selon l'indice d'octane utilisé: avance supérieure pour du 98 qui est moins détonnant que du 95.
Il est clair qu'en 1962-1978 on trouvait en UK un super 4 stars à 100 de taux d' octane, parfait pour le meilleur rendement étudié de ce moteur. Par contre entre 95 et 98, j' aurais cru l'inverse.
SI l'on ajoute à tout cela, le fait que le moteur de Jean-Pascal est un LC, alors on peut supposer que les 10° d' avance en plus sont justifiés?
Cela étant, si un jeu existe qui oblige à forcer le doigt d' allumeur en butée ( sens horaire), on a vite de 2 -4° d' écart dès le départ de la courbe.
Un petit peu de prose sur ce sujet..., avec un bémol, un constat assez dur et quelques vérités d'une dizaine d' année:
LUCAS Distributors, Why they need to be remapped, to correct
the original mapping problems and to run well with todays fuels.
Most of us now drive Twin Cam 4 valve cars without even knowing, or caring what is under the bonnet. Manufacturers are totally interested in efficiency, emissions etc, as well as power, torque and smoothness. Even the “El-Cheapo” cars are wonderful compared with the lousy cars that we had in the early days of driving. Then a Twin Overhead Cam 2 valve engine was a car to drool over, or a racing car.
In the 70’s, with the world wide need to make fuel efficient low emission engines came new ignition systems, fuel injection, high compression etc. With the huge improvement in engines came an equally big improvement in fuels, to operate these engines. Who would have thought that cars would run 10 to 1 or higher CR with 91 RON fuels, back when the MGB pinged at 8.8-1 CR, unless Methyl- Benzine was added to the normal Super Grade fuel.
As fuels changed so did the mapping of ignition systems and air/ fuel ratios to minimise emissions and improve economy. This leads into what I want to say regarding the necessity to re-map all old distributors [PARTICULARLY LUCAS] to operate properly with modern fuels, which have very different burning profiles, [they need less low end and midrange advance and less overall mechanical advance], to the fuels which were available when the MG was a common sight on our roads.
I have known of this problem for years, but who would one trust to try to re-map the advance curves of a distributor on a 30 to 50 year old car using a simple points and capacitor ignition system.
I tried to get the “C’s” distributor remapped when the problem became so bad that even on 8.6-1 CR, that I was using, the car would not run well on 97RON fuel. I had previously used BP100 or 100/130 Avgas for years until it was no longer easily available. I took the distributor to Kevin Baker [Beam Electrical] and Kevin tried to get it to conform to the workshop manual settings, which were vague and very imprecise anyway. This allowed me to just run on 97RON fuel, but pinging around 3,000 RPM was still critical under power. The distributors in the early “C’s” had the wrong advance springs fitted, by guess who, LUCAS. The correct springs were written up in “Safety Fast” magazine in August 68 but were not available here, or the UK, by the time I tried to locate a set. I expect that they were all snapped up by UK “C” owners, or replaced under warranty in the UK. My car was the only “C” in Australia until 1970 and the “C” ceased production in late 69 early 70.
I just accepted this and put up with it, not knowing what to do or how to do it. A few years ago I was looking at the MG Motorsport web site [UK] and was surprised to see that they were claiming that their fast road engines in “B’s” and “C’s” ran well up to 11-1 CR on 95RON fuels with their re-mapped distributors. I E-Mailed MG Motorsport [they refused to provide any information on this remapping or to supply spring sets etc.] and they replied that their re-mapped distributors were available for 65 Pounds on an exchange basis only. Too long away and much too expensive in AUD plus air freight both ways and ‘X’ weeks off road with no guarantee that the distributor would not go missing.
While buying “EBC” Green Stuff pads for the “C” I talked to Steve at Greg Tunstall Mechanical, at Cleveland, about the problem with the Ignition and he said, “The first thing Greg does when he rebuilds a Triumph or any motor with a Lucas distributor is to send the distributor to “Performance Ignition Services” in Victoria, to be rebuilt and remapped for use with the current fuels.
This resulted in me contacting Dick, the owner of Performance Ignition Services, I asked how long they had been servicing Lucas distributors and he said “I have had the business for 25 years. I bought it when the original owner retired and he set it up 25 years before that”. So here was a company that has been rebuilding and remapping distributors for 50 years. To add irony he said we are about 50 metres from the Victorian MGCC clubrooms.
Dick says that when the MGA Twin Cam engines are remapped that they will never burn holes in pistons again, it would seem that the real problem with the Twin Cam was not the cylinder head design or the compression but the ignition mapping was all wrong right from the start, just the same as the MGC. [It is no surprise to me that the UK lost their Motorcycle and Car industries to countries that could and did do their engineering homework properly]. The MGB-V8’s also benefit from remapping, as would all MGA’s & MGB’s. I would expect higher compression X-PAG and X-PEG motors would all benefit from having current knowledge applied to get the best results, from current fuels, or just to have the mapping correct for the first time. Lucas did not pay a lot of attention to accuracy or precision in their designs, cost was always the primary motivation, if it worked that seemed good enough. Fortunately Bosch did pay attention to precision, cost was secondary.
I discussed the problem with the distributor. Dick said they knew all about the mapping problem with the MGC. I mentioned this to a friend in the GCMGCC who has a C-GT. He then sent down his distributor [this unit was not even a “C” distributor but a Lucas Australia 6 Cylinder without vacuum advance] to be rebuilt, with electronic ignition and remapping. A group of GCMGCC members then went for a little tour to Karumba, on the gulf, and back to Maryborough a distance of 2,000 miles. The C-GT on 95 RON fuel got 30.8 MPG. Standard MGC-GT’s do not run on 95 RON fuel and certainly never get 30.8 MPG and this car is Air Conditioned as well, he is very pleased with the result.
