Today I checked out the compression on a 7 I am lookin at buyin. Only took out the top two spark plugs, there was a bracket in the way of one of the bottem ones, and I was really pressed for time. The one at the front of the engine was 80 , and the rear 110. I am thinking thats not too bad.... What should those number be?

 

110 is excellent, 80 is dying!

 

 

DYING???? for real???? I thought 80 was ok....

 

Should run Ok. but will be a bit low on power.

 

105-150 is Mazdas spec for a good engine. Below 70 in my experience will be impossible to start. Make sure your gauge is accurate. I checked mine against a calibrated gauge and found it read 10lbs low. So I have to add 10lbs to my readings.

 

I test drove a 3rd gen that felt like it was running really well and it's compression was 61 and 63. No probs starting at all.

 

I've read a few threads where we're saying you should have compression numbers around 100 lbs, that your car is almost dead if it's at 80, etc...

According to the Mazda manual ('83), it quotes a compression pressure limit of 85 lbs., with a maximum difference between chambers of 21 lbs. (@ a cranking speed of 240-250 rpm).

Why the different numbers? Are we talking about two different things?

 

They have low compression rotors since they're turbo. I don't know what their compression #'s should be, but it is supposed to be quite a bit lower than n/a engines.

 

well, I know my gauge isn't working right. I press the check valve on mine and I get 30lbs of pressue and when I leave it closed it's less than 60lbs!

Kinda impossible or I'd hate to see how it runs at 100% considering I can roast the tires in 2nd.

 

what about the other 2 sides of each rotor?

you really need a rotary compresion guage reading...

If any other sides of the apex seal of off too, it wont run for ****, even tho both rotors may have a 110 reading on one side of the rotor. If such is the case its indicative of a bad corner seal or side seal.

 

The turbos aren't spooled when the car is starting. If 60 is too low to start, it shouldn't matter if it's a turbo or not.

 

Lower Static compression (Compression ratio) = Lower reading on the gauge. Higher static compression (NA engines) = Higher reading on the compression guage. Turbos help a low compression engine by raising the Dynamic compression ratio.

 

What's your point?

 

N/A engines have a compression ratio somewhere around 9.5:1. Turbo engines have a compression ratio around 7.5:1. So if you're doing a compression test on 2 perfect condition engines, 1 N/A, and 1 Turbo. It's obvious that the turbo engines compression test will read significantly lower.

 

80 is low but driveable. 110 is very good!

 

Well, my gauge is bad. I took the check valve out and get 30 for each side and when it is in I get 60. It can't be right because the car does run strong. Don't think I would be getting 2nd gear rubber with compression like that or even starting the car.