OPRV impact on hot oil

Towner · May 11, 2014

Hello,

there are a lot of threads about the oil pressure relief valve. It is always stated this valve has only impact on cold oil, because when oil is hot the valve wouldn't open anymore. But has anyone tested, whether the maximum oil pressure controled by the valve is the same with hot and cold oil ? With raising pressure the little piston travels until the holes are reached, but the amount of oil that can escape through the holes is much higher with hot oil. So I suppose that if you set your valve to 55 psi with cold oil it could be possible that the valve could restrict the pressure of hot oil e.g. to 45psi.
I am just discussing this topic with some guys here, whether it is better to have the pressure on the higher side (e.g. 80psi with cold oil) to get better pressure with hot oil. What is your opinion ?

Ralf

olChris · May 11, 2014

My MK3 has 80+lbs cold (80lb gauge) and sits around 45+lb hooooot...,... 30-40lb hooooooot idle is here nore there to me.. you got oil pressure you got oil!!! .... And i run the "original" black plastic oil feed lines to the head.. The naysayers will condem these pipee but with a bit of research you will find there capable of 250lb pressure...

L.A.B. · May 11, 2014

Towner said:
With raising pressure the little piston travels until the holes are reached, but the amount of oil that can escape through the holes is much higher with hot oil. So I suppose that if you set your valve to 55 psi with cold oil it could be possible that the valve could restrict the pressure of hot oil e.g. to 45psi.

Yes but theoretically I think it should take the same level of pressure to open the valve regardless of whether the oil is hot or cold.

If the oil escapes more quickly from the valve because it is hot then, as I see it, the valve would not open as far as it would when the oil was cold.

Towner · May 11, 2014

L.A.B. said:
Yes but theoretically I think it should take the same level of pressure to open the valve regardless of whether the oil is hot or cold.

Yes, but I think that a part of the job is done by the size of the holes (eg. 35 psi to move the piston + 20 psi restricted by the holes)

L.A.B. said:
If the oil escapes more quickly from the valve because it is hot then, as I see it, the valve would not open as far as it would when the oil was cold.

Yes, but if hot oil doesn't open it as far as cold oil, wouldn't that imply that the pressure ist less ?

texasSlick · May 11, 2014

As LAB says, theoretically the valve will open at the same pressure regardless of oil temperature.

However, when the oil is hot, the oil will flow thru the engine passages, channels, and tight places more readily, and the pressure does not build up to the "cracking" pressure of the OPRV. This lower pressure with hot oil is due to the limited oil pump capacity, and not related to the OPRV.

Slick

Towner · May 11, 2014

texasSlick said:
This lower pressure with hot oil is due to the limited oil pump capacity, and not related to the OPRV.

That's my question. Has this been proven by someone ? E.g. by checking the valve outside the engine with cold and hot oil.

L.A.B. · May 11, 2014

Towner said:
L.A.B. said:

Yes but theoretically I think it should take the same level of pressure to open the valve regardless of whether the oil is hot or cold.

Click to expand...

Yes, but I think that a part of the job is done by the size of the holes (eg. 35 psi to move the piston + 20 psi restricted by the holes)

I don't know how you could have arrived at that conclusion without some testing?

As I see it, the point of opening is basically controlled by the spring pressure, personally I don't honestly see how the holes could be adding much additional restriction as the volume of oil supplied by the Norton pump isn't all that great when compared with other oil pumps.

From previous discussions about oil pumps/pressure/OPRV, etc. and the consequent drop in pressure as the oil gets hot, I think it's likely that the OPRV basically stops working once the oil is hot.

texasSlick · May 11, 2014

Towner said:
texasSlick said:

This lower pressure with hot oil is due to the limited oil pump capacity, and not related to the OPRV.

Click to expand...

That's my question. Has this been proven by someone ? E.g. by checking the valve outside the engine with cold and hot oil.

Sorry Towner, my explanation requires an explanation....my bad.

The oil pressure is lower than the CRACKING PRESSURE of the OPRV with hot oil because the pump cannot raise the pressure to the cracking pressure of the OPRV, NOT BECAUSE THE OPRV is taking control. Hot oil just escapes thru the engine more readily, and the pump cannot keep up. When this happens, the OPRV is just sitting there doing nothing.

Cold oil: oil flow thru engine is retarded...pressure builds up....OPRV takes control and limits pressure.

Hot oil: oil flows easily thru engine...pressure limits out less than OPRV set point because oil pump capacity maxes out.

I hope this is clear.

Slick

L.A.B. · May 11, 2014

Towner said:
texasSlick said:

This lower pressure with hot oil is due to the limited oil pump capacity, and not related to the OPRV.

Click to expand...

That's my question. Has this been proven by someone ? E.g. by checking the valve outside the engine with cold and hot oil.

Again, if the OPRV actually caused a restriction, or it wasn't capable of handling the volume of oil then as I see it, the oil pressure would continue to rise as the revs increased, but from my own oil pressure test gauge readings, this doesn't seem to happen even with cold SAE50, the oil pressure rises as revs increase to a point where the OPRV opens, the pressure then drops back a little and remains stable even if the revs continue to increase.

Hobot · May 11, 2014

Boils down to how easy the oil is to flow out from the fixed hydraulic flow rate of gear pump. Turn pump 100 rpm you get that much set oil flow regardless of temperature. If thick oil can't move through fastest then pressure rises till it does or something blows out. If hot thin oil then same flow can occur at less pressure. Crank spin sling is enough to supply rod shell oil wedge so pump only has to feed enough oil not to starve the rod journals and a little left over to trickle feed the head. Any more pressure than that is taking power away from road performance. You should pick oil grade so oil pressure stays up enough when full heat thinned not by restricting pump out put to up pressure. If rods let oil out so fast it don't build pressure then put in engine honey or tighten up the internal engine clearances.

