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  1. #1
    Elite Member johnk's Avatar
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    Default Starter Torque

    My son and I got in a discussion about what is harder to crank over. A diesel of 30 HP or a 6 cylinder car gasoline engine. Would it take more torque to turn over the 22:1 30 HP diesel or the 7.5 to 1 gas engine. I figured it would take a higher torque starter for the diesel and my son says no. Who is right and why???????Thanks for any info to helpsettle a discussion...........


    Making misstakes is learning. Not making them again is knowledge........

  2. #2
    Veteran Member BillyP's Avatar
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    Default Re: Starter Torque

    Due to the more than twice the compression of the diesel engine verses gas, the diesel.

  3. #3
    Elite Member johnk's Avatar
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    Default Re: Starter Torque

    He says with the gasoline engine and more cylinders there is more mass to get in motion and therefore more torque needed for the higher displacement gas engine......... [img]/forums/images/graemlins/blush.gif[/img]

  4. #4
    Epic Contributor jinman's Avatar
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    Default Re: Starter Torque

    John, I think you are going to have to add a lot more specifications for both engines. For example: What are the relative size of the pistons? What is the size of the flywheel (starter motor to flywheel gear ratio). I think the starter/flywheel gearing is designed to show equal torque to either starter.

    Now my diesel truck has two batteries, but it also has high current air preheaters, not to mention that it's a 6-cylinder, 5.9 liter engine with 16:1 compression when the turbo boost is zero. All that is plenty for two batteries to handle.

  5. #5
    Elite Member johnk's Avatar
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    Default Re: Starter Torque

    I was just speaking in general terms. A 35 HP tractor diesel and say a 6 cylinder 3.7 liter Jeep engine. What would take more torque to crank over? Generally speaking..........I say the diesel.... [img]/forums/images/graemlins/smile.gif[/img]

  6. #6
    Veteran Member Nasty135's Avatar
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    Default Re: Starter Torque

    The diesel requires more torque to over come the compression stroke, the gas engines lower compression can be turned over much easier.Unless your talking about a 13:1 Bigblock thats just run the quarter and is a bit warm and tight that will take considerably more torque to restart,which would be closer to the diesel at that point .The starter for a stock dodge cummins puts out about 4hp no car starter puts out that much, [img]/forums/images/graemlins/wink.gif[/img]Dads are always right! GEE I hope my dad doesn't read this!! [img]/forums/images/graemlins/tongue.gif[/img] [img]/forums/images/graemlins/blush.gif[/img] [img]/forums/images/graemlins/grin.gif[/img]

  7. #7

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    Default Re: Starter Torque

    He is wrong on two counts. First, it is unlikely that the mass of that 6 cy gas engine is greater than the mass of the 30hp diesel (probably 4 cyl). The pistons, rods and crank of the diesel are much beefier and heavier that the pistons, rods and crank on that gasser.

    Two, it really doesn't matter that there are 6 cylinders. What matters is how many of those pistons are on the compression stroke at any one time. At best, one is near the top of the compression stroke and one is on the way up. Of the other 4, two are going down and two are on the exhaust stroke.

  8. #8
    Gold Member uhmgawa's Avatar
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    Default Re: Starter Torque

    </font><font color="blue" class="small">( My son and I got in a discussion about what is harder to crank over. A diesel of 30 HP or a 6 cylinder car gasoline engine. Would it take more torque to turn over the 22:1 30 HP diesel or the 7.5 to 1 gas engine.)</font>

    This is quite complex and depends upon many factors
    including:

    - displacement of the engine
    - number of cylinders
    - where each piston is located within its cycle
    - cylinder compression ratio
    - mass of cylinders, linkage, and camshaft
    - aspiration mechanism (eg: gasoline vs. diesel)
    - minimal self-sustaining speed of the engine

    Note it is assumed the engine is at rest and needs
    to overcome resistance of the compression stroke
    as well as vacuum of (what would normally be) the
    combustion stroke. Add to this the partial
    vacuum resistance of the intake stroke for a
    gasoline engine.

    All of this ignores frictional losses which vary due to oil
    temperature; valve restriction for intake/exhaust strokes;
    fuel injection pump loading; outboard mechanical loads,
    etc.. I'd expect approximate mathematical models to
    exist for the truly curious. However it isn't as simplistic
    as gasoline vs. diesel.

  9. #9
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    Default Re: Starter Torque

    I think it's basically going to depend on how many square inches of piston area are under how much compression at any one time.
    Back in the days before motorcycles had electric starters on them, 500cc, (30.5cid), single cylinder engines and 74cid twin Harleys were so hard to kick start they were equipped with compression release levers, wheras 650cc, 40cid, twin engines, with same compression ratio, did not need them.

  10. #10
    MMM
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    Default Re: Starter Torque

    Definitely diesel,, which must come to combustion speed as there is no spark to start combustion. A tuned gasser should start at the touch of the key, where the diesel must overcome repeated high compression strokes before ignition occurs. I service large 16cyl CATs with air starters that simply will not cold start unless spinning 400rpms or more.

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