![]() I find the engine compression ratio specification and multiply it by 18 (I don't measure intake manifold vacuum). I know that for most engines, vacuum at idle is usually about 18 to 22 in.Hg (although some may produce only 15 to 17 in.Hg at idle). Once you've found the compression ratio, then you multiply it by the intake manifold pressure your vehicle's engine is producing at idle.įor example if a 2009 4.6L Ford F150 has a compression ratio of 9.8:1 and its intake pressure (intake manifold vacuum) is at 17 in.Hg, then you can expect a cylinder pressure in the 170 PSI range (17 x 9.8 = 166.6, which I then round to the nearest 10 and call it 170). This involves doing a little research online or in a repair manual to find out the compression ratio of your particular vehicle's engine. If your online search doesn't turn up a 'good' compression specification for your vehicle, you can calculate an estimate of a 'good' compression value by multiplying your intake pressure (intake manifold vacuum) by your engine's compression ratio. It used to be that repair manuals had a factory specification for what the 'normal' cylinder compression should be. What Should The Normal Compression Value Be For My Vehicle? Looking at the table of the compression test results, I can conclude that cylinder #1 is the one causing a misfire due to low engine compression. Any cylinder with this compression (or lower) value will misfire. STEP 2: 26.25 = 26 (rounded to nearest one).Here's the math I would do to calculate the minimum allowable value: ANSWER: The result of this subtraction is the lowest possible compression value the other cylinders can have.įor example, let's say I got the following compression test results (from a four cylinder engine): Cylinder.STEP 3: Subtract the result (the number that was rounded) from the highest compression value.STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).Here's the method I've used to figure it out: If you'd like to know the math that determines the lowest possible compression value an engine cylinder can have, this is the section for you. How To Manually Calculate The Lowest Possible Compression Value What the low compression calculator does is find out what the 15% compression value is compared to the highest compression value you got. ![]() ![]() If a compression value varies by more than 15% of the highest compression value, then that engine cylinder will cause a misfire and/or rough idle. Low Compression Calculator Low Compression Calculator: What Should The Normal Compression Value Be For My Vehicle?. ![]()
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