Porting an 090

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Hey Pioneerguy, I think I just got the twin of the saw "Lawrence the Enabler" sent you from BC!! I just picked this one up in the BC interior (thank you CL!) It's got the original 5 foot stihl duromatic bar, original (never sharpened) stihl full deck chain, and a 24" bar (a Draper Penfield "Wear Tough"). The guy was the original owner and cut 12 slabs with it and put it away..., maybe 3 hours running time, and the Stihl and 090 labels on the saw top still have their protective tape on them!
Great thread on porting, but I think this one's gonna stay stock.:cheers:

That is a beauty - better and cheaper than a new one you could buy overseas. Welcome to the 090 club and I'm jealous.
 
I don`t think there is a formula for leaving enough cylinder wall for the piston ring end gap to be supported on. I will leave around 100 thou or 1 mm out past the pins for the rings to ride on.

I don't know that I've read a hard fast rule on this either. But I'm much more cautious about leaving plenty of surface for the ring end to ride, than I am about skirt edges. The reason being that you'r leaving such a small area of NiSi to ride on. I fear that if it's too narrow, the ring may wear through it prematurely. Maybe not. Reguardless, I like to leave .100" when I can. I'm expect you could go a little less than that, but 1mm is only .039". That seems a little narrow to me.
 
FWIW, I have no experience with the 090; scrutinize my comments and make your own decision. But as a few general comments:

I like that you are using port maps. It is good to have a plan when doing a project, and IMO the best way to go about making a jug perform better.

The engineers aren't dummies. They can design these motors to do exactly what they want, though there are always compromises. What they do not have is much (any) oversight in quality control over modular components. Parts may even be built by different plants, maybe sometimes even different companies. The result is a motor, that once random parts are assembled, passes spec, but it not exactly what is on the drawing table. Alignments may be of a few thousandths here, casting flaws there, cutter marks, port mismatching, etc. etc. Point being, a basic blueprinting of the motor, simply getting the motor to its original spec, will make an significant difference in performance. Throw is an exhaust mod to correct for emissions standards, and you have a whole new baby.

That said, my conjecture is that your transfers are a bit aggressive. You've got a lot of experience, so you know it only takes small changes to make big differences. I think you are risking a loss of flow velocity (or at minimum, designed scavenging) by making such a drastic change. Ostensibly, it would be fine if you were trying to make a race saw, but you said you wanted it to stay in the 8500 RPM range. You can fill the combustion chamber with air using low volume and high velocity transfers or high volume and low velocity transfers. This saw is already turning low RPM, so increasing the volume of the paths the air is traveling, so drastically, at such low RPM, is potentially going to slow the velocity. Like said, if this were a 12-14K race saw, then the rules change. Metal is easy to take off, but a PITA to put back on :laugh:

When it is all said and done, whether you are running 8500-9500 RPM, I think you'll notice the best performance gain in cutting by getting a much larger rim. That is, unless you do a lot of cutting of 5-6' trees, in which case I am very jealous :laugh:



Very grateful for your input and information on this mod, I have done many smaller bores that rev higher and many bigger bores in MC`s + car engines. This engine is a slower turning variety and as I am studying up on it along with others who have been kind enough to submit info I have rethought this port job and changed my thinking several times. I certainly will include your thoughts and info into the future port job, many thanks.
Pioneerguy600
 
I don't know that I've read a hard fast rule on this either. But I'm much more cautious about leaving plenty of surface for the ring end to ride, than I am about skirt edges. The reason being that you'r leaving such a small area of NiSi to ride on. I fear that if it's too narrow, the ring may wear through it prematurely. Maybe not. Reguardless, I like to leave .100" when I can. I'm expect you could go a little less than that, but 1mm is only .039". That seems a little narrow to me.

I agree completely, once the lines are laid out on the cylinder surface I will know just by looking at shapes and sizes what to leave and what to take out.
I will most certainly leave over .100 out past the side of the ring locator pins. I have rethought the upper transfers and have changed my mind more than once since posting that last port map, actually that is why I am playing around with this paper map, it is much easier to make changes on it than on the cylinder once I start to cut. Good thing that I have several other saws undergoing P@C port work right now, its giving me lots of time to rethink the whole porting job.
Pioneerguy600
Pioneerguy600
 
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