I started a project to evaluate oil filters for SHOs, both V8's and V6's, and learned that many of the lubrication rules of thumb I learned 30 years ago needed updating. Before I could evaluate oil filters for the SHO I needed a better appreciation of how things have changed in the last 30 years, both engines and filters.

One old rule of thumb is that an engine needs 10 psi of oil for every 1000 rpm. So an engine with a 6800 red line needs 68 psi of oil at redline.

Above is a chart of the actual oil pressure as a function of engine RPM for a warmed up V8 SHO. The red line represents actual field data and the blue line is a 6^th order polynomial trend line. Note that below 3000 rpm the pump creates about 15 psi per 1000 rpm then output pressure drops to 55 psi at 5000 rpm where the relief valve evidently limits the maximum pressure to the specified 55 psi limit. (Thank you Bob Fijal for the field information.) 5W30 is the recommended viscosity oil for 96-99 SHOs. The Helms shop manual calls for 20-45 PSI oil pressure at 1500 rpm.

Why the change in slope, and odd behavior between 2500-3000 RPM? I suspect the oil filter by-pass valve opens in this car between 2000 - 3000 rpm trading full flow filtering for quantity. The big question is what happened to the 10 PSI per 1000 RPM rule?

It is still a good rule, up to a point. With very tight engine clearances, outstanding production control, a unique over built split block design that is dimensionally rigid and stable and high performance low viscosity SJ motor oil, this is one rule that can be bent if not broken above 5000 RPM. The pump exceeds this rule by 50% up till 3000 RPM. One may speculate that during normal highway operation when the engine RPM tends to stay below 3000 RPM the "excess" oil may contribute significantly to steady state engine cooling. Above 3000 RPM the design emphasis shifts to abundant lubrication without the unnecessary parasitic HP loss that 65 PSI at 6500 would require. The information I gather from reading my books on lubrication for street is that running excessive pressure requires more hp without benefit.

The SHO V6 pump is rated at 12.1 GPM at 43 PSI at 6400. Since output is near linear for the positive displacement internal gear, one could approximate this as 2 GPM per 1000 rpm. Pump specifications for the V8 are absent but assumed to be similar if not slightly greater.

With a total oil capacity of 6-7 quarts at 12 GPM ~ 48 quarts per minute, infers the capability to suck the oil pan dry about 8 times a minute or every 7.5 seconds when the engine is near red line. And the oil pan would be dry if the oil can not make the trip through the pump, filter, lubricate the appointed task and return in the prescribed time!

One can see the trend in tight tolerances and improvement in manufacturing quality control. This also reflects an improvement in block dimensional stability with the very strong and rigid split block design of SHOs. When looking over the clearances with some one much more knowledgeable than I he remarked:

"I'd like to see one more piece of data, bearing journal diameter. I'll bet that the Yamaha bearings are smaller in diameter than the small block equivalents. Smaller diameter journals increases the bearing foot-per-minute speed, and lubrication films must be thinner in order to remain stable at high FPM's and RPM's."\

(NOTE: with respect to the unnamed "knowledgeable" person quoted above, it has been pointed out to me that smaller journals actually DECREASE bearing speed -- Ed.)

Interestingly the SHO journals are almost the same diameter or larger in diameter as the larger displacement Chevy engine! These are massive and rigid crankshafts for the engine displacement and one could wonder if they aren’t so heavy as to have a significant flywheel effect of their own? Also from the same source:

There is good technical sense for tighter bearing clearances in these engines. One has to do with the sheer number of bearings that must be supplied with oil. In addition to the main and rod bearings, there are probably 20 or more cam bearings in the V8 SHO motor, I know there are 20 or 21 in the V6 engine, and all these must be supplied with oil. The rate at which oil leaks out of a shear film bearing is directly related to oil pressure and the bearing clearance. The sloppier the bearing, the more oil that must be supplied to it. Couple large bearing clearances with a great number of bearings and you have the need for a gargantuan oil pump and oiling system to feed all these hungry bearings. Oil pumps use up valuable horsepower, so its advantageous to only have as much as you need.

What looks interesting here is the main bearing clearance is probably small enough to be affected by oil filtration efficiency for very fine particles. With more pedestrian bearing clearances, a 20 micron filter is plenty, but it appears that the V8 SHO engine might benefit from improved fine particle filtration.

This last point is the reason for this focus in clearances. Conventional wisdom is that particulate contaminates in motor oil smaller than 10 microns are too small to hurt anything. Say for example we have 30 micron clearance between moving parts separated by a 30 micron film of motor oil. If one or two 10 micron specs get in there who cares, they are in the oil bath and can not harm anything? Add a third 10 micron spec and if they line up and bridge the film they can scratch a bearing or cylinder wall. The old rule of thumb, particles smaller than 10 micron can't hurt anything may be obsolete! Troubling ain't it? Microscopic soot and metal particles work harden as they travel increasing their ability to scratch softer bearings and mark cam lobes, shims and cylinder walls. Because of their high surface area to mass ratio they also deplete the oil anti-wear additives without benefit to the engine.

One could also see the problem with oils heavier than the recommended 5w-30 weight. Pumping 15w50 through a tight engine is like pushing a barrel of molasses through a microscopic hole. Expect high effort (HP loss) and starved bearings. Remember the rule - A revving SHO engine can empty the oil pan every 7.5 seconds at red line on a continuous basis? The quantity flow essential for proper cooling and lubrication can not be maintained with thicker oils and the HP drain of the pump would be much higher.

One old trick is to over fill the pan by one quart on the theory that at least one quart will always be in transit somewhere upstairs. Several problems become evident with this trick. The worst problem is with the crankshaft and rod ends beat the oil into a froth and that makes a poor boundary layer lubricant. The probability of losing a bearing is much higher. One other reason is the HP lost using the crank to churn the oil. Smokie Yunick writes about experiments with transparent oil pans and fat taffy like strings of oil getting wrapped up on the crankshaft like ropes fouling a boat propeller. That would have to slow a car down. Lastly the oil bath would coat the cylinder walls with a butter of oil making the job of the bottom oil control rings and piston skirts that much more arduous.