Jet feed, pressure feed difference

[Timmäää]

Hi everyone. 
Please can anyone explain the difference between jet feed and pressure feed lubrication. I am too stupid to understand. Do I have both possibilities when I order a new Phoenix crank?
Please help 
Regards Timo

[Ivor Hawkins]

Jet feed is the standard “spit and hope” set up found on the vast majority of Austin Sevens and works surprisingly well, pressure fed is found on some sports models and most modern racers and is identifiable by the external oil feed usually going into the nose piece.

If your car is just a standard road car the jet feed (splash fed) I’ll be fine, if you are building a special or a racer you’ll need the pressure feed and all the other parts to go with it!

[Austin in the Shed]

Pressure feeding the crank on a 2 bearing A7,or  Rosengart requires a new or modified front timing cover to feed the oil into the end of the crankshaft.
If you intend to supercharge the engine it is much simpler if it is splash feed to drive the supercharger directly off the front of the crank.

[Bob Culver]

A little background if you are curious. Very early cars had basic lubrication systems where oil was dripped or dribbled to the slow turning bearings. Then many had some form of low pressure circulation so oil flowed down to the main bearings and reservoirs were filled for the big ends and  the rotating big ends scooped into these ie the Model T Ford and very many/most others. The system continued in Chev cars into the 1950s. When the scoops on the crank encounter oil it is smacked into the big end bearing with some force. The Seven is closely modelled on the Belgian FN motorcyle. The rolling main bearings need no special quantity supply. For the bigends, instead of filled reservoirs,  low pressure oil jets point down to small scoops in the rotating crank. The system was also used on a model of the unusually small engined Bugatti Brescia sports/ racing car of about 1920 and earlier.
Many cars and all moderns adopt full pressure system with oil conducted to most bearings at considerable pressure. Involves drillings s in the crank and not very convenient for 4 cyl 2 bearing engines with rolling main bearings as  feed to the bes cannot be from those. And centrifugal aspects complicate the somewaht long tortuous flow through the crank. The basic Seven system sometimes called "spit and hope" has been proven to remarkable rpm. A few sport and racing models were fully pressure fed and many of the current Seven racers favour it.
In cars before universal filters and modern detergent oils, one disadvantge of pressure feed was that a lot of oil was forced through the bearings and consequently, more dirt. Neverthelss cars as "recent" as the VW Beetle had no filter.

[dickie65]

I run a pressure fed engine in my EB 65.
It requires a lot of modifications to work well.
This is a Two Bearing Engine.

Front cam bearing needs to be either modified or a new one made with a restrictor in its oil feed It also has two O rings on the outside of the bearing.
The oil feed to the rear cam bearing needs a restrictor
The oil jets need to be blanked off, or the complete oil gallery needs to be plugged with the oil coming out of the engine at the oil pressure gauge which needs to be opened out and threaded to 1/4 BSP then the oil needs to go back into the engine at the front of the oil gallery via an oil filter and external oil pressure relief valve and oil also needs to be fed to the front of the crankshaft.
Uprated oil pump is also useful.

[Ian Williams]

"If your car is just a standard road car the jet feed (splash fed) I’ll be fine, if you are building a special or a racer you’ll need the pressure feed and all the other parts to go with it!"

I am sorry Ivor this is simply not true, myself and numerous others have raced very successfully with jet fed cranks! I have cars with both set ups and in all honesty do not see the need to add the complication of pressure feed unless it is an original set up or you intend to use you car under exceptionally demanding circumstances. It is true that long distance races at sustained high RPM, such as existed at Brooklands, showed weaknesses in the jet feed system however modern oils and small modifications to the Jet set up negate these issues under most modern sprint and race situations.

[David Stepney]

I can vouch for the efficacy of the standard 'spit and hope' lubrication. My own car, admittedly an unmodified RP saloon has now done 110K miles, which I believe to be genuine. The big ends are as they were made in the factory and the crank is standard. There are no funny noises.

[dickie65]

Ian is quite right that a well set up splash feed system is perfectly fine for speed events !

In my case the car came from my father and he bought the pressure fed crankshaft in the 1980s.

[andrew34ruby]

To those who have little knowledge of the Austin seven engine it might be confusing, so lets start by saying... Both systems have an oil pump and both systems create oil pressure.

The 'oil jet' system squirts a lot of oil at very low pressure through jets that 'squirt' at holes in the crank as it passes by. These holes lead to the big end bearings. It seems a poor system but works very well.

'Pressure fed' engines run at a much higher oil pressure with oil from the pump going to the bearings at pressure all the way. it seems to be a better system, but if you don't need it then you don't need it.

[Ian Williams]

I can not tell you precisely what the big end oil pressure is with Jet feed, but it would appear that with carefully aligned jets and well designed crank oil pickups/pockets the effective PSI created by the centrifugal forces is more that sufficient for it least 6000 RPM. That has proven to be the case with my car and those belonging friends of mine. That said I am not sure I would attempt to drive a 100 miles or more at a sustained 6000 RPM on jet feed, I have however often driven long distances on motorways at a sustained 5000 RPM with no adverse effects at all.