Consequences to reducing relief valve pressure?

[RangerGress]

What happens if I put on an oil filter with a relief valve pressure of 12-15psi, when the OEM filter has a relief valve pressure of 25-35psi?

I'm not even sure if I understand what the relief valve does. My car has a hot idle pressure of ~10psi. If there is some valve in the oil filter that doesn't open until 25psi, then oil won't flow. And since that doesn't make any sense, I must be missing something.

This is a BMW (1987 325) race car. It has an external oil filter adapter on it so I can fit any oil filter with the right threading on it. Purolator Pure One's seem to have good flow so I thought I'd try that. But the only oil filter that Purolator makes for my car is their lower tier filter.

Since I thought I'd try to find a different Pure One that had the right threading. And that's when I figured out that there is a problem with matching the (apparent) OEM spec of 25-35psi.

So how does the relief valve pressure work and am I ok to go with a filter that has a lower-then-spec valve actuating pressure.

 

[Scott_Tucker]

The pressure bypass valve in the filter opens when pressure across the filter is whatever the valve is set at. In the case of the OEM filter, when the pressure drop across the filter medium is about 25 psi it starts bypassing the filter, which means that the oil is no longer going through the filter but around it instead. In reality not all oil goes through the filter on each pass, which is OK. Changing to a different filter will have no effect on your hot oil pressure so you would be better off searching for the mechanical reason your oil pressure is low at idle. In reality, 10 psi should be enough at idle. What is your oil pressure at higher rpm?

 

[Nitrox]

CM Filters don't contain a bypass valve. They claim their filter media filters down to single digit levels, and flows enough a bypass valve isn't needed. I think it's around $100 to buy the spin on filter, and replacement cartridges are around $10 a piece, good for 10k miles.

 

[bepperb]

" In reality not all oil goes through the filter on each pass, which is OK."

No.

The bypass is for when the pressure across the element is above a certain level, not when the oil pressure is at a certain level. In the real world, a filter is almost never bypassing any oil. In a perfect world, a filter would never bypass any oil. Your bypass oil pressure really has very little effect on anything unless the element is clogged, of you're doing very cold starts.

 

[RangerGress]

That helped. Thanks a lot guys.

I looked up the CM filter. Interesting. Canton says 16gal/min for my 3/4-16 filter with hot oil. Any idea what a quality spin-on flows/min with hot oil? There's some cold flow charts around here, but I didn't see hot info.

Scott, I may have phrased my question poorly. I have no oil pressure problem. I just wanted to better understand how that valve worked.

 

[SuperBusa]

Originally Posted By: RangerGress

So how does the relief valve pressure work and am I ok to go with a filter that has a lower-then-spec valve actuating pressure.


The filter bypass valve only opens when the PSID (pressure delta - PSID means "PSI" Differential) across the media is higher than the bypass opening pressure.

If you ran a filter with a lower bypass pressure rating than the OEM filter, make sure it is a filter that flows well. Don't know if you saw this thread, but the PureONE seems to flow very well.

http://www.bobistheoilguy.com/forums/ubb...451#Post1619451

Other data I have seen indicates that most oil filters will flow pretty much the same with hot oil, and the PSID spread will be about 2 PSID at around 5 GPM.

Hot Oil Flow vs PSID:
http://www.acdelcotechconnect.com/html/tas_filt_oil_hot_img.jsp

There is a much wider PSID spread among filters with cold oil flow:
http://www.acdelcotechconnect.com/html/tas_filt_oil_cold_img.jsp

Based on the PureONE flow data I got directly from Purolator, I would say it would work just fine ... but if possible try to get one that is avg or larger in size just to be safe. If they make one in your size in the Classic model, it would be good too, as I believe the Classic even flows better than the PureONE based on river_rat's testing.

As with any filter, you don't want to do high revs until the oil is somewhat warmed up of course.

 

[SuperBusa]

Originally Posted By: RangerGress
I looked up the CM filter. Interesting. Canton says 16gal/min for my 3/4-16 filter with hot oil. Any idea what a quality spin-on flows/min with hot oil? There's some cold flow charts around here, but I didn't see hot info.


If you put enough pressure on any oil filter it will flow a lot ... and of course the PSID across the filter will increase accordingly. If you look at the Flow vs. PSID chart for that PureONE, they flowed up to 18 GPM in the bench test.

The maximum flow volume your engine will allow is based on 3 factors:
A) The restriction factor of your engine's oiling system (including filter PSID).
B) The maximum oil pump outlet pressure.
C) The viscosity of the oil.

