One of the first gauges we look at while we’re racing is oil pressure. Without sufficient oil pressure, your engine is at risk for severe damage, and road racing is a notoriously harsh environment for oiling systems.
To learn more about the finer points of engine lubrication, we caught up with John Schwarz, president of Aviaid, a manufacturer of dry-sump oiling systems. Aviaid was started in 1961 by a boat racer named Tom Davis, who began building oil pans for all kinds of racing applications. When Bruce McLaren wanted to lower the engine in an early Can Am chassis, he found there wouldn’t be enough room for a wet-sump oil pan. So he worked with Davis, who recalled that Allison aircraft engines used scavenge pumps in the valve covers to return oil to a gimbal-mounted dry sump tank during inverted flight. That’s when Aviaid started building dry-sump systems for automobile racing.
It’s easier to get away with wet-sump oiling systems for other applications, such as drag racing. The car blasts through the gears for up to 15 seconds or so, and oil control is only needed on one axis. Road racing is different. Not only are the cars accelerating, but they’re also braking and turning, which tosses oil around in the pan in every direction.
“Usually the problem is you’re just uncovering the pickup in the pan because you’ve got such high g-force issues you can’t keep the pickup covered in the pan,” Schwarz said. “You can’t control the oil,”
Another unique issue with contemporary wet-sump systems is oil-pump speeds. In older engines, the oil pump was driven off the camshaft, which spins at half of engine speed. Modern engines, such as General Motors’ LS architecture, Ford’s modular V8s and many imports, are equipped with gerotor-style pumps driven off the crankshaft. Spinning at engine speeds is acceptable at lower engine rpm, but at higher, sustained rpm, it can become problematic.
Whether the high rpm becomes an issue is a function of how long the engine stays at that rpm and other factors such as oil temperature, oil viscosity, and inlet size, which also have a marginal impact. All those factors can add up to cavitation inside the oil pump itself.
“Oil has mass and it can only be accelerated so fast, and you get to the point where it can’t accelerate fast enough to do what you need it to do,” Schwarz said. “Our experience is that it usually happens at about 5,000 to 6,000 rpm. That’s a rough estimate, but we know that once you approach 7,000, 8,000 rpm with a lot of these engines, you’re starting to see cavitation. Basically, you’re kind of doing kind of a reverse champagne effect. You’re creating such a high vacuum that you actually start getting the dissolved air, the dissolved gases and the more volatile dissolved liquids that are in the oil to come out, and some of this stuff can get destructive in the gear chamber. I don’t know that we necessarily see a lot of that, but when you start popping holes in the oil, you get some interesting things going on. The biggest thing is you don’t pump oil, and what you end up seeing is really erratic oil pressure, to the extent that you’ve got a fair amount of air in the oil.”
When pressure drops occur, one of the more obvious cures is to add an oil pressure accumulator tank. If you’re unfamiliar with accumulators, here’s how they work. Essentially, the oil pump charges the accumulator cylinder with surplus oil against a sprung diaphragm or a pressurized piston. If the engine starts to lose pressure in the main galley, such as when the oil sloshes away from the pickup tube, the spring or pressure forces the surplus oil into the engine to maintain a certain level of pressure. However, even accumulators have their limitations.
“They’re good for one or two turns, and then what happens is your oil pump spends all its time to trying refill it,” Schwarz said. “Once it discharges, there’s nothing more to backfill the system. Not only that, but because of the way it’s plumbed into the system, the oil pump is now not only trying to pressurize the engine, but it’s also trying to refill the cylinder.”
When trick oil pans and accumulator systems aren’t enough, it’s time to step up to a dry-sump oiling system. Dry-sumps are a more complex and expensive solution than oil pans or accumulators, but the costly engine damage they prevent justifies the expense. The C6 Chevrolet Corvette Z06 is a perfect example. GM engineers realized that the cornering loads the car was capable of and rpm the 7.0-liter V8 could sustain necessitated a dry-sump system from the factory.
If you need a dry-sump system, Schwarz recommends finding a manufacturer that makes a product for your application. That may or may not be Aviaid, but he recommends finding a competent shop or engine builder who is familiar with the systems. Essentially, what you’re doing is taking the stock oil pump and moving it outside the engine and then taking the reservoir off the bottom of the engine and moving it to a remote location. The reservoir provides an uninterrupted supply of oil to the pressure pump. Schwarz said even a basic three-stage dry-sump setup — two scavenging pumps to feed the reservoir and one feed pump to pressurize the engine — will outperform any trick wet-sump oil pan.
Key considerations for a dry-sump system are the oil pan, pump location, reservoir location and routing all the plumbing. Schwarz explained that it might be possible to modify the oil pan currently on your car.
“That can be a very standard kind of system and we take a lot of stock sumps and convert them for dry-sump applications,” he said. “Most stock sumps are designed for oil to accumulate in one location so we take advantage of that. So you get a couple of pickups you can weld in there and connect them to your tank. There’s no need for fabrication or billet in most instances.”
Then you need to find a way to mount and drive the pump. The pump can be driven off the crankshaft or even exposed cam gears. The good news is that you have a choice of pulley sizes so you can achieve the appropriate ratio to drive the oil pump at the correct speed and avoid cavitation altogether. Once the pan and pump issues are solved, it’s just a matter of locating the reservoir and plumbing the system.
“It’s one of those areas where an engine builder who knows what he’s doing is really worthwhile,” Schwarz said. “We try to be as flexible as possible, but it’s still a more complex system than bolting a wet-sump pan on, so somebody’s got to dig in there and figure out what kind of space you’ve got in the engine compartment and see how stuff starts to fit together.”