Degreed my saw, now what?

[buck futter]

Thanks

thank you for your comments I have a lot to share with you but I am very busy today I will touch base with you on tuesday. any q's don't hesitate to pm me. or post it here and then others can learn.

 

[buck futter]

one idea worth of thinking when planning on porting, is to drill holes to sides of exhaus instead of widening them... makes those rings last alot longer and also decrease the risk of jamming the rings into exhaust...

I don't think this would be effective but unless people keep thinking like you did we will never get ahead.

 

[buck futter]

after I create a chamfer with a stone I take dremel bit that looks like wide scotch brite and run it over the chambe area, you can buy different grits but I haven't seen it do anything to the nikasil. You could cheap out and make one with a mandrel and a piece of scotch brite.
I say on your intake you need to blend the cylinder wall opening back to the intake manifold more.

 

[buck futter]

something I didn't explain before but I touched upon. when you measure a port opening the open and close should be the exact same from a reference point of tdc or bdc. to make it simple lets say a exhaust opens at 90 atdc it should also close at 270 atdc for a total of a 180 duration. What do you think the actual duration and port opening would be if you showed that it opened at 87 atdc and closed at 267 atdc? if you average the difference from tdc (87+93=180) and divide that in two you will see that you are actually opening at 90 and closing at 270 but you tdc is not perfectly aligned. you got it? this is why I recommended you check you numbers again. what if the intake opened at 76 btdc and closed at 84atdc, what is the duration and actual port opening/closing at?

so just figure your duration and divide it in half. you still need to bring you exhaust up at least 6 degrees. intake sounds ok.

 

[blis]

I don't think this would be effective but unless people keep thinking like you did we will never get ahead.

why it wouldnt be effective? ive seen such even stock engines (well, motorcycles, but 2 strokers still)... it gives you bigger exhaust area while not reducing lifetime of rings much (unlike widening exhaust port)...

 

[Four Paws]

Buck,

Thanks for the input. I was thinking along the same lines as you with the degree measurements of port open and close times and that my numbers were a little off - TDC is kind of tricky to find - the piston is moving so slow that I was kind of guessing at TDC.

As far as the intake - you think I should widen it more in the manifold area itself? I guess I didn't quite follow you. I am as wide as I want to go (for now) at the cylinder bore - and I have blended it evenly back to the intake opening at the carburetor boot surface without changing the shape of the hole at that surface.

Okay, I have a question - might seem silly, might not. Instead of blending the lower transfers on my cylinder, like I did in the following picture -

Would it be possible to completely remove the material I have outlined in red in the following picture -

In an effort to create a transfer set-up similar to the Dolmar 7900 cylinder pictured below?

I am just curious what the effect would be. I am assuming the material I would remove would then lower the crankcase pressure, causing a slower velocity of flow through the upper transfers. The case on my saw looks like it is machined for it, and the piston doesn't need any support in this area -

Again, this is just a question - like I said, I don't know, and would like some theory/logic behind what removing the material would do and why. I have seen some 372 Husky cylinders that have had the lower cylinder skirt cut WAY back and the lower transfers enlarged a huge ammount - just wonder if it would work in this case or not. Thanks for the help and replies.

Josh

 

[blis]

by shortening the skirt of piston you can increase the transfer time (time which transfers stay open), therefore allowing more mix into transfers...

and about the intake, the manifold side of intake should blend seamlesly into manifold itself to create optimum flow and to maximaze the flow you will want the intake port to be slightly narrow down towards cylinder...

ps. you might want to think about filling the bottom of piston with epoxy or something like that to increase the crankcase pressure (-> better flow trought transfers)

 

[buck futter]

why it wouldnt be effective? ive seen such even stock engines (well, motorcycles, but 2 strokers still)... it gives you bigger exhaust area while not reducing lifetime of rings much (unlike widening exhaust port)...

the reason is.........most of the time the skirt is the limit so you won't widen any more than .1" on each side, You could drill a .080 hole or two on each side but wouldn't you get mor area by taking .1 x .75" assuming the height is somewhere around 3/4 of an inch.
you also have to think about how the transfers scavenge the cylinder and push out the exhaust, misplaced holes will not help that flow.

hope this helps

buck

 

[buck futter]

by shortening the skirt of piston you can increase the transfer time (time which transfers stay open), therefore allowing more mix into transfers...

don't do this



and about the intake, the manifold side of intake should blend seamlesly into manifold itself to create optimum flow and to maximaze the flow you will want the intake port to be slightly narrow down towards cylinder...


This is exactly what I mean't thank you


ps. you might want to think about filling the bottom of piston with epoxy or something like that to increase the crankcase pressure (-> better flow trought transfers)

don't do this either.


just my opinion.

 

[blis]

well, they work well with motorcycles, but ofcourse, everyone is allowed their opinion and these are just suggestions and ideas how to get more power + every engine is unique...

not everything works everywhere...

 

[blis]

the reason is.........most of the time the skirt is the limit so you won't widen any more than .1" on each side, You could drill a .080 hole or two on each side but wouldn't you get mor area by taking .1 x .75" assuming the height is somewhere around 3/4 of an inch.
you also have to think about how the transfers scavenge the cylinder and push out the exhaust, misplaced holes will not help that flow.

hope this helps

buck

point taken, didnt think about transfers...

yet, the biggest point with those holes is that you can go beyond safe width of exhaust (normally around 75% of bore)... just imagine that exhaust as 3 port exhaust (2 holes + exhaust port)...

 

[Four Paws]

I got more time to work on the saw today. I will be putting the saw together tomorrow and *hopefully* getting it started and cutting some cookies. If I had the correct tools for the job, I would have been done by now, but I figured I would give porting a go on this saw to see how it turned out - if I had success I could justify getting the proper time-saving tools rather than slugging along with my Dremel.

I blended the intake port into the throat of the manifold more. I took the ports out to about .040" from the piston skirt. I also widened the exhaust out to about 62% of the bore diameter and made nice rounded edges, and chamfered the port at the cylinder bore - hopefully the port won't hook a ring.

I have determined that degreeing a saw is vague at best. There is lots of room for human and mechanical error - that is why I degreed the saw several times and averaged the numbers to come up with the final timing numbers below. The transfers are the hardest to degree as they are very difficult to see. I ended up using very thin steel wire to assist me in determining the open/close times. The duration numbers below are likely a touch longer than actual - but pretty darn close.

Intake - 170* duration
Exhaust - 170* duration
Transfers - 114* duration

The crown of the piston slightly blocks the upper transfers now that the cylinder has been cut. I don't want to raise the transfers as I would like to get a little greater blowdown than the 28* I currently have. At the same time, I don't want to increase the exhaust duration anymore than it is as I am afraid I will lose torque. After I get the rings in it and do a compression test, that will help me decide about raising the exhaust some more.

Josh

 

[Four Paws]

It's alive...

The saw roared to life today. Throttle response is overwhelming, and the gyroscopic effect of the torque when holding the saw is much more noticable. Unfortunately, the saw needs clutch springs - BAD! If you remember, I bought this saw with a burnt piston and rings, so I have never had it running before. I should have checked the clutch, but never thought about it. I decided to wait until I rebuild the clutch before I make any timed cuts.

Just wanted to post an update as I got some time to work on her today- so far so good! Not only did it start (goal #1) it seems to run dang good (goal #2). Cranking compression checked out around 180 PSI. As time goes on, and I get some run-time on it, I will go back in and maybe change a thing or two. Hope she is a real MONSTER in the cut.