So I then decided to send down my distributor to be rebuilt and remapped, I already have a Lucas electronic ignition kit in it. When it came back I fitted it and timed it to 10* BTDC as instructed. Whammo a whole different car. For the first time ever the idle was smooth and at constant revs, not plus and minus 200 RPM around 800 RPM as it used to be. Much improved low down torque, and power from 1,500 to 2,000 RPM. Acceleration in O/D 4th [at 2,000 RPM] is nearly as good as direct 3rd before. What a pity that I had never heard of this company, when the car was new, I would not have reduced the compression in 1986 from 9.5 to 8.6-1 to run on the then, at best, 97RON fuel.
When I first rang Dick and told him of the problem and the current state of tune and that to minimise pinging around 3,000 RPM I set the static advance at 6* BTDC. He said, “I am surprised you can even start it at that setting”. When I rang him to check that they had got the distributor OK he said that they had already started on it and the first thing they found was that the distributor was starting to advance from 250 crankshaft RPM. That is why it starts. The distributor is advancing on the starter motor. We set them up to have NO advance until 1,000 RPM. I asked where they get their springs from and he said we always make our own. They add metal to the tongue on the cam to reduce the maximum mechanical advance [which is usually 4* to 6* too much]. They also check and rebuild the Vacuum unit, when fitted, as they have found that the diaphragm develops small holes in it and some Vacuum units stick on and do not release. More brilliant 1930’s Lucas [lowest cost] engineering. In my case they found that the Vacuum unit was sticking on, intermittently. After remapping it did not have enough advance for the remapped distributor. They found a unit that was more suited, in their box of rebuilt spares. They obviously have a lot of knowledge about earlier distributors. There are two different standards of Lucas units [cheap and even cheaper]. Some with steel bob weights others with soft metal weights which wear badly. These are unable to give accurate or consistent timing. If you have one of these it would be necessary to get a new Bosch distributor. The best that a Lucas unit can give is a variation of about 2*. If accuracy if required Dick can supply a small body short neck Bosch unit [similar outside to a Lucas unit] which does give consistent accurate timing, these come with electronic ignition. Nobody should use points these days as apart from the constant maintenance they do not and cannot provide accurate timing. Carbon HT leads should not be used and solid wire leads are not suitable for electronic ignitions. Dick suggested using inductive leads as they have constant resistance and are reliable [unlike carbon] with no ignition interference. I had never heard of these and gladly had a set copied, from my carbon leads. They require special crimping tools so you cannot buy the cable and make your own. They cost $70 for the “C” made up, bloody good value. They work perfectly no radio interference at all.
Here are the before and after figures for my distributor: [crankshaft RPM].
Before Retiming. After Retiming.
Total advance with 6* static. Total advance with 10* static.
250 RPM 6* No advance below 1,000 RPM.
600 RPM 12* 1,000 RPM 10*
2,000 RPM 26* 2,000 RPM 18*
3,200 RPM 34* 3,000 RPM 26*
4.000 RPM 34* 4,000 RPM 32*
Vacuum Advance 12* Vacuum Advance 18*
This over advance at 3000 to 3500 RPM [Max BMEP is @ 3,000 RPM] was the cause of the heavy pinking under full power at maximum Break Mean Effective Pressure [BMEP]. At 4,500 RPM the engine was normal. This is what people meant when they said that you could drive through this pinking period around 3,000 RPM. Had the distributor timing been correct from the factory no problem would have existed. Once again not enough homework was done.
Rebuild distributor and remap $160. Inductive HT lead set $70. Plus Post. This is for my car, with existing electronic ignition.
A new Bosch distributor with electronic ignition and mapping costs $400. This is a short neck unit to replace the dreaded 1930’s Lucas design. These are recommended for competition and serious cars. I used Express Post, both ways. Allow about 10 to 14 days.
Performance Ignition Services,
P O Box 464 Brentford Square Nunawading Victoria 3131
Talk to Dick or if he is not there, Peter. [03] 9872 3644.
Dick said he could not get my Lucas exactly right but it is within 2* of what they wanted. It is a great improvement in performance, driveability and economy.
Dick says that if I want to or need to I can run the car on standard unleaded 91 RON fuel. I think my car is about 8.7- 1. A standard MGB is 8.8-1. Most MGA’s and MGB’s will be running higher compression due to work on the head over the years.
A standard MGC at 9-1 CR runs perfectly on 95RON fuel. Dick suggests that if I refit flat top pistons I will have no difficulty with 95 RON even at 9.6-1 CR. I am sick of pulling the bloody huge lump out, so it will probably stay as it is. It is so easy to drive anywhere and with our pathetic restrictions I think it is good enough. If I were younger I would not hesitate to bring it back to 180 BHP at the flywheel.
I had Peter Liddle convert the head to use unleaded fuel and I really should try 91RON just to see, but I won’t unless no other fuel is available. RON 98 is 10 cents a litre more expensive than standard unleaded but is a much better quality fuel with carbon solvents etc. and it gives up to 10% better economy, so it is actually cheaper for me to use 98 rather than 91 or 95 fuel.
The MGCC of SA sent a number of MGC distributors to Dick for remapping [4 units] apparently they were all different and all wrong as well, so a lot of people have done a lot of harm over the years. I expect nearly all old Lucas units will be way off the correct settings now, after 40 or so years use.
Bruce Ibbotson, #600. 2 October 2008.
), et régler l' avance de façon à mesurer 14° @ 600 tr/min à la lampe stroboscopique.
