Ignition timing differences

The ignition timing for a Commando according to the manual is 28 degrees BTDC with the advance mechanism locked in the advanced position. Boyer go for 31 degrees. Trispark 28 degrees. Why the variation and what are the fast/race bikes set at?

gripper said:

The ignition timing for a Commando according to the manual is 28 degrees BTDC with the advance mechanism locked in the advanced position. Boyer go for 31 degrees. Trispark 28 degrees. Why the variation and what are the fast/race bikes set at?

Click to expand...

Fast race bikes normally have high compression -so 26 to 28 degrees is common.

Of course I should point out that the needed advance is also determined by porting, camshaft and piston dome.
Big lazy flowing ports or domed pistons usually require more advance. Jim

TriSpark advance curve follows the standard AAU. Whereas Boyer has a straight line advance curve and it keeps on advancing, and is a bit of a compromise.

Dave

You can do the manual version of modern electronically controlled ignition advance. Advance to the point where the engine pings, then retard slightly. Just use worst case such as hot outside temperature and under engine load such as high gear up an incline.

I think there are two things to consider : The static advance and the advance curve. If the advance curve is a straight line from 1000 RPM to 7000 RPM, it probably doesn't compensate for the geometry of the rod length and stroke and the fixed time ignition event. Moving the static advance moves the whole curve and if the curve is a straight line, provides a compromise at best. The other thing to consider is the shape of the carburettor needle. To maintain a stable combustion temperature (where conditions provide max power) , it takes a balance between mixture, advance and compression ratio. To my mind it all takes fiddling with the advance curve and carb jetting to get max power across the whole of the rev range and throttle openings, and that is after you have established which cam and timings and exhaust system you are going to use.
The alternative is to copy what somebody else has done, if their bike is a good performer. I wouldn't hesitate to copy what Herb Becker or Ken Cummings has done with their racer, ever for a road bike.
As far as static advance is concerned, advancing the timing until just before you get the ping might not be the best way to go. Sometimes at a given throttle opening, best power occurs quite a bit before pinging occurs. In any case it is a bad habit - if you ever tune two strokes, going that way is expensive.

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:

Click to expand...

that's because the different ignition boxes achieve maximum advance at different rpm's. But all want to have circa 28 degree at 3000rpm.

acotrel said:

I think there are two things to consider : The static advance and the advance curve. If the advance curve is a straight line from 1000 RPM to 7000 RPM, it probably doesn't compensate for the geometry of the rod length and stroke and the fixed time ignition event. Moving the static advance moves the whole curve and if the curve is a straight line, provides a compromise at best. The other thing to consider is the shape of the carburettor needle. To maintain a stable combustion temperature (where conditions provide max power) , it takes a balance between mixture, advance and compression ratio. To my mind it all takes fiddling with the advance curve and carb jetting to get max power across the whole of the rev range and throttle openings, and that is after you have established which cam and timings and exhaust system you are going to use.
The alternative is to copy what somebody else has done, if their bike is a good performer. I wouldn't hesitate to copy what Herb Becker or Ken Cummings has done with their racer, ever for a road bike.
As far as static advance is concerned, advancing the timing until just before you get the ping might not be the best way to go. Sometimes at a given throttle opening, best power occurs quite a bit before pinging occurs. In any case it is a bad habit - if you ever tune two strokes, going that way is expensive.

Click to expand...

This reminded me of one of my favorite 3 Stooges routines. Watch the part at 2:50

https://www.youtube.com/watch?v=RHDta63IfDU

Danno said:

acotrel said:

I think there are two things to consider : The static advance and the advance curve. If the advance curve is a straight line from 1000 RPM to 7000 RPM, it probably doesn't compensate for the geometry of the rod length and stroke and the fixed time ignition event. Moving the static advance moves the whole curve and if the curve is a straight line, provides a compromise at best. The other thing to consider is the shape of the carburettor needle. To maintain a stable combustion temperature (where conditions provide max power) , it takes a balance between mixture, advance and compression ratio. To my mind it all takes fiddling with the advance curve and carb jetting to get max power across the whole of the rev range and throttle openings, and that is after you have established which cam and timings and exhaust system you are going to use.
The alternative is to copy what somebody else has done, if their bike is a good performer. I wouldn't hesitate to copy what Herb Becker or Ken Cummings has done with their racer, ever for a road bike.
As far as static advance is concerned, advancing the timing until just before you get the ping might not be the best way to go. Sometimes at a given throttle opening, best power occurs quite a bit before pinging occurs. In any case it is a bad habit - if you ever tune two strokes, going that way is expensive.

