Just completed a compression test and ??

HoofnIt

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#1

• Mar 7, 2004

• #1

I did at three tests per cylinder trying very hard to turn the car over the same number of tries (3). My readings varied from the lowest @ 119 lb. to the highest @ 128 lbs. Can anyone tell me if these numbers are OK? I know that you shouldn't have a large difference between two consecutive cylinders but my concern is not a blown headgasket or broken piston. I just wanted to see what condition the motor is in before installing my H/C/I.

PS - the engine is stock with 82K miles and uses a little more oil than I'd like.

 

S/CBlack95GT

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Apr 6, 2003

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Mar 7, 2004

#2

• Mar 7, 2004

• #2

Those numbers are a little low. Typically anything below 100psi justifies a rebuild. Your car probably doesn't need a rebuild already just b/c your comp tests are low, but of course it will make less power than the same engine with 180psi. As a comparison my 28,000 mile shortblock makes around 170psi when cold.

 

HoofnIt

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#3

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How many revolutions do you need to do for an accurate reading? I noticed that the more times the car 'turned over' the higher the reading was. The gauge I used records the highest reading.

 

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smokediver27

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#4

• Mar 7, 2004

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Hey Hoof,

How difficult is the compression test to do , given that you have descent access to all your plugs??

 

VibrantRedGT

"STANGNET'S PENGUIN SMACKER"

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Mar 7, 2004

#5

• Mar 7, 2004

• #5

Did you do the test with the throttle plate wide open? It's like a 10-15 psi difference. Also leave a battery charger hooked up while you do the test. This will insure the same voltage every time.

 

Rootus

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Mar 7, 2004

#6

• Mar 7, 2004

• #6

If those are real numbers, 119 to 128 spread isn't bad, and sounds about right for a stock setup. I saw around those same numbers before I did my H/C/I, and 170-175 on all cylinders after.

Dave

 

H

HazMat404

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#7

• Mar 7, 2004

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i thought it was 4 or 5 times you are supposed to turn it over. i cant remember right now but you definately need to open up that tb when you are doin it. well you are supposed to mash the accel pedal to cut off the fuel anyways. when i bent/broke a valve and was gonna rebuild, my other 7 cylinders were puttin out about 140ish (give or take 5 here and there) after about 120k miles. and i beat on her a ton (thats how i threw a valve ). i say do it again and make sure you follow the directions exactly like it says then if it is under a 110 again then its probably time to rebuild.

 

mo_dingo

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Aug 26, 2003

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Mar 7, 2004

#8

• Mar 7, 2004

• #8

And you may be burning oil because one of your valve guides is bad. A new H/C/I will cure that.

The piston rings will be the only variable left in the equation.
Scott

 

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HRT11

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Mar 7, 2004

#9

• Mar 7, 2004

• #9

turn it over about 5 times
have your throttle open
lowest to highest is pretty close thats about all your looking for

 

Zero Signal

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Feb 24, 2003

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Mar 7, 2004

#10

• Mar 7, 2004

• #10

Yeah I had that problem too. After doing it with WOT, I went from 120's to upper 130's. Still kind of low, but about normal for the mileage.

 

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Fast94Cobra

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Mar 7, 2004

#11

• Mar 7, 2004

• #11

i just had mine done and got 142-151 on my 132,000 mile long block. i was excited enough to slap some N20 on there!

 

2muchtime

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Mar 7, 2004

#12

• Mar 7, 2004

• #12

crank the car till the compression guage stops climbing,, otherwise your stopping in the middle of the test.. 110 is very low you should be seeing around 150 on all 8 cylinders give or take 10..

 

HoofnIt

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Mar 8, 2004

#13

• Mar 8, 2004

• #13

Thanks for all of the input guys. Turns out that I did three things wrong.
1. Engine was not warm enough.
2. I did not have the throttle body opened..at all!
3. I only cranked it over 3 times.

More bad news is that I won't be able to repeat the test until the end of May when I get back from my work assignment!

How much of a difference do you think these corrections will make given the fact that it was around 120 before?

 

GTJake

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Mar 8, 2004

#14

• Mar 8, 2004

• #14

VibrantRedGT said:

Did you do the test with the throttle plate wide open? It's like a 10-15 psi difference.

Click to expand...

I would expect with that and the engine being warm and cranking it until it peaks will produce good numbers.

Jake