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HELP! How much does machining base change timing?

Shane¹

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I think I have seen this before but I am trying to figure out how much you need to machine off the base to gain 1 degree of intake timing? 42 mm stroke. I thought I have seen @srcarr52 talk about it but may be wrong about that.
 
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Powerstroke Cowboy

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I don't think there's a number that can be nailed in stone.

Wouldn't that all depend on length of stroke?
 
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srcarr52

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It also depends on the rod length and it's only an estimate as it will vary some as the intake timing changes. The equations to go from port height to crank angle are in A. Bell "Two-Stroke Performance Tuning".
 

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This is what I have noted, if incorrect someone please let me know…

To find thousandths per degree:

Piston stroke in mm / 25.4 = aaa / 180 = thousandths per degree of timing



Example:

36mm / 25.4 = 1.417 / 180 = .00787” per 1 degree of timing…

So…
42mm/25.4= 1.653 / 180= .00918” per 1 degree of timing
 

huskihl

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This is what I have noted, if incorrect someone please let me know…

To find thousandths per degree:

Piston stroke in mm / 25.4 = aaa / 180 = thousandths per degree of timing



Example:

36mm / 25.4 = 1.417 / 180 = .00787” per 1 degree of timing…

So…
42mm/25.4= 1.653 / 180= .00918” per 1 degree of timing
That doesn’t work because as the crank approaches 0° and 180°, it still rotates but the piston barely moves vertically. And then at 90° the piston moves the same amount as the crank.

It is likely the average, which only takes place at exactly 45° and 135° before/after TDC
 

Smoke_Run

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That doesn’t work because as the crank approaches 0° and 180°, it still rotates but the piston barely moves vertically. And then at 90° the piston moves the same amount as the crank.

It is likely the average, which only takes place at exactly 45° and 135° before/after TDC
What's the right way to do it then?
 
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huskihl

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What's the right way to do it then?
There are programs available where you enter the stroke, length of the rod and several other parameters to give you the distance from the squish band to a port opening, but there is no time where a degree can be repeatedly measured in thousandths of an inch if the crank is at a different spot in its rotation. Near top dead center and bottom dead center, .010” may be 5°. When the crank is at 90°, .010” might only be 1°
 

Smoke_Run

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There are programs available where you enter the stroke, length of the rod and several other parameters to give you the distance from the squish band to a port opening, but there is no time where a degree can be repeatedly measured in thousandths of an inch if the crank is at a different spot in its rotation. Near top dead center and bottom dead center, .010” may be 5°. When the crank is at 90°, .010” might only be 1°

I'll bow to your expertise in the matter, maybe i just got lucky?
Have any links available or a name of an app that I can try for this calculation? I'd like to play around with it some more.
Thanks much
 
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Paul Fithian

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srcarr52

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I'll bow to your expertise in the matter, maybe i just got lucky?
Have any links available or a name of an app that I can try for this calculation? I'd like to play around with it some more.
Thanks much

With a quick google search here are the top three, all are just for exhaust or transfer timing. The intake duration takes a different equation.



 
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Ketchup

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If you want to know how much a port moves for a specific saw it’s simple to measure it with a ring. In the case of porting, a ballpark is usually all you need. Then you grind the ports to the degree you want.

I’ve messed around a tiny bit with 2stroke software, some chamber volume/shape formulas and even drawn up some stuff in Solidworks. It hasn’t gotten me closer to performance than simply measuring and listening to people with experience. Maybe the calculations are too related to bikes, maybe I’m not good at using the tools. Either way, it hasn’t saved me time or made stronger saws.
 

Smoke_Run

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With a quick google search here are the top three, all are just for exhaust or transfer timing. The intake duration takes a different equation.



S, Thank you, I’ll give these a try, much appreciated for the links!


If you want to know how much a port moves for a specific saw it’s simple to measure it with a ring. In the case of porting, a ballpark is usually all you need. Then you grind the ports to the degree you want.
I’ve messed around a tiny bit with 2stroke software, some chamber volume/shape formulas and even drawn up some stuff in Solidworks. It hasn’t gotten me closer to performance than simply measuring and listening to people with experience. Maybe the calculations are too related to bikes, maybe I’m not good at using the tools. Either way, it hasn’t saved me time or made stronger saws.
Yep, that’s what I found in searching around here, and it works (I’m glad to see I’m not the only one)…I used the info I posted to figure how much to cut base to get the exhaust to 102*. It worked like a charm and was right on the money for the 2 saws I tried it with?
Once I started using the ring method it made me question all the previous ones I’ve ever done. Made sense then as to why some things I tried worked and some other things didn’t. I went back in and “fixed” a 372xp and it made a world of difference. I’m a Stihl guy, but that 372 makes me smile. Thank you much
When I mounted the 44 mm 026 cylinder on my 024 AV, Doc shaved the cylinder base 0.016" to get it to 0.021" squish without a base gasket.

I then adjusted port heights after taking measurements with the new shaved cylinder installed and modified the ports to get target timing numbers.

Check this thread: https://opeforum.com/threads/stihl-...rade-to-ported-44mm.27931/page-2#post-1442901
Thank you, I’m kinda doin the same thing, but I enjoy seeing all the builds here, picking things from each and trying them to see what works for me. I’ll make sure to check this one out, Appreciate it!
 
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