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Testing RPMs

posted by  Don Foster on  Sat Apr 13 01:17 CST 2002
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last visit: Sun Oct 5 11:53 CST 2014
[RELATED]

Are you sure the '87 had shifted into overdrive?



Here's a dumb, simple, but effective trick you can use to verify your tach.

Paint a bright white spot on the edge of the front pully -- maybe right at the timing index mark.

Illuminate the front pulley with a FLUORESCENT light.

With a helper watching your tach, run the engine and manipulate the throttle (by hand, under the hood) while watching the front pulley.

The white dot will appear to stop (stroboscopic effect) at multiples of the 60 Hz frequency (120 Hz, actually). 900 RPM, 1800 RPM, and 3600 RPM would be good reference speeds. Your helper can tell you when you're approximately at 900 or 1800, and then you juggle the throttle to "stop the dot" -- and then helper can read the tach.

The dot will appear to stop (and also appear in multiple spots simultaneously) at various speeds that are harmonics of 120 Hz (remember that the fluorescent light flashes twice per electrical cycle).
--
Don Foster (near Cape Cod, MA)






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New cruising rpm's
posted by  someone claiming to be kswan  on Fri Apr 12 16:20 CST 2002 >
  • New cruising rpm's
    posted by  someone claiming to be Bob Conolly  on Fri Apr 12 16:27 CST 2002 >
  • New Testing RPMs
    posted by  Don Foster  on Sat Apr 13 01:17 CST 2002 >
  • New cruising rpm's
    posted by  someone claiming to be kswan  on Sat Apr 13 19:33 CST 2002 >


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