compression, are these numbers good or bad? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Blue_71 · 06-20-2002, 01:00 AM

checked 7 cylinders today, couldnt get to number 6 because of the danged header...
cyl|lbs/sq.in|plug|gap
1|115|wet|40
2|155|wet|40
3|135|wet|40
4|132|wet|40
5|125|wet|40
6|n/a|wet|40
7|130|wet|40
8|130|wet|40

notice those are in square inches, is the like 9. numbers square feet? and how could i be getting too much gas in the cylinders? thats what i was told as to why they were wet (showed them to my grandpa who has worked on cars 60+years and thats what he said)

*Gee_Emm* · 06-20-2002, 01:10 AM

The main thing to look for in compression readings is the difference between the numbers and how they could be related to a problem. Shouldnt the wet numbers be higher then the dry?

*Gee_Emm* · 06-20-2002, 01:12 AM

Oops I read your chart wrong. Did you take "wet" readings, as in putting oil in the cylinder then taking the reading after you take the dry readings?

Blue_71 · 06-20-2002, 01:14 AM

lol, i dunno, all 8 were wet, i was just trying to find out my compression, and then i discovered the wet plugs, which i doubt have less then a mile on them. (just moved truck around yard)

and the thing about getting too much gas, how do i get rid of that?

the cylinder wasnt wet, that i know of, i just took the plug out and screwed the tester in and turned the motor over a few times.. it was the plugs therselves that were wet when i took them out

*Gee_Emm* · 06-20-2002, 01:26 AM

No I was actually referring to doing a wet test by squirting oil into the hole. I have no idea why your plugs would be wet, sounds wierd. Try doing a google search for a website that explains the compression testing process, you would get alot of good info in one spot that way.

johnnyp · 06-20-2002, 10:01 AM

Numbers aren't too bad for engine with some miles. I think stock numbers are in the 150-155 range. The main thing is they should all be close - # 1 is the one out of line. If you do the test again, using the wet method (pour couple of tablespoons of oil in plug hole) and you get higher numbers, then it's rings/cylinder. If numbers are same, then it's exhaust leak, possibly valves.

If the plugs are wet with gas, then your carb may be leaking gas from the bowl - could be bad plug in bottom of bowl. Whatever is causing the plugs to get wet needs to be address ASAP - gas mixing with the oil is a very bad thing.

Blazer1970 · 06-20-2002, 10:41 AM

Did you have the throttle open when you were cranking the engine for the compression tests? If not, it will suck fuel through the idle circuit, and since there is no spark, it just gets sucked into the cylinders. It (throttle closed) will also affect your compression test a little, but it will just show lower compression across all cylinders, so it does not matter if you are just comparing to see if they are even.

hanssolo · 06-20-2002, 10:56 AM

The uniformity of the readings are more important than the actual readings, unless they're all real low. Hotter camshafts will bleed off compression at cranking speeds also so I think its normal for those to be lower. i.e. more overlap.

Blue_71 · 06-20-2002, 04:32 PM

so oversized cam will make the readings lower? i was told when i bought the motor that it had a big cam in it, not like huge but a little over stock.

throttle was closed, so i need to do it again open?

i was told quadrajets are bad about leaking gas, so im goign to check that out in a few minutes

would having open headers on it mess with any of this? the open headers is why im not positive about the cam being over stock

WB72 · 06-20-2002, 06:20 PM

Having open headers will Not affect the readings. If the readings are more than 10% different, there could be problems with your valve seats or rings.

06-20-2002, 10:01 AM	#6
johnnyp Keepin' It Stock Join Date: Aug 2001 Location: Grand Island, Nebraska, USA Posts: 554	Numbers aren't too bad for engine with some miles. I think stock numbers are in the 150-155 range. The main thing is they should all be close - # 1 is the one out of line. If you do the test again, using the wet method (pour couple of tablespoons of oil in plug hole) and you get higher numbers, then it's rings/cylinder. If numbers are same, then it's exhaust leak, possibly valves. If the plugs are wet with gas, then your carb may be leaking gas from the bowl - could be bad plug in bottom of bowl. Whatever is causing the plugs to get wet needs to be address ASAP - gas mixing with the oil is a very bad thing. __________________ 72 Sierra Grande - Maych Forget about "do it right the first time". Do it wrong the first time to learn how, then do it right the second time.

06-20-2002, 10:41 AM	#7
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,507	Did you have the throttle open when you were cranking the engine for the compression tests? If not, it will suck fuel through the idle circuit, and since there is no spark, it just gets sucked into the cylinders. It (throttle closed) will also affect your compression test a little, but it will just show lower compression across all cylinders, so it does not matter if you are just comparing to see if they are even. __________________ Tim

06-20-2002, 10:56 AM	#8
hanssolo Registered User Join Date: Sep 2001 Location: central Ill. USA Posts: 447	The uniformity of the readings are more important than the actual readings, unless they're all real low. Hotter camshafts will bleed off compression at cranking speeds also so I think its normal for those to be lower. i.e. more overlap. __________________ 69 C10 longbed: 357c.i. /TH400/3.73s/AC/ beige primer/saving $ for paint ______________________________

06-20-2002, 06:20 PM	#10
WB72 Before and After Join Date: Feb 2000 Location: Leduc, Alberta CANADA Posts: 632	Having open headers will Not affect the readings. If the readings are more than 10% different, there could be problems with your valve seats or rings. __________________ Not Grandpa's C-20 anymore... 383 Stroker 425HP, 485Ft/Lbs Lowered 3.5" Front / 5" Rear 66,000 miles. Alberta, CANADA

06-20-2002, 01:10 AM	#2
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	The main thing to look for in compression readings is the difference between the numbers and how they could be related to a problem. Shouldnt the wet numbers be higher then the dry?

06-20-2002, 01:12 AM	#3
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	Oops I read your chart wrong. Did you take "wet" readings, as in putting oil in the cylinder then taking the reading after you take the dry readings?

06-20-2002, 01:26 AM	#5
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	No I was actually referring to doing a wet test by squirting oil into the hole. I have no idea why your plugs would be wet, sounds wierd. Try doing a google search for a website that explains the compression testing process, you would get alot of good info in one spot that way.



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