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Discussion and questions related to the course EFI Tuning Fundamentals
So I was watching the “Lean Tuning Myths” module and came across the portion where Andre was talking about how to tune for applications that use a very lean mixture like F1, and his comment was, “Running lean can make you more prone to detonation so we need to run less ignition advance to leave a larger safety margin.” Well that statement dumbfounded me as it uprooted what I thought I understood about ignition timing this whole time. I’ve been under the impression that the more you RETARD timing the more prone you are to detonation and not the other way around. I mean, later in the compression stroke the overall temp of the charge will be higher as the piston squeezes the mixture more and more so I figured if you then fired the plug at this elevated point of pressure and heat it would only create MORE pressure and heat making the charge highly unstable and hence more likely to detonate. Where as if you ignite the charge earlier in the compression stroke, the gasses have more room to expand and thus create lower pressures as well as heat making it LESS likely to detonate. Please help me understand what I’m missing 🙏🙏
Well when you ignite the mixture it begins expanding and HEATING. If the pressure keeps increasing from the piston rising, you could reach the pressure/temp that allows detonation. So if detonation is caused by too much timing (too much ignition lead, or igition advance), retarding the timing will prevent it. If the detonation is caused by too much pressure (boost?), hotspots in the combustion chamber (carbon buildup, a glowing spark plug that isn't cold enough to remove the heat), then the detonation will happen regardless of the spark advance.
Below are two pictures with pressure and temperature graphs representing two ignition points 45 degrees BTDC and 0 degress at TDC. As you can see although piston and cylinder wall see much higher exhaust gas temperatures at the end of exhaust stroke with ignition point at 0 degress (or TDC), they receive approximately about 15 percent more heat under curve with ignition point of 45 degrees BTDC. That additional heat makes more power and can bring influence on detonation threshold. The more you heat the combustion chamber the more prone to detonation your engine becomes. And as David said the earlier you ignite the air fuel mixture the bigger pressure and temperature raise in decreasing volume as the piston going up the bore trying to compress expanding explosion thus getting additional heat ( Ideal Gas Law) which makes it prone to detonation again.