Tech Talk #98 – Oil Is Everything


By David Reher, Reher-Morrison Racing Engines

“It’s better to allow a little trash to circulate inside the engine than to risk a catastrophic failure due to insufficient oil flow.”


Oil is the lifeblood of a racing engine. I know this isn’t breaking news for most racers, but that doesn’t diminish oil’s importance. The majority of catastrophic engine failures are caused by lack of lubrication.

In 45 years as a professional engine builder, I’ve performed autopsies on many expired engines. I’m usually skeptical when a racer tells me, “It broke a rod.” The reality is that connecting rod failures are very rare. The real culprit is usually a seized bearing. When a crankshaft journal is black, a broken rod is the result, not the cause of the problem. And the root cause of most spun bearings is insufficient oil.

It’s ironic that components intended to enhance lubrication sometimes have the opposite effect. For example, an oil filter with a superfine screen that’s capable of removing the smallest particles might seem like a good idea – unless the fine mesh impedes the flow of oil. Cut apart a conventional paper element oil filter and you will find about six square feet of pleated filter material. In comparison, a small diameter screen might have only a few dozen square inches of filter area.

In reality, there is a lot of “trash” floating around inside even the cleanest racing engine. Multi-piece valve springs slough off metal as their coils rub against each other. The springs wear against their retainers and seats, pushrods wear against the rocker arm cups, lifters scuff their bores as they are subjected to thrust loads, pistons rub on cylinder walls, crankshafts flex and bend against bearings . . . and all of these (and many more) interactions between moving parts create the shimmering metallic flakes that are visible in used oil.

Any filter that is filled with this debris can restrict the flow of oil, and that can be a prelude to disaster. I certainly don’t advocate running without an oil filter, but I am suggesting that an effective filter should catch the big pieces and let the little ones pass through. In practice, it’s better to allow a little trash to circulate inside the engine than to risk a catastrophic failure due to insufficient oil flow and lubrication.

It’s not just metallic debris that can impede flow through a filter. Beware of excessive use of additives such as molybdenum and zinc. I’m not a chemist, but I know that these complex molecules function like sliding discs to improve lubrication – and since they are larger than oil molecules, they are also more prone to restrict flow through a superfine filter. Since zinc is no longer blended in most oils, it’s common for racers to pour in a bottle of additive. Unfortunately, the concentration of zinc is then higher than it ever was in over-the-counter oil.

Here’s a real-world example. When Bruce Allen and I were racing in Pro Stock, we tested a moly additive in the oil – and promptly collapsed the dry-sump system’s inline filter. The filter had clogged, and our research subsequently revealed that the element was rated at 60 microns (a micron is one millionth of a meter, or 0.00003973 inches). So we replaced it with a 75-micron filter, and never had another problem. I’ve recently seen problems in engines equipped with much finer filter elements; in my opinion, a 75-micron filter is a better choice in the long run.

Current Pro Stock engines use oil that’s as thin as water, but I advise sportsman racers to avoid extremely low viscosity oil. Remember that Pro Stock racers typically chill their engines to 60 degrees before every run, and they have plenty of time between rounds to cool them. If you are a sportsman racer making hot laps between the finish line and the staging lanes in the late rounds of an eliminator, you need oil with higher viscosity and more film strength. A wet-sump system provides very little oil cooling – the heat just builds and builds, round after round.

Racing in the cool months, 10W-30 oil is a good choice for most sportsman engines. However, in summer when the weather is broiling in most parts of the country, I recommend 20W-40 or 20W-50 viscosity. You might give up a little power compared to a super-thin oil, but engine durability will be much better. When we switched to high-viscosity oil in our Pro Mod engines, the reduction in wear in highly stressed components such as wrist pins and pushrod seats was dramatic.

The importance of oil level is obvious, but it’s not always easy to check because few racing engines have dipsticks these days. While it’s easy to check the oil level in a dry-sump tank, a racer can get behind the curve on how much oil is actually in a wet-sump engine. Oil is relatively inexpensive, so I recommend frequent oil changes. This eliminates any doubt about the volume of oil in the motor.

And where is this precious oil? A wet-sump engine equipped with a crankcase vacuum pump can trap a lot of it in the cylinder heads if the system draws from the valve covers. We tested this by installing transparent oil return lines on an engine during a dyno test, and we observed very little oil flowing through the drainbacks. After a run, as the vacuum bled off, oil began to flow through the return lines. Holding a volume of oil under the valve covers isn’t necessarily a bad thing because it helps to cool the valve springs, but it may require an extra quart of oil in the pan to keep the oil pump pickup fully submerged.

I expect that not everyone will agree with my views on filters, oil, and additives. However, I think every racer will agree that a constant supply of oil is absolutely essential to keeping an engine alive.