General opinion appears to be that advancing timing to 14 degrees BTDC is worth doing to improve top end free revving and to give a bit more verve, but don’t expect much of a BHP gain. Then I stumbled across the “Mazda Miata Performance Handbook” (link below):
“…You can get 20% more torque at 1000rpm by increasing your base ignition timing from the stock 10 degrees BTDC to 18 degrees BTDC…The 18 degree BTDC setting will, however, require the use of high octane fuel…If you shift most of the time at 6500rpm and are looking for more top end power, advance your base timing to only 14 degrees BTDC…”
A table in the handbook indicates that the 18 degree BTDC mod gives a power boost of 7 BHP, whilst the 14 degree mod gives an extra 9 BHP.
These figures appear very impressive. Is this because we are dealing with a US spec 90 BHP 1.6 litre engine, or are they just exaggerating? My Eunos 1.6 is being reset to 14 degrees BTDC today, and whilst I’m not holding out for anything like an extra 9 BHP it’s going to be interesting to compare before and after.