From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Lehanover@aol.com
Sent: Sunday, July 17, 2005 7:54
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Startic
Timing / TDC
In a message dated 7/17/2005 5:50:54 AM
Pacific Daylight Time, finn.lassen@verizon.net writes:
Hi Mark,
I have read this a dozen times; I cannot make sense of it!
What do I do after I count the teeth? And what tooth do I place the mark on?
Once you have marked the two teeth, the
TDC mark should be added half way between those two marked teeth.
So, mark the two teeth that correspond to
the apex seal in the center of the spark plug hole. Count the teeth between the
two marked teeth. Divide that number by 2.
For example, say there are 17 teeth
between the marked teeth.
Half way will be the full tooth 8
teeth from each of the marked teeth.
If the number is even, say 16 teeth, half
way will be the throat between two teeth.
Just put a paint mark there in the
throat.
Make the TDC paint mark a different color
from the first two marks.
With the TDC mark under the pointer, at
the flywheel or flexplate, sight along one of the water pump mounting studs and
across the timing pointer on the front case to the pulley.
Move your head back and forth so that
when looking past the correct stud (like sighting a rifle) half of the pulley
will appear on each side of the pointer on the front case.
Make a paint mark on the pulley. ( I file
a groove in the pulley flange and fill it with paint ).
That takes care of the TDC mark.
Count the total number of teeth on the
flywheel/flexplate.
For example, let us say that there are
110 teeth.
Divide 360 (degrees in a circle) by 110
teeth to get 3.27 degrees per tooth.
So, now determine what total advance
you want to use.
Tracy has the most time on a 13B in
aircraft so check his site for the number.
In a racing, 12A NA engine 20 degrees is
fine up to 9,000 RPM. In a racing 13B NA engine 25 degrees is plenty up to
9,000 RPM is plenty.
These numbers may sound conservative, but
the rotor (piston) is moving at 1/3 crank speed, so there is ample time to get
a full burn and develop the highest cylinder pressure at the ideal crank
angle, which is the whole idea behind correct ignition timing.
For example, let us say that you want 25
degrees of total advance.
Divide 360 degrees by 3.27 to get 7.64
teeth.
Count off 7.64 teeth from your TDC mark
on the flexplate/flywheel, in the direction of rotation.
You may have to turn the engine backwards
slightly (top of the flywheel moving toward the intake side) to get to this
tooth.
In this case, the total advance mark with
nearly be identical to one of your first apex seal marks. (8 teeth). Your
actual tooth count may be different.
Make that tooth red and white to stand
out as your total advance mark. Leave this mark in front of the pointer.
Go back to the front pulley. Sight along
the pointer/ mounting stud combination and again mark the pulley with the full
advance mark. Using the same color as on the flywheel full advance mark would
be good clear thinking.
Timing from the flywheel end marks will
be a bit more accurate. Timing from the pulley marks will be further from the
prop. In the event you want more distance there.