Clocking Surefly to reduce max timing

[SteveR]

This is in reference to my parallel valve IO-360:

The concensus seems to be that standard advance curves on the Surefly are too aggressive, with max timing up to 38 degrees. This increases CHTs with no benefit. It seems like it would be simple to set the engine to 0 degrees TDC, then turn the flywheel forward 5 degrees before setting the final mag position. Using the 25 degree variable advance setting on the dip switch, I would get base timing of 20 degrees, and max timing of 33. This seems less aggressive while still giving the advantages of advancing at lower MAP. Thoughts?

 

[lr172]

This is in reference to my parallel valve IO-360:

The concensus seems to be that standard advance curves on the Surefly are too aggressive, with max timing up to 38 degrees. This increases CHTs with no benefit.

Sorry, but that is a huge generalization and not likely accurate. Optimal advance is determined by many factors: RPM, MAP, AFR, cyl head shape, sparkplug position, etc. 38 degrees is probably pretty close to optimal with a MAP of 17" and 100* LOP with no CHT penalty. Optimal advance requires lots of instrumentation and testing. Sorry, but don't buy into whatever consensus yoiu have subscribed to. Every modern engine has an advance curve that tries to achieve optimal adv for every set of variables, while creating det margins in high load regimes.

I agree that 38* is a lot for the way most of us fly, but that doesn't mean that surefly got it wrong with a max of 38. You probably don't know the RPM/MAP combo that nets 38* in the surefly. Without that, you can't really say it is too much. Nor does it mean that the advance is too high at 23"/2500.

 

[DanH]

Steve, there is some limited benefit for a parallel valve, given the right circumstances. See Nigel's data, here distributed with the author's permission:

https://www.danhorton.net/Misc/Nigel Speedy - Ignition Advance .pdf

It is true to say there is little value in advance past the low 30's if you typically run peak EGT or richer. Lots of advance is useful to recover some (but not all) of the speed lost to running very lean.

Be aware angle valve models optimize with less advance; 38 degrees would far too much.