port timing for modded saws

[JasonBmac]

It's not hard to get dead nuts on with the degree wheel.

Glue on the wheel "close". Insert an object (I use a piece of copper wire) into the exhaust port. Rotate the flywheel each way, and note where the pointer points at each contact with the wire. Bend the pointer wire until each "stop" happens at the same number of degrees each side of bdc.

If you can't be within a quarter degree, you're not trying.

J

 

[Tzed250]

It's not hard to get dead nuts on with the degree wheel.

Glue on the wheel "close". Insert an object (I use a piece of copper wire) into the exhaust port. Rotate the flywheel each way, and note where the pointer points at each contact with the wire. Bend the pointer wire until each "stop" happens at the same number of degrees each side of bdc.

If you can't be within a quarter degree, you're not trying.

J

.25 deg. on a 4" wheel is .0084"


.

 

[gink595]

I just drew a to scale model of a rotating assembly and timed the exhaust at 100* for example, this has a 40mm stroke and a 54mm bore, same as a 066, I guessed on rod length, but the longer the rod the greater the error. But for what I drew if a guy was .060 off of center the margain of error showed 3.5 degree's. I'm not sure how much 3* off would effect the total picture but 3* here and 3* there would start to add up, especially if your trying to do more than one saw the same way.

 

[wigglesworth]

I just drew a to scale model of a rotating assembly and timed the exhaust at 100* for example, this has a 40mm stroke and a 54mm bore, same as a 066, I guessed on rod length, but the longer the rod the greater the error. But for what I drew if a guy was .060 off of center the margain of error showed 3.5 degree's. I'm not sure how much 3* off would effect the total picture but 3* here and 3* there would start to add up, especially if your trying to do more than one saw the same way.

That is some good info Gink. It sounds like it should be secured to the flywheel via the flywheel nut, not just glued on.

 

[volks-man]

I thought so

A couple things to think about. Go too high with the transfers, and you start pushing exhaust down them and hindering flow and RPMs. Raise the exhaust too high, and you loose compression and torque. Lower the intake too far and you start loosing base compression. It's all a balancing act. And some saws like what another saw doesn't. For example, a 7900 likes more blowdown than say a 372.

quick post while i'm home for lunch:

so, what i gather is that a big jug-small case saw will turn high rpms easily.

so what i want to do is keep the exhaust low to maintain torque. dropping the cylinder just a bit would help this.
what about intake? with the big jug-small case saw i will still have the tendacy for a lot of spitback? short of installing a reed valve (bad at high rpm) i don't know how to compensate except to raise the transfers so the case pressure bleeds in to the cylinder more than back out the intake. just don't raise them so much as to force exhaust down the transfers.

am i on track here?

 

[Meadow Beaver]

i wouldn't even expand the ports on the saw. i am concerned with port timing only. if the timing needs to be adjusted then i would go ahead and port for gains as well.

basically lets say i want to drop a 066 jug on an 026. with that tiny case volume, when the piston drops, there is a lot of fuel/air displaced. i would assume i would need the intake to close as soon as possible so not to spit it all back to the carb.
i also assume i want the exhaust to close as soon as possible to retain the charge before it is forced out by the high pressure of the tiny case. i can't do both at the same time without a TIG.
i am not real sure about the transfers though. just extend them all the way down the cylinder (sorta increase case volume) and make them bigger and hope for the best, i'd guess.

thoughts?

You're not even talking work saw then, an 066 p&c on an 026 would be a huge money pit.

 

[gink595]

That is some good info Gink. It sounds like it should be secured to the flywheel via the flywheel nut, not just glued on.

Yes that I would think is the only way to do it, guessing will only get you so far. I've never understood how that can be accurate, maybe that is the draftsman in me and overthinking this but I know when I'm off a little bit in ACAD it shows up big time on other measurements, I figured this to be no different on degreeing a saw

 

[troutfisher]

I use a magnet to attach the wheel to the clutch. I need to be able to reset the wheel to different zeros, so glue won't work for me.

 

[CHEVYTOWN13]

I do the same. I use a couple of rare earth magnets and triple check the readings from different starting points.

 

[ronT2]

Thanks gink for pointing out the error that having the degree wheel off center can cause. I’ll still use my hot melt to attach the degree wheel and a copper wire and piston stop to find TDC. But I’ll switch to the degree wheel below so that I can center it on the end of the crank better.

 

[Jacob J.]

Somebody told me about making a degree wheel out of an old CD and then attaching it to the PTO side of the crank. It works quite well.

 

[Terry Syd]

Tdc

I bought a new piston for my Husky 450 to try some further timing mods. The first piston had a wrist pin offset of .02mm and essentially split the timing either side of TDC.

The new piston was a different casting and machining from the first, it had a wristpin offset of .16mm. Because of the rod angularity it shifted the opening and closing times of the ports 1.5-2 degrees.

I was only looking for 2 extra degrees of blowdown, so it was critical that I had the proper rotation of the crank when I measured the timing.

 

[parrisw]

A ball park number, that is not for all saws would be:

Exhaust 100
Transfers 120
Intake 80

Is that a duration? or is it actual opening timing

 

[Jacob J.]

Is that a duration? or is it actual opening timing

I'm pretty sure those are port opening times after TDC. Durations are usually in the 150-165° range on exhaust for a lot of modern saws.

 

[volks-man]

jacob,
am i on track in thinking that small case-big jug saws need the intake to close quick as possible?

excessive blowdown should not be too big a concern in that situation?

 

[Jacob J.]

jacob,
am i on track in thinking that small case-big jug saws need the intake to close quick as possible?

excessive blowdown should not be too big a concern in that situation?

Well...

In my mind (which is coagulated with the remains of 10,000 P-nuttles), yes in some cases. It would depend highly on the stroke length vs. the bore diameter of the motor.

Take the case of the PM275/Skil 1680. These are very large bore/short stroke saws with small mixing areas in the case. They have flat, wide intake and exhaust ports with short durations. They have a narrow power band but when you're in it, they're dead on.

Now in the case of many of the Dolmar saws- these have a smaller bore with a longer stroke (with large mixing areas) and they have wide power bands which are very forgiving at a wide RPM range.

One thing I've noticed on the Bailey's big bore cylinders- they almost always have less blowdown and shorter port durations than the equivalent OEM cylinders, which leads me to believe that Bailey's is shooting for a medium power band with the saw essentially making its peak power at a lower RPM.

 

[volks-man]

you are above my pay grade a bit again.

i'm thinking i should just slap 'er together with widened exhaust, forward aimed transfers, and leave the intake alone. if i wind up dropping the jug i'll have to hope for the best.

it never really hurts to open the lower transfers all the way down, does it?
i would think in my scenario it would help increase the effective volume beneath the piston at mid travel.

 

[Jacob J.]

What's the transfer arrangement now? Quad ports or the standard dual closed transfers? Aiming forward as in towards the exhaust? Is this is a windowed or slab-sided piston?

Dropping the lower transfers will buy you some mixing time but slow your velocity a bit. However if your case mixing area is really that small then it would help for sure (in my cloudy judgment.)

 

[volks-man]

i don't want to be too specific as this project is still in the skunkworks......
but i did mention it to you in a pm a week or so ago... remember? hush, hush.

anyway, i meant aim the transfers at the intake.

 

[Jacob J.]

Look, I told you already that you can't put an 090 cylinder on a Poulan 53 case. Beside the fact that it's a lot of work, Hans-Peter Stihl would probably show up wanting to confiscate your saws...

I think your ideas are solid and I would say go for it.