Again an area which requires further investigation, however for now the McCulloch instructions are given.
In order to eliminate detonation which can occur in supercharged engines due to the higher pressures in the combustion chamber it is necessary to retard the ignition spark as boost pressure increases. Generally this involves modifying the ignition advance curve such that it is raised slightly at lower rpm, and cut down by as much as 20% at higher rpm. Ignition curves tend to be specific to individual engines, and Iíve included the curves I have at the bottom of this page, however the following ignition adjustment method tends to give pretty good results with supercharged engines:
1) With the engine at normal operating temperature, test the car (with the distributor set at normal timing) as follows:
2) Maintain a forward speed of 30 to 35 miles per hour in high gear and then apply full acceleration by depressing the throttle to the floorboard. If detonation, or ping is present, retard the distributor to the point where detonation is removed.
3) The ideal setting of the distributor is at the point where maximum engine performance is obtained without detonation, or ping. Do not run a supercharged engine with detonation present at any time other than for the short period required for adjustment.
Note: In the case of of the mid fifties Fordís, where lack of vacuum in the manifold during high speed conditions (with a supercharger) makes the vacuum unit inoperative, giving a resultant retarded spark, McCulloch supplied a new vacuum advance unit which replaced the stock unit. Obviously it is unlikely that one of these units will be available, so in this case it is recommended that the distributor us replaced with a later Ford distributor (1957 of later), and the distributor re-curved as per the tables. The unit that McCulloch originally supplied had an additional nipple which was connected to the supercharger boost outlet. This allowed the back side of the advance unit diaphragm to be pressurized which balances the pressure in the intake manifold and allows the advance unit to operate correctly. A further modification that was made to the Ford distributors was installing a sleeve on the retard spring post, to increase the retard spring pressure.
Replace the existing spark plugs with colder spark plugs. This is required to prevent premature detonation of the fuel/air mixture by hot plugs. This should not be necessary on multiple carburetor setups where the richer fuel supply should be enough to keep the plugs cool.
The following are the only curves I have, although they may prove useful as a guide for your set up.