Compression check

[Shimy87]

Did a test, drivers side front to back 146,155,152 pass side 155,160,155. Should I be concerned about the 146 hole? I don't think I should, it's within 10% but figured I'd ask you guys....thanks!!

[gusszgs]

Those numbers are good i believe. Mine was 145-155 across the 6, and runs fairly strong.

[Steve Wood]

looks very reasonable to me...maybe better than I would have expected on a stock engine.

[Scoobum]

Check the 146 every couple of months.

[earlbrown]

Put a can of Restore in there and run it a few hundred mies


Also: I'm not kidding   It actually helps in real life

[wmsonta]

It seemed high to me also. 80's oem pump premium car at 160#.


Turns out it is valve timing. There is a 78 and a 87 V6 Buick in my engine development software. Both turbos. Both listed at 8-1 compression. One cam shows a 106 CL and the other a 107 CL. One shows an estimated cranking com of 156, the 78 shows an incredible 170. Wow.

Op, without symptoms, I would pay 146 no mind. Too many variables with a compression test. Especially if the 146 was the last cylinder tested. JMO.

[earlbrown]

There's more to it than just the installed centerline.   It's the exact point in degrees ABDC the intake valve closes that matters.   The earlier the closure the more cranking compression (and higher dynamic compression).


Not to mention the older a car gets, and the more the timing set develops slack, the longer it takes to close the intake.    That's why ''loose'' engine runs so well.   When the cam gets retarded as hell, the bottom end goes away and it wakes up just before time to shift.

  Soft squishy humans interpret that data incorrectly and think a supercharger just kicked in.

[wmsonta]

There's more to it than just the installed centerline.   

That is true

This is not the installed CL. This program shows 106 and 107 as the camshaft CL. If true, I find that unusual for an oem 80's camshaft. Just as I find 117 CL unusual for the LS3 vette motor. The LS3 is true.

The 78 in the program would have the early 'low' port heads. Both were small profiles.

[earlbrown]

117 is wide as hell.   I can't think of any pushrod OHV V engine that needs that much.  Nowhere close.

Even considering that much separation means the lobes are waaaaaaaay to fat.  (and the correct fix is to put the right lobes in there)

[earlbrown]

Also, depending on your opinions of David Vizards dyno experience, he came up with a formula for choosing the optimum LSA...   On a non-canted head it's 128-(cubes per hole / intake valve D in inches X .91)

for a 231 V6 with a stock intake, the match comes out to 107.3911.
a .30 over with an oversized 1.77 valve comes out to 107.8634


Granted that's for an N/A and I have no idea how it converts over to turbocharged if there's any adjustment for being supercharged. Modern turbos are so efficient these days that the cam choice is damn near the same as N/A.

Plus, the other cool thing about turbos is that cam choice isn't that critical.  Pressurized air with blow right past the throttle body and cracked open intake.  Atmospheric just can't do that.

[wmsonta]

117 is wide as hell.   

Well, there are reasons.

Earl, you appear to have a good grasp on this subject. If I could figure PM's out, I might have something you would have interest. Dunno. If PM's have a reply button, you could message me.

To Op, I am sorry if I have helped to derail your thread.

I am out-

[Shimy87]

The 146 was checked first and then again after the rest...hoped it might go up some...stayed the same....gonna just run it and keep an eye for changes

[TexasT]

Get you a leakdown tester if you have an Air compressor it gives more info than just psi.

[wmsonta]

a leakdown tester

This.

If you get a leak thru the intake or exhaust, tap the top of the valve. I lay a ball peen hammer on the valve stem and hit the hammer. Centering the valve in its seat, can show bad guides.

[Scoobum]

With the cylinder pressures all over the place the valves are likely leaking a tad.