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Author Topic: Aftermarket Cylinder kit for 268 with interesting timing numbers.  (Read 635 times)

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Offline weimedog

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Aftermarket Cylinder kit for 268 with interesting timing numbers.
« on: January 22, 2014, 09:29:56 am »
Building a saw with an aftermarket Cylinder kit....an open port 268 top end from an Asian source. Put it together and measured things.....

With NO base gasket:

Squish .017"
Intake: 76 degrees BTDC
Exhaust: 103 degrees After TDC
Transfers: 132 degrees ATDC

Blow down.....29 degrees???

Going to run it as built....the way most would in the same situation. Curious what that long blow down will do and is it worth bringing it to the 25 degree range. Would be a PITA! Might be able to get a couple of degrees modding the piston crown w/o killing it.... ::)
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Offline winland

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Re: Aftermarket Cylinder kit for 268 with interesting timing numbers.
« Reply #1 on: January 22, 2014, 09:47:13 am »
Quote from: weimedog on January 22, 2014, 09:29:56 am

With NO base gasket:

Intake: 76 degrees BTDC
Exhaust: 103 degrees After TDC
Transfers: 132 degrees ATDC

Blow down.....29 degrees???

Do not want to hi-jack but is there someplace I can learn what these numbers mean and how they are measured?
I thank you for your knowledge.
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Offline srcarr52

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Re: Aftermarket Cylinder kit for 268 with interesting timing numbers.
« Reply #2 on: January 22, 2014, 10:03:33 am »
Long blowdown will hurt upper end HP.  I wouldn't mess with the piston crown for transfer timing as it will hurt the loop scavenging flow.
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Offline danders

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Re: Aftermarket Cylinder kit for 268 with interesting timing numbers.
« Reply #3 on: January 22, 2014, 10:25:29 am »
I get the part about a high blowdown limiting rpm but I'm not understanding what loop scavanging is. Does it have to do with the angle of flow coming out of the transfer?
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Offline srcarr52

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Re: Aftermarket Cylinder kit for 268 with interesting timing numbers.
« Reply #4 on: January 22, 2014, 12:48:59 pm »
Yes, angling the piston could interfere with the way the new a/f flow into the cylinder and could disrupt the whole process. 
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Offline danders

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Re: Aftermarket Cylinder kit for 268 with interesting timing numbers.
« Reply #5 on: January 22, 2014, 02:46:36 pm »
Okay, that makes sense to me. Thanks

To the OP - it's nice to see am kits with a tight squish; a little compression should add some grunt to the saw. See what it runs like as-is before altering the transfers, you may like it well enough not to bother with it.
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