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MG MGA - Timing and Engine Temp

This point has been around 60 messages into Asymmetric Fan thread, which as is often the case, rather wandered so I will repeat here.

Dominic made the point that when unleaded petrol came in the advice was to retard timing a couple of degrees. If I ever knew this point I had forgotten it, although I only went unleaded on the A 2 years ago. My point is that many of us have been advancing timing as far as possible which may be exactly the wrong thing to do if we are running a bit hot and or/suffering fuel vapour locks.

Correct Dominic?

Paul




Paul Dean

Paul. The early recommendation to retard the ignition timing when running unleaded fuel was because the available unleaded fuel was only available in a single grade, "regular". As unleaded fuel completely replaced the older, leaded fuels, "premium" and "mid-grade" fuel was offered for sale and the problem of using a lower octane rating than specified was done away with.

Ignition timing is a complex problem basically due to the burning rate of the fuel being used. Either too little ignition advance (retarded) or too much ignition advance (over advanced) will cause the engine to run hotter than intended. Ignition advance is to start the mixture burning before the cylinder reaches top dead center, but only to the extent that the most efficient burning begins to take place just as the piston is rotating over top dead center. This allows the maximum energy to be used to force the piston downwards, providing as much energy as possible when forcing the piston downwards.

When the ignition timing is overly advanced, the full burning begins to take place before the piston reaches, and dwells at, top dead center. Thus, the momentum of the piston has to fight its way upwards against the pressure of the expanding gasses. This damages the piston and raises the engine temperature.

When the ignition timing is excessively retarded, the mixture does not begin to fully burn until the piston is moving downwards, not allowing the mixture to burn as effectively, and, perhaps, may still be burning when the piston tries to rise upwards on the exhaust stroke. This, too, results in excessive engine temperatures.

Timing at idle and steady speed operation can be significantly more advanced, with benefit. This is why most engines have a vacuum advance system associated with the distributor. On the other hand, when accelerating, the fuel is not being drawn into the cylinders as efficiently, burns in a more erratic manner, and will only accept a certain amount of ignition timing advance without detonation being a distinct possibility (pinging/pinking which can sound like shaking a number of nuts and bolts in a coffee can).

The only exact method of finding the ideal ignition timing is through empirical testing, either on an engine test stand or a chassis dynamometer (rolling road). From those results, an experienced technician can develope and ideal ignition advance curve for that engine, using any particular fuel. That ideal advance curve is, then, translated into a mechanical advance curve on a distributor which will approximate the ideal as closely as possible without chance of damage to the engine in road going vehicles.

Les
Les Bengtson

Les says it all really, but I have certainly found that running with retarded ignition will result in the engine running frighteningly hot, as I discovered when my timing slipped back a few degrees! I have also found that pre-ignition knock and running-on is more likely when running with NGK BPR6ES than with the BR6ES that NGK recommends for the 1500 engine.
I have set my timing with a Snapon Tachostrobe, and if my memory serves me correctly it is about 12 degrees at tickover and 32 degrees at 3000 rpm with the vacuum advance disconnected. My engine is a bog standard 1500 and it runs very sweetly with this setting, returning about 30 mpg. It does tend to run a bit on the hot side in the summer though, 200-212 is frequently seen when it is pushed, but I just ignore it, it continues to run sweetly and has done for the last 15,000 miles, so it can't be doing it too much harm!
Lindsay Sampford

I became aware of this min my early days of working with the Judson. Getting timing or mixture wrong caused massive overheating or running problems, so I had to read up on the subject.
There is a nice write up here that covers the basics here:

http://classicmechanic.blogspot.ch/2011/05/ignition-timing-with-modern-fuels.html

With a little too much advance the car runs great but overheats (and in a supercharged engine may also be suffering from predetonation which quickly devours bearings and piston crowns). Retarding the ignition just a little has quite a dramatic effect on temperature. Conversely with too much retard it will be sluggish . The fuel and mixture also has a huge effect. Too lean - too hot, too rich - runs badly and fuel consumption is too high. The advance curve is also important to get correct.

It's a three dimensional puzzle, and the only way to guarantee getting it right is with professional equipment on a rolling road. That's why Peter Burgess has such a great reputation.
dominic clancy

This thread was discussed between 06/02/2016 and 09/02/2016

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