Author Topic: 42cc Poulan Clamshell Case Inserts  (Read 180 times)

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Offline 3000 FPS

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #20 on: November 25, 2015, 04:52:47 pm »
That is not bad.  10,500 in the cut is pretty good.    What is the compression on it now.
PP 505, 475, 445.

Offline Chris-PA

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #21 on: November 25, 2015, 05:13:27 pm »
That is not bad.  10,500 in the cut is pretty good.    What is the compression on it now.
Thanks!  I felt like it was turning a bit faster later on but was too lazy to set up the camera for another video to check.  The wood was reasonably hard stuff.

I actually don't have a compression gauge, as I don't really think that low speed cylinder pressure readings mean much of anything other than to tell you if it's shot (which is usually obvious anyway).  You just can't infer much about the cylinder pressure at 10,000rpm by taking a pressure reading at 100rpm.

Offline Chris-PA

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #22 on: November 25, 2015, 05:30:12 pm »
What I liked was that it hold the rpm well under load.  The images are audio spectrum plots of the entire 1st and 2nd cuts.  The lowest spike is quite narrow, showing that the rpm did not vary much during the cut. 

Offline 3000 FPS

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #23 on: November 25, 2015, 07:05:17 pm »
I would have to disagree with you on the compression testing.   
PP 505, 475, 445.

Offline Chris-PA

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #24 on: November 25, 2015, 07:14:12 pm »
I would have to disagree with you on the compression testing.   
That's OK - I'm sure most everyone else does too!

Offline 3000 FPS

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #25 on: November 25, 2015, 07:23:49 pm »
I would have to disagree with you on the compression testing.   
That's OK - I'm sure most everyone else does too!

We both like to modify our saws and I like to know how those modifications affect the final out come in specific ways.   I do not use a compression gage to tell me how much torque I am getting, but if I raise the exhaust port I would like to know how much compression difference there is.   Just an example of not wasting time.
PP 505, 475, 445.

Offline Chris-PA

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #26 on: November 25, 2015, 07:53:59 pm »
I would have to disagree with you on the compression testing.   
That's OK - I'm sure most everyone else does too!

We both like to modify our saws and I like to know how those modifications affect the final out come in specific ways.   I do not use a compression gage to tell me how much torque I am getting, but if I raise the exhaust port I would like to know how much compression difference there is.   Just an example of not wasting time.
That's a good illustration of why I made the claim I did though - I have several saws that have a notch in the top of the exhaust port.  It bleeds off pressure at low starting rpm, but doesn't really do anything at operating rpm.  The time and pressure at operating speed are too different from what you see on a gauge. 

There's no doubt that raising the top edge of the exhaust port must reduce cylinder pressure and shorten the power stroke (at least if it's more than a notch), but you can't measure what the effect is at operating speed with a typical compression tester.  So for example a 10% change at low speed can't be assumed to make a 10% difference when it's running.

Without dynos it's tough to quantify the results of our modifications - we're left with cut times, rpm under load and "feel". 

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Re: 42cc Poulan Clamshell Case Inserts
« Reply #27 on: November 25, 2015, 10:54:46 pm »
That is true.
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