by Michael Shiffer
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At higher engine speeds, larger throttle openings and greater loads than idle, you need ignition advance. There are two reasons for this. First, you are burning more fuel so complete combustion takes longer. Second, the combustion time, as a percentage of the time the piston is at or near top dead center is much longer because of the piston speed. What this means is that you have to ignite the charge earlier, while the piston is still coming up, in order to get the full benefit of the pressure against the piston at the right time. Too early or too late timing will have a similar effect at speed as at idle, but greatly magnified and with far more destructive potential. Too retarded timing will give low power, lousy emissions and excessive bore wear. Timing too advanced will cause pinging (a rattling noise usually heard on acceleration), overheating cylinder heads and other problems too scary to contemplate.
Excellent
article but it is all about
VW vans.
I had a VW back in 1976 and put a VW van engine with a big bore kit into it.
I would have loved to have had that resource, but it has very little to do with our 2 strokes.
Back before East German
Ernst Degner snuck Walter Kaaden's secret of the tuned pipe over to the west, 2 strokes did need more advance, just like 4 strokes. You can still see this in older 4 stroke outboard motors that have a long spark advance mechanism built into the throttle because they do not have tuned exhausts.
The tuned pipe changed everything, 125 engines went from making about 6 hp to today's 40+ hp. How they did this is by improving cylinder filling at higher speeds. This increased the rpm the engines could run and vastly increased the cylinder pressures they run at. Typically most 4 strokes have a decreasing "volumetric efficiency" (cylinder filling) as rpm goes up, with maybe a slight advantage at their torque peak, due to cam timing, intake and header tuning. Still, unless they have a supercharger or turbocharger, they always have less than full displacement in their cylinders, and less and less as rpm goes up. "Thin air burns slower" is a fact known to anyone in the mountains, and is true in the cylinder as well. The less air available, the less complete cylinder filling and the less time available (due to increased rpm) mean that a lot more timing advance is needed as a 4 stroke revs up.
Our tuned pipe 2 strokes break all the rules, thank you Walter Kaaden. The tuned pipe draws air and fuel mixture up into the headpipe and just before the piston closes off the exhaust port, jams it back into the cylinder. This increases the volumetric efficiency (cylinder filling) hugely, often more than their static displacement, and 2-3 times more than a comparable 4 stroke. This increases cylinder pressures. Fuel and air under pressure burn faster. More pressure, faster burn. Faster burn, less timing advance needed.
Back in 1976, besides working on VWs, I also had a 1974 CR250 and bought
Gordon Jenning's wonderful book: Two-Stroke TUNER'S HANDBOOK which is still very valid 40 years after it was written. Unfortunately it doesn't have much information on timing advance, but I did find this interesting gem:
"While most tuners would agree that much depends on the basic tuners' arts, they
are nonetheless inclined to seek magical solutions to any problems not instantly resolved
by changing a main jet or replacing a set of points. Sadly, seldom do mechanical exotica,
magic-box ignitions and the like, truly solve a problem. More often they merely replace
the existing difficulty with another - or multiple, even more mysterious problems. For
instance, most engines have ignition systems entirely capable of producing sparks at a
rate adequate for the speed range anticipated by the engines' makers, but may collapse
into a fit of misfiring at higher revs. Most tuners will assume that the root cause of this
distress is that they have built themselves such an incredibly vigorous engine that nothing
weaker than God's-Own-Lightning is enough to make it run properly, which sends them
scurrying away in search of some transistorized, magnetically-triggered system with
enough sheer zap to fracture atoms."
Which is entirely pertinent to this thread!
Back to timing advance on 2 stroke engines. A bit more up to date is
AG Bell's 2 Stroke Performance Tuning book, which starts to get into the subject on page 129, but has this quote from page 130:
"With four-stroke engines it is usual for the modified engine to require considerably
more advance than standard. However, from the foregoing you can see that this does
not apply in the case of two-strokes. In fact, it is quite unusual for a two-stroke engine
to need more advance than that specified by the manufacturer. It is difficult to say just
how much advance a modified engine will require, but as they can be easily damaged
because of too much ignition advance, I would suggest that you reduce the
recommended timing by 20% to begin with. "
The timing curve on most performance 2 trokes is a slight advance in the mid-range and then retard in the power band. I haven't put a timing light on a Blaster, but does this agree with what you saw Turbowrenchhead?
. I wish there was more info such as Jennings and Bell redily available.
