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F134 Cruise Rpm

Discussion in 'Early CJ-5 and CJ-6 Tech' started by kenb, Mar 27, 2019.

  1. Mar 27, 2019
    kenb

    kenb Member

    Detroit
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    Hello,
    I have my M38A1 plated now and I'm ready to start driving it. It has a well running F134 engine with unknown miles or rebuild history. What I'm trying to decide is how fast to cruise it. The tires are 30" and it has the 5.38 axles.
    Some internet searching yields some recommendations of 2700 or 2800 RPM for max continuous. Does that sound reasonable? Per the calculator I found that should yield about 45 MPH.
    I would like to be able to drive it at a reasonable speed but don't want to toast my engine. Any thoughts are appreciated
    Thanks,
    Ken
     
  2. Mar 27, 2019
    Keys5a

    Keys5a Sponsor Sponsor

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    With 30" tires and the stock 5.38 gears, I would think 50-55 mph would be resonable as top usable speeds. These Jeeps were not designed to be used on modern multi-lane roads at modern speeds. Also keep in mind the brakes are marginal, at best, at the speeds mentioned.
    -Donny
     
    Alan28 likes this.
  3. Mar 27, 2019
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member Sponsor

    Tantallon, Nova...
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    45 is spot on for cruising speed. 50 to 60 is doable on flat ground but the engine noise is painful. :(
     
    scoutpilot likes this.
  4. Mar 27, 2019
    PeteL

    PeteL Member Sponsor 2019 Sponsor

    Hills of NH
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    That is my policy - ever since I blew up my dad's F-head on the Interstate when I was sixteen. 50 years ago, and I'll never forget the sound of the rods knocking.

    I know others here will defend the F-134 as a good engine - and I agree! But why risk it with an unknown older unit?
     
    Twin2 likes this.
  5. Mar 27, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member Sponsor

    Apopka, Fl
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    I'll certainly agree with 45 mph being a comfortable and very easily achievable cruising speed. Anything over 55 starts getting to "it sounds like it's gonna blow", but it's really the fan that is making the most noise.
     
    Twin2 likes this.
  6. Mar 27, 2019
    Twin2

    Twin2 wasn't me Sponsor 2019 Sponsor

    Virginia Beach, VA
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    I have been around jeeps with the L head and F head . I wouldn't feel comfortable pushing it too hard past 50mph for any extended time
    they were made for wood/dirt going slow
     
  7. Mar 27, 2019
    Cowboyjeeper

    Cowboyjeeper Member

    Show low az
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    That’s where the overdrive is handy
     
  8. Mar 27, 2019
    Alan28

    Alan28 Sponsor Sponsor

    Châtillon en...
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    I made many miles on highway at 45 miles per hour if flat.
    Above it is too noisy and risky for the engine.
    In fact 40 is good, anyway it is slow for the today trafic.
    Brakes are correct, it is the brakes of other new cars which are too powerful with the Abs. Better to keep distances.
    Better to let people pass you.
    And better to drive in curbs with attention, morever if the road is wet. Like on a motorcycle, slower before, some gas to exit but cool. The choice is roll over or make a full turn... It is like skiing in fact.
    I like artistic driving so it is happiness. People say hello.:clap:(y)
     
    Cowboyjeeper likes this.
  9. Mar 27, 2019
    kenb

    kenb Member

    Detroit
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    Thanks for the replies. Sounds like 45 is pretty reasonable.
    An overdrive is on my to do list at some point. Maybe in the fall. It's a big investment for sure. Where I live a lot roads are 55mph speed limit so I would sure be more compatible with traffic if I had the overdrive. Otherwise I think I'll have to use mostly the gravel roads.
     
  10. Mar 27, 2019
    jeepstar

    jeepstar Sponsor Sponsor

    Sheboygan
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    45 is best. Let the people pass. They either thumbs up you, or flip you off.
     
    tcfeet, Jrobz23, Rick Whitson and 4 others like this.
  11. Mar 27, 2019
    Cowboyjeeper

    Cowboyjeeper Member

    Show low az
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    There’s a guy who drive an old Willy’s on the i 10 in Phoenix all the time. And Jeepstar is right those are the two reactions you see most
     
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  12. Mar 27, 2019
    mickeykelley

    mickeykelley Active Member

    Republic of Texas
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    I go 45 and that's in the 27-2800 range. Occasionally I will take it to 55 but just for short periods. I did the Geo disc front 11" Bendix rear and while better, it's not a good setup for modern tailgating. So I'm always driving 50 yards ahead and pull over on the shoulder (allowed in Texas) all the time when on the 55-60 mph roads I have to get on to get anywhere. I do have modern 218/85/16 radials but handling is not great. And 45, let alone any faster, can be scary if you have a deer (or idiot) do something stupid in front of you. I've made the conscience decsion that hitting the deer is better than flipping by trying to steer around trouble like you can a modern car. Just my opinion, but OD to get faster is not where I'd spend my money.

    As for the above, I just assumed they were not Aggies so didn't know which one was the proper thumb to give a Gig'em sign. Had no idea I was being flipped off.
     
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  13. Mar 27, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member Sponsor

    Apopka, Fl
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    OD is more of a lesser rpm benefit generally, and if you do need to go faster yet obviously the lesser rpm is still a benefit.
     
  14. Mar 28, 2019
    70cj5134f

    70cj5134f Member

    East Tn
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    I have a tack on mine, no speedometer.
    25-2700rpm max cruise.
    I hit 3000rpm in 1st and 2nd, on way to 3rd.
    Doesn't feel safe over 40-45mph.
     
  15. Mar 28, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member Sponsor

    Apopka, Fl
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    Don't forget with tires larger than stock your speedometer is going to read slower than your actual speed.
     
  16. Mar 28, 2019
    70cj5134f

    70cj5134f Member

    East Tn
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    That's why I have a tack.
    Use my smartphone if I need mph?
     
    Cowboyjeeper likes this.
  17. Mar 28, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member Sponsor

    Apopka, Fl
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    GPS is the new speedometer? :) And by the way, assuming you are replying with your smartphone, does it know you're talking about a tach and not a tack? :D
     
    ITLKSEZ likes this.
  18. Mar 28, 2019
    kenb

    kenb Member

    Detroit
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    Speaking of tachs, does anyone have a recommendation for a unit that I can mount on the steering column? I was hoping to find on with range 0-4000 so I have better resolution than the common 8000 rpm units
     
  19. Mar 28, 2019
    70cj5134f

    70cj5134f Member

    East Tn
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    Sun or sun pro still makes a 6k I think.
     
  20. Mar 29, 2019
    elawson

    elawson New Member

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    I have a 1950 CJ3A with a 4L-134 engine and an overdrive, a 1964 CJ5 with a 4F-134 engine with no overdrive and a 1957 FC-170 with the 6L-226 engine with no overdrive. The CJs have 5.38:1 axle gears and the FC has 4.88:1 axle gears.

    The CJ3A will do 55mph with the overdrive and 60mph without the overdrive. Most likely this is due to a lack of horsepower in the 4L engine. The CJ5 and FC will do 60mph. HOWEVER, I set my maximum speed at 45mph because these vehicles likely have a IIHS crash safety rating of 0 stars and I don't want to out run my brakes, steering and suspension.
     
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