A Slippery Subject
Mike O’Brien; Brooks, KY: I have a question on oiling systems regarding the difference between a high pressure oil pump and a high volume oil pump. Assuming the oil pump is working fine and pumping to capacity, does your engine actually see any increase in oil flow from a high pressure oil pump or high volume oil pump if its internal passageways are not capable of passing any more oil? It would seem that a high volume oil pump would push the relief valve off its seat and bypass the oil. On the other hand, it would seem that a high pressure oil pump would flow more oil because the relief valve would stay shut longer and the increased pressure would push more oil through the constricted passageways. Even if the engine is turning a high rpm, only the oil passing the internal passageways would arrive. I think the only time an engine might benefit from a high flow oil pump would be if the OEM pump were providing insufficient oil to begin with. What say ye?
Jeff Smith: The great thing about car crafters is their insatiable appetite for information. While this might seem like an obscure question, it’s actually relevant to all street engines. The easiest way to think about moving fluid is understanding that pressure and volume are inversely proportional. That means if you wish to move a greater volume of oil, pressure will be reduced. If pressure increases, volume will be reduced. The classic example of this is a garden hose filling your Saturday morning car-wash bucket. With an open garden hose, you get high volume at low pressure. Place your thumb over the end of the hose, and the pressure inside the hose increases. The water squirts over a greater distance, but the volume is reduced since it will now fill your bucket more slowly.
An engine lubrication system operates much the same way. Because the oil pump can supply far greater volume than the restricted oil galleries can support, pressure begins to build in the system. At idle, for example, a small pump spinning relatively slowly still creates sufficient pressure. Since the pump is spinning slowly, most of the time the pump is supplying only enough oil to maintain a given pressure. As rpm increases, pump output also increases to the point at which the pressure pushes the pump’s internal oil pressure relief spring to open the relief valve slightly. The amount of oil that is bypassed is directly proportional to what we could call the internal oil leaks in the engine. Main and rod bearing clearances are major contributors to this leakage, along with the volume of oil that reaches the top of the engine to lube the valvesprings and the guides. The greater the number of internal oil leaks, the more volume it requires to fill that demand. If the volume demand is great enough, oil pressure will drop if the oil pump cannot supply sufficient volume to maintain the pressure. This rarely happens because the design engineers have done their homework. What is more common is when a high volume oil pump is used in an engine with tight clearances. At higher engine speeds, the pump has far more capacity than the engine needs, and a greater volume of oil generally ends up at the top of the engine, flooding the valvetrain. In certain circumstances, this can result in the high volume oil pump actually sucking the pan dry. We’ve heard of instances of this occurring with high-rpm LS engines with stock oil pans. One reason this occurs with LS engines is because the factory gerotor oil pump spins at crankshaft speed instead of half speed, as is the case with most older engines where the pump is driven off the camshaft.
The simplest way to create a higher pressure oil pump is to merely increase the spring pressure required to open the relief valve. This can be accomplished by either adding higher-rate spring or shimming the existing spring. To move a greater volume of oil requires taller pump gears. For example, Melling’s stock-volume, small-block Chevy oil pump uses a pair of spur gears that are 1.20 inches tall. A high- volume Melling pump increases the height of these gears to 1.50 inches. These 25-percent-taller gears can move a greater volume of oil. Of course, there is also the potential to build a high- volume / high pressure oil pump that combines the attributes of both styles.
So you might see that an engine with standard or slightly tight clearances (which reduce the internal oil leaks) really doesn’t need a high-volume pump. Therefore, a typical performance street engine doesn’t need a high- volume pump if the clearances have been properly set. When employing a high-volume and/or high pressure oil pump, the only thing that is really achieved is greater parasitic power losses required to spin that bigger pump. And if a higher pressure is employed, the only real result might be higher oil temperatures. The classic Smokey Yunick recommendation of 10 psi per 1,000 engine rpm is certainly OK, but we’d be willing to offer that your typical street engine could survive just fine on 50 psi of max oil pressure, 7 when spinning to 6,000 or 6,500 rpm. Idle oil pressure is even less critical, since there is no load on the engine. Many car crafters freak out when idle oil pressure drops below 30 psi at idle, but anything above 10 to 15 psi is probably acceptable. This is because there is very little load on the engine. Automatic transmissions place a slightly higher load on the engine—but that can be 40 lb-ft or less, which is still pretty low.
To put this in perspective, I called Jon Kaase Racing Engines (JKRE) and asked them what those 800-plus-cubic-inch Pro Stock engines run for idle oil pressure. JKRE’s Cliff Moore told me idle oil pressure is often well below 10 psi, and on some of their engines the pressure is actually zero. Moore says, “We just put a piece of tape over the warning light.” The reason they are not concerned with this idle pressure is that they know the pump is circulating oil (much like the water hose filling the wash bucket), but there is no pressure indicated because the clearances are wide enough that they do not present a restriction. This is an extreme example, but it does illustrate the point. As an additional example, the engine in this month’s 406ci small-block buildup generated more than 70 psi of oil pressure at 6,000 rpm, and we think if we had spent the time to reduce the oil pressure to around 55 psi, we might have been able to increase power slightly.
Oil viscosity plays a big part in this discussion. The basic premise is that tighter bearing clearances increase bearing load capacity but also reduce oil flow past the bearing. As a result, the temperature of the oil at the bearing increases. The opposite is also true: Wider clearances reduce load capacity but increase oil flow volume with subsequent lower oil temperature. Building an engine with tighter bearing clearances allows the engine builder to use thinner-viscosity oil (assuming the load capacity is sufficient with high-pressure additives and high-shear stability). Lighter-viscosity oil will flow more easily through tighter clearances and reduce the temperature rise. Conversely, wider bearing clearances demand a thicker-viscosity oil. We hear rumors of NASCAR teams experimenting with SAE 0–viscosity engine oil combined with tighter tolerances all in search of reduced engine friction and parasitic loss required to drive the oil pump.
In the Nov. ’09 issue, Westech’s Steve Brulé and I did an oil-pump test on a small-block Chevy (you can find this story archived on CarCraft.com). We tested four Milodon pumps on a Dart 372ci small-block Chevy ranging from a standard-volume unit, a high-volume unit, a high-volume high-pressure pump, and even a big-block Chevy pump. The average horsepower numbers tell the story
As you can see, the stock-volume and pressure pump produced an average of almost 5 more horsepower over the high-volume pump. The odd thing was the only slight power loss from the high-volume/high-pressure pump. But these results clearly support the concept that a stock pump works pretty well. The exceptions to these low- pressure rules are engines like the 351C and 429/460 Fords that feed the main bearings through the lifters. These engines require more oil pressure than priority main-fed engines because the lifters act as a restriction, so more pressure is required.
Melling Automotive Products; Jackson, MI; 517/787-8172; Melling.com
Milodon; Simi Valley, CA; 800/828-8224; Milodon.com
After being sidelined by the scuttled Super T10 in his Chevelle, Tech Editor Smith was offered an ’11 Cadillac CTS-V wagon to drive on Sunday at the Spectre 341 Challenge. Jeff’s best time in the Caddy was 3.51, and the experience left him longing for an LS-A engine in at least one of his Chevelles.
Spectre hired HeliTahoe to get aerial footage of the 341 Challenge on Sunday. You can see the videos on Spectre341Challenge.com. BTW, want to get married in a helicopter? HeliTahoe offers HeliWeddings—get married in a chopper over Lake Tahoe. Check that out at HeliTahoe.com