# How to perform an engine compression test on your Miata

> You might want to do a compression test on a Miata engine for many reasons, such as to narrow down possible issues or determine the engine's overall health.

Source: https://help.flyinmiata.com/en_us/how-to-perform-an-engine-compression-test-on-your-miata-HJrIlAggh

Last updated: 2025-10-24T21:46:17.612Z

* * *

  

You might want to do a compression test on a Miata engine for many reasons.  For example, an engine may run poorly, so you need to narrow down the possible issues.  Or maybe the engine runs great, but you're about to add a turbocharger or supercharger, and you want to have a baseline of its health.  Whatever the reason, performing a compression test is a good way to get more information to determine the general health of that engine.

  

![](https://cdn.kustomerhostedcontent.com/media/5eaadd5b7ea1da00197e0989/d0aac2286bca77a027976c859adc5784.jpg)

  

* * *

  

### Before you start

  

First, you need a healthy battery.  When performing the compression test, it's essential to keep the procedure consistent for all cylinders.  To keep your battery topped off, put a trickle charger on it overnight or at least during the test.

Also, ensuring that the fuel injectors and ignition coils are deactivated is crucial.  This seems easy enough since you don't want the car to start; however, we found in the past that removing power to the ECU by pulling the main relay doesn't always get the job done, and the ignition coils can still be damaged.  To eliminate any possibility of damaging the coils, it's best to completely remove power from them.  The ignition coils are usually on the same circuit as the ignition switch, so pulling that fuse will keep power from the coils, but it will also prevent the engine from cranking over.  So, the best solution is to unplug the ignition coils from the main harness to prevent any power from reaching them.

  

* * *

  

### Compression testing procedure

  

1.   Warm up the engine to normal operating temperature.
2.   Stop the engine and allow it to cool off for about 10 minutes.
3.   Remove the fuel pump relay.
4.   Remove the spark plugs.
5.   Disconnect the ignition coil connector(s).  Some generations will have a relay for the coils, which would be an easy way to make sure they don't get power while cranking.
6.   Thread the compression tester adapter into the spark plug well and connect a compression testing gauge.
7.   Fully depress the accelerator pedal (this opens the throttle body to prevent air from being restricted) and crank the engine over at least five full revolutions.  Do this three times per cylinder to average the results.
8.   Record the maximum gauge reading.
9.   Rinse and repeat for all cylinders.

  

* * *

  

### What compression numbers are considered acceptable? 

  

Remember that the numbers below are based on sea-level barometric pressure.  Your compression will be somewhat less if you're at a higher elevation, so factor a reduction in compression numbers relative to your current barometric pressure.  

In the list below, the first number after the date range refers to a new engine's compression, and the second number represents the minimum acceptable compression value.  NA, NB, and NC Miatas have a maximum allowable difference of 28 psi between the cylinders, while the ND shouldn't see a difference greater than 21.5 psi between cylinders.  Also, the engine should be spinning about 300 RPM when cranking to achieve good and repeatable numbers.  As long as you have a good battery that is fully charged, you shouldn't have issues achieving a high enough engine RPM.

*   1990-93:  192/135 psi
*   1994-97:  182/128 psi
*   1999-00:  209/146 psi
*   2001-03:  196/146 psi
*   2004-05:  198/146 psi
*   MSM:       209/146 psi
*   2006-15:  249/174 psi
*   2016-25:  128/103 psi 

  

* * *

  

### What if the compression numbers aren't great?

  

If the compression in one or more cylinders is low, or if the difference in compression values between cylinders exceeds the maximum of 28 psi (for NA-NC, or 21.5 psi for ND), pour a small amount of clean engine oil into the cylinder and test the compression again.

*   If the compression increases, the piston, the piston rings, or the cylinder wall may be worn, and an engine overhaul is required.
*   If the compression stays low, a valve may be stuck or improperly seated, and an overhaul of the head is required.
*   If the compression in the adjacent cylinders stays low, the cylinder head gasket may be damaged, or the cylinder head is distorted, and an engine and/or head overhaul is required.