Ignition advance curve

[arcticXS]

Hi!
I have decided to upgrade the charging and ignition on my -77, and ended up ordering a "Powerdynamo" PM alternator kit from Heiden, with a flywheel that has a "lump" for external triggers. Then I will use a fully programmable "Ignitech" ignition black box, which has 2 channels, so can be used with a single pickup and dual coil for stock 360 crank, or with 2 pickups and 2 single coils for any rephase angle you like.

Part of the reson for going with crank trigger instead of Pamco or other cam mounted systems is that I want to use some "Mule" cam caps, that replace the advance mechanism and points housings and covers.
I guess that crank mounted triggers are more precise also, removing the cam chain from the equation, and the triggering is done on a larger radius.

Anyway, here is my question: Do any of you have some info on suitable advance tables, for a stock engine with aftermarket pipes, filters and carbs. Or for a 750 with 9:5 CR and a street cam, plus the above mods?
The "Ignitech" box is programmed in 500 rpm steps, and interpolates between these points.

The Ignitech box is also full of other tricks, like tacho output, shiftlight output, rpm limiter. They have been sold for a few years, and a lot of guys with carburetted Ducatis use them and recommend them.

I am thankful for any ignition timimg info, curves, tables or general dscription!

Dagfinn

 

[Tomterrific]

The stock mechanical ignition becomes fully advanced by 3000rpm or even lower. The reason it doesn't need to advance farther as the rpms increase is the mixture motion of the combustion chamber increases in turbulance with rpm and the burn speed increases accordingly. A fancy programable ignition would be an advantage for a high compression XS due to the archaic combustion chamber design.

What I'd like is a way to advance timing at low throttle, high vacuum conditions (cruise). I think this would give a dramatic increase in fuel mileage even on an otherwise stock engine.

Tom Graham

 

[pamcopete]

Small note: The issue of crank driven vs cam driven ignitions comes up all the time with the usual comment that the crank driven ignition is somehow better because the timing is directly related to the position of the piston. That is true, but one should not forget the fact that the cam operates the valves and valve timing is just as important as ignition timing. It has even been theorized that if the valve timing is out due to things like a stretched timing chain, then it may be appropriate to have the ignition timing out by the same amount.

As for having a sophisticated advance curve, the engine reaches full advance at 3,000 RPM and spends most of its productive life above 3,000 RPM so there is not much point in having an advance curve that goes beyond that unless you have a method of measuring the load on the engine, such as a vacuum sensor as Tom suggests, that would allow you to increase the advance without the danger of excessive detonation. There are some motorcycles that do have a vacuum sensor that is fed into the advance computer along with ambient temperature, barometric pressure etc.

I think for the XS650 it is more convenient to thing of the advance mechanism, such as it is, as a retard mechanism that allows the engine to start easily and idle smoothly.

 

[howardsmed]

high compression engines need a slower advance,
roller bearing engines will not tolerate high roller pressures at low rpm, if you do the big ends go