Long story short, I purchased an 05 RX-8 about a month ago. It had a blown engine. The compression results were 6.0 on rotor 1 for all three faces. Rotor 2 had 6.6 as the highest on one face. Either way, Mazda said the minimum is 6.9. What unit of measure is that?

 

Wow those figures are really bad.

It's a ratio of total volume in a combustion chamber at the beginning of the cycle i.e. before combustion to the volume at the end of the combustion cycle.

Higher is better because it means your combustion chamber is relatively small compared to a fully open chamber. This small space means that the air and fuel are mixed better and the combustion is more efficient.

I probably did a horrible job explaining this so maybe google it for videos on YouTube.

 

Btw I should mention too that lower figures mean that your rotors are warped or the chambers are. This will lead to incomplete combustion.

 

The acceptable range of compression @ 250 RPM as follows:

KPA:
830 standard
680 minimum
150 diff / chamber
100 diff / rotors

Kg F / cm^2:
8.5 standard
6.9 minimum
1.5 diff / chamber
1 diff / rotors

PSI:
120 standard
98.6 minimum
21.8 diff / chamber
14.5 diff / rotors

Remember values need to be normalized for 250 rpm.

 

Huh?............

 

Yeah, I kind of figured the engine was super toast when it took 7 quarts to go 400 miles. Probably one of the worst days of my life that... But now it's back with a new(ish) engine! :D

 

Footman is correct. The reading they were using is Kg/cm2 also known as Bar. The question is; what rpm was the test performed at. Slower cranking speeds produce lower numbers, hence the 250 rpm reference.

Paul.

 

A brand new RX-8 is 10:1 right?

 

Btw there is no unit for compression ratios other that volume.

You are coverting those into pressures....

Am I missing something? I would not know how to calculate it for a rotary since the chambers are so odd shaped and the volume is probably some crazy integral.

We know it's 1.3 liters with two rotors and 6 chambers. (3 per rotor)

 

I'll have to look at the RPM. I'm going to say 250, since that seems to be the standard, but I couldn't say for sure.

 

We are not discussing the compression ratio but rather the compression itself. This is the standard in the automotive industry for assessing the health of an engine. When an engine is not sealing well (Any engine), it's compression figures will be reduced. This is normally a result of wear and or stuck components.

Paul.

 

10:1 is the compression ratio. It refers to the static change of volume of space from maximum intake to the volume at point of ignition.

i.e. if the maximum space in the chamber holds 650 cc during intake, and at the point of ignition, that same space is only 65 cc, then you have a 10:1 compression ratio.

When you are performing a compression test, you are not verifying whether your engine still has 10:1 compression or not. It would always have 10:1 compression as that is a function of the shape of the rotor and the rotor housing. What you are doing is putting a pressure sensor in the spark plug hole and verifying the pressure exerted on the sensor by the action of the spinning rotors which causes the air to compress. The quality of the seals play into this because if you have poor sealing, gases will leak to the next chamber thus decreasing your pressure you can exert onto the sensor.

In the real world, this translates to gas mixtures and exhaust gases to leak from chamber to chamber producing poor fuel economy, dirtier emissions, and a weaker engine.

 

Ahh thank you. This makes more sense now. It does make me wonder what my own compression is now.

 

Sad but true.