The answer lies here: if you were to have the engine just sitting (not running) and put a vacuum to the crankcase, would it suck the oil out of the pan? The answer is NO. Vacuum does not negate gravity!

I don't think so based on hydraulic pressure with atmospheric pressure on the oil versus oil pressure with a vacuum in the oil pan and motor. The crank, roids and pistons are pumping up and down in both cases, the ring seal should better with the vacuum creating less crankcase pressure, correct

So, i’ve followed “71Drmon572”’s post on the 580” engine recently. A comment he made brought up a a question I have had for a long while, and it may just be the answer. I didn’t want to convolute his post wth my question, so i made this new post. You guys can give your opinions.

About 2 years ago, I put together a lowdeck stroker motor with Indy-1 325 heads. Ran on the dyno and made nice power. However, i noticed the oil pressure was hovering around 60 for the entire pulls. Engine made peak hp at 7500 and pull to 7900. Vacuum was at 13-14”. This was a dual ext line with Milodon billet pump and super stock milodon pan (hole for centerlink). I always questioned why the pressure never climbedup to 70-80s. Added data** Dyno run from 4900-7900 rpm. Vacuum was 11.6” at 4900 climbed to 13.4 at 7900. Oil pressure was 62 lb at 4900 and tapered to 58 lb by 7900 rpm.

We even had a discussion in the dyno control room about the theory of whether crankcase vacuum can reduce oil pressure in the engine. Both sides of the argument were supported by different guys in the room. The engine was only run with vacuum, so I don’t have o p readings without vacuum.

So, does vacuum in crankcase reduce oil prssure that the engine sees, or readings,or it is totally unaffected by crankcase vacuum.

I just thought of this in a different way……both ends of oil (the inlet[pickup] and the outlets [oil passage exits] are all subject to the same vacuum suction. Like having a pipe with both ends subject to the same vacuum. The net effect within the pipe is zero.

This is correct?
Thx.

So, i’ve followed “71Drmon572”’s post on the 580” engine recently. A comment he made brought up a a question I have had for a long while, and it may just be the answer. I didn’t want to convolute his post wth my question, so i made this new post. You guys can give your opinions.

About 2 years ago, I put together a lowdeck stroker motor with Indy-1 325 heads. Ran on the dyno and made nice power. However, i noticed the oil pressure was hovering around 60 for the entire pulls. Engine made peak hp at 7500 and pull to 7900. Vacuum was at 13-14”. This was a dual ext line with Milodon billet pump and super stock milodon pan (hole for centerlink). I always questioned why the pressure never climbedup to 70-80s. Added data** Dyno run from 4900-7900 rpm. Vacuum was 11.6” at 4900 climbed to 13.4 at 7900. Oil pressure was 62 lb at 4900 and tapered to 58 lb by 7900 rpm.

We even had a discussion in the dyno control room about the theory of whether crankcase vacuum can reduce oil pressure in the engine. Both sides of the argument were supported by different guys in the room. The engine was only run with vacuum, so I don’t have o p readings without vacuum.

So, does vacuum in crankcase reduce oil prssure that the engine sees, or readings,or it is totally unaffected by crankcase vacuum.

The oil pressure doesn't change, but the reading on the gauge does. The oil pump is still pushing the same amount of oil thru the bearings.

All due respect but I don't think that's true. If there is less force applied to the oil in the pan pressing it into the pump, the pump wont draw as much oil as the pump creates a negative pressure to operate. The question is how much does it effect it ?
For info 14 inHg = - 6.88 psi, negative OR 7.82 psi of pressure in the crankcase / pan instead of 14.7psi (at sea level).

That most likely is not true at anything other than hot idle since oil pumps are almost always regulated by the relief spring, not the input suction.

All due respect but I don't think that's true. If there is less force applied to the oil in the pan pressing it into the pump, the pump wont draw as much oil as the pump creates a negative pressure to operate. The question is how much does it effect it ?
For info 14 inHg = - 6.88 psi, negative OR 7.82 psi of pressure in the crankcase / pan instead of 14.7psi (at sea level).

The oil pressure doesn't change, but the reading on the gauge does. The oil pump is still pushing the same amount of oil thru the bearings. The oil pressure gauge is "recalibrated" by the vacuum pump. Instead of reading from 0 to 100 it is now reading from -10 to 90.

I can understand that the oil pressure does not change. The gauge reading change I am having a little trouble understanding. Can you or anyone explain why? The way I am thinking about it, it the oil passages are full of pressurized oil and the oil sending unit is at the end of that passage, if the pressure itself doesn’t change, how would the reading change? I inderstand the unit is referenced off atmospheric pressure externally

Thx for all the replies.