ski-doo ignition timing
andy c
nippondenso ignition engine fitted to some ski-doos around 15 yrs ago - i
understand it's in the region of 2mm BTDC but can't find anything accurate
the timing is actually advancing towards TDC as revs increase instead of
retarding as i was expecting - can anyone comment on this ? does it sound
like a cdi fault ?
many thanks
andy
David Courtney
ignitions "advance" (spark occurs further BTDC) up to about 3,000rpm... then
begin to "retard" (spark occurs closer to TDC) as the rpm's increase.
Keep in mind that both the stroke of the engine and the rod length
determine "how many mm = how many degrees" so you can't just pick a number
of millimeters from an engine with the same ignition system but a different
stroke or rod length... you need the correct measurement for your exact
motor.
Degrees are always degrees... but the piston height at "x" degrees BTDC
is different if you have a different rod or stroke length.
For example, 20 degrees BTDC:
112mm rod and 60mm stroke; piston is 2.2802mm below TDC
130mm rod and 70mm stroke; piston is 2.6631mm below TDC
Conversely, setting the timing on both of these engines to the same
height BTDC would mean the actual timing is off by about 1.5 to 2 degrees
between them.
Hope that helps a little,
David
"andy c" <a...@nospammm.com> wrote in message
news:102198967...@iapetus.uk.clara.net...
andy c
sorry, i made a mistake - i meant to say it was retarding towards TDC at
high revs
> I'm not sure I understood your question correctly, but most snowmobile
> ignitions "advance" (spark occurs further BTDC) up to about 3,000rpm...
then
> begin to "retard" (spark occurs closer to TDC) as the rpm's increase.
yes, this is exactly what mine is doing - if i set my timing to 2mm BTDC at
2000rpm, then by 6000rpm it has retarded to approx 1.3mm BTDC
but why would it do this ? in all other engines i've worked with (cars!) the
ignition constantly advances (sparks earlier) as the revs increase to
compensate for the time taken for the fuel combustion to spread
if this is the correct design, what is the rationale behind retarding at
higher revs ?
my 'guess' of 2mm was based on hearsay and internet searches for this
engine, but i was hoping someone would have something concrete
many thanks,
andy
David Courtney
There are a couple of things going on at once that conspire against
"conventional" reasoning.
First, as the rpm's increase beyond a certain point, the turbulence in
the cylinder increases greatly... which allows the flame front to travel
much faster. So, it takes less time to burn all of the fuel in the
cylinder... and the pressure would increase too rapidly and cause detonation
if you kept increasing the amount of ignition advance as the rpm's increase.
(Even on four-strokes... the advance is normally "all in" by something like
5,000 rpm because of the increased turbulence beyond that point.)
This is especially true in a "squish band" cylinder head like nearly all
snowmobiles use... where the air/fuel around the perimeter of the cylinder
is "squeezed" into the center of the chamber by the reduced clearance of the
head around the perimeter. Squeezing the fuel inward at very high rpm
forces it into the center of the chamber at very high (squish velocity)
speed and turbulence... which requires less time to burn, so you have to
light the fire later (closer to TDC).
The second "factor" is that the porting and especially the expansion
chamber are really only effective over a fairly narrow rpm range, especially
on a high output engine like all snowmobiles. That means that as the rpm's
increase and you feel the motor come "on the pipe" as the dirt bikers say...
it is being "filled" much more efficiently.
Consider that the crankcase pressure can easily exceed 1.3 atmospheres
in a normally aspirated two stroke when it comes "on the pipe" and you can
see that it's a lot like running a supercharged or turbocharged four-stroke
with 10 pounds of boost coming on as the rpm increases. That would require
the timing to be reduced as the boost came in also.
Hope that helps a little,
David
"andy c" <a...@nospammm.com> wrote in message
news:102201793...@eurus.uk.clara.net...
andy c
:-)
a surprising number of 'experts' seemed puzzled at the retarding and
suggested i buy a new cdi
can anyone confirm that 2mm BTDC is the accurate initial timing for this
engine ?
thanks again,
andy
"David Courtney" <adv...@powercom.net> wrote in message
news:acek1n$p9p4r$1...@ID-70857.news.dfncis.de...
andy c
yes it's going on a hovercraft :-)
thanks for the link - unfortunately everything i can find on the rotax 503
relates to the ultralight late ducati dual ignition version or the early
points/ condenser version -
mine is single cdi nippondenso ignition which i believe was uniquely fitted
to the 503 ski-doo's made in the 1980's, so i was hoping someone here might
have an old manual or data sheet (i think mine came from an MX5500 blizzard)
thanks again for your help.
andy
> Hi Andy,
> I noticed that you checked with some of the ultralight guys... so I'm
> guessing you're planning to use the motor for something other than
> snowmobiling... which is fine.
> There's an article here:
> http://www.800-airwolf.com/pdffiles/ARTICLES/part22.pdf that you might
find
> helpful on timing ultralight motors.
> The 503 Rotax is listed at .086" BTDC which I believe is 2.2mm BTDC.
> Hope that helps, it looks like a good article.
> Good luck,
> David