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What is good compression pressure?

Yesterday a customer called me and told me a sad story. Her car, Nissan Rogue, got serviced in one of the local quick lube shops here (nope, I'm not pointing my finger, but I know the business's name). Along with the oil change, the air filter got replaced as well. For whatever incomprehensible reason, she got the air filter for Nissan Juke installed in her Rogue. And because it was of the incorrect shape or size, it wasn't filtering air at all. Unfortunately, this got discovered only after 16 000 km, when the car just stopped on a highway. The customer stated the air filter was still pretty much like new by that point.

So, this customer asked, "can you check the engine condition with a borescope and see if there is damage caused to it?" I replied that, actually, no, I don't have a borescope camera with good optics to do such a test. The borescope camera at my disposal does not have a clear enough picture to visually evaluate the cylinder bores' surface condition. But I would suggest doing a cylinder compression test for sure; this would give at least some idea of the state of wear of that engine.

The customer replied that the compression test was done, but different shops interpreted the results differently. One shop told her that this engine was shot, and the other two said it was OK. So, what are the results of that compression test?

The result was 170 psi; is this a good compression or not?

My first answer was, "well, I need to check the service manual data for this particular engine, but just off the top of my head, this compression seems acceptable." To be honest, I rarely need compression tests to evaluate the state of engine wear these days. Most engines get all sorts of other problems before they wear down to the point of low compression - therefore, there is no longer a need to remember what compression pressure is considered good. Out of curiosity, I checked how good my memory is on Google. Yep, indeed, the joint mechanics' knowledge agrees with me: 170 psi is a good or even "excellent" compression.

Is this good enough, is the question answered, and I'm happy? Nope, let's check what the Nissan Shop Service Manual says about compression pressure. And here we go:

The standard compression pressure on this particular engine should be 204.5 psi, and the acceptable minimum is 176.9 psi. The conclusion is, therefore: that this engine is shot. Sad story, considering the car has only 70-something thousand kilometers on the clock.

Now, why is there such a difference in compression pressure? See, a multitude of factors determines the absolute compression pressure on a healthy engine. Most important here would be a compression ratio, but the valve timing also affects the compression pressure big time. On modern engines, variable valve timing exists pretty much for a reason to increase compression pressure at a low RPM range, therefore producing more torque and better fuel economy. As a result, the old "rule of thumb" regarding "good compression pressure" might not be applicable anymore.

Always check the OEM data on a subject in question. In other words: RTFM - Read The Manual!

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