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Engine Rpm

Discussion in 'Early CJ5 and CJ6 Tech' started by 1970CJ6, Feb 2, 2006.

  1. Feb 2, 2006
    1970CJ6

    1970CJ6 new mexico air

    rio rancho, new...
    Joined:
    Nov 29, 2005
    Messages:
    630
    HI
    I am new to this. i have a 1970 CJ6 with F134,T90,D18, with 5.38 gears.
    also warn over drive. I would like to know what RPM the engine is safe to run at. I have the engine out rebuilding it. do not want to hurt the rebuilt engine. i am also thinking about changing the gears to 4.27.
     
    1970CJ6, Feb 2, 2006
    #1
  2. Feb 2, 2006
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Aug 10, 2003
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    23,596
    Opinions vary about peak RPM and the F134. The chart of power and torque from the factory ends at 4000 RPM, so I think that's a good value for absolute maximum RPM. Certainly, in terms of power, it's all over by that RPM so anything beyond that is pointless anyway.

    If the engine is in good condition, you are probably safe at around 3000 RPM. I would be very reluctant to go faster than that for any length of time.

    Welcome from Boston! We like pics here...

    <edit> You already have the overdrive, and I think that you won't do any better in top speed with the 4.27 gears. As speed increases, air resistance increases mightily and you won't be able to overcome it without more power. If you like your F134, I'd suggest you be happy with its current top speed.
     
    timgr, Feb 2, 2006
    #2
  3. Feb 2, 2006
    jpflat2a

    jpflat2a what's that noise?

    Hermosa, SD
    Joined:
    Jul 30, 2003
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    8,524
    break it in first
    it will run at 3000 RPM all day long, have no worries, higher if necc.
    F4s are not fragile engines by any means
    they are very tough and durable
    drove mine to Colorado and back several times
    have no worries, keep the 5:38s
    my own experiences and my opinion
     
    jpflat2a, Feb 2, 2006
    #3
  4. Feb 3, 2006
    Fhead Guy

    Fhead Guy Member

    Northern VA
    Joined:
    Apr 29, 2003
    Messages:
    83
    I had the same set up in Percival (5.38s with an O/D) and swapped the 5.38s out for 4.27s with P/Lock (kept the O/D). For what I do with the jeep, I'm glad I did that. I run on the highways (65 MPH) as well as off road. The 4.27s do not offer any extra speed, as has been noted before, but you are running the engine at a lower RPM, which I think is good - again for how I use my jeep.

    Do I have to downshift (go to 3rd direct) going up hills? Yes. Do I lose sleep over it? No.

    If you DON'T do any highway type travel, I don't know that I would spend the money to swap them out. I typically drive 50 - 60 Mi on the highway until I get to a place to lock the hubs, pull out the fishing rod, or favorite thunder stick.

    FWIW-
     
    Fhead Guy, Feb 3, 2006
    #4
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