my home and so we trailered the car down and they duly set about sorting out the engine setup.

I cannot praise enough the work that the 2 guys did on the engine. Pete Lander is regarded as the guru on building and setting up straight-6 Jaguar engines for performance but I believe that they also see quite a few little MGs. He mentioned that he hadn’t seen one of these with a supercharger fitted since Friday  …and this was Monday!

In addition to the strobe, the engine was also instrumented for fuel pressure (this is important so that the fuel level relative to the jet is maintained) and the lift of the dashpot is monitored with a mirror. The first run confirmed the level of richness and another needle was selected. This turned out to be a bit too lean with spitting back into the intake and an inability to rev. The 3rd needle was spot on and, after resetting the timing to 37 BTDC fully advanced and checking the points and distributor bob-weights, we ended up with clean combustion throughout the rev range, confirmed by the much lower levels of CO, and maximum power of 42.6 BHP at the crankshaft (the losses through the transmission and tyres is estimated from a coast-down test).

Although I was quite happy with this result, Pete was disappointed and expected more power. I was satisfied because we had more than the 36 BHP that is quoted for the original N/A engine (although I have also seen a figure of 34.9 BHP quoted) and we had clean combustion. It seemed that the amount of boost provided by the blower was only ~3 psi and this would be reason for the lack of power, but without changing one or other of the pulleys there is not much that can be done about the boost pressure. Without a baseline measurement of power from this engine before the supercharger was fitted there is no way of knowing exactly the power increase; it can only be surmised by reference to the quoted power output of this engine type from various sources.

If we take a value of 36 BHP for the original engine the increase in power at 5000 rpm is 18% and if a value of 34.9 BHP is used then the power increase is 22%.

I could not find any references to the expected power increase for a Marshall supercharger on a PA but I did find here advertising data for a Marshall supercharger fitted on TD and XPAG engines. Running at 6 psi boost pressure these gave increases of 24% and 26% respectively at 5000 rpm. This is not that different from the figures obtained from this project.

The litmus test, of course, is how it drives on the road. Although I have only so far taken the car out on one decent run (I am reporting this the day after its rolling road outing) the performance increase is very apparent. I had been driving this car for more than 4 years before fitting the supercharger so I have a pretty good feel for what it will and won’t do. The increase in torque at lower engine rpm is very noticeable; I can now accelerate up hills in a higher gear than would have ever been possible previously. The boost pressure at a steady 3500 rpm is between 3.5 and 4 psi; this is quite close to the data on the XPAG engine quoted previously which showed 4.5 psi at that speed. As boost pressure is increasing with engine speed this could easily translate into 5 – 6 psi at 5000 rpm on this engine. The car has also not lost any of its driveability and is just as “sweet” to drive as before.

So, how to summarize this project:

Cost: I don’t keep exact records but if I exclude the cost of another exhaust manifold (which would probably not have broken if the engine had not been lowered so far to fit the new pulley) and the first rolling road outing the total cost is probably about £8,500.

Effort to fit and commission the supercharger: More than I thought initially and I hit of lot of “gotchas” along the way. I have well equipped workshop facilities at home, I enjoy engineering challenges and I have learned a lot along the way but it was more effort than I anticipated.

Would I do it again – was the required effort and money spent worth it? It wasn’t cheap and quite a bit of effort but I am very pleased with the eventual result. Based on my limited testing so far, it has transformed the performance of the car. So, Yes, I would do this project again.

….except that I don’t have another MG PA, so now back to building my AJS V-twin world record attempt replica…

 Again, Thanks to Oliver Richardson for invaluable advice and for sorting out various things on this project. I’d still be stuck in the mire somewhere if he hadn’t helped.

Also thanks to the guys at Sigma Engineering for applying their knowledge in the final stage of getting this to run properly. I would wholeheartedly recommend them to anyone that needs real “old school” experience to set up their engine.


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