Ignition timing differences

acotrel said:

So the optimum curve will always be tailored to suit the stroke and rod length.

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An over simplification with significant omissions.

The appropriate ignition curve will have much to do with the fuel burn rate, combustion chamber turbulence, intake charge temperature and combustion chamber shape.

In "the ideal world", the ignition curve should be tailored (mapped) to the throttle opening for each rpm. The goal is to maximize torque (without detonation) for each point on the rpm continuum. And each point on that rpm continuum can operate at anywhere from near closed throttle to WOT. So, in essence, you are mapping ignition against a rpm-throttle opening/load three dimensional curved surface map.

Does the burn rate of the fuel affect the SHAPE of the advance curve, or only the static advance ?

L.A.B. said:

The Boyer (analogue) advance curve isn't straight (how can it be "straight" if it's a "curve" ? :? :wink: ).

commando-timing-advance-curves-compiled-reva-t6488.html

rick in seattle said:

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Looks like the Pazon offers the least chance of kickback at kickstart.

rx7171 said:

Looks like the Pazon offers the least chance of kickback at kickstart.

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The Tri-Spark Classic Twin has a built-in anti-kickback feature.

My Boyer must have been getting tired because starting was becoming a pain. (it probably is well over 30 years old) Whilst I waited for my Trispark to arrive I re-fitted my points with a AAU sleeved and lubed with about 0.0015" clearance. Strobed on both cylinders, it started quite easily and ran stronger than the Boyer. The Trispark almost started itself, it's that much better. Haven't ridden it yet, hopefully the best is yet to come.

L.A.B. said:

rx7171 said:

Looks like the Pazon offers the least chance of kickback at kickstart.

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The Tri-Spark Classic Twin has a built-in anti-kickback feature.

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Is that different than just having zero advance like the Pazon?

rx7171 said:

L.A.B. said:

rx7171 said:

Looks like the Pazon offers the least chance of kickback at kickstart.

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The Tri-Spark Classic Twin has a built-in anti-kickback feature.

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Is that different than just having zero advance like the Pazon?

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I don't know how the Tri-Spark achieves this, but according to the graph, the Tri-Spark doesn't retard below 5 degrees BTDC.

L.A.B. said:

I don't know how the Tri-Spark achieves this, but according to the graph, the Tri-Spark doesn't retard below 5 degrees BTDC.

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The Pazon on the graph is presumably the surefire system, the Altair has a curve which looks very similar to the Trispark.

acotrel said:

What makes me wonder is that some manufacturers offer a switchable choice of advance curves. And one of the Dynatek (?) systems uses manifold vacuum to switch the curve if vacuum drops off at wide throttle openings. I think Jim Comstock's approach of changing the static advance and jetting by using a knock sensor doesn't quite get there. It might be 'a dog chasing it's tail'. The stroke and rod length in the motor affects the 'rock-over time', and the advance curve is there to correct for the changes as the revs rise. So the optimum curve will always be tailored to suit the stroke and rod length.

To my mind the two things which should be easily adjustable are the static ignition advance and the jetting. Then jet to the curve which suits the engine geometry of rod length and stroke, and the static advance which suits the fuel.
My question is whether manufacturers sell systems originally intended for shorter stroke Triumph engines and use the same curve for those systems intended to be used on Norton Commandos ?

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The ignition advance on my bike does not simply advance until it detects ping. There is a 144 entry 3D table to set timing and that is where the timing advance for any combination of load and rpm is determined. That table is modified by the engine temp and ambient air temp as well as the ambient air pressure. The table was developed on the dyno.
This would be all I would ever need if there was never any variation in the fuel octane. Since you can never depend on the fuel, the knock sensor is needed. It comes into play when spark knock is detected and retards the timing a couple degrees per stroke until the knock goes away. That table entry will be temporarily modified. The modification will be forgotten when the bike is shut off. That way it can start over after refuelling.
The knock is never heard since the knock sensor is much more sensitive than your ear. The only way I know when the table is being modified is by a little red light that flickers on the dash.
This works pretty well although there are new systems on some cars that actually measure the air pressure in the cylinder by measuring the resistance across the plug gap and computing the necessary advance on a per-stroke basis. These systems are obviously more advanced and expensive and are not generally used for custom applications. Jim

Hi LAB

I don't know how the Tri-Spark achieves this, but according to the graph, the Tri-Spark doesn't retard below 5 degrees BTDC.

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I'm sure that's just the set point, If the ignition was retarded by 5 degrees the start would be 0 degrees ie the whole advance curve would retard. If it were retarded by what looks like 3 degrees the curve would almost mimic the AAU. My Tri-Spark is currently set at 26 to 27 BTD and the motor pulls like a train.

Dave