If I have the specifications for cylinder compression can I convert that to how much intake vacuum the engine should be pulling?

For example, if I have a 4 cylinder engine that is supposed to have about 180psi per cylinder can I convert that to an expected inches of mercury ( vacuum )?

1 Answer 1


As far as I'm aware, it cannot be done. Vacuum is a direct relation to valve lift and timing, does not relate to compression ratio. The vacuum is drawn during the intake stroke. In a 4-stroke (cycle) engine, the intake and compression events are completely separate.

  • OK, so I would have to look for a vacuum spec for a specific engine separately. Is there usually a way to find out how much vacuum a specific engine is supposed to draw? Commented Jun 26, 2015 at 14:49
  • There are so many factors involved WRT vacuum draw. I mean, just in how the vehicle is timed will affect the vacuum of an engine. They used to say you could tune a car by how much vacuum it draws. You'd change the timing until it was drawing the most vacuum and that was where your get your best performance. Though, when I did this, I ended up with a large amount of engine knock (pinging) going on during acceleration, so I moved the timing back to where it should have been in the first place. Commented Jun 26, 2015 at 14:58
  • Looks like I'm going to need a compression tester after all. Commented Jun 26, 2015 at 15:56
  • @RobertS.Barnes What problem are you trying to solve? Commented Jun 26, 2015 at 21:45
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    @Movemorecommentslinktotop - I believe he is trying to find a way to do a compression test without pulling the spark plugs. He's trying to get as much out of a vacuum test as possible so as to figure out if an engine is in good shape. Way too much fun. Commented Jun 27, 2015 at 2:05

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