ayousef

Well, I cant seem to find this piece of information, should the needle on a compression test gauge STICK to a certain value after you stop cranking, or is it normal for the needle to fall off slowly after the cranking stops?

Ive been told that its only important to see where the needle shoots to during cranking and not when cranking stops, someone else told me the cam specs have an effect on this as well.

Can someone verify?

PS. all cylinders do exactly the same.

Gen414

iTrader: (38)

Well, you should only have to crank it 3-4 times to get a reading. It should hold pressure for awhile though, and bleed off slowly. How fast are you talking about??

Cam specs WILL affect compression yes...as will a low voltage battery (a motor that turns over slow, will NOT have teh same readings as one with a properly charged battery).

And while every gauge is different, as long as your readings are pretty consistent, then yes, you are ok (unless they are ALL LOW of course)

CamaroSS22

iTrader: (1)

The needle should stay in one place, if it bleeds off fast you either have a valve open or have some serious issues with your rings.

ayousef

bro, if i had serious issues with my rings, the car wont hit a good reading while cranking, my only concern is the needle dropping off AFTER we stop cranking, and again it drops slowly.

I know the cam specs will effect compression, but can the cam specs cause compression to bleed and slowly drop the gauge needle during a compression test?

and again, all cylinders do the same, so its obviously not "A" valve, piston rings all worn out also seems odd, but I wanna figure out why it does this.

ayousef

my 9:1 motor with a turbo cam, was reading in the 100-120 PSI on that gauge, a stock C6 Z06 reads about 150, and a z06 with 11.5:1 CR reads about 170.

Ill say it again, all the readings are consistent, BUT the needle drop issue is whats concerning my, although the guy at the shop says he isnt concerned,

he showed me how a car with excessive carbon buildup read (also smoking white smoke badly), the needle falls way fast to zero, yet even that car's problem was cleared by pouring some oil, and some carbon cleaner into the plug hole and cranking the car over etc...

Andrew91GT

iTrader: (6)

Not true. The schrader valve in the compression tester prevents the pressure from bleeding off. There is no pressure in the cylinder when you quit cranking, as nonzero leakdown will relieve it. If the needle is falling the compression tester has a leak in it.

ayousef

I dont think thats completely true either, some gauges maybe, but not all. If thats the case, you wont be seeing the gauge readings pulsate as you crank the engine, it will just stick to the highest value until you manually bleed it off.

In my case, its a little different. All of the compression test articles talk about the reading on the gauge, whether its low, high, and how to differentiate between a worn ring or valve issue, but none speak of what happens AFTER you stop cranking...