Hi all.
Posted on this site are some numbers in regards to spark timing BTDC for rotary engines. On the charts that I see would these be the actual total advance firing times? The reason I ask is that I have a racing friend that has over 44 years with racing rotary engines. He states a fact that when you hit 26 BTDC the apex seals will go pop. All his engines run max. 25 Deg. BTDC. Some charts in this forum show from 26 up to 42 in some areas. Do you subtract 10 because it is set up in the program in spark settings with a -10 or some thing? Sorry I can’t understand the charts the way they are set up.
Kim

 

Kim,

The numbers you see are spark timing in degrees BTDC. The 25 degree BTDC limit is a typical one used for naturally aspirated engines at high manifold pressures. I suspect your friend racing rotaries doesn't use any method of vacuum advance on his engines, so the 25 deg. is the maximum advance his engine will see. For street-driven engines, where much time is spent at low load, some type of vacuum advance is used to increase advance above the 25 deg maximum when in reduced load conditions. Take a look at the MAP and RPM bins associated with that table, and you'll see what I mean. FWIW, the table is quite similar to the stock timing map created by the combination of mechanical and vacuum advance on a stock 1st gen distributor.