threw another #3 rod, how to prevent this from happening again. |
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threw another #3 rod, how to prevent this from happening again. |
crash914 |
Oct 29 2019, 03:17 PM
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#1
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its a mystery to me Group: Members Posts: 1,826 Joined: 17-March 03 From: Marriottsville, MD Member No.: 434 Region Association: MidAtlantic Region |
So blew up my #3 rod. wondering what caused this. lack of oil, due to rpm? poor return? oil pump sucking air?
last time it was #2 rod. I really don't want to live with this again. Motor specs, Dual oil coolers with full flow plumbing 102 bore by 80mm stroke long rods, ham heads and valve train. Running to 6800 rpm g loads to 1.5 windage tray modified at pushrod tubes for oil flow tuna can with 1/2 extra oil Pics of horror below. First are this failure, then old failure. |
Superhawk996 |
Oct 29 2019, 07:12 PM
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#2
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914 Guru Group: Members Posts: 5,900 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch |
On a more constructive note.
Play with this centripedal force calculator. https://www.engineersedge.com/physics/centrifugal_force.htm In this specific case, RPM's have increased from 5500 rpm to 6800 rpm. RPM is directly related to the velocity term. The force on the rod bearings is going up as the SQUARE of velocity. Said differently, the force increase isn't linearly proportional to the crank journal velocity, it is increasing exponentially. Double the velocity term in the calculator and watch what happens to Force, it goes up by way more than double. Bottom line, although the top end was likely beefed up to control the valve float, the stock lower end rod bearing journals are getting overloaded. This squeezed out the very thin oil film that separates the rod bearing from the journal and prevents metal on metal contact. First part of this occurs as dramatically increased bearing material wear. Eventually it results in steel on steel contact and the micro-welding Lew mentioned. If you are going to run this type of engine at these elevated power levels I'd recommend that you do professional oil analysis after every race. If you establish a baseline immediately after engine break in, you WILL be able to tell when the bearing material is being shredded off into the oil more rapidly than usual. At that point, you need to stop running the engine and replace the bearings at predetermined intervals. Ultimately you will likely get a crank failure in the long term after some unknown number of bearing replacements. |
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