Click to expand...

This reminded me of one of my favorite 3 Stooges routines. Watch the part at 2:50

https://www.youtube.com/watch?v=RHDta63IfDU

Click to expand...

Well done that man !!

Towner said:

that's because the different ignition boxes achieve maximum advance at different rpm's. But all want to have circa 28 degree at 3000rpm.

Click to expand...

If you are programming an ignition system, a straight line is still a curve. I don't believe there would be any ignition system ever made which had a true straight line of advance across all operating revs.

Cars typically had ignition advance linked/operated by manifold vacuum.
Modern fuel injected whizzbangs also have throttle % control input.

When you think about it, a purely mechanical/rpm advance is pretty unsophisticated.
But works....

I was always told the factory recommended 28 degrees BTDC ignition timing at full advance and makers of electronic ignitions recommended 31 degrees because the points-and-bobweight-and-springs advance mechanism was less precise than the programmed curve of a black box. Made sense to me.

After being rubbished by a couple of you guys, I've decided to get off my backside and do the maths involved in deriving the curve which compensates for the geometric effects of rod length and stroke on a fixed time ignition event from 29 degrees BTDC to 15 degrees ATDC at 1000RPM, as the revs rise above that. If I can find my scientific calculator, I might be able to give the theoretical advance versus the revs relative to a static advance of 29 degrees. You can then put the static advance wherever you like to compensate for fuel differences.
I apologise for being lazy. I haven't yet bought a programmable ignition system, mainly because I use methanol fuel and the margin for error compared with petrol , is so much greater before you get a performance loss.
It is secondary school level maths and you guys should be better at that than I am.

As one of the guys who probably stirred a bit!

My reasons are along these lines:

- Firstly the original post was about full advance - not about the curve shape.
- Secondly the arguments are all pretty academic until the actual machine has been checked for the accuracy of the degree indicator on the chain case. These are often in error. (Use a degree wheel)
-thirdly the "perfect" advance is a function of all the things comnoz said (including piston shape and squish) plus fuel type, carb tuning, probably spark plug, compression and the weather on the day - seriously!
- fourthly in my experience except when there is a redundant spark system and both side fire together BOTH cylinders must be checked. Yes I was shouting.

If you read for example AG Bell he describes deriving the perfect ignition curve versus carburation , air temp humidity etc from multiple dyno runs to define a multidimesional ignition curve which is then programmed into the ignition computer. This is probably derived these days by engine simulation software. (And modified for the all import environment emissons - Volkswagon :shock: )

The simple answer to the original post has already been given. Race bike are more around 28 deg full advance because of high compression. Street bikes up to 31 deg depending on fuel, compression etc.

If you want to get more detailed than that then get into the details of the specific machine and its environ.

acotrel said:

Towner said:

that's because the different ignition boxes achieve maximum advance at different rpm's. But all want to have circa 28 degree at 3000rpm.

Click to expand...

If you are programming an ignition system, a straight line is still a curve. I don't believe there would be any ignition system ever made which had a true straight line of advance across all operating revs.

Click to expand...

I didn't want to answer to LAB's post (our posts have been nearly at the same time, so I saw it too late). But the chart explains what I wanted to say. Most curves nearly meet at 28 degree (especially Pazon and Trispark), which is the factory setting at 3000rpm. The difference of degree between the boxes depend on the rpm for the setting. And it is recommended to set it at the rpm where the maximum of advance is achieved. That's the cause for the difference in degree (IMO).
E.g.: If you set the Pazon at 28 degree with 3000rpm is the same as 32 degree at 5000. By the way I strobed my Pazon at 4000rpm with 31degrees instead of 5000.

LAB
One question about the graph. It would seem like to produce the curves that one point on the curve has to be fixed as a reference. When you set the timing on one of these systems you set that reference point ie a certain degree of advance at a certain rpm by moving the plate either points or ei. Is the fixed reference point the manufactures recommended rpm and degrees advance?
Htown

htown16 said:

Is the fixed reference point the manufactures recommended rpm and degrees advance?

Click to expand...

Yes, although the full advance figure and the recommended RPM for checking varies according to the particular electronic ignition manufacturer.

Thanks, that explains why for manufacturers using the same references point, say 28 degrees at 3000 rpm, the curves stack.

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 ?

I have a friend who has adapted a Honda VT750 ignition system to a 750SS Ducati. He says it is the ant's pants. He was one of our top A grade road racers in the 1960s and has been a motor mechanic his whole life. He is extremely technically proficient. In my own case, I am still thinking about which way to go. I'm lucky - using methanol fuel means the need to improve is not urgent.