ignition timing on modded saws

[Artsi_L]

Hello

I've read this forum on saw modding. Many threads revolve around cylinder porting and tuned exhaust pipes.

The little understanding I've accumulated on 2-stroke engine tuning points towards taking into account factor of fine tuning of spark ignition event.
As tuned exhaust pipe elevates cylinder's volumetric efficienfy, needed spark advance should decrease. In other words, we'd need to retard spark advance where the engine is happy.

I fear some modded saws out there with stock ignition timing are "knock limited" as it comes to power/torque delivery.

I am yet to check my saw's stock ignition advance, but I reckon it'll be more than 2-stroke motorcycle engines with tuned pipes.

Comments please.

 

[Artsi_L]

No responses in this thread. Maybe I should have written shorter.

Question: Does the saw benefit from moving the ignition sensor for slightly retarded timing when installing tuned pipe exhaust ?

 

[mtngun]

I've never played with saw timing, or for that matter, with a pipe, so I didn't respond to your original question, even though it's a very interesting subject.

I'm not sure that there is a simple answer. Optimal ignition advance, in theory, should depend on the design of the combustion chamber, and on RPM.

One problem is that it is difficult to use a timing light with a running saw. I believe the procedure is to cut a window in the starter cover.

 

[Artsi_L]

You are right. Combustion chamber style and rpm are goverining factors in ignition advance. There is at least one more important factor: cylinder filling (i.e. volumetric efficiency).

Either in 4-stroke or 2-stroke engines required ignition advance is reduced when cylinder filling is increased.

In 4-stroke engines this is evident by looking at turbocharged engine's ignition advance tables when the engine is on turbocharger boost. Ignition advance gets reduced by about 15 degrees when turbocharges comes into play.

In 2-strokes we have the same "turbocharger" effect through tuned pipe.

Stock saws have only simple exhaust muffler box which has zero effect in elevated cylinder filling. I believe manufacturers have engineered ignition advance to max out power delivery with this combo.

Slap on tuned pipe on such saw, and I believe our stock ignition timing is holding back our saw. More power can be had by retarding the ignition event.
What stock timing does, is to ignite combustible mixture far too soon, and pressure peak exists too soon, where crankshaft is not yet in it's most ideal position (at about 17 degrees past TDC).

Please have a look at enclosed Yamaha 2-stroke motorcycle engine ignition advance curves. By my wild guess, the tuned pipe seems to come on at about 11 000rpm and finishing at about 14 000rpm. Have a look at ignition advance requirements what the tuned pipe does to the engine.
Ignition advance requirements are way different at lower rpm's when the engine is not "on the pipe".

There are many different ignition curves. I believe that this is a comparison between different available Yamaha ignition boxes, suited for different combos of tuned pipes and differently ported cylinders.

I believe many piped saws would benefit from retarding the ignition sensor by say 5 degrees (or perhaps even more).

 

[cheshire cat]

timing mod

I guess there's not much responce as no one has tried, on a sealed system I supose the only easy option is to move the coil radially, however I understand it's not that easy, to "mill" out the screw holes in order to do this as it will short out the layers of plates, room for any more a more complex movable fixing would seem limited. I'm new to this stuff but have already thought about it altho' it's not high on my agender as yet, keep us posted if you take it further cheers Nic

 

[cheshire cat]

forgot about stepped woodrough keys:redface:

 

[Terry Syd]

On many of the saws the 'woodruff key' is actually cast into the flywheel. If you want to change the timing you have to nip just a bit off the side of the cast 'key'. It doesn't take much to get a couple of degrees of change.

Another approach to changing the rise time in the cylinder is to try a projected tip plug. It may be that a different position of the spark in the combustion chamber will advance the timing (a stock saw usually has some 'safety factor' built in, so it may respond to a bit of advanced timing). However, as stated above, if you increase the the cylinder filling or trapping efficiency, you may end up retarding the timing a bit to get it in the sweet spot.

 

[trappermike]

I rather doubt that many of today's saw ignitions have any type of advance/retard built in. On some sophisticated racing engines there is a retard built in at high speed.
Initial ignition timing can sometimes be altered slightly(By slotting the ignition unit mounting holes or an offset and hardened woodruff key),about your only option is to experiment with that...
Some of the older points ignition systems can be upgraded with an electronic triggering module and they usually work good.

 

[one.man.band]

old old dirtbike trick was to remove woodruff key.....coat shaft with valve grind compound then turn flywheel in direction of spin. not the best way. offset key would be better. but key does not really come into play unless motor takes a very large jolt. could be of benefit however to find offset for new key pattern.

with just larger holed muff.....5 degrees maybe much.... never tried but will if time permits.

i will leave the piped hot saws to the pros.

