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 )?


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? – Robert S. Barnes Jun 26 '15 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. – Pᴀᴜʟsᴛᴇʀ2 Jun 26 '15 at 14:58
  • Looks like I'm going to need a compression tester after all. – Robert S. Barnes Jun 26 '15 at 15:56
  • @RobertS.Barnes What problem are you trying to solve? – Move More Comments Link To Top Jun 26 '15 at 21:45
  • 1
    @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. – Pᴀᴜʟsᴛᴇʀ2 Jun 27 '15 at 2:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.