Timing advance

[spudulike]

Yes, ported saws are a bit different but I tend to tune them within 500rpm of the max to be safe. The spark plug colour has the final say!

[Echo]

The faster the engine turns, the shorter the available window in time

becomes in which to get fuel mixture in, ignite it, burn it and go on to repeat 

the process, so, light it too early, and you will be forcing the piston 

back down before it even reaches the top, this robs the engine of useful

power, and if taken too far, will wreck it too late with the spark, and other forces take over, the temperature rises in the engine due to more unburnt fuel being even more compressed, heat builds to dangerous levels, and pre ignition occurs, if this cycle is repeated, you will have an aluminum nugget.

 

As said already, it takes a certain amount of time to ignite the fuel mixture, and for

the fuel to burn, the end product of the burning is that the released gases

expand and push the piston down, its called phase shifting, when the fuel goes

from a liquid to as gas as it is compressed and burned.

The process of phase shifting also removes the heat that was present under

compression thus reducing that momentarily very high temperature that existed

as the fuel became compressed.

 

So the advancing of the spark is to get the fuel ignited and burned in time so the

engine can move on to the next rotation, and it depends on the fuel being used,

the compression ratio is also a big part, for if it is so high the fuel can ignite under the pressure and resulting heat,

this is bad,  if the engine is so abnormally hot this too causes pre ignition,

in both cases we now loose control of when we light the mixture, and we really need to be in control of when to spark things off, sparking things off at the right time also helps reduce stress on the engine, the spark at the right time allows the fuel mixture to burn in a timely manner, any other method of ignition such as too much heat or pressure or both that lead to pre ignition causes very abrupt burn, which puts the engine under much more stress, it can even shatter the spark plug insulator amongst other sorts of damage,

bearings get an awful wallop, so do rings, and piston.

 

There is a lot to it, I am just scraping the surface.

 

 

Edited January 31, 2019 by Echo

[Echo]

On 26/01/2019 at 16:28, Gerbutt said:

Yeah it’s easy enough with diamond files, and I just did it a little bit. It does feel more nippy, and I’m sure it started a tad better too

What width was the key, and if you had to measure, how many mm or thou did you take off,

if you could convert what you took off into degrees it could help understand what your timing is now at. What was the reason you altered the timing, did you loose out on revs or gain.

[bmp01]

On 26/01/2019 at 09:49, Gerbutt said:

 

Hiya

 

I’ve been pondering about timing advance and I understand what happens when you do it, it sparks a little earlier. What I don’t understand is why this is an advantage and how come it seems to give more power? I’ve seen vids where a 200/201 had been done and it looked quite a lot better, faster in the cut and holding rpm better. Why is this?

 

Why does saw go better with a bit more advance ? Here's my go at a simple explanation. ...

1) There's an optimum igntion advance for performance. 

2) What's optimum for one engine might be too much for another dependant on engine tolerances,  engine maintenance,  bad fuels,  bad tuning., etc, etc

3) Manufacturers don't like their saws exploding,  gives them a bad reputation, sales don't do too well, so they design in a bit of safety and retard the ignition a bit.

4) Advancing the ignition a small amount is just getting back to optimum.  

[Gerbutt]

15 hours ago, Echo said:

What width was the key, and if you had to measure, how many mm or thou did you take off,

if you could convert what you took off into degrees it could help understand what your timing is now at. What was the reason you altered the timing, did you loose out on revs or gain.

Interesting read above dude, thanks for that.

 

The key is built into the flywheel so i couldn't measure it.  I only took a smidge off, probably a 1/4 or less. Id read that folk take a 1/3 off but didn't want to risk going to far.  Infact, when fitting the flywheel again i couldn't really tell if there was anymore play when moving it forward it was such a little amount.  I advanced the timing just because I'd read that's what people seem to do, and I was interested to see if it made a difference. The saw seems a bit more responsive, and it's probably just my imagination but it also seemed to start better - not that it really had a problem starting before.  I'd just ported and raised the comp so it was just part of that.  All seems good though, will have to wait until i've something large to cut up to see how it performs. At least it works hey

 

[Echo]

Good to hear all went to plan, especially if you improved starting

by advancing. Some engines definitely have room for improvement,

read about people modding the Makita 6100, timing was advanced

and those engines did not like it at all, they were put back to where they

were coming from the factory, I guess they got the timing right.

 

Thats sneaky building the key into the flywheel, I have little love left

for Stihl.

[Stubby]

4 hours ago, Gerbutt said:

Interesting read above dude, thanks for that.

 

The key is built into the flywheel so i couldn't measure it.  I only took a smidge off, probably a 1/4 or less. Id read that folk take a 1/3 off but didn't want to risk going to far.  Infact, when fitting the flywheel again i couldn't really tell if there was anymore play when moving it forward it was such a little amount.  I advanced the timing just because I'd read that's what people seem to do, and I was interested to see if it made a difference. The saw seems a bit more responsive, and it's probably just my imagination but it also seemed to start better - not that it really had a problem starting before.  I'd just ported and raised the comp so it was just part of that.  All seems good though, will have to wait until i've something large to cut up to see how it performs. At least it works hey

 

Did you shim the the same amount back in the slot top prevent it shifting ? 

[spudulike]

No need, you have to be careful in the way you retighten the flywheel nut otherwise the flywheel will rotate slightly and the timing will go back to standard. The key just gives you the correct ignition timing, it doesn't hold the flywheel in place, the fit to the crankshaft and the nut does that.

You also have to take down the correct side of the key otherwise you will retard the ignition timing.

[Wonky]

Umm.., ask me how I know that..?

[Wonky]

A better pic...of my mishap

 

Well not really  a better pic, so I edited it...

 

sorry.??

Edited February 1, 2019 by Wonky