Compression Test Numbers

F

FastSVO

Founding Member

Jun 28, 1999

193

3

18

Los Angeles

Jun 4, 2007

#1

• Jun 4, 2007

• #1

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

"How long does it take to get help in here?

15 Year Member

Nov 29, 1999

31,179

33

129

Jun 4, 2007

#2

• Jun 4, 2007

• #2

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

 

Maryland Stang

Active Member

Aug 21, 2002

1,656

30

39

Greenville, NC

Jun 4, 2007

#3

• Jun 4, 2007

• #3

I agree, those numbers are excellant.

 

91 HATCH

Banned

May 17, 2007

262

0

0

illinois

Jun 4, 2007

#4

• Jun 4, 2007

• #4

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

 

R

RioRed4v

New Member

Aug 1, 2003

24

0

0

Las Vegas

Jun 4, 2007

#5

• Jun 4, 2007

• #5

not bad at all.

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

 

Spyderd68

Founding Member

Jun 25, 2000

935

0

26

Syracuse, NY

Mar 10, 2008

#6

• Mar 10, 2008

• #6

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

Founding Member

May 19, 2002

1,437

1

38

Orlando

Mar 10, 2008

#7

• Mar 10, 2008

• #7

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

New Member

Jun 20, 2005

515

0

0

pottstown, PA

Mar 10, 2008

#8

• Mar 10, 2008

• #8

91 HATCH said:

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

Click to expand...

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

 

Foxfan88

My Grandpa has great wood.

Sep 13, 2004

2,487

4

0

Miami, Ok

Mar 10, 2008

#9

• Mar 10, 2008

• #9

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

"How long does it take to get help in here?

15 Year Member

Nov 29, 1999

31,179

33

129

Mar 10, 2008

#10

• Mar 10, 2008

• #10

Spyderd68 said:

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

Dave

Click to expand...

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

 

E

El_CUCUY

Member

May 1, 2006

95

0

6

RGV

Mar 11, 2008

#11

• Mar 11, 2008

• #11

gcwh02 said:

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

Click to expand...

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

 

Spyderd68

Founding Member

Jun 25, 2000

935

0

26

Syracuse, NY

Mar 28, 2008

#12

• Mar 28, 2008

• #12

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

"How long does it take to get help in here?

15 Year Member

Nov 29, 1999

31,179

33

129

Mar 28, 2008

#13

• Mar 28, 2008

• #13

Spyderd68 said:

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

Click to expand...

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).