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.
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
