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Old 08-30-2009, 02:03 PM   #1 (permalink)
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Compression test oops

I did a compression test on my car and got very high readings. (Book says 192PSI, my gauge showed around 220PSI)
Afterwards I realised I didn't hold the throttle open while doing it.
Now, wouldn't that have given me readings *lower* than expected?

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Old 08-30-2009, 02:09 PM   #2 (permalink)
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Quote:
Originally Posted by markweatherill View Post
I did a compression test on my car and got very high readings. (Book says 192PSI, my gauge showed around 220PSI)
Afterwards I realised I didn't hold the throttle open while doing it.
Now, wouldn't that have given me readings *lower* than expected?
With the throttle blade open your compression readings should go up a little.

As far as the higher comp. numbers I would be happy. Compression gauges tend to vary, so as long as the cylinders are reading close to each other your fine.
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Old 08-30-2009, 02:50 PM   #3 (permalink)
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Carbon build-up on the crown of the pistons will raise your compression as well.
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Old 08-30-2009, 04:46 PM   #4 (permalink)
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It could have carbon buildup, the head could have been shaved, could have been bored. etc etc
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Old 08-30-2009, 06:12 PM   #5 (permalink)
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If you suspect carbon buildup, try using a spray bottle to put a little water into a vacuum connection. Water injection works surprisingly well to keep an engine clean.
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Old 08-30-2009, 06:16 PM   #6 (permalink)
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Not to start an argument,
Boring a cyl normally leaves the compression pressure the same, unless the head has a squish band.
O.P. , I guess you were checking an older engine to see if it has excessive blow-by? As in wore out low compression?
Ecco driving tends to build carbon on the piston crowns. Don't worry if the comp is high as long as its not pinging due to detonation.
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Old 08-30-2009, 06:35 PM   #7 (permalink)
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Jason, how so? (the fact that comp stays the same)
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Old 08-30-2009, 11:24 PM   #8 (permalink)
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Quote:
Originally Posted by JasonG View Post
Carbon build-up on the crown of the pistons will raise your compression as well.
very true. the high number is actually headed towards evil. the purpose of the test is to be within specs. Remove EGR anytime...
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Old 08-31-2009, 09:17 PM   #9 (permalink)
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Jason, how so? (the fact that comp stays the same)
If you compress a cyl 80mm X 80mm to 10mm X 80mm or compress a 82mm X 80 mm to 82 X 80mm the compression ratio stays relatively the same. The actual (piR)2 math is slightly in favor of the smaller Dia but once you get into the slight dome shape of most heads it evens out.
With a squish band type head (typ for 2 strokes and some diesels) the cyl volume is compressed into a small dome 1/2 the size of the cyl Dia. Boring the cyl doesn't change this volume, just increases the starting volume that gets compressed into the pocket. In that event the final pressure really goes up.
Now you're thinking "But Jason, every engine I've ever had bored came back with more compression!"
That is due to the fact that the head was at least cut a bit to clear it up, usually some more to raise the compression ratio for a few more horses.

Either way, the higher compression can cause detonation issues and needs to be watched.
Dumping the EGR will help, as will a bit of ATF misted down the intake at half throttle. You may want to disconnect the cat before doing that as it throws ALOT of black carbon out the exhaust.
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Old 09-02-2009, 05:52 PM   #10 (permalink)
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Math time:

94mm bore X 71mm stroke, 1mm deck height, 55cc combustion chamber, flat-top pistons.

Deck volume = 94 / 2 * 94 / 2 * 3.14159 * 1 ~= 6939.77 mm^3, or 6.9 cc.
Swept volume = 94 / 2 * 94 / 2 * 3.14159 * 71 ~= 492723.83 mm^3, or 492.7cc

Fixed volume = 55 + 6.9 = 64.9
Fixed + swept = 64.9 + 492.7
Compression ratio = 492.7 / 64.9 = 7.591 : 1

Change the "94" in the above to "96"--
96mm bore X 71mm stroke, 1mm deck height, 55cc combustion chamber, flat-top pistons.

Deck volume = 96 / 2 * 96 / 2 * 3.14159 * 1 ~= 7238.22 mm^3, or 7.2 cc.
Swept volume = 96 / 2 * 96 / 2 * 3.14159 * 71 ~= 513913.86 mm^3, or 513.9cc

Fixed volume = 55 + 7.2 = 67.2
Fixed + swept = 67.2 + 513.9
Compression ratio = 513.9 / 67.2 = 7.647 : 1

So an increase of 2mm bore (0.080") on a very mild two-liter four-cylinder (in this case going from 1971cc to 2056cc) would raise compression by one-twentieth of a point. The effect should be larger if the CC volume is lower, giving a higher initial compression ratio.

It's not a huge effect, but there is an effect.

-soD

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