Low oil pressure?

Small Block Mopar Engine

  1. Duane

    Duane Well-Known Member

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    Agreed needs to move toward drivers side to get 70atdc.
    1/4 inch would do it.
    no need to slot the block if the counterbore is there. Just slot the bearings.
    Sanborn claims the mod was reliable for super speedway rpm of 7800 rpm. More than that with a 5/8 slot in block and bearing shell with dry sump. And this is almost continuous rpm.
    These mods cost very little except your time. What's your motor worth lol.
    Not too many stroker motors Reving that high lol.
     
  2. brian6pac

    brian6pac Well-Known Member

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    A'm I looking at this right, I think I need to get my degree wheel out and assemble 1 piston and rod but you want 70* of the rod and crank not necessarily 70* ATDC because the rod is not straight up and down it's also at a angle, so 20* before the crank and rod are at 90*.
     
  3. Duane

    Duane Well-Known Member

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    I might be wrong Brian but I believe it is 70 atdc crank position.
    I believe the theory goes that we might ignite the fuel mixture at say 35deg btdc, the burning mixture produces its maximum downward pressure on the rod journals at 70degrees atdc. This is deemed the ideal time to introduce a full pressure full flow gush of oil to get hydro dynamic wedge under the rod bearings.
    Because the oiling hole location is different in a sbm versus a sbc the timing is not ideal.
    Perhaps yellow rose will chime in here and correct me if I am wrong.
     
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    • yellow rose

      yellow rose Doctor of Thinkology.

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      Yup.
       
    • brian6pac

      brian6pac Well-Known Member

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      But what I am saying is that the most push on the crank is when the crank is at a 90* angle from the rod, It may have more pressure on it at a different time like as the fire goes out and the space gets bigger It gets less but the 90* crank vs rod angle doesn't happen at 90* crank rotation from TDC it happens before, not sure if it is at 70* that is why I would have to check on it.

      It also depends on the wrist pin being on center or off center like stock 340
       
    • yellow rose

      yellow rose Doctor of Thinkology.

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      You are correct. The most load is when the crank is at a 90* angle from the rod.

      The reason you want full flow and pressure there by 70* ATDC is so the oil is already there. The groove in the main bearing essentially starts some flow out to the rod. By 70* ATDC, you now have the feed hole in the block lined up with the feed hole in the crank and full oil is going to the rod bearing BEFORE it reaches max load. After the hole in the crank goes past the hole in the block, you still have the rest of the groove providing some oil to the rod.

      That's why a full groove bearing helps these engines, and not a Chevy. You are getting some oil to the rods all the time and it helps cover up the timing issue.

      I can't say for sure when you'll run into oiling issues, but I know it is RPM and load affected. I could easily turn 8500 RPM's when the engine only made about 540 HP. When it went over 650 HP at RPM's above about 7500 would start grabbing the bearings and at 8000 the rod would exit the block. It gets worse from there.

      If you can make 700 HP at 7000 RPM you probably will get by with a HV pump and full groove bearings. I never built long stroke engines that made that much HP with a small block.
       
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      • Wyrmrider

        Wyrmrider Well-Known Member

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        there is little pressure on the block bearing saddle- grind away, slot, whatever
        lie I said I slot all the way to the parting line for lower main hydrodnamic wedge oiling- not for rod oiling but it don't hurt
         
      • Duane

        Duane Well-Known Member

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        Any pics to show what you mean.
         
      • Wyrmrider

        Wyrmrider Well-Known Member

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        no sorry
        it takes about a 3/32 x3/32 square channel to equal a 5/6 round hole cut in the saddle from the oil feed to the parting line- 1/8 works
        round the intersections
        then drill the bearing halves into where the rectangular oil resouvior / spreader is
        oiling the mains at the top looses lots of oil
        you can put a hole wherever best to oil rods
         
      • nm9stheham

        nm9stheham Well-Known Member

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        Actually... that is not the reason. All of this great info stimulated me to study up a lot on this in the last 24 hours and have found this is not the case.

        The most 'turning torque' on the crank is exerted by the piston+rod around 70 to 90 degrees. This is what Brian is thinking of above. People misinterpret this as the same as total rod force, but it is not. To see why:

        The rod force can be divided into 2 parts: tangential and radial. The tangential part is going through the rod journal in a direction that is 90* from a line between the crank center and the rod journal center. Your can think of that like the force you exert on a bicycle pedal and it is what turns the crank. Tangential force peak generally around the 70 to 90 degrees range and THAT is the 'turning torque', like what Brian identifies. But it is not the only force on the rod.

        The other part of rod force is the radial part, which is what is going straight into the center of the crank. If you put a piston at TDC, and pushed down on it, all the force would go straight down into the crank, and the crank will not turn; that is radial force. The piston+rod's radial force has NO leverage to turn the crank, BUT it is still force that the rod and rod bearing and oil have to deal with.

