Compression Test Numbers

[FastSVO]

Hey guys,

I have a 91 GT with 57k original miles and just performed a compression test. Here are the numbers:

CYL
1 149
2 150
3 145
4 145
5 150
6 149
7 145
8 149

I was using a cheap tester from Harbor Freight so the darn could not be 100% accurate. In any case, aren't these numbers a little low?

The motor is bone stock except for a GT40 intake and 1.7 rockers. Could the rockers high ratio be causing the low the compression numbers?

Thanks for the help.

-Eric

 

[HISSIN50]

The numbers actually look good to me. And the spread (lack thereof) looks real nice too.

 

[Maryland Stang]

I agree, those numbers are excellant.

 

[91 HATCH]

150 is about 9:1 or so it all looks fine and within 15%

 

[RioRed4v]

not bad at all.

my 90 w/ 98k miles with the Harbor Freight tester came in at 162avg across all 8.

 

[Spyderd68]

Is the compression supposed to be tested with the engine warm or cold? Does Ford give a recommended rate for warm vs cold?

Dave

 

[RYC CUKR]

I have always done them with a warm engine and the TB blade open.

BTW the numbers look good especially the how close they are to one another.

 

[gcwh02]

150 is about 9:1 or so it all looks fine and within 15%

is there a way to know your compression by the numbers from the test?? what compression would 230-240 a cylinder be?

 

[Foxfan88]

the numbers look good, the important thing is that they are all close, rather than how high the actual numbers are, since there are many things that will affect the numbers.

 

[HISSIN50]

Is the compression supposed to be tested with the engine warm or cold? Does Ford give a recommended rate for warm vs cold?

Dave

Hot engine, especially if you use forged slugs (which fit the bores looser when cold than a hypereutectic piston would).

 

[El_CUCUY]

is there a way to know your compression by the numbers from the test?? what compression would 230-240 a cylinder be?

I would like to know this as well my car will hold 220 on all 8

 

[Spyderd68]

I have read that you have to give it full throttle when cranking the motor over during a compression test. Anyone know why this is?

Dave

 

[HISSIN50]

I have read that you have to give it full throttle when cranking the motor over during a compression test. Anyone know why this is?

Dave

It's easier to get air into the chambers if there's not a huge impediment in the way (throttle blade). It also should shut off the injectors (backfire concerns).