Discussion in 'Chainsaw' started by NickC, Oct 31, 2019.
if you look at an 038 mag cyl,you will notice this on the intake port
He's not talking about the intake
@huskihl , pay attention.
With the piston at the tdc and the pointer at tdc 0°, it takes 97° of rotation to crack open the exhaust port.
So that's 83° more to bdc, then 83° back up to just shut = 166° of open duration.
The exhaust could be 97 but I doubt it. I don’t even take 70cc saws or larger that high.
The intake doesn’t make sense. I would assume a saw that small would have numbers around 110/128/70
On an 026 it is
Intake opens at 109 atdc, which is 71 btdc
The intake number suggested the OP was measuring wrong. 97 is surprising. How do these saws run stock?
Ahh there we go. Never seen someone word it the way the op did. I certainly don’t use that lingo to describe the intake.
so OP we have 97/121/71???
Yep, I was reading the intake aBdc not btdc...... Sorry about that....
Are you going to be machining on the jug? I’m curious how it would run with the exhaust more around 100-102. 18-20 blow down and still 78 intake. Take .040 off the band which would set your exhaust and get the intake closer to 78.
Maybe huskihl has more knowledge on the subject.
I've skimmed the base to get 0.02 squish.
I could skim the squish band and the base 0.04 to stay at the same squish, was hoping to avoid cutting the band as I'd have to make a fixture.... And the 026 has a domed piston and cylinder so I'd need to cut the piston I suppose, or try and follow the dome.
Oh I didn’t realized it was domed. Some of those domed cylinders you can get away with taking 0.020 flat off the band and still maintain an even squish. Otherwise just use the compound rest to cut the band at an angle. You’ll want to cut the base even more to get the intake down.
it’s up to you. I’m no expert on porting but I do have a good bit of experience just not much with small saws. Seems most the saws that come my way are around the 70cc range
I’d have to check my notes. I believe I take .050-.055 out of the band, cut the dome flat, and cut the base to get .018” squish
What’s your favorite exhaust height for this saw?
With a view to cutting the band, this is to lower the ports? These new jugs have a closer fit to the piston than the original OEM, I haven't checked the compression as is yet.
The sketch shows what I mean and the solder shows it clearly.
To lower the intake, increase compression. Most saws in order to get the exhaust where you want it you have to lower the jug in order to have decent compression after you raise it back up. If you took the exhaust from 104 to 98 compression won’t be that great afterwords. Make the combustion chamber smaller and you’ll actually gain compression even with raising the exhaust. I would imagine if you took the 026 jug and cut .040 from the band and base then don’t raise the exhaust at all compression should be really high afterwords and you’ll gain intake duration.
The advantage of closed transfers is that the runner can be routed away from the piston to better direct flow when it exits. Open transfers can work fine, but nobody tries to rout them far from the piston. To just hog out the separator will greatly increase case compression and runner volume. You'll have a lazy engine that won't rev - you'll have low transfer velocity and poorer mixing & loop scavenging.
As low as it’ll go
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