Most high performance engines might flow upto 10 GPM unless the oil pump and/or oiling system has been modified to flow more for some reason (track use, etc).

 

[RangerGress]

Ya the 16gal/min in the absence of more info is a pretty useless factoid.

That 10gal/min info is useful, thx. Our engines are stock and since their only 2.5 liters, as opposed to a big V8, I would imagine that they put out no where near 10gal/min.

Unsolicited background:
We find that our oil pressure drops significantly as we experiment with lowering our oil visc. My experience with other race cars is very limited so it's hard for me to put our race car oil pressure within a wider perspective. My tow vehicle, however, sustains ~75psi at relatively low rpms. The BMW will only get OP that high if the oil is cold. The oil pump's pressure release valve is 70-75psi. So apparently BMW made that engine to run happily on lower pressures.

In this '87 BMW 325 20W50 gives about 55psi@redline, xW40 40psi, xW30 28psi. Those are rough numbers because obviously oils are different and bearing state-of-wear are different. But our motors have to be entirely stock, so the difference in motors are only differences in wear. Many of us are racing very high mileage motors. A 150k motor racing competitively is common.

Since I can't replace the pump with a higher pressure pump (per the rules), I thought I'd look at minimizing flow resistance. And that was the genesis of this thread.

 

[PT1]

I would stick with the oil filter speced for your car by the manufacutrer. If it the regular tier purolator then so be it.

 

[RangerGress]

Originally Posted By: PT1
I would stick with the oil filter speced for your car by the manufacutrer. If it the regular tier purolator then so be it.

German outfits tend to spec pretty good stuff. But I think that someone racing in a Ford or Chevy, for example, would bust out laughing if folks suggested that they race with the same parts as are put on the common daily driver.

I would agree that a reasonable default solution is to go with the OEM recommendation, but one should also be willing to question authority. That is to say, seek to understand the engineering, and consider alternatives that might be better suited to their usage.

 

[Gary Allan]

The deal with the high bypass tends to revolve around the spec'd oil and the spec'd oil pressure on the Euro's. It stumped me for a while figuring why they did it. Doug Hillary finally filled in the missing link. The way they set them up they may be above the relief level or in relief (pump) for a long time. There are still some limitations on fluids and requirements at the full sustainable power outputs that the Euro's pride themselves on. The starting event may have a 100psi pressure limit, but due to the weight spec'd and the volume pumped (trying to) and the starting temp, it may be in pump relief for 20 minutes and much of it @ 135+psi. That's where you'll see elevated PSID. If not for the 30-35psid filter bypass setting, the thing could be passing under filtered oil for most of its life. They do this even where volumes aren't THAT high.

I imagine in the evolutions, the longer drains and the bigger sumps might somewhat balance themselves out, but if you look at the Wix spec's for some Euro filters, they're kinda loose. Not quite sieves ..but not the tighty whiteies that we seem to want.

 

[SuperBusa]

Originally Posted By: RangerGress

That 10gal/min info is useful, thx. Our engines are stock and since their only 2.5 liters, as opposed to a big V8, I would imagine that they put out no where near 10gal/min.


If that is the case (and I agree I doubt that your engine puts out 10 GPM; probably more like 5 at redline), then almost any filter would be safe IMO with hot oil based on the flow curves I've seen for filters. One reason I would suggest the Purolator (or any other filter similar in specs) is because they have a relatively high bypass setting - usually around 14~16 psi which gives some added safety that it will not go into bypass as easily.

Originally Posted By: RangerGress

Unsolicited background:
The oil pump's pressure release valve is 70-75psi. So apparently BMW made that engine to run happily on lower pressures.

In this '87 BMW 325 20W50 gives about 55psi@redline, xW40 40psi, xW30 28psi.


What oil viscosity does the BMW spec call out for that engine?

Keep in mind that the thicker the oil is, the less volume is flowing to the filter/engine oiling circuit. But based on your numbers above, even with 20w-50 at redline the oil pump is not at max pressure relief mode. In the case of all the different viscosities above, at redline you are probably getting about the same oil volume in all cases (because constant volume OP is not in relief). Therefore, I'd stick with the 20w-50.

 

[RangerGress]

Origonal spec was 20W50. A decade later (1999) BMW said that 10W40 was ok too.

Re. sticking with 20W50. Racers experiment. There are race-oil manufacturer's that recommend xW10 for qual and xW20 for racing with bearings
Some racers make a hobby of attempting to optimize things.

But, this was really just an oil filter question and you guys had the answers. Optimum oil types and viscosity is a whole 'nother can of worms.