11 replies to this topic

[28hodge]

I’ve just done a full engine rebuild. I wasn’t aware that the std oil filter housing head has a bypass valve in it that opens at low psi differential between incoming oil to the filter and outgoing oil pressure. My engine has always made good pressure 60psi when started and 50psi when hot and running At 3000rpm and 35-40ish at idle when hot. The mains that were in the engine were replaced prior to me buying it and hadn’t done huge mileage, maybe 7000. I have a receipt for them so it wasn’t just sales pitch. However they were about ready to start doing damage with copper starting to show.

Anyway I think that the oil filter housing bypass has been to blame. I get why it’s there but I change the oil a lot, we use the car for historic rallies and I bet it’s never done more than 1000 miles between oil changes and I always change the filter at the same time.

I was going to get a med or swiftune billet oil filter housing head that has the bypass valve omitted. Other than the obvious fact that the engine would get no oil if there was a blockage is there any other reason why not to?

Or to save me a few quid, anybody brazed up the original with success?

Edited by 28hodge, 16 July 2020 - 09:56 AM.

[28hodge]

Additionally I was searching and found a post advising maybe a bigger oil filter would be good

http://www.theminifo...remote-filters/

https://catalog.mann... Filter/W 712~9

This was the filter advised and just gives a greater surface area. Mine is a clubman so room shouldn’t be an issue.

Edited by 28hodge, 16 July 2020 - 10:02 AM.

[whistler]

All the engines I have built have made 75 psi on cold start up and run at about 55-60psi when hot. About 25 psi on idle when hot. In the last 12 months I've rebuilt a mk1 850, a 1275 metro and an overbored 1098 and they're all the same. My wife's 1330 and my 1293 all run the same pressures. Am I doing something right /wrong or is it magic.

My previous engine builds in the late 60's were a 997 and a 1275GT. They had the same pressures.

[whistler]

Have tried changing the oil pressure relief valve spring and cup? https://www.minispar...ssic/6K853.aspx

 

If the cup sticks then that could be the cause of your problem. Don't opt for the ball bearing version, stick with standard.

[28hodge]

I think this is the problem, mine also made good pressure and I was happy, the problem is say you are providing 70psi to the filter, if the filter can only allow say 55psi through it then the difference in pressure between incoming and out going at the filter is 15psi. This means the pressure relief valve In the filter housing head will open and allow unfiltered oil to the oil galleries with the obvious consequences

[28hodge]

It’s got a new relief valve and spring in it after rebuild but like I say oil pressure wasn’t an issue, it was rebuilt as I had time and I wanted to check the clearance on the crank thrust primary and idler gears

[nicklouse]

AC Dodd brazes up the valve in the head. It is quite a common thing to do. 
 

be careful when thinking about a bigger filter as many have this valve in the filter.

[Spider]

Just be aware that while you can plug up the Filter By-pass in the Filter Head, the Oil Filter itself has one built in.

[28hodge]

Just be aware that while you can plug up the Filter By-pass in the Filter Head, the Oil Filter itself has one built in.

Yeah but I think it’s a bit higher psi differential than the PRV in the filter head

[Spider]

 

Just be aware that while you can plug up the Filter By-pass in the Filter Head, the Oil Filter itself has one built in.

Yeah but I think it’s a bit higher psi differential than the PRV in the filter head

 

 

I think you'll find they are similar. I know the Ball Bearing in the Filter Head only has a light spring, but as the Surface Area is small, it's much higher than you might think.

 

On some filters, and you may have to cut some apart to find the right ones, you can crush the bottom of them a little to increase the By-pass lift off pressure.

[28hodge]

Hmmm the post I linked to suggested that the psi differentia to cause the Mann large filter to bypass was 36psi whereas the filter housing bypass was more like 8psi.

http://www.theminifo...remote-filters/

I have no way of verifying any of that though?!!?

[Spider]

Hmmm the post I linked to suggested that the psi differentia to cause the Mann large filter to bypass was 36psi whereas the filter housing bypass was more like 8psi.

http://www.theminifo...remote-filters/

I have no way of verifying any of that though?!!?

 

8 PSI ??

It's higher than that, you'll get that and more just because of the pressure drop across the Filter in normal operation.

 

And when the Oil is cold, it would be higher again.