Compression Test Score Help - RX8Club.com



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Old 09-11-2010, 04:28 PM   #1
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Compression Test Score Help

04 standard - Make a long story short, having the same old stalling when idle, loss of power type problems. Failed the initial compression test with the stock crappy starter, didnt get the numbers at the time. They claimed the rpms were too low to get an accurate reading. Fair enough. Suggested putting in the updated starter, agreed.

Replaced the starter with the updated one and passed with scores more or less 7.8 @ 320 rpm on all six faces. Now according to them the min spec is 6.9 @ 250 rpm.

I dont know the math or the measurements involved, but do my results not need to be normalized or whatever from 320 rpm to 250 rpm to make an accurate comparison? Or does that value take into account the rpm, and my engines compression is actually fine?
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Old 09-11-2010, 04:42 PM   #2
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7.8kgf/cm2 (110.94psi) @ 320rpm = 6.67kgf/cm2 (94.78psi) @ 250rpm normalized
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Old 09-11-2010, 04:50 PM   #3
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I take that to mean it failed somewhat miserably. What does 8.2 work out to, that was the highest value?

Much obliged for the quick response, you have a moment to explain how you obtain those values in that calculation? Or at least so I could do it myself when I tell off my mazda tech?

Hrm, apparently I made a mistake there, test was done at 310 rpm.

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Old 09-11-2010, 05:01 PM   #4
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8.2kgf/cm2 (116.63psi) @ 320rpm = 7.08kgf/cm2 (100.69psi) @ 250rpm normalized

I use a Rotary Compression Calculator to normalize the values:
http://foxed.ca/foxed/index.php?page=rotarycalc
Now for some reason this calculator doesn't have kgf/cm2... it only has kgf/cm3.
So I need to use psi in the calculator, which I turn to this site to convert kgf/cm2 to psi.
http://www.sensorsone.co.uk/pressure...onversion.html
Once I have the psi, I plug that into the calculator to normalize.
Once the psi is normalized, I use the converter site to convert back to kgf/cm2
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Old 09-11-2010, 05:05 PM   #5
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Quote:
Originally Posted by CW905 View Post
Hrm, apparently I made a mistake there, test was done at 310 rpm.
HA... well you can practice with the instructions I've given to normalize for 310rpm
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Old 09-11-2010, 05:20 PM   #6
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Any other day my friend. Id appreciate if you continue the favor and save me the trouble one last time. Trying to avoid going into a murderous rampage after dealing with my stealership over this for the last several months. I think trying to battle my computer might just put me over the edge. =)
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Old 09-11-2010, 05:29 PM   #7
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7.8kgf/cm2 (110.94psi) 310rpm = 6.7kgf/cm2 (97.70psi) @ 250rpm
8.2 kgf/cm2 (116.63psi) 310rpm = 7.3kgf/cm2 (103.62psi) @ 250rpm

Mazda's Specs:
Standard: 8.5kgf/cm2 (120psi)
Minimum: 6.9kgf/cm2 (98.6psi)
Standard difference in chambers: Within 1.5kgf/cm2 (21.8psi)
Standard difference in rotors: Within 1.0kgf/cm2 (14.5psi)

Last edited by Jon316G; 09-11-2010 at 05:33 PM.
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Old 09-11-2010, 05:36 PM   #8
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Many thanks. Looks like my first rotor is gone... with any luck the dealer will finally stop trying to get me to throw money at them and actually fix the problem.
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Old 09-11-2010, 05:39 PM   #9
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Now... without seeing the readings for all three rotor facings and on each rotor, I can only go off the two values you gave me.
But you're just barely under the minimum value.... the car should still run fine.
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Old 09-11-2010, 05:52 PM   #10
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Should is the key word I guess. Replaced plugs, coils, MAF sensor, cat is clear, throttle body is clear. Still struggles to maintain an idle in warm weather or after prolonged driving, or stalls out completely if you come to a stop. Significant loss of power in warm temperatures. Give it gas, the rpms dont rise, or at least lag significantly.

Problem is the intermittent nature of the problem. The dealer has, untill recently, struggled to duplicate the problem. He even went so far as to suggest it was something I was doing, although I have no idea what that could possibly be, seeing as how he and the other techs routinely drive it and claim to have no problems whatsoever.

Come to think of it now, I dont think they have ever actually performed a vaccuum test, to my knowledge, despite me asking repeatedly to have it done. Think its time to try a different dealer.

Edit - oh, and the first rotor was, as stated, 7.8 7.9 7.9
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Old 09-11-2010, 05:58 PM   #11
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I was actually going to suggest getting a vacuum reading... usually taken at the VFAD nipple behind the throttle body.
Do you ever receive a CEL?
Other items to consider is a dirty Eccentric Shaft Positioning Sensor (which then you should clear the NVRAM) or fuel pump.
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Old 09-11-2010, 06:03 PM   #12
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Nope, no CELs. Only one I ever had was the P2070 SSV stuck open probably 6-8 months or so before the problems started appearing. SSV appears to be functioning normally at this time - according to the dealer that is.
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Old 09-11-2010, 06:07 PM   #13
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Oh, and I do have the problem that others have reported in that having the A/C on tends to be the catylist for the problem rearing its ugly head.
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Old 09-11-2010, 06:10 PM   #14
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The SSV has been known to cause problems, such as idle and stalling issues (I know first hand).
Here is a TSB you can give to the dealer:
http://www.finishlineperformance.com...9-08-1929a.pdf

And here is what we've been doing with it:
DIY: Remove SSV (beta/teaser video)
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Old 09-11-2010, 06:16 PM   #15
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Yeah, I had them refer to that TSB when I brought it in for the specific code. I have asked repeatedly about the SSV in particular, as I have always considered that my most likely cuplrit, and he has repeatedly said that it was fine. Ultimately I think I have lost faith in the competancy of this dealership, to actually diagnose anything, and its time to try a different one.
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