Oil Pressure Concerns

Dff Moriarty · 06-04-2014, 08:03 AM

Just installed a fresh balance shaft delete race engine in an 87 NA and running Amsoil break-in oil. Oil Pressure less than 25 PSI at idle but jumps to 75 PSI under throttle. Our other similar race car sits at 75 PSI all the time. Looking for comments here as we are a bit concerned about the Low oil pressure. Could it be the extremely thin break-in oil? Was hoping to put this car on the track this Saturday.

9FF · 06-04-2014, 08:41 AM

That seems normal to me, are your other cars running a higher rpm at idle?

Techno Duck · Techno Duck's Garage

What is the oil pressure on a cold start?

Dff Moriarty · 06-04-2014, 01:07 PM

Initial start up pegged the stock oil pressure gauge a 5 bar. Gauge then fluctuated, either 0 or 5 bar so we decided to get another gauge. The new gauge measures in PSI and we have not tried it out "cold" as of yet. Readings noted above reflect new gauge with stock sensor. Thanks for the responses.

333pg333 · 333pg333's Garage

Tap in an independant gauge.

Techno Duck · Techno Duck's Garage

25psi will show as around 1.75bar on the gauge; that is not unreasonable on a warm engine at idle with a light weight oil. The pressure should increase with the RPM though.

AaronM · 06-04-2014, 08:31 PM

The old rule of thumb used to be that you wanted 10 PSI for every 1,000 RPM.

For a 944, hot idle should be around 1.5-2 bar (~21-28 PSI), up to 4-5 bar (~60-75 PSI) on revving the engine.

333pg333 · 333pg333's Garage

We see 55-63psi hot with 25w/60 oil, dry sump and temps below 230f. Lower than a normal motor but of course the scavenging is better with d/s system.

v2rocket_aka944 · 06-05-2014, 07:22 AM

Hot idle for me with 20W50 dino-blood is 3-3.5 bar...yours seems pretty low.
What weight is the break-in oil?

Dff Moriarty · 06-05-2014, 11:12 AM

Believe it is 30 weight. Seems a bit thin

sausagehacker · sausagehacker's Garage

I highly recommend that if you use an SAE 30 oil, you do not rev it high and do not put a high load on the engine - rod bearings do not like low oil pressure.

If you want to see how different oil viscosities affect pressures in a 944 engine, you can use the equation: P = .0232 * v * RPM^(.288), with P in bars and v (kinematic viscosity) in cSt. The manufacturer should publish the values at 40C and at 100C.

SAE 30 oil has a viscosity of 10.4 cSt (on average) at 100C, which yields 1.76 bars of pressure. It sounds like your readings are normal.
http://www.wolframalpha.com/input/?i=.0232*10.4*%281000%5E.288%29

If you want more info (because that probably sounds sketchy as hell), it's from some research I did for a class, and the constant terms are unique to an NA 944 engine.
https://dl.dropboxusercontent.com/u/12102615/Report.pdf

Dff Moriarty · 06-05-2014, 12:25 PM

Please elaborate on how high you think we should let it rev prior to changing over to our regular race oil. I had intended to run some break-in miles this evening (3rd gear rolling on and off the throttle) before changing oil and putting on the track on Saturday morning.

sausagehacker · sausagehacker's Garage

Quote:

Originally Posted by Dff Moriarty

Please elaborate on how high you think we should let it rev prior to changing over to our regular race oil. I had intended to run some break-in miles this evening (3rd gear rolling on and off the throttle) before changing oil and putting on the track on Saturday morning.

I honestly can't give you a number, I have never run SAE 30 in a 944 but have read the #2 rod bearing stories. I would add a quart of cheap 20W50 and keep it under 4k just to be safe.

Dff Moriarty · 06-05-2014, 01:01 PM

Thanks for the reply, I will go easy on it.