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built4sound's Avatar
Join Date: Nov 2002
Location: Clinton, CT
Posts: 51
oil pressure vs volume...

sorry a little long winded

Question one of two....

Some time ago I was having a major problem with oil flow through my 2.7 turbo. I did receive a lot of great advice here but eventually got very discouraged and the car has been sleeping for quiet some time. Well now I am ready for action.

One if the pieces of advice I had received was to add a rescrictor in the oil line going to the turbo. With that came the discussion.... will a restrictor reduce the oil going into the turbo or since the pressure is still the same will it just change the flow and jet the oil into the turbo at the same rate.

I never really pursued this idea as I already had an 1/8" reducer inline with the turbo and was skeptical if this was the way to go.

The other day I had a revelation and found an adjustable oil diverter that would send the extra oil back to the motor while limiting flow to the turbo. I figured this was a good idea and planned on ordering it and also an oil pressure gauge to install on the output side of it before the turbo so I could make sure that I didn't limit flow to much.

I ordered the gauge and sender but when I went to order the diverter valve they thought it was not the best approach and suggested I adding a restricter..... We then had a long discussion about oil pressure vs oil volume going to the turbo.

So after that long winded history report here is my real question..... since I already ordered an oil pressure gauge to put before the turbo, is monitoring your oil pressure relevant to the amount of oil I send to the turbo. Should I bother to install it and use it as a tool to see the effects of different size restrictors in the input side of the turbo?

Oil drainage question will be part 2, thanks in advance
74 911s
2.7 Turbo
Old 05-07-2010, 09:46 PM
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Max Sluiter
Flieger's Avatar
Join Date: Mar 2006
Location: So Cal
Posts: 19,572
Everything on a main oil galley or line sees (basically) the same pressure: the same pressure that your cockpit gauge measures. After a restriction or opening such as a bearing, the pressure is low and it spills down to the sump to be sucked up into the tank then re-pressurized.

Because your turbo sees this constant pressure, a restrictor in the line will decrease the flow to the turbo if you maintain the same pressure you had before the restrictor.

The restrictor will speed up flow until the fluid reaces its critical velocity (like the speed of sound) for its particular viscosity. This is because the oil pump is a constant-displacement type so it wants to pump a certain volume and will use as much pressure as needed to do so. Once the critical velocity is reached in the restrictor, it is effectively stalled or plugged and pressure goes way up for no increase in flow. Before this point, your oil pressure limiting bypass piston will have come into effect, just as if you were revving a cold engine. This is why the oil pressure is constant past a certain point and why the restrictor limits oil flow volume. It is the same principle as the cam box oil line restrictors, which I do not use.
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
Old 05-08-2010, 12:01 AM
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