        This radial force peak soon after TDC, when the combustion pressure peaks (ideally around 14 degree ATDC) and is considerably higher than the peak tangential rod force in the 70 to 90 degree range... for most engine uses. The 'primary' force being 2-3 times higher, or more, than the peak tangential force at 70-90 degrees, would be a good guess.

        Sooooo.... while you are at lower to mid RPM's, the rod force graph actually peaks soon after TDC due to the radial force component. If you only think of the turning force (tangential force) from the rod, then you will miss this higher peak of radial force acting on the rod and bearing and oil.

        HOWEVER..... when you get higher and higher RPM's, then another effect starts coming into play, and a 2nd peak emerges in the rod force versus crank angle graph.
        • Imagine #1 piston and rod moving down the bore at high speed; at 8000 RPM for a SBM, this moves as fast as 1500 inches per second (over 100 mph), so the piston/rod has a LOT of energy in it at that speed. And know that if you increase the RPMs from 6000 to 8000 the RPM's, this rod+piston energy goes by by 1.8 times more...it increases as the square of the RPM's.
        • As the piston and rod moves down the bore, something else is happening: the next cylinder in the firing order is on the compression stroke, and that work put a 'backwards' force on the crank; that wants to slow the crank down. Yes, the flywheel/TC try to keep the crank turning at a constant speed, but regardless, the crank DOES slow down a bit as the next cylinder's charge is compressed.
        • Now piston and rod #1 also HAS to slow down, and that deceleration has to come from the crank 'fighting back' and exerting a force 'backwards' on the #1 piston and rod. At higher and higher RPM's, that reverse force has to rapidly get larger and larger because the piston and rod inertial energies are growing as the square of the RPM's, and it takes a more and more of reverse force on the rod to take out the energy needed to slow the piston and rod.
        • That reverse force adds to the normal combustion force (tangential + radial) on the rod and produces a 2nd peak in the 70 to 90 degree crank angle range, that grows rapidly with RPM's and eventually becomes THE BIG peak in the rod force, bigger even than the one just after TDC.
        So that 2nd rod force peak nearing 90 degrees of crank angle, that shows up with any significance only at high RPM's, and which gets bigger and bigger when you to higher and higher RPM's, looks to be the reason for rod oil timing.

        I never would have dug into this if this thread did not turn this way. So I'll express my thanks to everyone and to FABO.

        As a side thought.... If you change from a stock 340 piston+rod weight down to something like a KB+SCAT rod weight, the weight drops around 18% and that drops that 2nd rod force component by the same %. I'd imagine that would be good for a few hundred more RPM before the oiling fails....
         
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        • yellow rose

          yellow rose Doctor of Thinkology.

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          I'm glad you did the research. I can't tell you how many times I've told guys not to build big RPM Chrysler stuff without a new block with correct oil timing or fixing the oil timing the way I did it.

          In fact, I refused to build them. I tore enough shit up myself making me look stupid learning how to do it that I didn't need customers helping me.

          This is also how I know many (if not most) of the guys claiming they were turning big RPM's and making power were either full of shit, had a bad tach, or were just smarter than me. Most likely the last one is correct.

          Interesting stuff. I hope everybody reads these threads even if they never build high a RPM engine. It's good to know that those who came before us, who didn't have computers, weren't as stupid as we thought. They had it sciences out.

          I also think the guys building stuff at the time they engineered this crap never thought anyone could make power at RPM ranges where oil timing would be an issue.
           
        • Duane

          Duane Well-Known Member

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          This is what I like about these threads too. It's ok too tell someone what mods to do, but I also like to know the whys, at least if it's not over my head lol to understand. Great info guys. I have had bearing troubles in the past as well and that is why I pay attention to theses oiling threads.
           
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          • Duane

            Duane Well-Known Member

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            Is this what you are trying to describe. Also here's a pic of a block that has a the counter bores. Does your block have these?

            image.png

            image.png

            image.png
             
          • Duane

            Duane Well-Known Member

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            Found this on the net out of a Chevrolet performance book lol.
            We're all these books put out by the same people lol.

            image.png
             
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            • Wyrmrider

              Wyrmrider Well-Known Member

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              I was referring to a groove from the oil hole to the parting line- feeding oil in at the parting line
               
            • nm9stheham

              nm9stheham Well-Known Member

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              Yeah, I saw that too. Seems like commonsense practical machining was used by everyone.
               
            • nm9stheham

              nm9stheham Well-Known Member

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              Honestly, I think this was all being figured out in the 1920's and 1930's, at least the thinking of what would happen and the math behind it. Heck, the Ford V8's started mass production in what... 1931?.... and building 1800 HP aircraft engines for WWII did not 'just happen'.

