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Killing a crank with torque, horsepower, or rpm

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  • Killing a crank with torque, horsepower, or rpm

    Suppose you have a cast crank. What are the ways to kill it?

    Torque?

    Rpm + rotating mass?

    A cam change moving the same torque to higher rpm? (aka horsepower)


    Put another way, should I worry about a cam change to increase horsepower killing the crank or pistons if I hold it at 6k rpm all day either way?

  • #2
    In the Nissan world, we worry about rotating mass when it comes to breaking cast iron cranks. People put crazy light flywheels on so they can reliably spin motors to 8,000+ RPM. These motors break cranks with under 200 HP, so I don't think HP has a lot to do with it.

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    • #3
      Yeah, that's a sticky issue. You're right on the cusp of what it will withstand. What are you revving to now, 5,500?

      Cast cranks aren't the best choice for sustained high-rpm operation. Don't they flex at sustained high rpm? Drag racing you could probably get away with a 6,000 rpm redline that you'll hit maybe twice on a pass, but a dirt car is another matter.

      Do you need more power?
      Do you mind finding out what the crank will take?
      Can you replace it with forged if it lets go?
      www.TrackHQ.com

      Comment


      • #4
        Ok, let's do details then:

        GM CT350 crank engine (aka "602") $3,500 350hp 395 ft/lbs, has the cam from a truck.

        Currently spending lots of its life hitting the limiter at 6,200 rpm.

        Considering a cam change to try to bump the output to 450-500hp.

        Hypereutectic pistons are supposed to be good for 500hp (maybe), nodular cast cranks supposed to be good for 500hp (maybe).

        Was thinking about this and started wondering why they are giving me horsepower limits instead of torque and rpm limits.

        Do I need more power? Yes.
        Sure, I can afford to blow something up.
        If it lets go, will there be anything left? I'd probably build a whole new engine.

        Comment


        • #5
          Cast cranks are not as stiff, and have less tensile strength compared to forged. If the crank is the first failure point at high revs, then it needs to be upgraded. In an SBC a flexing crank can kill the oil pump, and timing gear/chain long before it finally comes apart.
          Jealous. SBC parts are so cheap compared to fancy Miata engine parts and the relative power between them.
          WWW.949RACING.COM
          SuperMiata

          Aside from their cost I never understood why people race them.
          But obviously I just dont get it. -fatbillybob

          Comment


          • #6
            Originally posted by emilio700 View Post
            Cast cranks are not as stiff, and have less tensile strength compared to forged. If the crank is the first failure point at high revs, then it needs to be upgraded. In an SBC a flexing crank can kill the oil pump, and timing gear/chain long before it finally comes apart.
            Jealous. SBC parts are so cheap compared to fancy Miata engine parts and the relative power between them.
            Max RPM and the time spent there isn't changing, this would be going from 350hp and 395ft/lbs to let's say 475hp and still 395 ft lbs. Bouncing off the limiter at 6.2K either way. The engine hasn't blown up yet, I'm just trying to figure out if a cam change with no increase in max rpm adds much danger for the bottom end.


            The only difference would be the 395 ft lbs shows up like 2,000 rpm later.

            Edit: I think those hp/torque math needs some work...
            Edit2: 450hp is theoretically possible if you hit your 395 ft lbs at 6k. Won't hold my breath on that, haha
            Last edited by robburgoon; 11-14-2016, 05:52 PM.

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            • #7
              Originally posted by robburgoon View Post
              Max RPM and the time spent there isn't changing, this would be going from 350hp and 395ft/lbs to let's say 475hp and still 395 ft lbs. Bouncing off the limiter at 6.2K either way. The engine hasn't blown up yet, I'm just trying to figure out if a cam change with no increase in max rpm adds much danger for the bottom end.


              The only difference would be the 395 ft lbs shows up like 2,000 rpm later.

              Edit: I think those hp/torque math needs some work...
              Edit2: 450hp is theoretically possible if you hit your 395 ft lbs at 6k. Won't hold my breath on that, haha
              Even though that build idea is a bust, I'm still interested if cranks care more about torque or hp if the time spent at various rpm stayed the same.

              Comment


              • #8
                Originally posted by robburgoon View Post
                Even though that build idea is a bust,
                Well, gaining 30% more power with cam only and no RPM increase looked a lil bit fishy to me

                Comment


                • #9
                  Originally posted by bawareca View Post
                  Well, gaining 30% more power with cam only and no RPM increase looked a lil bit fishy to me
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                  In this example you have 3 engine packages, each putting out the same torque, but that torque happens at a different RPM.

                  Now, suppose you start out with the pink curves, and you set your limiter at 4k to protect your weak bottom end.

                  Now after a cam, intake, head change, you have the black curves, but you still have that weak bottom end so you set your limiter at 4k.

                  The oval track you race is very circular and tacky, so it's full power 100% of the track with rpms only varying from 3900-4000.

                  Engine 2 is making 100 more horsepower at the limiter at 4k rpm. Could this extra horsepower snap the bottom end?

                  Note: you have also run engine 1 at the torque peak 3000rpm for sustained periods of time when you were running a different gear ratio, so you know that 590ft lbs won't snap the crank.


                  You guys all with me so far?

                  Comment


                  • #10
                    Well, I don't know about your theoretical engines, but if the engine you have is already spinning 6,200 and you're just trying to add a cam that will make more power without increasing the rpm, that seems like a safe bet.

                    The added power output will increase wear on the upper rod bearings and lower mains, though.
                    www.TrackHQ.com

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                    • #11
                      Originally posted by ucfbrett View Post
                      The added power output will increase wear on the upper rod bearings and lower mains, though.
                      You sure? The torque IS staying the same... :-D

                      Comment


                      • #12
                        Coming from an sbc in my truck with 4-bolt mains, if I'm making over 390hp that crank will be replace by a forged unit. I personally have never broken, bent, or scrapped a crank, but I wouldn't try spinning a cast one over 6k. A forged crank can be had for less than $500. That's cheap insurance

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                        • #13
                          Rob, I understand what you're asking, but perhaps it's the wrong question? I see it as a test to pass vs test to fail proposition. Which crank offers you the higher "cushion"? Cost surely comes into it, but how much time and expense do you lose if the cast crank fails during competition? I would be asking if the cast crank is worth the cost to handle to power upgrades versus a cast crank failing.
                          Yer pal,
                          Force

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                          • #14
                            Originally posted by Force McCocken View Post
                            Rob, I understand what you're asking, but perhaps it's the wrong question? I see it as a test to pass vs test to fail proposition. Which crank offers you the higher "cushion"? Cost surely comes into it, but how much time and expense do you lose if the cast crank fails during competition? I would be asking if the cast crank is worth the cost to handle to power upgrades versus a cast crank failing.
                            The trouble comes when you apply that reasoning to the entire engine, the cost goes up dramatically. You gotta draw the line somewhere.

                            Old thread btw.

                            New ideas though. Maybe the horsepower hurts since you're flexing the crank much faster with the same torque. So the torque would change how much flexing is happening due to the force the engine produces, and the rpm changes how much flexing happens due to being whipped around and how fast the flexing happens.

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