Shimming oil pressure relief spring

[Buffalo Bill]

You're right…

It is my impression (and I could be wrong here) that the oil pump will pump whatever it can, and it is constantly pumping against a spring-loaded relief system. If the pressure ever gets too high, it overcomes the spring and the pressure is reduced to acceptable levels. OK, shimming the spring will increase the pressure at which it will open, but I thought that the root desire for this mod was to increase the operating oil pressure, not the upper limit. Am I wrong?

An intelligent observation.
The Haynes GSX750F manual agrees: “If the pressure is lower than the standard, than either the relief valve or regulator is stuck open, the oil pump is faulty, or there's other engine damage.”, (bearing oil clearances).

[haggisbasher]

The oil pressure spec for my bike is 43-78 psi. If the relief valve is undersprung and releasing pressure at say 50psi then surely no matter what the pump can do the operating pressure will never exceed 50psi. Based on the top end issues experienced in this engine design maybe it needs closer to the 78psi end of the range.

Hopefully by measuring the operating pressure stock and after shimming I can shed some light on the merits of this mod for everyone. I am expecting the pressure stock to be in the lower end of the range and the post shim pressure to be nearer the top of the range leading to better top end oiling. Equally I will not be too surprised to find that nothing changes.

On a good note I got the motor running last night so just have to figure out a pressure gauge attachment and teh experiment can begin.

Tom

[gearhead13]

If you run heavy oil like 20w50 then the shimming would be more beneficial IMO. Thin oil would never allow the pressure high enough for the relief valve to open. Also worn bearings or a worn out pump wouldnt allow the pump to build pressure. Starting and revving it cold would have the relief open, but wouldnt come into play on a hot engine with thin oil, when you actually would need the extra oil pressure.

[Steve]

The oil pressure spec for my bike is 43-78 psi. If the relief valve is undersprung and releasing pressure at say 50psi then surely no matter what the pump can do the operating pressure will never exceed 50psi. Based on the top end issues experienced in this engine design maybe it needs closer to the 78psi end of the range.

This is very possible, but nobody has ever led me to believe that is what is happening.

Hopefully by measuring the operating pressure stock and after shimming I can shed some light on the merits of this mod for everyone. I am expecting the pressure stock to be in the lower end of the range and the post shim pressure to be nearer the top of the range leading to better top end oiling. Equally I will not be too surprised to find that nothing changes.

Yes, measurements would be a very good thing. If you have the time, please note the pressures at a cold startup, then again at full operating temperature for stock and shimmed setups. Pressure readings at idle, mid-RPM and somewhere near max-RPM would be interesting to note.

Good to see that you are approaching this with an open mind, realizing that you might not see any change, let alone improvement.

If you run heavy oil like 20w50 then the shimming would be more beneficial IMO. Thin oil would never allow the pressure high enough for the relief valve to open.

I do not agree with the overall concept of using thicker oil to raise the oil pressure. True, pressure will go up, but that is not necessarily good for the engine. The reason pressure goes up is because there is more resistance to flow. The thicker oil has a harder time going through the small bearing clearances and other small passages, so pressure is higher at the pump.

Also worn bearings or a worn out pump wouldnt allow the pump to build pressure.

True, but it's not only pressure, but FLOW through the bearings that keeps them happy.

Starting and revving it cold would have the relief open, but wouldnt come into play on a hot engine with thin oil, when you actually would need the extra oil pressure.

Starting and revving a cold engine makes me cringe anyway, but that's a topic for another thread. I prefer to maintain the lowest possible engine speed to allow the engine to run smoothly while warming up.


Regarding the "need" for high oil pressures: I have found it interesting that the only bike that has this oiling problem is a high oil pressure bike. I have not noted any such failures on the bikes with roller bearing cranks, where pressures are typically in the single digits after warm-up. What keeps those cams lubricated with 6 psi oil pressure? Certainly not pressure, must just be a lot of oil flow.

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[haggisbasher]

Good to see that you are approaching this with an open mind, realizing that you might not see any change, let alone improvement.

This seems to be a commonly accepted problem with a less commonly accepted cure. I dont know how big a problem it is or how good the cure is. I obviously can't answer whether it prevents a premature engine failure but if I can show the pressure figures before and after maybe people with these motors can have a better idea whether this is a mod worth doing for peace of mind. I have received untold knowledge and wisdom on this site so if I can contribute a little to the collective knowledge thats awesome.

Tom

[1980GS1000E]

...
Regarding the "need" for high oil pressures: I have found it interesting that the only bike that has this oiling problem is a high oil pressure bike. I have not noted any such failures on the bikes with roller bearing cranks, where pressures are typically in the single digits after warm-up. What keeps those cams lubricated with 6 psi oil pressure? Certainly not pressure, must just be a lot of oil flow.

.

I was wondering that myself. I have seen wear on the cam bearings for the roller bearing engines. Are the cams happier with higher pressure? To have good flow, you need higher pressure to start with.

[Steve]

To have good flow, you need higher pressure to start with.

Or larger clearances.

Or thinner oil.

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