Crankcase Vacuum and effect on Oil pressure

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 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

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 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.

But, isn’t this the same as an example of a straw in a glass? Sucking the straw draws the liquid up against gravity. Just saying.

Yeah….this is one aspect I didn’t even consider. The effect, on oil pressure, of the change in ring seal.

Mulled over the same issue and if you apply a little science it does because the transducer measuring oil pressure is referenced to atmosphere.
At sea level, air pressure is 14.7 psi (one atmosphere). So fundamentally you have the oil pressure "pushing" against 14.7 psi on the gauge but also 14.7 psi of atmospheric pressure effecting the oil in the pan. Removing some of that air pressure in the pan by using a vacuum pump creates less atmospheric pressure on the oil so the transducer "sees" less oil pressure and the gauge indicates less oil pressure (Gauge Pressure).
If the transducer could use the vacuum source as a reference there would be no change on the gauge (Absolute Pressure).

HTH's

Yup. Oil pump suction and discharge are both at the same internal engine pressure, whatever that is.

Chargerfan68, at one point I turned the vacuum down to only 9”. The oil pressure did not fall off, but power did, which we knew would happen. After that pull I tweaked it a little bit higher, power came back, and oil pressure moved a little bit on higher RPM. The two were directly correlated. The 572 we were dynoing has a Milodon pan with two external lines, swivel pickup and Milodon pump cover.

We will have to see what it does when going down the track if we lose any oil pressure or not, and adjust from there.

I know personally, on my own car, when Tim installed a vac pump on it a few years ago. My first day out I was making 1/4 mile runs, at WOT, 7700rpm, 8.80 @ 152mph, it was pulling 15”. My oil pressure though, was 90lbs. When turned the vac down to 10” which is our target number. Oil pressure was 95lbs. Now I had ton of oil pressure and since then we turned the oil pressure down a little bit, but didn’t adjust by cranking the vac up.

I’m not an engineer or engine builder to explain it, I just know that the vacuum pump can affect oil pressure if cranked up too high.

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.

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.

As stated it does not affect oil pressure but it does affect the gauge reading. It will drop that number by whatever the vacuum reading is

Ahh...of course. The pump is already supplying way more the the oil system can use

I can't say I have the answer, but the pump's relief valve balances discharge pressure against ambient (external) pressure.
If the pump is external and sees atmospheric pressure (14.696 @ sea level), it won't change with reduced crankcase vacuum.
If the pump is inside the crankcase, the relief valve sees vacuum, which reduces the pressure against the relief valve, lb. for lb.
If the pump is big enough, viscosity high enough, blah, it won't affect pressure at the bearing at racing RPM: the expected pressure minus the vacuum is still a safe figure.

Did I miss something?

AndyF is correct. The pump suction and discharge are both referenced to the internal engine pressure. The gauge is referenced to atmospheric.

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.

Because the gauge is operating under a vacuum with the pump working. Disconnect the pump and make pass/pull and see what happens..

If you really want to know what is happening then you need to reference the oil pressure gauge to the crankcase. There will be a small port on the back of the pressure gauge that is open to the air. You need to solder a small fitting on there and then use a piece of vacuum hose to hook that port up to the valve cover. Once you do that you'll be able to read the actual oil pressure inside the engine relative to the crankcase.

[quote]But, isn’t this the same as an example of a straw in a glass? Sucking the straw draws the liquid up against gravity. Just saying./quote]

Well, I suppose if you were to run the pickup from the vacuum pump into the oil pan ... !!!

I don’t think the vacuum would reduce it. Oil pressure is a reading of differential pressure vs atmosphere.

But if it does reduce it would that mean blowby would increase oil pressure?

Unplug your vacuum pumps and feel free to report back...