Fram Bypass Filter


For an authentic looking engine bay a Fram or Mann style bypass oil filter is a good nod to the original 356 engines and will definitely help give your engine bay that vintage Porsche look. Bypass filters are often dismissed as old hat and inferior to full flow style filters, but is that actually the case? 

A bypass filter works by tapping off of the main oil gallery and passing the oil through a filter canister before returning it directly to the sump. The return path has a restrictor fitted so that only a small amount of oil is bled off and the pressure in the main oil gallery is not adversely effected. On Pre-A engines the return line fed oil back into the front of the generator stand. A full-flow filter on the other hand filters all of the oil and returns it to the main oil gallery so that it can continue on to lubricate the bearings. 

As the full flow filter has to be able to flow the full output from the oil pump the holes in the filter media are actually fairly large so as not to restrict flow. This is not the case for a bypass filter where the filter media does not have to be capable of passing the full output of the oil pump, the media can therefore be much finer. 

The bypass filter is therefore able to filter the oil to a much cleaner state, however it is still possible for larger particulates to make their way to the crank bearings as the oil bypasses the filter

Bypass filters are still used today but are often installed as secondary filtration in addition to a full flow filter setup. It is even said that such a setup can massively extend the period between oil changes as the oil is much cleaner in fact they are often installed for exactly this reason, especially on dirtier engines like diesels.

After looking at original and repro Fram style filters for a while I decided that they were a bit too expensive for my miserly taste and went looking for an alternative. I ended up finding an almost identical filter sold for Vintage Willis's for a fraction of the cost. (you can buy them from ebay)

This filter is ever so slightly different from those installed on a 356, the main difference being the bracket. There's also some debate in Porsche circles about the Orange colour too, but from what I've managed to figure out is that Fram filters were orange and Mann filters were green and both appear to have been used at one point or another on the 356. It's comical to read some of the purists comments about 'tractor filters'. 

The housings are supplied with drop in filters and the mounting brackets but no oil hoses. Those will need to be sourced separately. You can also buy repro water slide decals to complete that authentic look.

It is actually possible to convert a bypass filter into a full flow unit, this requires drilling the metering hole in the central shaft out to flow the full volume of oil from the pump (or cutting some slots with a Dremel). The hole is about 2/3'rds of the way up. I've seen at least one engine where the filter canister has been used as a full flow setup so they will apparently handle the full pressure. The only real consideration is the diameter of the oil lines as the original units only have provision for a relatively small oil line. For anything other than a 35hp engine you will need to weld in larger fittings.

My next challenge is to find some suitable Knecht style air filters, but that is proving to be a little harder.

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