              The dynamic forces get quite interesting when the RPM's get high enough. I found a rod force graph computed for a simple 1 cylinder crank. It looked normal at 4000 RPM with a single peak just after TDC. At 12,000 RPM, that peak disappeared almost completely, I suspect due to the inertial forces on the piston+rod going over the top just about equaling the compressive force from combustion.

              If I had to guess, your solution to the problem worked mostly due to the fact that you put full pressure 'right at' the bearing, and skipped all the pressure losses in-between. Plus, you probably got rid of all that oil mass that has to be accelerated down that 4-5" passage from the gallery to the main bearing. The time opening for oiling to the rod gets down to 2 tenths of a millisecond at 8,000 RPM for the standard hole size... yes, that's .0002 seconds. It takes a finite amount time to accelerate any oil mass. That all has less to do with timing per se, but just plain getting the oil where you want it.
               
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              • brian6pac

                brian6pac Well-Known Member

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                My question is still at what degree is the crank when the rod and crank are at 90* or better at 70* if that is where you want the oil holes lined up, and there going to be in different places for right and left banks. as you can see # 2 has fired and #1 is firing and is at TDC #2 is at 90* from TDC and past rod to crank 90*

                A-engine45 rod angle.jpg
                 
              • yellow rose

                yellow rose Doctor of Thinkology.

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                It was easier for me to look at it on a real block rather than a picture.

                That's why I always say about 70* ATDC, because there is a bit of fudge room. As RPM goes up the fudge room goes down.
                 
              • nm9stheham

                nm9stheham Well-Known Member

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                Interesting you should ask.... I started writing a spreadsheet for this, before I realized that it was so complex. But I did get as far as the rod angles.

                The rod-to-crank angle reach 90 degrees as follows for different strokes, all assuming a standard 6.123' rod length:
                3.31" stroke: 75* crank angle
                3.58" stroke: 74* crank angle
                4.00" stroke: 72* crank angle

                BTW, I am measuring crank angle referenced to the centerline of each bore so that 0* is when each piston is at TDC for its bore, not the standard 0* TDC for #1 cylinder.

                An interesting outcome: Thrust loading of the pistons on the walls as the rod angle increase does not increase rod force as much as you might think. Below is peak piston side loading as a % of a constant downward piston force is listed first, and then the resulting % increase in rod force for that same constant downward piston force:
                3.31" stroke: 28% side load % causes a 3.8% rod force increase
                4" stroke: 34.5% side load % causes a 5.8% rod force increase

                The above peak numbers occur when the crank angle is 90* from TDC for given cylinder, where the rod angle is maximum; the rod-to-crank angle has gone well past 90 degrees at that point.

                These are 'static' numbers that should apply up to the RPM's where the dynamic loads start rearing their ugly heads.
                 
                Last edited: Aug 13, 2018
              • nm9stheham

                nm9stheham Well-Known Member

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                FWIW, all the articles I read on this reference the crank angle to the TDC of the bore being computed/considered. I think the 70* ATDC recommendation is referenced the same. So the oiling angle is referenced to the journal for the piston in question; I assume, without looking at a crank, that the 8 different oil passages are drilled to accomplish this angle. Not sure that answers the question.....which seems like a very good one.
                 
              • Cope

                Cope Fusing with fire

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                Holy snit, I seen some threads take a turn but good lord this one for once took a turn in a positive direction.

                If a mod cares to remove the first half and just let it run that's fine with me.

                I wish this was posted before I built my mill.
                This winter when i pull it for inspection she may see some oil timing mods...

                Thanks for all the hard earned info.
                 
              • brian6pac

                brian6pac Well-Known Member

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                I'm trying to get some pictures together of a crank that I have and I have rods through the oil holes on # 1 & 2 and degree plates I might change to protractors to show the angles and the oiling points, right now I have found that the right bank will have to be oiled through the cap as YR said but the left bank needs to be oiled through the block were the stock oil holes are or close to them, havent got that far yet.
                 
              • yellow rose

                yellow rose Doctor of Thinkology.

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                Interesting. I like this. It's called learning and it involves both book work and hands on stuff. We could turn the country totally around in one generation if we put this kind of learning back in our schools.


                Here is something else to consider. I always killed the number 3 rod bearing first. After that 4 wasn't far behind. If I bone headed it, 3 and 4 would get off the crank. 1 and 2 were always in great shape, while 5 and 6 looked like they would be next if it lived long enough.

                I never figured out why 3 came off first. Once it was fixed I stopped dealing with it. All the bearings would make it 100 runs then I chucked them. The mains always looked perfect.

                It will be interesting to see what you find when you start looking at the actual geometry on both banks.

                Very cool.
                 
              • Duane

                Duane Well-Known Member

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                We could give it a new title and sticky it lol
                 
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