oil pump relief valve

[Inline__engine]

what is the function of the oil pump internal relief valve when there is a separate relief valve after the pump connected to the main gallery? is it just added redundancy? the spring in the pump feels a lot stiffer than the other one that is screwed in the main oil gallery in the block

[tapkaJohnD]

If the oil pump is driven by the crank, pressure and strain at the pump are proportional to crank speed. At high speed, the relief valve prevents the pump from being overloaded, by dumping excess pressure and oil back into the sump. It will also do this when the oil is cold and more viscous, when pressure is higher at lower speed.

John

[Inline__engine]

wouldn't the relief valve slightly down stream (after the pump) already do that?

Edited by Inline__engine on Sunday 21st October 11:12

[Mignon]

I've never seen an engine with a pressure relief valve other than the one inside the pump.

[stevieturbo]

No idea what engine you're talking about or the configuration to even chance a guess.

[Inline__engine]

It's a bmw m20 engine

[Inline__engine]

[227bhp]

Are you sure it is what you think it is?

[GreenV8S]

According to the workshop manual that is indeed the oil pressure relief valve. There is only one, and that is it.

[stevieturbo]

A little googling does suggest it is a relief valve.

Seems a little unusual.

[gordmac]

Some engines have a valve that bypasses the filter if it becomes blocked.

[Inline__engine]

Isn't that part of the filter itself? Both are pre filter

[stevieturbo]

Inline__engine said:

Isn't that part of the filter itself? Both are pre filter

Not always. Some filters have a bypass, some dont. And some engines will have a filter bypass in the engine galleries or filter housing.

[Inline__engine]

I'm going to do a pressure test to see what pressure the pump valve opens. My calcs show it might actually be lower than what the other relief is (~70psi). based on the used pump I have seen there are signs that it may operate in service.

[Richair]

The filter relief valve is completely different; there's some very unhelpful comments in here...

As above, the valve is there to regulate pressure during cold start ups and high RPM. It's proper name is an 'Oil Pressure Relief Valve' (ORPV). Without it the pump will make too much pressure (yes, there is such a thing) and at best will cause minor leaks and at worst will lead to catastrophic seal or pump failure. Many older engines have auxiliary ORPV's, including Porsche 944's and 968's, on which it's fairly common for them to stick causing very high pressure which literally blows the oil filter (which has one of the aforementioned bypass valves, different thing) seal or splits the canister and rapidly dumps all the oil. Very important component!

I believe many newer engines have this built in to the oil pump, so it goes unnoticed.

[Mr2Mike]

Richair said:

The filter relief valve is completely different; there's some very unhelpful comments in here...

As above, the valve is there to regulate pressure during cold start ups and high RPM. It's proper name is an 'Oil Pressure Relief Valve' (ORPV). Without it the pump will make too much pressure (yes, there is such a thing) and at best will cause minor leaks and at worst will lead to catastrophic seal or pump failure. Many older engines have auxiliary ORPV's, including Porsche 944's and 968's, on which it's fairly common for them to stick causing very high pressure which literally blows the oil filter (which has one of the aforementioned bypass valves, different thing) seal or splits the canister and rapidly dumps all the oil. Very important component!

I believe many newer engines have this built in to the oil pump, so it goes unnoticed.

That does not really help with the OPs question i.e. why does this engine have two oil pressure relief valves, one built into the pump assembly, and this remote valve. I suppose there is one before the filter and one after it, though spin on filters normally have a bypass valve built in so what would be the point of having both?

[spitfire4v8]

Don't bmw engines traditionally have a paper element that fits into a canister on the top/side of the engine? As such that may not have an in-built bypass for the filter .. but then a filter bypass still normally allows oil flow to just bypass the filter and still feed the engine, not dump oil back to the sump.

Another thought I just had .. is the system set up to provide high pressure to crank, lower pressure to tensioners / vanos / hydraulic tappets ? maybe there's twp relief valves for two different pressures required in the engine ?

[Mr2Mike]

spitfire4v8 said:

Don't bmw engines traditionally have a paper element that fits into a canister on the top/side of the engine? As such that may not have an in-built bypass for the filter .. but then a filter bypass still normally allows oil flow to just bypass the filter and still feed the engine, not dump oil back to the sump.

That could well explain it then.

spitfire4v8 said:

Another thought I just had .. is the system set up to provide high pressure to crank, lower pressure to tensioners / vanos / hydraulic tappets ? maybe there's twp relief valves for two different pressures required in the engine ?

I thought about that, but oil is incompressible so the pressure in the entire system would be set by the relief valve with the lowest pressure.

[spitfire4v8]

True, but if you needed a differential pressure system you could have a pump with two sets of rotors in one housing, but i've absolutely no idea about this engine, just idle thoughts

[GreenV8S]

Mr2Mike said:

I thought about that, but oil is incompressible so the pressure in the entire system would be set by the relief valve with the lowest pressure.

I suppose it could be done using an orifice to feed the low pressure part from the high pressure part, with the low pressure blow off valve bleeding off enough excess flow to get the required pressure drop across the orifice. Not suggesting it's what's happening here, but it could work.