A Postscript on Oil Pressure Diagnostics
by Geoffrey Healey
Originally published in the Austin-Healey Magazine, February 1989
In the January issue we published a technical article by Roger Moment on how oil pressure is controlled in the engine, and how different symptoms can be used to diagnose lubrication system problems. While preparing the article, Roger wrote to Geoff Healey and asked what experience she might have recorded from the record runs at Bonneville Salt Flats. They were operating for long periods of time in an area known for high ambient temperatures in the summer. Geoff had a number of comments to offer which should be of interest to Healey owners.
I don't think that an oil pressure around 40 psi should cause any problems. I think that the 20w /50weight. or SAE 40 would be the best oil for summer use in the Colorado area. I suspect that if you filled your engine with Castrol XK of the I950s vintage you would have an oil pressure around 50psi. It is important to obtain a good oil flow at low temperature on starting, and for this reason oil viscosity is lower today. US manufacturers always used lower viscosity oils than the British. This does not mean that British and US ratings are different. We use SAE ratings today, but in the early50s I think you would find that Castrol made XL and Castrolite which did not carry an SAE rating. Also, branded oils available in Europe were generally "thinner" than the same specification UK products. To be sure of what we were using, we always took supplies of the UK product with us when competing in Europe.
There was much more high speed operation in Europe than in the USA. Clogged or restrictive filter elements were notorious for low oil pressure on the 3000. The Tecalamit felt elements gave less restriction than the Purolator paper elements, and oil pressure would be higher.
The oils originally specified for temperatures between 32 and 90 degrees F were nominally SAE 30and the oils were generally at the upper end of the SAE rating. For ambient temperatures above 90degrees F the next heavier grade, i.e. SAE 40 was recommended. When multi-grade oils became available, a warning was made that they were not suitable for worn engines. 20w /30 oil was at the lower end of the SAE 30 rating at 210 degrees F and often resulted in lower oil pressure. If a multi-grade oil is preferred, a 20w /50 is probably the most suitable for older engines.
Most racing, rallying, and record-breaking was done using castor-based oils which approximate to SAE 40 grade. Occasionally a straight 50 grade or an extreme-duty 40 grade diesel engine oil was used. These oils cannot be recommended for normal motoring. Oil temperatures in excess of250degrees F were often measured under racing conditions.
The gauges used on the Austin Healey were accurate to plus or minus 5 percent. On the face of the gauge the maker's calibration points are marked below two or more pressure readings by two white dots. The needle should lie between these two points when subjected to a true pressure corresponding with the figures indicated. When gauges require calibrating the dots should be used. An actual pressure of 50 Ibs/sq.in should give a gauge reading of 47.5 to 52.5 psi.
I found my notebooks with comments on the Utah runs. Under the "Stock Car" notes [the stock car was taken directly from dealer stock and speed tested in carefully tuned but unmodified form. ed.]"Car tested 10 am - 12 am II Sept. 26 degrees C. Ending ran 75-80 oil pressure, High, Gauge faulty. Mean RPM 4000 ... " This high reading was probably caused by very high pressure starting from cold which strained the gauge. Under the notes for "International Car" (with 100S engine) "Oil temp= 215C ... Water 82C. Air intake temp 30C." The actual oil pressure was not recorded in the notes, so it had not been a problem. The oil was Castrol XL which was an oil having a viscosity at the top of the SAE 30 grade. Don't forget that these were new engines-the stock car had covered only slightly over 1000 miles before starting the record runs.
Only two cases of bearing failure were experienced with the 100/4 engines used for engine development, endurance road testing, and competition. Both failures occurred on starting a cold engine and were traced to filters that were heavily contaminated, resulting in the excess pressure valve in the filter bypassing debris into the oil-ways.
Oil pressure at idle with a hot engine was often around 5 psi without any adverse effect on the engine. Many of today's engines have a low oil pressure light that comes on with pressures below 4 psi.
Pressure should be around 50 psi above 2000 RPM. Excessive pressure can cause bearing wear due to high oil flow through the bearings, as the oil has minute particles of contaminants that pass through the filter.
The RPM at which the pressure relief valve starts operation can be used as a guide to engine wear. This assumes that the oil and filter are relatively new. If the oil has "thinned" with usage (cold running can result in un-burnt fuel dilution of the crankcase oil) or the filter is obstructed, the point of operation (RPM) would obviously be higher. I remember competing in the Tour de France Rally with the Nash Healey when we suffered from dropping oil pressure. We used to change the engine oil before every timed stage. This ensured satisfactory oil pressure during the period when the engine was used hard. After the stage the oil pressure would be much lower, with zero pressure below 1000 RPM.
Many engines ran thousands of miles with oil pressure well below the manufacturer's specification without any problems developing. At Sebring it was found that on one of the fast bends the needle would drop to zero and only return to normal after exiting the bend. This was due to oil in the sump surging away from the pump pickup. On stripping the 3000 engines after the race, the bearings and crankshaft were found to be in excellent condition showing no evidence of oil starvation. Some of the most durable engines of old had very low pressure oil systems.
Our thanks to Geoff Healey for taking the time to add some additional information to the topic of oil and oil pressure.
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