Low friction bearings don’t require the pressure that a plain bearing engine does. A pre ’07 Big Twin (pre ’06 Dyna) would probably run just fine with around 10 psi of oil pressure under the right conditions. A plain bearing engine like the VRod would experience a catastrophic failure under the same circumstances.
A little background on the H-D oiling systems: 1973 and later Shovelheads and all EVO’s produced were designed to stop pressurized oil from reaching the connecting rod bearing below approximately 12 psi. At low engine speeds pressurized oil was diverted from the bottom end in order to maintain an adequate supply of oil to the lifters and rocker arms. I am not aware of an H-D overhead valve, V-Twin engine built from 1936 to present day, including the Twin Cam, that has pressure fed crankshaft main bearings; they are all indirectly lubricated by splash.
It’s kind of funny to think about it now, but the folks that ran the big gorilla pressure relief springs, or the adjustable pressure relief in the EVO’s were actually reducing oil flow to the connecting rod bearings at lower engine speeds. Higher engine RPM's were required to overcome the additional spring pressure in order to move the valve past the crankshaft port in the relief tower.
Back around 1977, I saw a crankpin from the rod bearing assembly of a’76 XL that was manufactured without an oil passage. This engine ran over 2,000 miles without direct oiling of the connecting rod bearings. The engine was taken apart because of excessive oil pressure. There is enough oil in the crankcase of a ’76 XL, that the connecting rod bearing was adequately oiled and did not fail; in fact the parts looked flawless. I am sure that ridden hard over time, the bearing would have eventually failed, but it looked great in the first couple of thousand miles; try that with a plain bearing crankshaft.
How many people here have fired up their Twin Cam, watched the oil pressure gauge hit the right peg, then have the oil light come on? These are symptoms of a sticking pressure relief valve. A piece of foreign matter, usually chain tensioner material, becomes trapped between the relief valve and its bore. When the engine is shut down the material becomes wedged in tight as the relief valve closes. When the engine is next started the relief valve is held shut by the foreign material until the oil pump develops enough pressure to force the valve open, pegging the oil pressure gauge. With the material wedged tightly between the relief piston and the bore, the return spring cannot overcome the friction, now the valve fails to close. The pressure relief valve can no longer control the oil pressure and with the valve stuck open, the light stays on.
I experienced a stuck pressure relief valve on my 2002 FLTR. I was in Albuquerque a few years back, and I rode it 1,400 miles home with the oil light on below 2,200 RPM. The rational behind the decision to ride it home was that as long as the lifters remained pumped up, the engine will receive adequate lubrication to operate without damage under low load conditions. A plain bearing bottom end like in the VRod would fail in short order if run under similar circumstances.
FYI: All Twin Cam “B”, ’06 and later Dyna’s and all ’07 and later Twin Cam “A” engines require additional pressure to run hydraulic chain tensioners for the balancers (B engines), the plain aluminum camshaft bearings in the cam plate as well as the hydraulic cam chain tensioners. A sticky pressure relief valve would have the potential to become a bigger problem in one of these engines.
Since the “full flow” crankshaft oiling introduced in 1973 through the last EVO built, H-D Big Twin engine internals have literally been swimming in oil. If my memory serves me correctly, H-D’s first significant attempt to better manage the engine oiling began in 1981 with top end and crankshaft oil restrictors, oil pressure relief modifications, valve stem seals and internal air flow mods resulting in improved top end oil return. The EVO incorporated all of these changes along with substantially improved internal air flow. The weak link in the EVO from an oiling perspective was the method used to scavenge crankcase oil. The flywheel scraper and internal breather passage was a holdover from before 1940. The method is inefficient and results in the flywheels running in a substantial amount of oil.
Enter H-D’s Twin Cam with the “clean sheet” approach to the new Big Twin. This engine was completely redesigned, and except for a few token parts like wrist pin retainers, pushrod tube springs and cups, etc., it is a completely new package. All of the leftover 1930’s technology in the EVO was replaced, including the oiling system. I was initially surprised to see the oil jets for the pistons in the Twin Cam; in the past the challenge was to reduce the oil under the pistons; now the oil supply is finely tuned with additional spray.
Having said all that, how much oil is too much in the Twin Cam engine? Your hot rod Ford, Chevy or Dodge may benefit from higher oil pressure to offset the increased load on the crankshaft’s plain bearings, but plain bearing lubrication concepts do not apply to the Twin Cam’s low friction crankshaft bearings. My sense is that raising the oil pressure in a high flow system like the Twin Cam’s, simply introduces additional oil into the engine without returning commensurate value. Excess oil adds overhead to various systems like piston rings, air/oil separators, seals, gaskets, etc.; not to mention the effect of increased pressure on the hydraulic chain tensioners.
Bottom line: I can think of worse things than being too well lubricated, but if there is no substantial value returned, why not spend the money on something else; JMHO.
djkak
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Topic: Baisley LMR-02 oil spring (Read 22433 times)