Is there a formula which can be applied to estimate what to anticipate when performing a cranking pressure test on an engine?

I don't think it is as simple as multiplying atmospheric pressure ( say 15PSI) by compression ratio because I have measured figures of around 110 to 120 psi on simple 6.5:1 or so side valve engines.

I can't find any references that I can understand on either the web or in my various books.

Regards

Barry R.

Over the many years of checking compressions on A7s, when I do it I just hope it has some ! No ,seriously, I have aways applied the criterior that if the comressions ,whatever they ,are within 8 to 10% of each other that's o.k.

Barry

You cannot use the STATIC compression ratio, because it is fairly meaningless for what you are doing.

The DYNAMIC compression ratio is what actually applies to the engine.

Explanation......

The 4 stroke cycle is not as you see in text books. The valves are opened and closed at points not neccessarily corrosponding to the start and end of the appropriate strokes, cylinder filling is thus affected by a number of factors including engine revs. Therefore the actual volume of gas present at the start of the compression stroke differs depending on various factors, and will result in a different dynamic ratio.

Hope this helps.

At the end of the day I would suggest that most people are looking for a fault when performing a compression test, so in that case you would expect one cylinder to be a bit down or something on those lines.

Steve

Thanks for the info. Have now done a web troll on "Dynamic Compression Ratio" and have a broad understanding of the matter. I definitely can't follow the mathematics on several sites and don't want to!

Another satisfied customer.

Regards

Barry R