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Working oil pressure and oil filter pressure

Posted: Sun Jan 06, 2013 1:36 pm
by warpc0il
This came up in another thread and has been copied here for future reference...
warpc0il wrote:
Bulldog wrote: That is what is happening on my new engine. The oil pressure is going to be increased from 76psi to about 100psi .. however we did think up a bit of a problem at the Yorkshire meet in that there are two oil filters the S1 which opens at 76psi and the R3 variant which opens up at 120psi .. neither are any good for me ... Anyone with an idea of an oil filter were the spring is set to around 100psi that will fit on the Rex?
The pressure quoted for oil filters isn't the working pressure, it's the bypass pressure.

This is the point at which the pressure drop across the filter element, due to collected debris, triggers the flow to bypass the element and circulate unfiltered.

If a filter with too low a bypass pressure is fitted, then oil will circulated unfiltered even though the element isn't clogged.

If a filter with too high a bypass pressure is fitted, then oil will continue to be directed through a clogged element and the flow with be restricted.

The required filter bypass pressure rating for any given engine design is determined by a number of factors, including the recommended viscosity, working pressure of the system, delivery flow rate of the oil pump and where the filter fits within the overall circulation.

Mazda increased the bypass pressure rating for the R3 as the oil pump has a high flow rate and the system is set to a higher working pressure.

However, I suspect their main reason is that the 12k mile service intervals means that oil is being circulated longer resulting in partial clogging of the element. At a 76psi bypass setting, 12k old oil may start to become marginal and there's a risk that unfiltered oil would start to circulate.

By increasing the bypass pressure point, circulation will remain within the element and the oil pump has been uprated to cope with the back-pressure (resistance to flow) from the clogged element.

Increasing the working pressure of the S1 system alone wouldn't have a significant impact on the bypass pressure rating.

If you're using good quality oil and changing it well within the recommend service intervals, e.g @ 6k miles or less, and always replace the filter every change, then the bypass pressure becomes irrelevant anyway as the filter would never become clogged enough for it to kick-in.

In short, there is no direct relationship between the working pressure of the oil system and the bypass pressure rating of the filter. So you're higher pressure pump/regulator should still use the standard S1 filter.

Note that cheap filters are often poorly constructed such that the elements can collapse, blocking flow, and the bypass mechanisms can trigger at widely varying pressures; either way too early or never, despite what it may say in the specification.

Re: Working oil pressure and oil filter pressure

Posted: Thu Dec 28, 2017 7:09 pm
by Delanor
You really need to be aware that you get the correct oil filter for your car, on the last service I used a Bosch filter from Eurocarparts which was correct, now now they list the same filter for the MK1 & the R3 but there is a difference, I have queried this with Eurocarparts but not received an answer.

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