Oil bypass mod

[246jaenick]

Can anyone help?
Reference oil bypass modification.
The engine case is Aluminium which is from my 1967 2.0 911S.
I have a 4 rib pump which I want to fit.
My question is:- Do I need to have the case machined/modified? I am not sure because the Aluminium case/casting appears to be different to a later Magnesium case?
If I need to have the case modified, is the machining procedure the same?
Regards,
Jim.

[chris_seven]

Jim,

The 4-Rib pump will fit right in but if the 'stock' pump is in good order it should be more than adequate.

In our experience fitting a 4 Rib pump into an early case can result in very high oil pressure.

The 4 Rib pump has a larger pressure section and a smaller scavenge than the original pump and this means that more oil will have to flow through the by-pass.

As the early case doesn't have any piston squirters we believe that the extra flow can be too high for the by-pass circuit and creates too high a pressure.

This means that the Safety Relief Valve controls the oil pressure from around 3000 psi and you will see pressures of around 140psi.

I think cycling pressures at this level will not be good for the engine mounted oil cooler.

If we go back to basics the pressure section of the standard early pump is more than enough for the duty required and we have never understood why Porsche increased this section of the pump.

The early pump has a much better scavenge section than the 4-Rib pump as it has to deal with the by-pass circuit dumping oil back to the crankcase.

When you modify the by-pass circuit the larger scavenge section reduces the volume of oil mist remaining in the crankcase which significantly reduces windage and helps to control oil temperature.

We are successfully running several competition engines to 8500rpm with the standard pumps.

[Fubawu]

This is great info Chris, i guess bigger isn't always better. ��