Vire 7 - Bulletin Board

Compression Readings

From the Webmaster: The compression ratio of the Vire 7 is well publicised: it's 6.5:1. But what reading should you get on a compression gauge when you connect it to a Vire 7 ? The answer can be in PSI, or atmospheres, or Hectopascals, or millibars - it doesn't matter - and it will vary slightly according to the local atmospheric pressure when the reading is taken. With a brand-new engine, or immediately after fitting new piston rings, the reading will be slightly below its best, because of leakage past the piston rings. It will be at its best as soon as the engine is completely run-in and it will then fall slowly as the engine ages and wears. A sudden drop indicates a more severe problem, such as a broken piston ring or a blown crankcase seal.

I posed this question on the website, and got a variety of responses. Most of them theorised about formulae that could be used to calculate the range from the known ratio, but one owner had actually measured the compression. JTK wrote from California: "After purchasing my short block from Crinan, I ran a compression check on it just to find out what it's supposed to be, and it produced 100 PSI. Note of warning: beware when screwing in the adaptor into the spark plug hole. If the adaptor is too long the top of the piston will hit it."

PB wrote from New York: "My reading last week was 120 psi; my engine is from 1986."

Paul Saunders reported: "I have just bought a 23 foot Halcyon sailing boat and on first impressions I thought the engine was lying in State awaiting a dignified burial, due to the fact it had been sitting in six nches of water for I don't know how long. Amazingly, now removed from the boat and after a very quick external clean-up it is in perfect working order. I started it yesterday on a bench test with a hose pipe connected to the water pump, and in just a few seconds it started and ran like a dream. Just prior to starting the engine I carried out three compression tests and got the same reading each time : 124 psi".

P. Guy reported from Canada: "I just did two compression tests on the Vire 7 in my viking 28 both times I got a reading of 120 psi"

I have also measured the compression on my own engine, which had been run for about 50 hours since new piston rings were fitted. At the time one of the crankcase seals was leaking but, despite this, at a local atmospheric pressure of 1015 Hectopascals and an ambient temperature of 2¡C the cold engine yielded a compression of about 95 PSI. When the engine had been run at idle speed for a few minutes to warm it up, the reading rose to 100 PSI.

As for the formulae, they differ considerably. The simplest just multiplies the local atmospheric pressure by the compression ratio, to deliver a single figure. More complex formulae allow for compression leakage and the fact that cold air is being drawn into a warm engine, and also describe an acceptable range. Here are some examples, all calculated at Standard Atmospheric Pressure of 14.7 PSI (1013 millibars or Hectopascals): -

  1. RS writes from Canada: The pressure should read 14.7 x 6.5= 95.6 PSI where 14.7 is the standard atmospheric pressure at sea level. That's not very different from JTK's and my own measured figures.
  2. DW from the Netherlands directed me to With a compression ratio of 6.5:1 and barometric pressure 29.92 mercury, the formula on that site predicted a pressure of 80.85 PSI. This is much lower than JTK's and my measurements.
  3. The other formula that I found came from (Webmaster's Note: This link is now dead). It says that Compression = (Compression Ratio x Atmospheric Pressure) + Atmospheric Pressure + 5 +/- 15%, where all values are quoted in PSI. This predicts a mean compression of 115 PSI, with an acceptable range of 98 to 133 PSI. JTK's and my measurements fall just above the lower limit of this range, but the other three readings are on the correct side of where they should be. JTK's engine had just been rebuilt, so a fairly low reading was to be expected (it hadn't been run-in, so the piston rings wouldn't have been sealing to the best of their ability). My engine was probably just suffering from old age, although my leaking crankcase seal probably didn't help.

A survey of just five engines is not very extensive. So, if you ever measure the compression on your Vire 7, please E-mail the Webmaster with your findings.

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