[ron cozzo]

Stock 67 GTO YS 400, 99K mi, apparently never rebuilt. Did cranking compression check today, results follow: no.1, 170psi; 3, 178; 5, 178; 7, 165; 2, 208(yes, I double checked that one); 4, 185; 6, 168; 8, 170. Not great, proceeded to do leakdown test. Numbers listed are percent leak: no.1, 75; 3, 90; 5, 80; 7, 95; 2, 80; 4, 85; 6, 85; 8, 95. Scary bad. The leak in all cases could be heard through the dipstick, not exhaust or intake. I just bought this leakdown gauge, it's the cheap, one gauge type (already regretting that). The question is, are these leakdown numbers possible with those compression numbers? The car still runs, doesn't smoke unless I get on it hard (then it puffs gray) although I'm trying to track down a miss and general lack of performance (not vacuum, not spark, not fuel, prior to the leakdown results I was thinking burnt valve or flat cam). Any opinions.

[ April 25, 2002, 11:42 AM: Message edited by: ron cozzo ]

[ron cozzo]

Stock 67 GTO YS 400, 99K mi, apparently never rebuilt. Did cranking compression check today, results follow: no.1, 170psi; 3, 178; 5, 178; 7, 165; 2, 208(yes, I double checked that one); 4, 185; 6, 168; 8, 170. Not great, proceeded to do leakdown test. Numbers listed are percent leak: no.1, 75; 3, 90; 5, 80; 7, 95; 2, 80; 4, 85; 6, 85; 8, 95. Scary bad. The leak in all cases could be heard through the dipstick, not exhaust or intake. I just bought this leakdown gauge, it's the cheap, one gauge type (already regretting that). The question is, are these leakdown numbers possible with those compression numbers? The car still runs, doesn't smoke unless I get on it hard (then it puffs gray) although I'm trying to track down a miss and general lack of performance (not vacuum, not spark, not fuel, prior to the leakdown results I was thinking burnt valve or flat cam). Any opinions.

[ April 25, 2002, 11:42 AM: Message edited by: ron cozzo ]

[stu]

Smoking on accel and a miss plus the unequal comp. readings sure looks like worn bores/rings. What do the plugs look like? I have found them to be a great indicator of cylinder condition. I would suspect one or more is fouling.

[Half-Inch Stud]

NORMAL.

The ring gap will cause the dominant noise to be from the dipstick tube, or within the valley area if exposed.

Do not be alarmed. Sounds like a valvejob is all you might need.

Want a shot in the arm????
Redo the compression test while all the plugs are out, AND lock the carburator's plates to Wide Open Throttle. Will get better numbers.

Need another shot??
Redo the leakdown test immediately aftr driving the car, with motor warmed and crank oil all over those 2nd & Oil rings, walls and skirts. Will get better numbers.

I do not bother with Leak down tests anymore. Total-Seal in one moter and Gaps in the other motor. Simply not a diagnostic for me.

H.I. Stud

[ron cozzo]

stu, all of the plugs are kinda white, with orangish spots. Never saw that before, I run 110 leaded fuel, mixed 50/50 with 93 unleaded. I was guessing it was lead deposits. Not sure.

[ron cozzo]

H.I. Stud

Thanks for the reply, I did the compression check while the engine was warm, and with the throttle and choke wide open. All the plugs were in except the one being tested.

The engine had pretty much cooled off by the time I got to the leakdown tests. Those were with all plugs out and the throttle and choke wide open.

I'll try redoing both tests warm with a little oil in the cylinders to see what I get, maybe if the cylinders will seal up with a little oil I'll be able to see if I can find valve leaks.

If not, the next thing I was going to check was cam lift with a dial indicator on the pushrod ends.

[PONTIAC DUDE]

N/M

[ May 03, 2002, 12:28 PM: Message edited by: PONTIAC DUDE ]

[Half-Inch Stud]

P-Dude..yea, forgot about the worn rings versus valveseats&guides test...thanks.

Reason I forgot; because I have convinced myself long ago that rings only get better with time..unless something breaks. I have a little theory that an old stock PONTIAC motor could be rebuilt for all aspects of high performance except for bore/hone work, reusing the old top 2 rings, (new oil control ring), and result with an excellent performance.

FWIW: My opinion of cylinder&head health is to work within 10% rather than 10Lbs...then again I'm kinda cheap that way. My compression test shows 265-300+PSI per cylinder...after 12 years. I'm not gonna sweat the >35PSI difference across 8 cylinders. I suggest nobody sweat that kind of difference. The readings get to be luck of the draw (3 gauge bumps)...3 complete draws.

OF interest to me is to catch the first bump number (150PSI), maybe the second(220PSI), and the final bump (265+PSI)to know the PSI jumps are consistent.

Those COMP numbers are 455+.06", 48heads, TRW Flats L2359F+.06, and the Lunati 320 cam with a questionable Valve lash.

For that last matter, I suggest the valvelash uniformity is a big factor in the Compression Test spread of a healthy motor.

H.I. Stud

[Lady Dude]

1/2", here a tip for ya.
Ya do know that all motors only have one compression ring, right?

[zedo]

Ron, use your own good common sense. You're on to something, go with your gut instincts here...

Cylinder pressure that gets past the rings into the crankcase, can't be used to push the piston down= blowby and lost power.

Your readings are indeed "normal", for a very worn out motor that is. It'll run, yes, just don't expect optimum performance out of it.

Those cars you see at Pontiac events running 8's and 9's ?? They don't leak like that. Try 225 psi and 1-2% on all cylinders. You make the call.

Another example, my old 455+.030", after 30,000 street miles, 9.5 CR. 175 cranking psi, 4.5% leakdown, all 8 cylinders. Of course, these were done with a TORQUE PLATE. Which begs the question, was your motor done using a torque plate ??

Do yourself a favor, bore/hone with a plate, new pistons, rings, have fun. Enjoy less that 5% leakdown for a long, long time...

[ April 27, 2002, 07:52 AM: Message edited by: zedo ]

[LenCaverly]

On the cylinder that had high compression I would look for a worn camshaft lobe,going to run compression test on my 458 today will post results and again after I advance the cam.
Len

[ron cozzo]

Thanks all of you very much for your replies. I've decided to go ahead and rebuild it. Based on the mileage and that's its numbers matching, I decided its time to ante up. Hopefully I'll have the pieces in the machine shop by Wednesday. And a lot more peace of mind for the road trips I've planned this summer.

[ April 27, 2002, 08:53 PM: Message edited by: ron cozzo ]

[LenCaverly]

Ran my compresion test today all cylinders between 210-220 didn,t really expect them to be over 200 with the cam I run but extremely happy as its run on street with 92-94 octane fuel at 36 deg total with no detonation problems.Am going to advance this cam 4 degrees to get a little more out of the bottom end but suspect that I could run into detonation.Will post results after trial.
Len

[Florian]

Try a detonation sweep under load. When audible detonation detected,retart 6 degrees.This is safe zone with out detonation protection.Good luck.

[Skip Fix]

Best way to detect bad guides is a leakdown, can even her it back out the intake or exhaust. My original 78 400 had 4 bad guides after 70,000 with normal maintenance oil changes.