Tech Tip - 308 Lubrication
By: Bill Pound
Tom Brockmiller called me last week with a question about his 308 losing oil pressure on right-hand corners. This is a common malady of wet sump 308s, especially those modified for track use. It is caused by oil surge in the pan.To better understand the problem in 308 engines, we need to define what "surge" is. Surge is the effect "G" forces have on anything with mass. The same forces that make loose items fly off the seat next to you under cornering loads also cause the oil in the oil pan to rush in the opposite direction of the car.
Surge is a problem in 308s because the oil pump pick-up screen is offset about 2 inches (50mm) to the right of the centerline of the pan. What this basically means is that there is less oil to the right of the pick-up than there is to the left. Therefore, hard right-hand cornering loads cause the oil to rush to the left, allowing the oil pick-up to suck air instead of oil, resulting in loss of oil pressure.
How long the pressure loss lasts depends upon the length of time the pick-up is exposed to air. In many cases, as the car exits the corner the oil surges back into the pan and pressure is established again, so there is just a momentary drop in pressure. An electric gauge such as the stock Veglia unit does not quickly reflect changes in pressure so the driver may not even notice the momentary drop.
On an extended right-hand turn the resultant lack of oil pressure can be disastrous. As a general rule the first areas in the engine to show damage from lack of oil pressure are the rod bearings, and in some cases the camshaft tunnels in the heads. Pre-1980 cars with carbs are interesting in as much as the "G" forces affecting the oil also affect the fuel in the fuel bowls. The end result is that, while the oil pressure may drop in the corner, the engine is usually suffering from fuel starvation so that it doesn?t generate enough power to do much damage. On the other hand, a 308 with modified cams and any decent fuel injection system can destroy itself in just a few seconds of hard cornering. So what to do?
I read an article some time back suggesting that a 3 quart Accusump system would eliminate this oil surge problem and would be an inexpensive alternative to a dry sump system. Accusump systems work by storing extra oil in a pressurized tank that evacuates itself into the engine when the oil pressure drops below a certain threshold. This works well in most applications but their capacity is far too limited to cope with the volume of oil the 308 requires. Besides, even if the capacity were increased to the point of being sufficient, you?d need to deal with the problem immediately created by all this extra oil in the motor. Tom?s car has an Accusump and he was disappointed to find that he still starved for oil in long, high-G turns such as turns 2, 8 and 9 at Willow Springs.
The main reason the Accusump system is not sufficient is the tremendous oil volume the Ferrari engine needs, especially if putting out upwards of 270 HP.
Unlike a great many engines which drive the oil pump off the distributor and therefore at half engine speed, the 308 oil pump turns at almost engine speed (90% of crank speed actually). Couple this with the massive main oil galleries in a 308 (19mm diameter versus 12mm in a small-block Chevy). Add to that the many oil pressure points in a 308 motor -- the 308 has 5 main journals, 8 rod journals, 20 camshaft journals, and 16 holes (actually 1mm diameter) in the flank of each cam lobe. To top it off, the 308 has an all alloy crankcase and heads. Aluminum has a hefty expansion rate which increases oil clearances and therefore requires even more volume to maintain the same pressure.
During dyno development of our 308 IMSA program several years ago, we experimented with various methods of dealing with oil pressure loss. At the time, we measured oil flow through the cooler lines at over 400 gallons per hour
For a complete look at Sempre Ferrari, you may want to check out the rest of the articles from Volume 3, Issue 6 - November/December 1996