Mag Timing

[Dialed In]

So I just got my right mag rebuilt (Bendix S 20) and I am going to install it under supervision.  We were setup to install it yesterday however my IA had a personal issue come up so I had some time. I decided to check the timing on the left mag so I hooked up the box, it was set to 20 degrees BTDC. The data plate on the engine calls for 20, the engine was new in 1983 io 360 A1A.  The previous owners was an a&p and had told me he timed it to 25 degree BTDC so it has been corrected at an annual at some point. My question is is there a way to get a data plate that calls for 25 degrees of timing?  I know there is an sb changing the timing from 25 to 20 but is there a way to retrograde it to 25 from 20?  I’ve never had an issue cooling my hottest cylinder is under 340 degrees and it sure would be nice to have just a little extra power. @jetdriven@M20Doc@N201MKTurbo TIA,

 

Chase

[jetdriven]

 

 

plenty written on this

 

[Dialed In]

I appreciate your response however I guess I didn’t see an answer. Can we time a lycoming to 25 degree BTDC when the data plate states 20 legally? If not is there an option to change the data plate to 25 degrees?  I’m not trying to debate the performance gains, I know I want 25 degrees BTDC. Can it be just up to the IA to determine if he wants to time it to 25 degrees?  

[carusoam]

5 minutes ago, Dialed In said:

I appreciate your response however I guess I didn’t see an answer. Can we time a lycoming to 25 degree BTDC when the data plate states 20 legally? If not is there an option to change the data plate to 25 degrees?  I’m not trying to debate the performance gains, I know I want 25 degrees BTDC. Can it be just up to the IA to determine if he wants to time it to 25 degrees?  

Lots of research to find out what you have, what year it was built…

Some data plates have been legally modified by being Re-stamped…

Most people with the option prefer the 25° BTDC…. Because they also like using an engine monitor… and a few more HP, and the improved efficiency that goes with it…

At least one of our members enjoys his 10:1 compression ratio cylinders…

See if you can find the prior owner/mechanic to find the logic they used….

Best regards,

-a-

[jetdriven]

1 hour ago, Dialed In said:

I appreciate your response however I guess I didn’t see an answer. Can we time a lycoming to 25 degree BTDC when the data plate states 20 legally? If not is there an option to change the data plate to 25 degrees?  I’m not trying to debate the performance gains, I know I want 25 degrees BTDC. Can it be just up to the IA to determine if he wants to time it to 25 degrees?  

You modify the impulse coupler of magneto so it has the right part number for a 25 degree engine, and then you make a log entry stating your un-complying with the "optional" service instruction, and then stamp the data plate to 25.