 

[WidowMaker]

old old dirtbike trick was to remove woodruff key.....coat shaft with valve grind compound then turn flywheel in direction of spin. not the best way. offset key would be better. but key does not really come into play unless motor takes a very large jolt. could be of benefit however to find offset for new key pattern.

with just larger holed muff.....5 degrees maybe much.... never tried but will if time permits.

i will leave the piped hot saws to the pros.

===

What was the purpose of this ???

 

[wigglesworth]

I rather doubt that many of today's saw ignitions have any type of advance/retard built in.

The stihl ms460 and ms660 are two examples of saw ignitions with a built in advance curve in them.

 

[one.man.band]

....advancing ignition in old dirt bikes using the method i mentioned, was used to move peaky rpm powerband hit of the motor lower, at the expense of high rpm roll off based on track type.

 

[gink595]

What if you are running ally with say 40% nitro, do you still think it would need retarded?

 

[Naked Arborist]

offset keys

Offset keys was the way we did the old dirt bikes in the 80's. Granted we were running bikes from the 70's with no built in advance curve in the system. As far as saws go, I would think most of the timing mods should be answered by the race saw builders if they are willing to let out some of their secrets on ignition timing.

Seems like it would be a slippery slope to tackle if your going to change the base ignition timing on any motor you not familiar with the factory built in timing events. IMO I would only retard the ignition timing so it would not cause any damage to the engine from a timing event that was too early. Too much advance on the timing always seems to be a bad thing for the bearings and the connecting rod. If you have ever ran too much overall advance at WOT on a piston powered engine you would understand the damage that can occur in just a few seconds. Trust me, it's all bad news for the bottom end of the engine.

If it was a question about the Generation 3 and 4 small block Chevys I could point you in the right direction.

 

[one.man.band]

well worded naked arborist!

after rereading my post....realize that my reply could be misinterpreted to mean advancing timing is necessary.

the 'METHOD USED TO CHANGE TIMING' was my intent.

most likely 'advancing' timing is either necessary or warranted for a chainsaw application. for higher rpm usage the opposite (retarding) would be seem to be appropriate. changing timing on a motor without variable timing advance, by this method, changes timing values by X degrees over entire rpm range as NA mentioned. a tiny change using this method goes a very long way.

by retarding ignition over the entire rpm range, the saw should still start as easily as before... and take more heat of piston at higher rpm since spark occurs closer to TDC.

can't think of any danger trying 'retarding' timing. advancing timing is a different story however.

motor familiarity with is of the 'wheeled' variety as well. those were the days as they say.

 

[NHlogger]

Ignition timing

Advance advance advance! Always run your timing as close to detonation without detonation. For saws remove your key use valve lapping compound on your crank lay your flywheel on crank and mate the two. Now all you need is a degree wheel and a timing gun and listen for knock through good ear muffs. Start with half a key way advance and a drop of red lock tite. Boosted applications only drop timing to prevent detonation due to the add compression. Alky or high octane 100 and up will always enjoy lots of timing. Better throttle response to help the weighted flywheels get going and excellent top end. More compression less timing more low end torque, low compression more timing better top end horsepower.

 

[RTK]

Advance advance advance! Always run your timing as close to detonation without detonation. For saws remove your key use valve lapping compound on your crank lay your flywheel on crank and mate the two. Now all you need is a degree wheel and a timing gun and listen for knock through good ear muffs. Start with half a key way advance and a drop of red lock tite. Boosted applications only drop timing to prevent detonation due to the add compression. Alky or high octane 100 and up will always enjoy lots of timing. Better throttle response to help the weighted flywheels get going and excellent top end. More compression less timing more low end torque, low compression more timing better top end horsepower.

That's pretty much the way we did it way back in the day when building street rods and pull tractors

 

[stihl038x2]

subscribing........ opcorn:

Steve

 

[steve316]

tuning

in drag racing it is still done the same way. you keep advance till your car slows down.displacement and compression plays a large roll.smaller engines always took more timing. if a dyno is used you can by pass about 80 percent track testing.as far as chainsaws would't a dyno be great.:smile2:Steve

 

[064AV]

064/046 timing

A part of what led me to the issue of timing was noticing that unlike my 064 or old 066, a half-*ss pull on my 046 will result with a violent removal of the starter handle from my hand. So I am in the process of replacing the 064 alloy flywheel with an 046 poly flywheel. As near as I can tell, this advances the timing by at least one half the width of the woodruff to maybe the full width. This is more than I can correct by moving the coil, so I'm working on modifying/relocating the key. I've got the flywheel on, and the saw is running; however I'm concerned with detonation and top end damage under load. I've heard detonation in a V8, what will it sound like in a chainsaw? I'm running premium fuel so I should have the ability to run more advance without detonation.