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If the engine has been modified, its requirements for ignition timing will probably be different from standard. Also, the distributor itself may have been modified. What I describe below is a first principles approach on determining the ignition set-up for your car, and may result in the need to reconfigure the distributor advance curve if you want optimum performance.
Before doing so, I will outline the procedure I would use so I know what to expect from a modified engine. You may have already done these, but for the sake of completeness I'll outline them:
First, check the compressions. Standard MGB is in the range of 170 - 180psi (hot engine, open throttle). Modified engines may be quite a bit more than this, depending on compression ratio and camshaft type, and may even have less cranking compression than standard. (High compression ratio raises it, "hotter" camshafts reduce it due to the later closing of the inlet valve) If your compression is less than 150psi, it will start to affect low end performance quite significantly.
Next, ascertain what type of camshaft you are running, or at least get a feel for it - refer to the page
Setting up & measuring cam timing for modified engines
Now onto the ignition timing.
When you modify an engine,you will probably also need to change the rate at which the ignition timing advances with engine speed, known simply as changing the advance curve (the term curve deriving from the way the data looks plotted on a graph). In general, high compression ratios will mean the engine needs lees advance at lower RPM, while hotter cam timings require more advance at lower RPM.
If you raise the compression and fit a milder cam you may get lucky and find that the two effects cancel out and the standard advance curve will do, but Murphy usually applies! Cranking compressions give a good guide as to whether your advance curve needs changing - if they are similar to the standard spec engine, then the advance characteristics usually don’t change.
Using the MGB as an example, the maximum total advance is 30 - 32 deg. The speed at which it should reach it varies with the compression ratio and cam timing, as noted above. For a standard engine it is at 2300 - 2500 RPM. For a high compression fast road engine it will be between 3000 - 4000 RPM, the higher the
compression the later max advance should be reached. The octane rating of the fuel is also a factor. Carrying out the
procedure below for lower speed advance will be a guide as to when the max advance should be reached. (Measure all the following advance readings with the vacuum disconnected.)
Without special equipment, the most practical way of setting up lower speed advance is to road test the car, driving it in top gear on full throttle at the target engine speed. Advance the distributor 2 deg between each run and note the setting at which pre-ignition is first heard at the target engine speed. Measure the total advance at that speed, note it down, then road test for the next target engine speed. . Do this at 1500 RPM, then at 2000 RPM and finally at 2500 RPM The optimum ignition timing at each target engine speed will be 1 - 2 deg less than the figure you recorded as when pre-ignition appeared.
Following this exercise, you can plot your ignition advance. This is usually a straight line on the B series, so extending the line through the three points you measured until it gets to 30 deg will show where max advance should be reached. Draw a horizontal line along the 30 deg line at engine speeds higher than this
Get your distributor recalibrated so that it matches that advance curve and it will be very close to optimum for your engine. This involves fitting different strength springs which control the weights on the distributor, and/or resetting their length to adjust the point of take-up. It is simple enough to do yourself, but without experience requires a lot of time and a selection of springs. new ones are hard to come by, but second hand distributors from other BMC cars of the period provide a good selection of them.
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