Tech Talk #74 – The Oil Pan Paradox: Power or Reliability?

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“The most powerful oil pan is not necessarily the best oil pan for a sportsman drag racing engine.”

It’s an article of faith among racers that there is “free” horsepower to be found in an engine’s oiling system. Reducing windage and cutting frictional losses can indeed improve engine performance. Unfortunately, the unintended consequences of these horsepower-enhancing techniques can be a loss of reliability, followed by a big repair bill.

The most powerful oil pan is not necessarily the best oil pan for a sportsman drag racing engine. In sportsman competition, the goal is a lubrication system that keeps oil in the sump, keeps the oil pump pickup submerged, and supplies solid, non-aerated lubricant to internal components. That sounds simple, but it isn’t.

Professional and sportsman racing engines have different lubrication needs because the requirements are very different. In an all-out category such as Pro Stock, Pro Mod, or Comp, every opportunity to improve performance must be pursued relentlessly. In sportsman categories that are contested with a dial-in or index, affordability, durability, and minimum risk are the highest virtues. For racers on a tight budget – which includes just about everyone these days – reliability must take precedence over horsepower.

Many of the power-producing oil pan designs were originally developed for dry-sump oil systems. Large volume pans with big kickouts are very effective in getting the oil away from the rotating assembly. However, this pan design can be a disaster in a wet-sump sportsman engine. There’s too much area for the oil to get spread out, and no number of baffles and dams is going to keep the oil pump pickup submerged under all operating conditions.

The vast majority of connecting rod and bearing problems are really oil supply problems. This doesn’t mean the amount of oil in the engine – it means the oil supply at the pickup and crankshaft bearings. If the crankshaft is black and the rod is broken, that’s a sure sign of an oiling problem.

I’ve often heard racers say, “I spun a bearing, but I had plenty of oil pressure.” Well, oil pressure is not necessarily a good indicator of oil system performance. If a bearing is damaged and the debris ends up in an oil passage, the oil pressure can actually increase even though the oil volume is dangerously reduced.

Insufficient lubrication often does the most damage during burnouts and shutdown. The oil pressure might drop after a burnout and then come right back up again – but every time the oil supply is interrupted even briefly, some irreversible damage is done. If a wet-sump engine has an oil pan with a full-length sump, it’s almost certain that the oil pressure is going to fall to zero during shutdown. In a race car equipped with an automatic transmission, the engine usually decelerates in gear – and while this is safer than shifting into neutral, the oil supply must remain constant as the engine slows down under load. Any damage caused by a loss of oil pressure is cumulative; bearings don’t repair themselves.

A simple experiment will show the effects of acceleration and deceleration on the oil supply in a wet-sump oil pan. Put four or five quarts of solvent in the pan to simulate the hot oil. Then tilt the pan forward and backward at a 45-degree angle and watch what happens to the liquid. That’s the equivalent of 1 g deceleration and acceleration – a race car may produce even higher g loads. If the pan doesn’t have a defined sump and baffles to control the surge, there will probably be very little liquid around the oil pump pickup.

We’ve simulated the effects of deceleration on a wet-sump oil system on our dyno by angling the front of the engine downward. While this doesn’t precisely duplicate the conditions under hard deceleration, it does allow us to see whether the oil pressure remains steady during a power sweep. Through such experiments we’ve learned that vertical fences around the pickup definitely help to stabilize the oil supply. We’ve also tested a new Moroso wet-sump oil pump with a horizontal shield above the pickup that measurably improves the consistency of the oil supply. In fact, we are seeing only 5 psi variation in oil pressure during a sweep from 5,500 to 7,800 rpm.

In addition to lubrication, oil also plays an important role in engine cooling. In some endurance racing engines, the top end is flooded with oil to cool the valve springs. Our Pro Stock engines were equipped with oil squirters targeted at the valve springs. But these techniques are best left to dry-sump oil systems. It’s not necessary to have excessive oil in the top end on a typical wet-sump drag racing engine that doesn’t exceed 8,000 rpm and doesn’t overwork the valve springs with extreme lift.

Oil return is also important. The oil has to be able to get from the cylinder heads back to the pan. Some aftermarket cylinder head designs trap a lot of oil under the rocker covers. Engines equipped with these heads need external drains to provide a path for the oil to return to the pan.

While Pro Stock and Comp motors use extremely low-viscosity oil to reduce parasitic losses, my advice to sportsman racers is to use 10W-30 or 15W-50 oil. This multi-weight oil is less expensive and it’s better suited to the requirements of a typical sportsman engine. I also strongly recommend using an off-road oil with a good high-pressure additive package. The oil refiners have virtually eliminated zinc and sulfur in street oils to meet government regulations. Unfortunately these additives contribute to the oil’s film strength, and they are essential for any engine with a flat-tappet cam and stiff valve springs. I also recommend off-road oil for engines with roller cams – we’ve seen rapid wear in pushrod tips and other highly loaded areas in engines lubricated with street oil.

Oil pressure in a typical big-block sportsman engine should be 60 to 75 psi, depending on the displacement and operating speed. An engine with a short-stroke crankshaft usually doesn’t need as much oil pressure as an engine with a long crankshaft stroke. I’m certainly not an expert on hydrodynamics, but it appears that the increase in centrifugal force and the angle of the oil feed holes in the crankshaft account for the higher oil pressure that’s needed in a long-stroke engine.

Lubrication is critical in any racing engine. For most sportsman racers, the risk of running a low-windage oil system simply isn’t worth the meager reward in performance.