Tech Talk #67 – The Hidden Cost of Free Horsepower

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“For sportsman racers, using low-viscosity oil is a lot of hassle, expense, and risk for a few horsepower.”

There are two paths to improving engine performance: increase efficiency or reduce parasitic losses. An engine is like a balance sheet – the power produced by burning fuel is an asset, while internal friction, pumping losses and windage are liabilities. The difference between the power produced in the cylinders and the total losses is the engine’s net output.

It’s very difficult for racers competing in an intensely competitive category such as Pro Stock to increase the output side of the equation. After decades of relentless engine development, horsepower gains have become small and very expensive. That’s one reason why Pro Stock engine builders have focused on the liability side of the balance sheet: by minimizing internal losses, the net power goes up. But since mechanical friction accounts for only about five percent of the power produced by a typical four-stroke engine, the potential rewards are still relatively small.

Reducing parasitic losses is often termed “free” horsepower because it’s unlocks power that the engine is already producing. But the true cost of free horsepower can be high. The use of extremely low-viscosity oil is a case in point: Thin oil makes more power than thick oil. Unfortunately this “free” horsepower has a price – reduced reliability, increased maintenance, and higher operating expense.

I’ll preface my comments with the disclaimer that I’m not a petrochemical engineer. I can’t describe the differences in the molecular chains of various oils. But as a professional engine builder, I’ve seen thousands of engines. My experience and observations have confirmed my belief that many of the characteristics that allow oil with extremely low viscosity to produce more power are usually negatives for long-term engine life.

Viscosity is an indicator of a fluid’s resistance to flow. While racers are familiar with standard SAE viscosity ratings such as 10W-30, the experts typically refer to the kinematic viscosity of liquids in centistokes (abbreviated as cSt). This is a measurement of the oil’s gravity flow through a calibrated tube at a specific temperature. For example, the kinematic viscosity of water at 68 degrees is 1 cSt, SAE 40 motor oil is 319 cSt, and honey is 2200 cSt. In comparison, the viscosity of some of the specialty oils used in Pro Stock is around 5 cSt at their operating temperature. That’s why it’s sometimes called “sewing machine oil.”

By definition, low-viscosity oil has less resistance to flow than oil with higher viscosity. Therefore low-viscosity oil requires less power to turn the oil pump and reduces windage losses in the crankcase. Lightweight racing oils are also formulated not to cling to metal surfaces, reducing drag on rings and other internal components. Combine all of these attributes in a highly developed Pro Stock engine under a best-case scenario, and low-viscosity racing oil might produce eight to 10 more horsepower than conventional 10W-30 oil.

Now consider the trade-offs. Pro Stock racers change oil frequently because the lubricant breaks down quickly. They rebuild their engines regularly, and inspect highly stressed components such as lifters and valve springs religiously. They prime their engines before start-up because the specialized oil doesn’t cling to springs, pushrods and bearings. They have to treat highly loaded parts such as wrist pins with expensive DLC or Casidium coatings to prevent galling. It’s not the power level of Pro Stock engines that causes wrist pin problems; our Pro Mod engines produce 2,400 horsepower and pull 22 inches of vacuum in the crankcase, yet almost never have problems with wrist pin wear because they don’t have to rely on super-thin oil to wring out every last horsepower.

Problems associated with inadequate lubrication are often apparent in the valvetrain. Thin oil doesn’t adhere well to the valve spring coils, so the resulting high temperature affects spring life. We see instances of galling on lifter wheels and needle bearings in engines run with extremely thin oil. It’s very rare for a lifter roller to shatter; usually the needles and shaft wear to the point of failure. As the lifter components are ground into powder, they throw steel fragments onto the adjacent pistons, wiping out the skirts and cylinder walls. I’ve found that heavier oil definitely helps to prolong lifter, valve spring, pushrod, wrist pin and bearing life.

For sportsman racers, using low-viscosity oil is a lot of hassle, expense, and risk for a few horsepower. There’s really no convenient way to prime the oil system with a wet-sump pan. Who is going to pull the distributor and spin the oil pump with a dummy shaft every morning before starting the engine? Who wants to change the oil and filter after every three or four runs? What racer is going to lubricate the valve springs every night to keep them from rusting when the lightweight oil drains off?

My recommendation for most sportsman engines is to use 10W-30 oil, either conventional petroleum-based or synthetic universal utility type oil. Yes, you’ll give up a few horsepower compared to water-thin oil, but you’ll have fewer problems with broken lifters, fatigued valve springs and burned bearings. If your car sits for a few weeks between races, you won’t have to pressurize the oil system to prevent start-up wear.

Increasing temperature lowers the viscosity of oil. An oil that has a viscosity of 66 cSt at 100 degrees F might have a viscosity of only 12 cST at 212 degrees F. A drag racing engine doesn’t need an extensive warm-up. Just run it until the water temperature gauge begins to register (typically around 100 degrees F). By the time the car is driven to the staging lanes and does a burnout, the engine will be as warm as it needs to be.

It’s in the nature of racers to pursue performance at any price. For a Pro Stock racer with an unlimited budget, super-thin oil is a good deal. But for the rest of us, I can’t justify the hidden cost of free horsepower.