Engine timing for high compression

[bob888]

I have 10:1 compression in IO540 that normally times at 25 degrees before TDC. I have heard that timing should be retarded for the higher compression. Anyone have opinion or advice on this?

 

[olyolson]

I have an IO-360M1B that was standard compression before an overhaul by Aerosport Power. They put 9:5:1pistons in during the overhaul and the required timing changed from 25 degrees to 20 degrees.

 

[andoman]

20 degrees max to keep you safe from detonation. You can go a few less than that to help control high CHT and lose very little additional power.
I have 9:1 and use 19 degrees for takeoff and climb here in AZ. SDS CPI II.

 

[jrs14855]

0 320 with 10-1 pistons, I used 22 degrees until rings were seated then 24 degrees. Plenum and cowl flap 350 hours with no issues.

 

[lr172]

A complex question. Everyone tells you 20* instead of 25, but none of that is based upon any data, except maybe the quote from aerosport. Compression increases by themselves have no inherent need for less timing. However, increased compression reduces detonation margin and so does timing advance. So clearly one needs to be more concerned with detonation as compression goes up. In the performance auto world, they throw the engine on a dyno, run it up to max torque (where detonation is most likely) and advance untill it detonates to find the edge. Then find advance for best power, while limiting it to a number that is a few degrees below the detonation point. Not a lot of folks doing that on lycs. I am sure someone has done it and you should seek them out for an answer.

The old salts in the hot rod world would set advance be observing the edge of the heat color change on the plugs electrode. They want it mid point on the flat area. While not a best practice, it does help. Detonation is from too much heat and the electrodes color and band placement help to identify the level of heat during combustion. I have built a few high performance V8’s and always used this as a cross check when optimizing advance. If you have auto plugs have a look.

The fact that angle valve engines require 20* shouldn’t sway your decision. Combustion chamber shape greatly impacts the combustion proces and therefore optimal timing advance will vary substantially across different chambers designs