Pre-turbo compression test -> uh oh
 

Hi gang,

At the moment, my 95M with 78K on the clock is a stock N/A car. I am planning to turbo it but wanted to ensure the engine was healthy enough, etc. Did a compression test (at a shop) and the numbers were 155 150 125 155 dry. Did #3 wet and it came up to 150... so obviously #3 piston rings are possibly bad.

My mechanic suggested running a cleaning kit since he said there is a slight chance of the rings simply being gummed up (the car was driven by an elderly gentleman for 20 years of its life, tracked for the next 2 and then owned by me for the last 3 where I did mainly spirited drives on the backroads) although not to hold my breath.

As we continued discussing my options - his proposal was to simply boost it, drive it as I would and when the engine start showing signs of failure, do a full rebuild (or hell get a new engine and rebuild that) - enjoy the car for another 25 years on a fresh block. He is of the opinion that at 8 PSI (Flyin Miata Stage 1 kit), it will be at least 2 if not more years before any issues pop-up... I drive ~3500 miles a year.

Thoughts? Opinions?

Guess I am just venting a tad... especially after putting a new clutch in :cjerk:

If you have the time and are doing the install yourself, I might be inclined to boost the current motor. I'd run a leakdown test though, just to further isolate the problem.

Adding an incompressible fluid to the cylinder is always going to increase your compression readings. It doesn't prove a ring issue. I can't believe how many people believe that. It's also absurd to make a guess about how long the engine is going to last.

A leakdown test should be your next move IMO.

Well - the point of sprinkling oil into the cylinder was more to see if we have a badly sealing valve. Considering that compression increased, it seems to indicate valves are not the problem, no?

The theory is that adding oil to the cylinder helps improve a worn piston ring's ability to seal. It may well do that but the oil is also going to decrease the volume of the composition chamber which will definitely increase your compression reading whether the rings are bad or not.

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​​​​​​I think a compression test is fantastic for quickly identifying a problem cylinder, but it's absolutely useless for identifying what the problem is. A leakdown test will show exactly where the problem is.

Send it. My low cylender improoved after adding boost.

Sarcasm? :p

Nope, checked it again after a year with the same tester and the low one had come up to be almost even with the other three. No idea why.

Huh - interesting! What were your original readings, do you recall?

Fairly sure they are in my build thread somewhere? Maybe?

I had the same experience. My cylinder 3 was low, and after a year of boosting it and driving the piss out of it it was the same as the other cylinders.

That is bizarre but encouraging! As the young'uns say nowadays - YOLO!

Did the negcat come from someone who disagreed or were they offended by the delivery? If the former I'd love to here a counter argument.

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It wasn't me - but I will admit when I read your post I thought "hmm how much could a teaspoon or so of oil really throw the reading off?"

Doing some quick math, adding a teaspoon of oil to a 1.8l engine with 9:1 CR raises the CR to 9.9:1. So, by my math, the answer to my question is "probably quite a bit, enough to make a difference".

I haven't tried it myself, but have to imagine it could raise it by a solid 10-15+ psi based on CR alone (not accounting to potentially sealing valves or rings better)

I shouldn't have to, but given the other info the shop felt qualified to guess at; did they do this compression test with the motor warm, all 4 plugs out, and throttle held wide open?