converting inches to degrees

[1hander]

hello guys,
im trying to figure out how to convert the numbers from the service manuals for timing settings
the conversion is .017 inch per degree...
for the 015L
maximum ignition advance is .090 inch BTDC
so .090/.017 is 5.29 degrees
is that right 5.29 degrees BTDC

 

[Old2stroke]

Something doesn't sound quite right. 0.017" of what? If you are shaving down the flywheel key, a ballpark figure depending on the diameter of the crank is, shifting the FW by 0.010" will produce 5 deg of timing change. When you say max advance is 5.29 deg, is that the max change from basic timing that they want you to make? Most saws with variable timing built into the ignition module will have a timing of somewhere around 10 deg BTDC at starting rpm and will advance to 25-30 deg at top rpm. Need more info on what that service manual is doing.

 

[1hander]

well. what im trying to do is cheat i guees and figure out what the actual timing us in degrees without making a degree wheel.
the service manuals for the 08 and 015lL give settings in inches and mm
the points should start to break open at 2 to 2.2 mm btdc
i want to conduct some testing rpms vs timing settings but i wanted to fo it with degrees instead of inches you know
i think the reason my number is so far off is that the rule of thumb of .017 inch per degree is based on a 1 inch radius circle
and the cicrle in the case of the 015L would be the stroke which is 1.12 inch
which would be a bit more than double my finding of 5.29 BTDC To about 10.5 BTDC or so which seems much more reasonable
im sure it would be much easier and better to just put an electronic conversion and get the best performance that way and leave it.

 

[SteveSr]

360 degrees = piston stroke but it is NOT linear! It is a Sin(x) function where the amplitude of the sine wave is equal to the piston stroke.

 

[1hander]

360 degrees = piston stroke but it is NOT linear! It is a Sin(x) function where the amplitude of the sine wave is equal to the piston stroke.

OH my goodness that sounds a bit over my head

 

[Canyon Angler]

Here, maybe this will help (just fill in any two values and it will calculate the others):

http://www.carbidedepot.com/formulas-trigright.asp

 

[rupedoggy]

Stevesr isn't the stroke only 1/2 of the 360 degrees?

 

[1hander]

I think it's gonna best that I make a degree wheel and mount it to the crank.. That way I could find the degree number that correlates to the 2.0 to 2.2 mm btdc that is recommended... Once I do that I'll ve able to easily figure out the correlation between the degree wheel and the dial indicator.

Seriously I love numbers.. But this one spu ds like equations that I don't want to tangle with..

The method I tried to use is actually dependant on the radius of the circle being used.. And as stated above wont apply to linear motion of the piston..

 

[Huskitoter]

Edit.... This was incorrect because I failed to divide the stroke by two.

 

[1hander]

Damn your some kinda genius
That number seems much more in line with what it would be

Could you break that down step by step for me... I'd really like to remember and use that math

 

[Huskitoter]

The stroke is 1.12" = 28.45mm (multiply by 25.4).

The radius is half the stroke. 28.45/2 = 14.225mm. This is the hypotenuse.

The length of the adjacent side is the hypotenuse minus the advance.
14.225 - 2.1 = 12.125

Cos (x) = adjacent side divided by the hypotenuse. 12.125 / 14.225
x = arccosine (12.125 / 14.225)
x = 31.5

I used this site to lookup the arccosine.
https://www.rapidtables.com/calc/math/Cos_Calculator.html

 

[Huskitoter]

For .090....

.09" x 25.4 = 2.286mm
Adj side length is 14.225 - 2.286 = 11.939
Cos (x) = 11.939 / 14.225 = .8393
x = 32.9

 

[Huskitoter]

I'm guessing the point that the contact closes isn't the "real" timing because the discharge delay. Right?

 

[1hander]

I was thinking of that as well there's a delay from the instant the points open and the time it takes for fthe current to reverse and build the voltage in the secondary high voltage coil
and then to make it to the spark plug..etc etc etc

Man I thought the fórmulas were the problem but the delay time is the actual bigger variable I think
just to be clear, would the above formulas mean that the points gap is opening at 31.5 deg BTDC

so the manufacturers recommended timing settings are
2.0 to 2.2 BTDC
30.7 TO 32.29 deg BTDC
do i have that right

 

[1hander]

if my numbers are right, ir=ts much simpler than i thought , but only of course thanks to cosine and arccosine converters on my phone
that is more than my head can tangle with but the
way you broke it down made it make sense boss, i cant thank you enough...
I OWE YOU A BEER or is it I OWE YOU A SAW

 

[Huskitoter]

That looks right. Glad to give it a try.

It would be interesting to measure it with a timing light.

 

[1hander]

That looks right. Glad to give it a try.

It would be interesting to measure it with a timing light.

Yes sir it would i want to verify.. Although I always say that besides the words actually coming from God's mouth... Numbers are as close to hearing the word of God we will get on this side of the grave.. They are absolute truth no more no less

I want to make a degree wheel as well.. They should all match up very closely

 

[1hander]

Another question... If I advance timing to the point of kickback during startup.. Does that theoreticaly I'm in I'm in the preignition zone during operation at wot..I would think yes but I'm not sure

 

[Franny K]

Not only is it a sin or cos function there is a connecting rod involved.
With a two cycle dirt bike I think 15 degrees is about what static timing would be if no advance either mechanical or digital exists.
It is pretty easy to get a repeatable spot for top center with a pencil, eraser end if the plug hole allows. The circumference of the flywheel divided by 360 will allow you to mark out 5, 10, 15, etc. A little white paint helps. A timing light that plugs into the outlet or a 12 volt battery works on dirt bike for me.

 

[1hander]

i was planning on marking the fywheel like you mentioned ...