Jaguar Repair: Ignition timing Spitfire modified, high octane fuel, lean mixture

Question
I spent the winter modifying my 1979 Spifire. De-smogged. 4-2-1 tubular header, milled head to 9-1 compression, ported intake manifold customized for a 45mm Mikuni carb (purists stop reading here!). With the higher compression and greater air flow through the cylinder head, should I retard the timing? The stock ignition timing of 10 degrees BTC worked great. It's my understanding you rotate the distributor counter-clockwise to retard timing. I am trying to prevent pre-detonation and damage to the head components on my inital start-up of this new set-up.

Answer
Hi Peter,

You are correct that you should pay close attention to ignition timing now that you have altered the engine. Your rotor turns counterclockwise so you are correct that you would rotate the distributor counterclockwise to retard the timing.

When you raised the compression ratio, you speed up the flame in the combustion chamber so you would normally retard the timing. However, you are committed to high octane fuel now with the higher compression ratio and high octane fuel burns slower then low octane fuel so that would set you to a more advanced timing. This also upsets the advance curve in the distributor.

It is a difficult process to get to the optimum setting all through the RPM and load range. But you can get in the ball park by just a few simple moves.

First you need to be sure your carburetor mixture of the main jet is slightly on the rich side. Always start rich and work your way toward lean never the other way. Keep in mind that max horse power as far as mixture goes is from 15% to 40% richer then stoichiometric (ideal mixture of a type of fuel)

The reason you must be sure of the mixture first is because a lean mixture can cause detonation and cover up the timing issue.

Low RPM loads will give you the detonation / pre-ignition sound to warn you that the advance is too far but there is a high RPM detonation that is very harmful to the engine (the pistons and rings not the head) and it is very difficult to hear. You have gone to great lengths to gain improved performance so you don't want to risk engine damage so you should consider a "Knock Sensor Display System" like the one from MSD. The most horse power as far as ignition timing is concerned is just before the point of detonation and the only safe method to gaining that, is to be able to "See" high RPM detonation. All your tests should be done with the engine up to full operating temp as increased temp also promotes detonation.

You can do a rough first setting by heating up the engine and advancing the timing until you get the "Ping" of detonation on a hard quick throttle opening (no load)Then retard the timing in 5 degree increments until it stops "Pinging". Then take it out on the road and put it in high gear and at a low RPM go to full throttle. If you get the "Pinging" retard by an additional 5 degrees until it stops. Then check the ignition timing. You may find that it ended up close to the factory 10 deg anyway due to the two main factors canceling each other out.

With the Knock Sensor Display unit installed you can then get closer to the max possible performance at the higher RPM range without the danger of damaging the pistons.

Let me know how it works out.
Howard