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cruising rpm's

posted by  someone claiming to be kswan on  Fri Apr 12 16:20 CST 2002 [RELATED]

Just installed tachs in my 84 B23 and 87 B230 240's the other day. I notice when cruising level on freeway at speeds of 65-75 the B230 reads about 3000 to 3200 rpms and the B23 reads about 2500 at the same speeds.

Just curious as to the cause in difference. Both are AT's.

And no I'm not driving them at the same time, haven't gotten to that level yet.

thanks in advance

kswan








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    cruising rpm's

    posted by  someone claiming to be kswan on  Sat Apr 13 19:33 CST 2002 [RELATED]

    thanks for the suggestions. I ended up swapping tachs and now the tach I put in the 87 reads properly so I know at least that it's not the vehicles but the tach. Haven't checked the one that was reading high in the other vehicle yet but if the tach is off I'll just pick up another at the boneyard.

    thanks again

    Kirk S.








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    Testing RPMs

    posted by  Don Foster on  Sat Apr 13 01:17 CST 2002
    Author: [PROFILE] [EVERY POST] [THREADS] [BEST POSTS] RSS Feed     [IGNORE ALL POSTS]
    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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    cruising rpm's

    posted by  someone claiming to be Bob Conolly on  Fri Apr 12 16:27 CST 2002 [RELATED]

    Kswan,

    My '85 240 with B230F and standard trans. (M46) cruises at 70mph at about 2600rpm.

    Are you sure that the tach that you installed in the '87 is not bad, or from a 6cyl car? You might want to try swapping them and see what happens.

    If both tachs are 4cyl tachs and check out 'OK', then the only thing that I can think of is that the '87 might not be shifting into overdrive.

    Assuming that everything else is equal, cruising rpms should be roughly the same for both cars.

    -=Bob=-







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