The Mysterious Red Line....

[Roadjunkie1]

A friend of mine recently purchased an RV-4 with an O-320-D1A that, as typical, has 25 degrees on the data plate as the advanced timing. The starter ring has the imprinted 25 degree mark but a drawn Sharpie red mark at about 26 degrees. It is currently timed at 25 degrees and runs well. He is wondering (and I can't answer his question) what the purpose of the red mark at 26 degrees would be. Two Slick mags, one impulse. 5400 foot field elevation. I did not know but told him there are a BUNCH of people who might. Anyone have any experience with advancing the timing to 26? One mag at 25 the other at 26? (Insert Shrug Emoji ....)

 

[Ralph Inkster]

My simplistic suggestion would be to run both at 25 deg & erase the other mark. It's running good there so why change it.

Splitting mag timing only means the engine would effectively be running on the more advanced mag, maybe not a great idea for cooling & high load climb out situations.

 

[Dan 57]

IIRC the tolerance is +/- 1°, so the question is rather moot...
And indeed, I suspect loads of people experienced out there flying with 26, or even 24° settings

 

[Taltruda]

Perhaps someone verified TDC, then marked the real 25degree mark? I would just use the factory mark and call it good..