Myford · May 11, 2014

A large part of my day job involves testing relief / safety valves, from a couple of psi up to 4500 psi. LAB is correct in what he says. It dosnt matter what medium you use, gas or fluid, hot or cold, it still takes exactly the same amount of pressure acting on the moving part of the valve to overcome the spring pressure acting against it. The thickness of the oil will have no effect.
Terry

Hobot · May 11, 2014

Cold oil piles up inside engine parts popping off relief while hot oil flows on through under pop off threshold. Alright Myford then the reason for not just blocking off OPRV is cold starts could damage pump drive and hoses but not other engine parts. Oil pressure beyond enough to reach head seems over kill on Nortons as long as the flow to rods is not blocked. I'm getting impression oil grade thickness is more important to Notons than oil pressure. IIRC the oil heat absorbing from internal passages and engine surfaces is under 10% of total flow away form combustion. The buggaboo is thick oil don't flow and carry off as much heat as thin oil yet thin oil don't hold parts separate as well as thick oil. ugh Mult grades makes some sense in wide temp of operation. All out racers that replace stuff to win at all costs use water thin oil but don't know how they keep engine oil pressure up to surf on, bigger higher horse power pump flow that's still less drag than thinner oil pays back?

Towner · May 11, 2014

Myford said:
A large part of my day job involves testing relief / safety valves, from a couple of psi up to 4500 psi. LAB is correct in what he says. It dosnt matter what medium you use, gas or fluid, hot or cold, it still takes exactly the same amount of pressure acting on the moving part of the valve to overcome the spring pressure acting against it. The thickness of the oil will have no effect.
Terry

Thanks for all your answers. Finally I agree with you all. The purpose of the valve is to limit the pressure of cold oil, that's for sure. On my Commando I read 55 psi max with cold oil and 35 psi max with hot oil. But I couldn't prove that the 35 psi with hot oil are no more controlled by the valve. I have always been sure that this 35psi is the limit of the pump, but some guys here want to tell me, that it is still restricted by the valve. May be the maximum pressure controlled by teh valve is a little bit lower with hot oil, but this can't make that difference.

Edit: So it is good to get an answer of someone who tests this valve - Thanks !

Ralf

Towner · May 11, 2014

hobot said:
Alright Myford then the reason for not just blocking off OPRV is cold starts could damage pump drive and hoses but not other engine parts.

I heard of inverted crank seals.

pete.v · May 11, 2014

Towner said:
[ The purpose of the valve is to limit the pressure of cold oil, that's for sure.
Ralf

I do not think that is the right way to say it.

I really think the PRV is to create a somewhat precise resistance in the system so oil is pumped to where is need to be pumped to without blowing out everwhere. Without this resistance oil would just flow back (bypass) to the tank. This is why if the spring gets weak with age or what ever, there are shims to boost its function.

I really like my gauge and although it reads 80psi cold and 40psi hot, it is really just to validate the systems function rather than tell me how well it is functioning.

Edit.
I am not sure what is wrong with this thread. Hobot, your post on the next page, which I cannot see, shows my cranial logic lock issues. Although the OPRV indirectly does what said it does, it is not the correct description of its function. I saw what you wrote by searching your posts and reading the abbreviated portion.

Jerry/LAB, something is screwy here.

pete.v · May 11, 2014

Towner said:
[ The purpose of the valve is to limit the pressure of cold oil, that's for sure.
Ralf

I do not think that is the right way to say it.

I really think the PRV is to create a somewhat precise resistance in the system so oil is pumped to where is need to be pumped to. Without this resistance oil would just flow back (bypass) to the tank. This is why if the spring gets weak with age or what ever, there are shims to boost its function.

I really like my gauge and although it reads 80psi cold and 40psi hot, it is really just to validate the systems function rather than tell me how well it is functioning.

Hobot · May 12, 2014

Duh, the pressure relief only functions to protect hoses and seals not to assure engine gets full oil flow. If you see oil pressure below PRV setting then that means there's more volume of hot oil flowing through engine at low enough pressure so nil needs to be bypassed at PRV. The more oil by passed-recirculated at the PRV the hotter it gets till thin enough to flow with out much pressure resistance through engine. Only seals too much oil pressure can blow is the TS oil pump nipple and crank feed end seal and head feed connections. These seals can not be inverted and expect to hold as much pressure inside. DS crank seal only needs to seal in blow by pulses that are vented before much pressure can develop or restist sucking in by too good a breather extraction system. Too often a dry Commando outside after a good run means its about dry inside too.

Towner · May 15, 2014

hobot said:
Duh, the pressure relief only functions to protect hoses and seals not to assure engine gets full oil flow. If you see oil pressure below PRV setting then that means there's more volume of hot oil flowing through engine at low enough pressure so nil needs to be bypassed at PRV. The more oil by passed-recirculated at the PRV the hotter it gets till thin enough to flow with out much pressure resistance through engine. Only seals too much oil pressure can blow is the TS oil pump nipple and crank feed end seal and head feed connections. These seals can not be inverted and expect to hold as much pressure inside. DS crank seal only needs to seal in blow by pulses that are vented before much pressure can develop or restist sucking in by too good a breather extraction system. Too often a dry Commando outside after a good run means its about dry inside too.

Mmh, Hobots post stays hidden ...

L.A.B. · May 15, 2014

pete.v said:
I am not sure what is wrong with this thread. Hobot, your post on the next page, which I cannot see, shows my cranial logic lock issues. Although the OPRV indirectly does what said it does, it is not the correct description of its function. I saw what you wrote by searching your posts and reading the abbreviated portion.

Jerry/LAB, something is screwy here.

Yes, well, hobot's post was there. :?

OPRV impact on hot oil

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