I carried out the overlap check using two dial indicators, one on valve #7, one on #8. After a bit of work to understand what was going on I came up with some readings but they confuse me. What I found was with the cam set by LCA as I had done earlier, the valve overlap occurred at 4 BTDC, not TDC. At 4 BTDC both cam lobes were providing lift of 0.031"
If I continued turning the crank until reaching TDC, the exhaust valve was providing about 0.020" of lift and the exhaust about 0.040". Based on Glen's text, that suggests the cam is advanced too much by about 4 degrees.
If I had found a difference of about 2 degrees between the LCA method and resulting overlap measurement I would write it off as measurement error. With 4 degrees I decided to do some additional checks and compare my data to what is available online. Below is a link to the reference data I was using (from Paul G.'s web site). The chart below is an Excel chart created from the lift data I measured with my cam set to 110 LCA. The chart is provided to show where my cam is starting to provide lift and where the max lift occurs.
Paul G's Data:
http://auskellian.com/paul/links_files/spitfire_cam_specs.htm
My chart:
My lift curve shows that if you base duration on measurements at 0.010" lift, you get about the right duration for a stock cam. The max lift is also correct at about 0.240" The max intake lift occurs at about 110 degrees on my chart which matches the LCA I set yesterday. The max lift on exhaust also is occurring at about 110 BTDC as called for in Paul G's data.
So... I am confused. The cam appears to be stock yet I have a 4 degree discrepancy between the overlap method and the LCA method. The max lift appears to be occurring at the correct angle(s) based on Paul G's data. No matter how many times I measure it, I keep coming up with these values. I have no idea what I should do. I can reset the timing to have the overlap at TDC and then see where the LCA lands. Maybe I should split the difference and move the cam by 2 degrees to "split the difference".
Any comments or suggestions?
Thanks,
Doug L.
--Previous Message--
: Glen,
:
: I know you have cut & pasted the
: text in your last post from an earlier
: thread or your notes. Since I do not
: have the head on my engine I see
: several areas where I have to interpret
: what is meant and adjust accordingly.
: Please read over my
: "head-off" interpretation of
: your instructions. Since you can edit,
: please delete or correct anything that
: is wrong so others don't use my
: head-off interpretation incorrectly.
:
: Head-Off Interpretation:
: Find true TDC for piston #1 and zero
: the degree wheel.
: Put dial indicators on the tappets for
: valves 7 & 8 (cylinder #4)
: Zero both the indicators when their
: tappets are truly resting on the cam
: lobe base circles.
: Turn the engine over slowly by hand
: until piston #1 is back at TDC.
: At true TDC both indicators should have
: the same non-zero reading.
: If the exhaust indicator shows more
: lift than the intake, the cam is
: retarded and vice versa.
:
: You said to turn the engine over until
: the exhaust valve for #4 is "just
: closed". With no head on the
: engine... is that just closed with the
: intake just opening... so piston #4 at
: the beginning of what will be the
: intake stroke... lobes 7 & 8 in an
: upward "V"?
:
: Is any of this correct?
:
: Thanks,
: Doug L.
:
:
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