A crash course in ignition timing
- Thread starter sicivicdude
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I gotta get my tachometer setup on it and figure out how to mark the flywheel with timing marks that I'll be able to read at high RPM's. I KNOW I can't just stick my degree wheel on there and run it, I'm afraid it might get out of balance and come flying off!
With good clear timing marks, a tachometer, and a timing light, I should be able to map the stock configuration timing advance (including the maximum advance and rev limiter) for general knowledges sake.
I do believe that the stock CDI is "safe" enough to use even with the wildest of aftermarket blasters but I'm sure there are areas that could be improved too...
I will be doing it in neutral probably. For everything but the REALLY high rpm's I can just hold it around a certain rpm (probably on the 1k's) and record the advance at that rpm. For the really high rpm stuff I'll probably rev it up and then let it back down and just do that a handful of times to get a "best guess" for the very top of the rpm range.
The CDI box has no clue at all about whether the wheels are loaded or not. It only knows current RPM and current "advance" (or delay as it's designed in!)
This is an older thread, but a good one. Found it on a search for something else.
Excellent info Sicivicdude, but more oriented to 4 strokes. 2 strokes have an important difference.
Most of them do not like timing advance at high rpm, in fact higher output 2 stroke engines typically have the timing retarded at top rpm.
Here is a good example. Our Blaster timing curve from the book:
The Blaster is a moderately tame 2 stroke, and yet notice the slight timing retard as the engine goes past 6000 rpm?
Let's compare that with the Yamaha DT125, a hot little powervalve engine making more hp (in un-governed form) than the Blaster:
Wild hey? Why the whacky zig zag?
Well, this is a powervalve engine so the zig zag is to take advantage of the powervalve.
The important thing for us is why the serious timing retard at higher rpm?
A tuned pipe 2 stroke engine builds cylinder pressure fast as rpm rise near the tuned point.
Like Sicivicdude pointed out in his first post, pressure increases burn rate, less timing advance is needed to keep the pressure peak in the sweet spot of the crankshaft's stroke.
There is another factor too. Exhaust temperature.
The pipe tuning is dependent on the speed of speed of sound which is itself dependent on the density and temperature of the gas involved. What this mean is that as the temperature inside the pipe rises, so too does your "tuned rpm" of your pipe.
Sweet! 'Cuz this works for you. You give it WOT (or WFO!), you accelerate, pipe comes on tune at 6000 rpm, max power! Exhaust gas and pipe get hotter, tuning effectively becomes 7000 rpm, now max power at 7000 rpm too! You feel a long pull of power, more over rev. As the pipe and exhaust gases cool, tuning drops to 6000 rpm again, ready for the next pass. This effect lengthens the tuned power peak of your pipe.
So what does this have to do with timing?
More advance pushes heat into the piston. Less advance (retard) pushes heat out the exhaust. Simplistic explanation, but trust it to be true. At higher rpm we want that heat in the exhaust to help stretch our tuned power peak.
Another point, Sicivicdude explained this but only briefly:
That "dip" where the timing is backed off for the first 500 rpm?
It is only there for kick starting. It is below idle speed and prevents kick back.
Those of us who rode pre-CDI bikes are most grateful for it. I was launched over the handle bars more than a few times.
Excellent info Sicivicdude, but more oriented to 4 strokes. 2 strokes have an important difference.
Most of them do not like timing advance at high rpm, in fact higher output 2 stroke engines typically have the timing retarded at top rpm.
Here is a good example. Our Blaster timing curve from the book:
The Blaster is a moderately tame 2 stroke, and yet notice the slight timing retard as the engine goes past 6000 rpm?
Let's compare that with the Yamaha DT125, a hot little powervalve engine making more hp (in un-governed form) than the Blaster:
Wild hey? Why the whacky zig zag?
Well, this is a powervalve engine so the zig zag is to take advantage of the powervalve.
The important thing for us is why the serious timing retard at higher rpm?
A tuned pipe 2 stroke engine builds cylinder pressure fast as rpm rise near the tuned point.
Like Sicivicdude pointed out in his first post, pressure increases burn rate, less timing advance is needed to keep the pressure peak in the sweet spot of the crankshaft's stroke.
There is another factor too. Exhaust temperature.
The pipe tuning is dependent on the speed of speed of sound which is itself dependent on the density and temperature of the gas involved. What this mean is that as the temperature inside the pipe rises, so too does your "tuned rpm" of your pipe.
Sweet! 'Cuz this works for you. You give it WOT (or WFO!), you accelerate, pipe comes on tune at 6000 rpm, max power! Exhaust gas and pipe get hotter, tuning effectively becomes 7000 rpm, now max power at 7000 rpm too! You feel a long pull of power, more over rev. As the pipe and exhaust gases cool, tuning drops to 6000 rpm again, ready for the next pass. This effect lengthens the tuned power peak of your pipe.
So what does this have to do with timing?
More advance pushes heat into the piston. Less advance (retard) pushes heat out the exhaust. Simplistic explanation, but trust it to be true. At higher rpm we want that heat in the exhaust to help stretch our tuned power peak.
Another point, Sicivicdude explained this but only briefly:
That "dip" where the timing is backed off for the first 500 rpm?
It is only there for kick starting. It is below idle speed and prevents kick back.
Those of us who rode pre-CDI bikes are most grateful for it. I was launched over the handle bars more than a few times.
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