I'm looking into purchasing an FC and was given this compression test by the seller. I've already gotten 1 vote for "comp is too low" by a mechanic, but wanted more opinions. The shop that did the test said it was done w/a mazda comp tester and assures me that it's only a little bit low.

What do you guys think? This is for a '90 NA w/~60k on a mazda reman motor.

Thanks.

 

According to the Mazda FSM, anything below 85 PSI, with the engine cranking speed of 250 RPM and the test location at sea level, is marginal. Since you apparently don't know the engine cranking speed during the test or the elevation of the location where the test was conducted, you can't really compare these readings with the Mazda specifications. They could be higher or lower depending on engine cranking speed and elevation. However, they most likely would not vary significantly from the recorded readings. Best to add in an engine rebuild or replacement to the price if you really want the car. That can cost you anywhere from approximately $1,500 to $4,000 plus depending on whether you do some or all of the work yourself or you pay someone else to do everything.

Aside from all that, you don't really know if these are, in fact, compression readings from the car you are interested in....do you? My guess is they are, but the seller is certainly not doing himself any favors by showing them to potential buyers. I guess ignorance is bliss, eh?

 

This doesn't look good very good. You buy this car and you may be looking at a rebuild very soon. If that is part of the plan, and part the budget then you are fine. You can afford to just drive it till you rebuild, assuming it runs.

How does it run and drive?

 

Thats pretty damn low. 80-85 is the borderline for rebuilding it, and thats the highest it gets, with the front rotor being quite low. Unless you have the money for a rebuild I wouldn't risk it.

 

id pass...unless you want to rebuild it

 

Get a second shop to do the check. That just seems awfully low for such a fresh motor.

 

This compression test does omit some critical information such as barometric pressure/altitude and engine rotating speed. (Not to mention that it may not be for this car.)

I would recommend checking the engine yourself.

You can buy a compression gauge for $24.99 at Autozone (you can return it when finished if you don’t want one.) You will need someone else to turn the car over and hold the accelerator down, you have to watch the gauge carefully. You will see three bounces per rotor revolution (1 per EC rotation), meaning that you will should see ~4 per second at 250 rpm. If one apex seal is bad, it will show up as one strong pulse and two weak ones. Obviously you need to check both rotors.

My engine checks at 90-95 psi per lobe on both rotors at 1100 ft elevation with ~ 55k on engine. I used an ordinary compression gauge, held the release, pulled the spark wires, held the throttle open and turned the engine over on the starter (~250 rpm).

(note: if you are testing the car at a high elevation, expect ~5 psi drop for each 1000 ft of elevation above sea level)

At any rate, if the car starts easily and appears to have good low end torque, I would not be afraid of it.

Even if the engine is bad, don't completely discount the car. Read the rebuilding section of the forum, a rebuild can be done for < $1000.00 if you do the work yourself, and it is not that difficult. (Just make sure you pay $1500 or so less than the car is worth.)

 

Thanks for the quick replies.

As for elevation, the city it was tested in is pretty much at sea level.

I test drove the car and it drove fine. I used to have a '91 NA and it felt the same to me. I may have another shop test it if it doesn't sell to someone else beforehand. The car itself has a lot of the things that I'm looking for, but I plan on using it strictly for the track, so I really want the engine to be strong.