Oil pressure relief valve spring, and by-pass valve spring

[Tom Markussen]

Hi, can anyone help.

I am assembling my 356a motor. And I noticed that the two springs (pressure spring valve, and by-pass valve spring) are the same type. Both the weak type. But according to the workshop manual (356b), they should be different. About the double spring force when compressed (for the pressure relief spring). I have measured them on a scale. Definitely the weak type both of them (close to spec). Checked another old motor, with the same setup (two weak springs). Isn't that wrong?

The first look at Hofman's 356 catalogue also suggests the same spring for both valves. Strange?

Hope someone can clarify.

Thanks, Tom

[Tom Markussen]

See Hofmann Classic Car catalogue specifies the same spring for both valves. But the Porsche manual specifies two different forces:

Releif: 4.7 kg at 49 mm compression
By-pass: 2,3 kg at 47 mm compression

A mystery.

[Martin Benade]

I thought the large pump engines had a more similar pair of springs which Porsche then changed to a matching pair, but I don’t think that was for an A.

[Garry Haussler]

Harry Pellow describes the springs dimensions in his books.

[Al Zim]

When restoring engines we always replace the springs and make sure the piston is smooth (in its passage in the case bore) Zim

[Mike Wilson]

Why would one spring have greater force; to overcome what?

Mike

[Dave Erickson]

Why would one spring have greater force; to overcome what?

Mike

I assume Porsche originally designed the engine with a PSI setting for pressure relief and another for bypass, and they were different. Or perhaps the distance the piston has to move is different for each function.

[Mike Wilson]

Thanks, Dave. Both make sense.

Mike

[Tom Markussen]

Hi all,

I have looked a bit more into this issue, since I believe this must be very important. Both the two engines (356A) I have disassembled had the same spring in both Relief Valve and By-pass Valve. My conclusion is now, that this is very problematic. The Relief Valve shall definitely have the hard spring, as specified in the manual.

I have been informed that at a later stage (356C?), the setup was changed, and the same spring could be used for both (soft/hard?). I have not looked into this in detail, since I work on A engines.

My (rough) calculations show that the opening pressure of the Relief Valve is 57.5 PSI. This seems to be a plausible maximum value for oil pressure.

If the soft spring was used (as in both my disassembled engines), the Relief Valve would open at 24.6 PSI. That is very low. I wonder what limit the pressure sensor (on/off type) is set at? My guess is that one would never notice the problem based on reading of the oil pressure lamp alone. An oil pressure gauge should have been a commodity (like most British cars).

Tom

[Jacques Lefriant]

Hi Tom
you are definitely correct to be concerned you must use the stronger spring in the oil pressure relief. i beleive this confusion was caused by the discontinued 52mm spring for the oil pressure and replaced by the 66mm spring for the pressure relief. the original bypass was 68 mm but had only 2.3kp vice the 4.7kp so if used in the oil pressure relief would only by your calculations 24.6lbs. Now the current practice is to use the same spring in both places to make it idot proof. Also this increases the oil flow thru the cooler.
j

[Dick Weiss]

Ditto, Jacques.

[Tom Markussen]

Another interesting thing is the different types of valve spring plugs. Notice the different depth of the inner bottom surface. Can easily be set-up in a wrong configuration.

The one to the left is for the external tube that goes to the by-pass valve.

[Vic Skirmants]

I have never seen different depths on the plugs. Two of those appear to have washers in them to boost the pressure; commonly done on race engines.