Oil Pressure - Race Engines

[Fast Westie]

I know that this subject has been discussed several times, but this was mainly for road engines.

 

My race Zetec engine uses a 2 stage dry sump pump running at 50% engine speed and I am using Shell 5W40 fully synthetic oil. When cold the oil pressure is 4 bar, but when hot is only 2.5-3 bar at about 4000rpm, and less than 0.5 bar when idling.

 

I spoke to a reputable FFord Zetec engine builder who told me they only run 8-10psi idle and 35-40psi full chat on their FFord engines with synthetic oil. Someone else suggested thicker oil.

 

question.gif Should I worry?

question.gif Should I run thicker oil, e.g. Mobil 1 5W50?

question.gif Should I get a new dry sump pump?

 

Many thanks in advance for advice

 

The car in front is a Westfie1d wink.gif

 

Edited by - Fast Westie on 8 Nov 2001 12:41:34

[Peter Carmichael]

Reputable engine builders with experience will tell you that two same spec engines can come out with different oil pressure characteristics. Not necessarily a problem, but worth looking at why this happens.

 

There are four aspects:

1. The appetite of the engine for oil.

2. The pump.

3. The viscosity characteristics of the oil

4. The oil pressure relief valve.

 

If the bearing clearances double, the appetite for oil goes up by a factor of four. i.e. if your race engine is built a bit loose on purpose, the need for oil may outstrip the original delivery rate from the pump.

 

The pump. You mention the dry sump pump, but aren't necessarily clear enough about its set up and whether it contains the pressure pump. Does the sump have two pickups? If so then you have two scavenge stages and the original pressure pump is in use; i.e. the pump is appropriate to a standard Zetec and you only have to worry if the clearances are very large and the appetite for oil has gone up a lot. If the pressure pump is external you have to rely on the engine engine builder who specced it. Pumps can wear. Pump rotor tip wear can cause a *significant* loss in pumping capability. External pressure pumps often have adjustable pressure relief valves and this could be just set too low.

 

The oil pressure relief valve can be opening too low and you can shim the spring with ~2mm washer and see if this picks up the pressure. If not then it wasn't the problem. This is very rarely the problem.

 

What about oil viscosity. Of the 5W40 spec, the 40 bit is the hot running viscosity and 40 is not that "thin" (I thought Shell Helix was 5W30 which possibly is a bit too thin). Viscosity is a double edged sword. What you need is an oil that reduces the bearings consumption of oil but doesn't reduce the delivery of oil to feed the bearing. The higher viscosity oil will resist flow through the galleries more and may generate a higher pressure that does not equate to more flow. This is the thing to try out once you know that the oil pressure relief valve is working correctly.

 

So how much pressure is enough pressure? First thing to remember is that the pressure is only a *symptom of oil being pumped through a series of restrictions at a given flow rate*. If the restriction is less, you need less pressure for healthy flow. The pressure of 40psi is only delivering the oil. Inside the bearing, the action of the bearing pressurises the oil to ~60,000 psi. The oil pressure only needs to keep the flow rate matched to the rate at which oil spills out of the sides of the bearings. Bearings rate of consumption of oil accelerates at higher revs. If you rev the engine (and I am talking about 6500rpm and above range) and you notice a distinct rpm where the oil pressure starts to drop off then you have a problem of oil demand exceeding supply. FWIW, 53psi has kept my engine alive at 9000rpm with Mobil 1 15W50.

 

 

So for a complete the diagnosis:

- try shimming/adjusting the pressure relief valve spring (if pressure goes up then you have found the problem)

- see if the pressure drops away at particular elevated revs value (if not then probably don't have a problem)

- if the *pressure pump* (not the scavenge pump) is external, gear up the pump and this should eliminate the pressure dropping away symptoms and should improve pressures at all revs (more so below the pressure relief valve opening pressure i.e. the pressure will come in earlier in the rev range).

- or try a thicker oil

- you could check the pump clearances, but you probably won't find anything conclusive

- you could rebuild with new bearings but there is no guarantee that the clearances will close up

 

Also take stock of what work was done on the engine in making it a *race* engine. If new crank/rods/bearings then all the oil characteristics will be out of the window. My race engine ran 75psi last year, but ~55psi this year but with new crank (known to have smaller big end journal sizes)/different big end bearings in a better material that don't cause this on anybody else's engine/a new pump rotor with slightly large clearances. I am going to retrofit the original pump rotor and see if that fixes it. If not then the crank is causing it.

 

If you want to know more and are anywhere near Surrey next Thursday, come to the Wotton Hatch...

 

[Fast Westie]

Peter

Many thanks for your reply

 

In response to some of your points

- The pump is an external 2-stage pressure+scavenge

- I have adjusted the pressure relief valve, but that only changes the cold pressure.

- I will check for wear when I take the pump off and also check the engine bearings

- I had also thought of running thicker oil and gearing up the pump, but I believe there is a maximum limit on pump speed?

- I haven't noticed the pressure dropping away at high rpm (max 7900) and I've never had a "rattly tappets" problem

- I don't know too much about the engine since I bought it secondhand, although it had only been dyno run. I think the clearances may be larger on this engine

 

The car in front is a Westfie1d wink.gif

[John]

Try thicker oil and see if there's a difference. If the VX engine is anything to go by, try 15/50 instead of 5/40.

 

John

[EFA]

Your pump is geared to high. This is why the pressure is so good at "idle speed"

 

Don't be surprised it you get aireation at 7900rpm!

 

You should gear the pump down.

 

 

 

Fat Arn

The NOW PROVEN R500 Eaterid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[Rob Walker]

The other critical factor is oil temperature a mates SLR was seeing 110C at donington on Tuesday and at this temperature his oil pressure was falling away like crazy and he was on thick Mobil 1 15W 50.

[Fast Westie]

Interesting about oil temps, mine is 110-120C while running

 

The car in front is a Westfie1d wink.gif

[Peter Carmichael]

Arnie,

 

Do you just say the opposite to everybody else to wind people up? If you read the question in its entirety, idle pressure is 0.5 bar. Pressure is low all over, so gear the pump up not down.

[EFA]

Peter, I just read the question wrongly - I interpreted 2.5 - 3 bar at warm idle.....

 

 

Sorry for the confusion.

 

Fat Arn

The NOW PROVEN R500 Eaterid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[Rob Walker]

Peter,

 

I fitted the steel oil pump rotor and was concerned during the build at the increased clearance rotor to annulus but have found my oil pressure to be slightly higher than the stock engine set up. I am also running with all grooved top main shells. When hot 4.5 Bar above 3000 rpm and a tad over 2 bar at tickover 1050 rpm. In my experience the Apollo that you have re-fitted looses in excess of 0.5 bar across the range due to the bleed off into the cam cover have you factored this in?