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Intake Closing and other Intake Valve Reader Tech Questions - Ask Anything

Pontiac Oil Pressure Problems
Brian Matte; Milwaukee, WI: I have a street/strip ’69 GTO that has low oil pressure when the engine is up to normal operating temperature (180-185 degrees) when idling. The cold oil pressure is 30-35 psi, then down to 8-9 psi at 180 degrees. I have tried three different oil pressure gauges, all reading the same. The engine is a ’69 Pontiac block that is stroked with an Eagle crank, steel H-beam rods, and forged JE dished pistons (9.5:1) to 468 ci with Federal-Mogul race bearings. The oil pump is a Melling M54DS with a welded-on Melling pickup set up to 3⁄8 inch clearance from the bottom of the stock Pontiac oil pan. The engine has maybe 200 miles on the new rebuild. I changed the conventional oil that I’m using from 10W-30 to 10W-40 to now straight 40 with Prolong in fear I’m going to ruin the crank again. Changing the oil has only raised the cold pressure a few pounds to what it is now. I’ve also tried shimming the oil pump spring 0.050 inch with no luck. I have a solid roller cam with restrictors in the lifters, aluminum Kauffman D port heads, a Victor intake, and a Holley 850 carb. The engine does not leak oil, and the main bearing clearances are 0.0024 to 0.0026 inch, while the rods are all 0.0019 to 0.0020 inch. The rod side clearances are between 0.024 and 0.026 inch. I only use 93-octane pump gas. The car is 3,500 pounds but runs hard, clocking an 11.51 at 115 mph on the motor in the quarter-mile. I also sprayed it for fun with a 100hp shot, and I was rewarded with a 10.84 at 125.

Jeff Smith: We talked to our Pontiac engine builder buddy Ken Crocie at H.O. Enterprises, and he clued us in on what Crocie says is a common Pontiac problem. There is an internal oil passage plug located at the rear of the engine that can be accessed (the good news) without having to disassemble the engine. But (here’s the not so bad news) you will either have to pull the engine out of the car or at least remove the trans. Yanking the trans might be easier, but that will depend on how easy it is to access the plug. Once you have the engine out, unbolt the flexplate/flywheel and remove the pressed-in 15⁄16-inch plug located on the passenger side of the camshaft. This will allow access to the 3⁄8-inch pipe thread gallery plug that is missing, but you will also have to remove the distributor to see the plug, or in your case the hole where the plug should go. This plug is usually removed by machine shops to ensure the block is adequately cleaned before machine work starts. It’s a common oversight to leave this plug out, which creates a massive internal oil leak. That you have some pressure once the engine warms up indicates that the oil pump has enough capacity to create a small amount of pressure despite the large internal oil leak. That means that once the leak is cured, the engine will make a lot more pressure.

The repair is simple. All you have to do is install the pipe plug, press a new 15⁄16-inch plug in the block, and you’re ready to go. We’d suggest leaving the distributor out and pressure-lubing the engine with a 1⁄2-inch drill motor to double-check oil pressure before you button everything back up. You should be able to generate 50 to 70 psi with the drill motor with cold oil, which will indicate that you have solved the problem. If the engine is out of the car, you might consider removing that 0.050-inch shim in the oil pump, as it will not be necessary now. We’d also suggest returning to the 10W-30 oil to reduce oil pressure. Even large, 3-inch main journals used on Pontiac and Oldsmobile engines do not require oil pressure in excess of 60 to 65 psi. Any pressure higher than that is merely greater load on the oil pump and camshaft distributor gears, which will be manifested in greater wear. If the oil pressure is higher than 70 psi with the oil at an operating temperature of around 210 to 220 degrees F, consider changing to thinner-viscosity 5W-20 oil. It will lower oil pressure with no sacrifice in lubrication protection.

Another related point concerns wide rod side clearances that are attributed to loss of oil pressure and excessive windage. The problem with this assumption is that the numbers don’t add up. Let’s assume a rod bearing clearance of 0.0025 inch on a rod journal of 2.5 inches in diameter. With a 0.025-inch rod side clearance (which is often considered excessive), if we do an area calculation of the circumference of the rod bearing clearance times the bearing clearance times Pi and compare that with the area calculation of the same circumference times the side clearance (we’ll spare you the boring math), the rod side clearance area ends up 10 times more than the rod bearing clearance (this is calculating one side of the rod with all the clearance on that side). So the rod bearing clearance determines the amount of oil flow (since it is the restriction) which means it should be obvious that the rod side clearance does not contribute to any additional oil flow from the bearing. You can use this little tech tidbit the next time some engine builder “expert” espouses on the evils of excessive rod side clearance.

More Info
H.O. Enterprises; Rancho Cucamonga, CA; 909/980-1451

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