Inliners International Forums General Discussion Engines 261 Engine - Oil Pressure Pump Performance

I've been talking with folks about increasing the oil pressure provided by a 261 engine oil pump.

The discussion started with comments about V-8 engine oil pump pressure being increased by shimming (increasing pressure) the spring to the oil pump pressure relief piston so the amount of pressure going to the block would be increased.

The system in my 261 engine's is the same so why not install a 1/8" shim to improve the oil pressure in the system?

Comments?

Thanks,

Ted

It's probably not a good idea to just randomly shim the pump assuming you need more pressure without having a baseline reading of how much pressure you have to begin with. Your bearing clearances are going to affect your oil pressure also, so you need to have a measurement of those as well to help determine what you might end up with for pressure.

The standard oil pressure I've gotten from a rebuilt 1959 235 with about 4,000 miles on it, after full warm-up, has been about 14 pounds at operating speed (30+ miles per hour) and down to almost nil (2-4 pounds) at idle of 850 rpm. The gauge pegs out at 30 pounds when the oil is cold at startup. My interest is in raising the pressure during idle when the vehicle is warm.

As I'm working on a new rebuild I've set up the main bearing clearances at 0.002 by installing the bearings, determining the dimension after torqueing down the bearings and then having the crank turned as needed to provide 0.002 clearance.

Any suggestions as to how to measure main and rod bearing pressure when the engine is running other than to read the standard gauge? Removing and re-installing the oil pump multiple times would get real old very quickly; trying to re-seal the oil pan after the engine is installed in the vehicle is a real pain. I have multiple oil pans, I could cut a door into the side of the pan for access to the oil pump.


Thoughts, suggestions?

Thanks,

Ted

That is really low for a 59 motor. Are you sure your gauge is reading correctly? Do you have an oil filter on the motor? do you know who rebuilt the motor? Are the rockers getting oil?

I'd be looking for where the pressureized oil is leaking past, bearing clearances, by-pass oil filter flowing too much, etc.
You have to put you finger over the garden hose to build pressure, same idea.

If you have a low oil pressure problem at idle, the shim trick won't help. It only raises the treshold pressure for bleeding out EXCESSIVE pressure, i.e. off-idle. It only raises the maximum oil pressure limit, it does nothing for the low pressure. If you raise your oil pressure, you need to know what other things it may cause, like 100% oil filter bypass (no filtration at all) or some other harmful side effects. And you would still have low oil pressure at idle.

IF that is your problem, then I would try to find out what the root cause is. Without knowing this engine, I can only give a generic best guess and say worn bearings. Maybe oil pickup tube getting air from somewhere, a loose fitting tube could do that. It would also aerate the oil.

Oaky, got it, adding a shim impacts excessive pressure so there is no need to go there. The 235 in my truck does not have an oil filter, I recently removed it and installed a braided metal line (same diameter as what was initially on the engine) because the heat from the headers was softening the rubber hoses. I have a new pressure gauge on the car for the 261 and I will take extra care to make sure all of the oil pump connections are well sealed to stop air being sucked into the system. Thanks for your comments.

I'll leave the 235 engine diagnosis for another time and stay focused on the 261. I have a filter for the 261, what should the oil line inside diameter be to maintain oil pressure? Is 3/16" too large?

I've learned the hard way - oil pump gasket THICKNESS is critical. I learned from a retired dealer mechanic that the GM gasket is .009" thick. I've mic'ed gaskets included in full sets - THEY ARE THICKER, around .014"!!! Of course, I learned this after spinning a rod bearing on a 3/4-race 235, with GOOD clearances - .002-.0025" across for rods and mains, but never has decent pressure at road speeds - even after replacing the pump with a near-new part in the car. That job was a PITA!!!

My current 235 in my '57 car has a "sloppy bottom" (rods at .003", mains at .004"+), but did have a GM oil pump. I made my own gasket from a file folder that mic'ed at .008". Idle pressure is on the low side like your '59 engine, but perks up to 28-30lbs HOT. Oil is Castrol 20w-50, with a bottle of STP to keep the cam and lifters alive.

Tom Langdon seems to prefer the GM pumps, and still might have the GM gaskets. I would check the gear to housing clearance (use V-8 specs, or contact Tom for advice) and stay with the GM pump. I used sealer on both sides of the gasket, then removed the cover to clean up the squeeze-out and back together. So far, SO GOOD!!!

I also have a 5-window, a '53 I bought in 1974 at 16, and my next build will be a 261 I bought in a basket that looks really good for wear and still at STD. Ha!!! Keep us updated with your build progress!

Good Luck, and MERRY CHRISTMAS! Tim