Without DA data from both days and wind conditions, it is hard to say if its timing or a combination of things. Everything else being equal, 4 mph sound like a lot to me. My iron headed-no quench 440 liked 40 to 41 total timing. The et fell off more than the mph down at 35 to 36. Next time you go, try both settings to see the effect and let us know.

IMO, most of the mph loss was the weather (temp, humidity, baro pressure, DA, and the wind). The ignition timing will effect the car/power/torque, but I don't think you will see that much from those two settings by itself.

Just to show how little it can effect things, and each car is different, this is my track testing. Now this engine doesn't need much timing; likely because it is flat top piston, 0.039" quench, good combustion chamber shape (Indy 440-1), 13.5 CR, on E-85 ethanol.

Interesting stuff, and I agree. I run flat tops, good chamber and a tight quench on my 408 and I have done some timing testing with the same results - little change in ET / speed. I think this scenerio can be different when you have something impeding the flame front, start getting in to detonation, run way rich or lean, etc.

Just to let you all know, I was back at the track today with the timing set at 38 degrees and the mph was back to normal. Had a slight tail wind this time so I'm not sure if it was the headwind or the timing or a bit of both but I'm happier than I was a few weeks ago.