Cummins 300 question

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by 85COE, Feb 16, 2020.

  1. 85COE

    85COE Light Load Member

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    Aug 6, 2019
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    Not sure what all info you would need to answer this, but I'm looking at doing some super loads from MI to OH, 165 miles 1-way loaded. According to a dealer, it's considered a "medium-duty" engine. 808,xxx miles, 9 speed.

    Permits in Ohio would allow me to be up to 120,000lbs on the roads. Tractor weighs in @17,500lbs, trailer will be around 15k-20k or so (extra axles).

    Just wondering if my engine could handle it on a daily basis. 165 loaded miles max 1-way.
     
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  2. x1Heavy

    x1Heavy Road Train Member

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    At 120,000 it's going to be about a 5 hours or so to get that thing 165 miles. There is a possibility your front end steering might go out and get too loose on you above 25.

    I have not had a 300 cummins at those weights, the closest engines ive had were the 200+ macks and less than 1000 torque and higher weights than that. The results were predictably bad for me and company at times.

    Ive had 300 cummins in the past and frankly with that 9 speed? you are going to be having dinner served to you by the time you shift the next one up to be honest with you.

    Can you find a bigger horse truck to do this work? I have visions of you coming off 6th into 7th and finding that you wont build RPM's to get 8th or 9th at all. You would have maxed out your engine's power.
     
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  3. tnpete

    tnpete Medium Load Member

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    Pulled loads of logs 110,000 to 122.000 with a 315 Big cam and 9 speed. It pulled it and did ok, but your not going to get in a rush. Now later on when we swapped out the button to a #9 and a new spring. Along with a few more items. It pulled well, but you had to drive it the EGT gauge with those heavy loads.
    It did that job 3 years before the other driver got the engine. He held it down and melted a few parts.:eek:
    Years ago most of us thought we had a left lane truck if it was a 400PH truck. Most in the 80's were 300 to 350 HP. And got the job done. Not fun but got it done.
     
  4. Dino soar

    Dino soar Road Train Member

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    What is your rear axle gear ratio?

    The lower your gears the easier to pull a larger load.
     
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  5. 379_largecar

    379_largecar Light Load Member

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    I had a formula 300 big cam in an 85 freightliner (back when freightliner was respectable and not plastic!) first thing I always do to a big cam is swap the button to a 5, drill the ball and crank that pressure waay up, get the spring to crank 2300 rpms install a pyro and run the lines to both front head and rear head. Swap out the steel fuel lines for high psi hydraulic lines (I had a problem splitting the steel). I thought it was doggy hauling 100,000 of sand. Ran it it for several years and only blew one head gasket on the number three head. Shoot about 5 hours and I had the thing tore down and put back together in my driveway. I then bought a Pete with a 525 red top and man did I wish I had my 300 back!
    Had to have a geek turn that n14 way up to even get close to the old shaker!!
     
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  6. 85COE

    85COE Light Load Member

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    Columbus, Ohio
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    No idea, still working on learning the mechanical aspect. Never worked on trucks, before just drove them. How would I find that out?
     
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  7. AModelCat

    AModelCat Road Train Member

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    Since when is 120k a superload?
     
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  8. Dino soar

    Dino soar Road Train Member

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    There should be a tag somewhere on both rear axles. Sometimes it is on the differential sometimes it is on the housing.

    You have to look very closely because if there is a lot of dirt and grease on the rears they blend in.

    I use a rag with thinner to wipe the tag to be able to see it better.
     
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  9. ODR

    ODR Light Load Member

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    Sep 30, 2013
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    Be careful of cranking the fuel pressure up on a big cam. Everything has limits. Larger injectors to handle the extra flow without flattening out your cam would be a good thing when upping the the power. Lower compression pistons and changing the timing may also be needed. Also clutch, trans and rear ends might not be able to handle extra power
     
  10. Snow Monster

    Snow Monster Medium Load Member

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    If your gears are too tall, (meaning fast), you're going to have problems, at 120k lbs.

    If your clutch is too light, you're going to have problems.

    If you have 34k or 38k diffs, you're going to have problems.
    I doubt it has 40k, Super 40 or 44k diffs, definitely not 46k.
    If you have a 9 spd OD, you're already screwed unless you have a gear ratio in the 5:x range.

    If your truck now goes over 60 mph at 2000 rpm in top gear, you're probably already screwed.
    The truck was built to run 80k lbs or less, not 120k lbs.

    You can go ahead, but I don't think it will last very long running that heavy.
     
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