Cam Operating Range & Highway Driveability

Mopar General Discussions

  1. Jesus Chrysler

    Jesus Chrysler Forgiving Sins Against Mopar Since 1983

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    If the advertised operating range is outside of highway cruising RPM, how does that effect driveability (I know that's a vague term)?

    If the cam & intake are designed to operate between 3000-6500RPM and your gearing is set up to run 75 @ 2500 rpm, will the engine be dogish or struggling under these circumstances? Or does it just mean you need to drop a gear and mash it if you need power on tap?
     
  2. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    Yes and yes to a degree.
    A lot depends on the overall package & the C.I.D. Bigger engines have less of a struggle.

    Another thing is how efficient it is and how bad it becomes. Also based on the above. How well the package is balanced in the taget area for a hot, bad ass street machine or drag racer will show more of an issue down low more so than a mild street strip engine.

    Here, a compromise of a smaller carb and/or smaller cam will help.
    Other things that can help the balance would be;

    A hyd. Lifter cam (SFT or roller) rather than a solid lifter.
    A slightly smaller carb, cam, intake runner, 4 hole carb spacer or a HVH Super Sucker (or equal), a dual plane or small runner single plane.

    Running at 500 below the advertised rpm range rated can be a lot on a small engine, (318) or not so much on a larger engine. (408+) The cam size is also a factor. Even if the duration @.050 is the same.

    Let’s say the cam has a min. range of 3K on a street grind. And at that magic rpm it picks up. With the Hwy. gears and operating below the min. rpm, the engine will have a soft spot until the system reaches that rpm where it all “catches on” and starts working.

    Hey. Gear hurt and the taller the tire, the worse (and longer) it will be.
     
    Last edited: Feb 28, 2019
  3. Oldmanmopar

    Oldmanmopar Going left turning right FABO Gold Member

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    Converter. That is what gets the motor to operating torque.

    Also Install a factory (Part throttle kick down servo).

    The servo makes the biggest difference in performance at highway speed . You find them on most older/6 or fleet/police cars. Three screws hold it to the side of the valve body. I put one in all my cars. Unbelievable difference in the performance on my Power Wagon's when plowing. extends trans life also. They are different from early models to later models but 904 and 727 are the same.
     
  4. mbaird

    mbaird mbaird

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    2.76 rear gears?
    That is an odd combo... What is your intended purpose ? Bonneville ?
     
  5. TrailBeast

    TrailBeast AKA Mopars4us on Youtube

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    I currently have 273 gears and the 42RH overdrive with a mild cam designed for 3,500-5,500.
    Bonneville was back there a few off ramps.:D

    I am finally about to start on my 8.25 locker with 355 gears though, which has always been the plan.
     
    • Agree Agree x 1
    • mbaird

      mbaird mbaird

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      Whats that like in a 35 or 45 mph zone ? Even with OD off...
       
    • TrailBeast

      TrailBeast AKA Mopars4us on Youtube

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      Yea, what's funny about that is when I was building my automatic shifting system I test drove it with only the manual on/off switch.
      I could idle on the cam lope at 35mph in OD.:D

      But to answer your question, it kinda sucks around town and in 45mph zones if it was on.
      With the automatic shifting system I built, it only goes into OD when it reaches 60mph and lockup at 75 or so, but I usually just keep OD and lockup both turned off manually unless I'm out on the hiway.
      I have everything adjusted where it tends to keep the motor in the 2,100 to 2,500rpm range in normal driving conditions all the way up to 100mph if OD and lockup are in full automatic mode.
       
      Last edited: Mar 2, 2019
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