Need input on my cylinder compression

goatnipples2002 · Post by **goatnipples2002** » Sun Apr 22, 2007 10:46 pm

Just looking for some input.

Are my compression #'s alarming. I have read that most peoples are in the 150-160 range.

My gauge only has 120 and 150 with 5 marks between the numbers so I came up with each mark at about 6psi.

DRY test
#1-126
#2-144
#3-126
#4-120
#5-132
#6-114-120

WET test
#1-138
#2-150
#3-138
#4-132
#5-156
#6-114-120

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Apr 22, 2007 11:20 pm

I think 6 is toast. probably bad valve since the numbers didnt' go up when doing the wet test.

Do a leakdown on that cylinder.

Other then that they seem normal for a lower compression ratio motor.

goatnipples2002 · Post by **goatnipples2002** » Mon Apr 23, 2007 12:56 am

yeah my exhaust seals are junk they are those shitty rubber bands. I realized they were wrong as soon as I started my motor for the 1st time. I need to use the premium seals on all my valves not just the intakes. I guess I will check the comp after some new seals. Thanks.

Series8217 · Post by **Series8217** » Mon Apr 23, 2007 1:45 am

Dude, valve seals have nothing to do with compression. Those are to keep oil from coming down around the valve stem...

The Dark Side of Will · Mon Apr 23, 2007 7:34 am

#5 isn't too strong either. It gained 24 psi wet, while the others only gained 12.

Definitely have a valve problem in #6.

What's done to the engine?

Blue Shift · Post by **Blue Shift** » Mon Apr 23, 2007 8:26 am

Hahah... you had to do math to find out what each tic on your gage is? And yeah, if your valve seals are causing low compression, it's probably cuz one made it's way inside a cylinder...

The uneven nature of your readings isn't good - I got more consistent readings on my 3.4 when it had a bad head gasket and a spark plug non-fouler in #3 when I got it. As for the overall low compression, cam timing and all that shit will change your actual compression readings. Is your cam(s) tweaked?

goatnipples2002 · Post by **goatnipples2002** » Mon Apr 23, 2007 10:06 am

I have a compcams 260 and my timing is retarded. If it is a valve problem would having my valves reseated solve this problem. I have switched valves and heads without having them reseated.

whipped · Post by **whipped** » Mon Apr 23, 2007 10:31 am

How are you testing compression?

Is this the reading on the 1st stroke? 3rd? 4th?

Is the throttle blocked open?

The Dark Side of Will · Mon Apr 23, 2007 11:51 am

Dunno how he does it, but when I do I just crank until the needle stops going up...

Series8217 · Post by **Series8217** » Mon Apr 23, 2007 1:19 pm

Make sure you put some oil on the o-ring of your compression tester, sometimes it doesnt seal well enough otherwise and you'll get abnormally low readings.

goatnipples2002 · Post by **goatnipples2002** » Mon Apr 23, 2007 1:23 pm

Throttle was closed and the o ring was dry...it is a new unit

Shaun41178(2) · Post by **Shaun41178(2)** » Mon Apr 23, 2007 5:28 pm

Looks like you put too much spray to your motor. Looks like not everyone can just slap a kit on and go fast without doing damage to it huh??

FYI I have always done my compression tests with the TB closed. It might make a little difference. No idea though

The Dark Side of Will · Mon Apr 23, 2007 7:14 pm

I believe the textbook method is with throttle open.

goatnipples2002 · Post by **goatnipples2002** » Mon Apr 23, 2007 10:47 pm

Live and learn as long as it last until the end of the year then I am good.

Blue Shift · Post by **Blue Shift** » Tue Apr 24, 2007 5:24 pm

They say throttle blocked open, and stop on the 4th compression stroke, IIRC.

The Dark Side of Will · Tue Apr 24, 2007 11:43 pm

Who is they?

I've never understood the logic behind trying to stop on a given stroke. The gauge goes up... and then stops. That's the compression reading.

EBSB52 · Post by **EBSB52** » Wed Apr 25, 2007 12:53 am

The Dark Side of Will wrote:Who is they?

I've never understood the logic behind trying to stop on a given stroke. The gauge goes up... and then stops. That's the compression reading.

I agree....

Why not get an aircraft differential compression tester. You pump 80 in, see what ya get out. You have to get a breaker bar on the crank, but it is a more rpeceise way to check, plus, you can check to see where the leak is.

Blue Shift · Post by **Blue Shift** » Wed Apr 25, 2007 6:10 am

EBSB52 wrote:
The Dark Side of Will wrote:Who is they?

I've never understood the logic behind trying to stop on a given stroke. The gauge goes up... and then stops. That's the compression reading.

I agree....

Why not get an aircraft differential compression tester. You pump 80 in, see what ya get out. You have to get a breaker bar on the crank, but it is a more rpeceise way to check, plus, you can check to see where the leak is.

They... are the hand in the shadows manipulating your readings to the benefit of the automotive rebuilders! Quick, hide me before they realize I found out!

...Or not. Yeah I usually just find myself cranking it to whatever ultimate pressure the cylinder can muster. It probably is more consistent/accurate trying to read a cylinder that's in motion and trying to lubricate itself anyways.

EB, I remember helping my dad do the same test on the IO-470's on his Cessna 310 (I actually have a piston on my shelf, it barely press fits into a Folgers Coffee can). I remember him saying something along the lines of "hold onto the prop and don't let it slip" [implications of death and dismemberment]. Isn't that basically just a leakdown test?

EBSB52 · Post by **EBSB52** » Wed Apr 25, 2007 9:39 am

Blue Shift wrote:
EBSB52 wrote:
The Dark Side of Will wrote:Who is they?

I've never understood the logic behind trying to stop on a given stroke. The gauge goes up... and then stops. That's the compression reading.

I agree....

Why not get an aircraft differential compression tester. You pump 80 in, see what ya get out. You have to get a breaker bar on the crank, but it is a more rpeceise way to check, plus, you can check to see where the leak is.
They... are the hand in the shadows manipulating your readings to the benefit of the automotive rebuilders! Quick, hide me before they realize I found out!

...Or not. Yeah I usually just find myself cranking it to whatever ultimate pressure the cylinder can muster. It probably is more consistent/accurate trying to read a cylinder that's in motion and trying to lubricate itself anyways.

EB, I remember helping my dad do the same test on the IO-470's on his Cessna 310 (I actually have a piston on my shelf, it barely press fits into a Folgers Coffee can). I remember him saying something along the lines of "hold onto the prop and don't let it slip" [implications of death and dismemberment]. Isn't that basically just a leakdown test?

Yea, it is a running leakdown, menaing that the 80 is maintained. Also, you can move the move the piston slowly to see if there is a big dropoff as you leave TDC. And the point you make that testing it stopped rathr than moving is right on. It is a bigger pain to diff test it, but more accurate IMO.

Yea, they're fuckin monsters, 470 cubes in 6 cylinders. And I used to think a 454 V8 was big.

goatnipples2002 · Post by **goatnipples2002** » Wed Apr 25, 2007 3:14 pm

Would switching valves into different heads make a difference on compression if they weren't reseated?