Hi Guys,

I am new to this forum and after reading this forum for a few months I've decided to try and pick up a used RX-8, Hoping some of you may help me out with some compression test numbers.

I recently took a 04 RX-8 GT with 90100KM (Sold by another small mom and pop dealer) I was interested to a Mazda Dealer here in Canada to have a compression test before deciding on if I should purchase it.

The results were:
Rotor 1: 7.5, 7.3, 7.3 @262RPM
Rotor 2: 7.4, 7.2, 7.2 @259RPM

Here's my question:
I asked the service at the dealership (Guelph Mazda) whether or not these numbers are normalized to sea level as Guelph is 1096ft in elevation. They had no idea what I was talking about; they said they simply recorded the values from their compression tester.

Do I have to normalize these results(This would make this engine seem like its in excellent condition)? Or are they already normalized? If they are already normalized is this still a worthy buy for the price since its getting awfully close to the 6.9 failing line?

I know between 7.5-7.9 could possibly mean another 40K-60K left on the engine from (New and Potential Owners START HERE!), but any estimates on results between 7.0-7.4?


Additional Info:
2004 RX-8 GT Blue with original engine, I was able to talk the dealer down to $8500CAD
Spark plugs recently changed, not coils or wires.
Cold Start Time: About a 1.5 to 2 seconds (Dead Battery, Needed to be boosted)
Hot Start Time: About a 2 to 2.5 seconds (Dead Battery, Needed to be boosted)
Mazda Tech said this was one of better compression test results for 2004 RX-8 he has seen, as many 2004's he has done failed the test.

Thanks for your help in advance.

 

If they plugged the tester into the WDS system VIA USB as they should have then yes the results are normalized. Did they give you a print out? If they did then they likely printed those results from the system.


And those numbers are barely passing, not great. But passing.

 

Just average yours out and you will see where you stand. as for how long it is will last, well it could last a day or a year, it depends on a lot of things. Personally I would not buy the car unless the cost ($5,000.00 or so installed) of having a remanufactured engine installed was factored in.

 

This is the invoice I was given, not sure if that helps what kind of tester system was used.

 

Yeah that was typed in so likely just a lazy tech reading straight from the tester, so not normalized. But it look slike where you are from is at sea level so it doesn't really matter............

 

It's about 334m in elevation above sea level (1096ft). If i convert the given test values to PSI and enter them into the foxed.ca Foxed.ca - Rotary Compression Calculator rotary compression calculator with 1096ft factored in I get some high numbers.

After converting to PSI, normalizing at 1096ft, then reconverting back from PSI
Rotor 1: 8.0 7.9 7.9 @262RPM
Rotor 2: 8.0 7.6 7.6 @259RPM


Mid to high 7's to low 8's, perhaps a bit high for a car with that mileage?

 

Ah, well I looked on Google Maps real quick and it looked real close to the water, I didn't realize that was a lake,

So yeah normalized those numbers are great The mileage is not that great, sounds like a healthy engine.

 

So the tech reading directly from the compression tester and manually entering the values would mean the value's aren't normalized?
As these testers I assume aren't calibrated for altitude compensation?

 

Foxed.ca - Rotary Compression Calculator

 

If they didn't correct for RPM, it's a good bet that they didn't correct for altitude. If they corrected for RPM using the tester's function, then it would correct for altitude too.

 

How does adjusting for RPM work? Does it mean they would have to adjust the test scores for 250RPM? They gave me 262RPM and 259RPM, does this mean its not RPM corrected?

 

The first RPM is for the first set of numbers, the 2nd RPM is for the 2nd set of numbers, thanks to them testing one rotor then testing the other rotor. Battery drain typically results in the 2nd rotor tested to have a slightly lower cranking speed.

Since the numbers are nearly always close in speed, often only 1 number is reported, not both, but that is acceptable due to the low variance.


If you used the calculator, then it would have corrected for the RPM as well.

 

Okay, so I take it. Chances are the Mazda did not normalize the compression test values; and that the new normalized values makes the engine look like its in decent shape.

Another kinda dumb question, would trying to start with a dead battery (being boosted) result in slightly long cold/start times then compared to a fully charge battery?

 

If the booster is another car, it should out perform a full charged battery.

If the booster is a battery pack, yes, it will still be inferior to a full charged battery.