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Author Topic: 1969 z28 dz 302 compression results low?  (Read 1851 times)
myty
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« on: November 17, 2011, 10:33:17 AM »

Ok guys, has anyone done a compression test on a dz302?  Mine average 125 to 130 after 3 turns of the motor and 150 to 155 after maxing out at 7 turns.  Is this in the ball park or a little low?  All plugs removed, throttle open, 11 to 1 pistons, comp 346 cam, rebuilt w new rings and broken in. Jerry, you there?  Thank you, Gary 
 
 
 
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myty
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« Reply #1 on: November 18, 2011, 10:08:29 PM »

 Oh yea, and the leak down test was good. Any one?
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JKZ27
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« Reply #2 on: November 19, 2011, 10:09:00 AM »

Compression specification for the 302 in the service manual shows 190psi with a maximum variation of 20psi between clyinders. Leak down was 10% or less? does it run poorly or lack power? If leak down test is ok, I would suspect cam timing or valve overlap and verify actual compression ratio (piston dome, 64cc chamber and head gasket thickness). I don't own a 302, just trying to help.

John
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69 RS/SS Cortez Silver, L48 MC1
68 RS Ash/Ivy Gold 327EFI M20
79 Z28 Brown LM1 M21
04 SRT 10
68Zproject
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« Reply #3 on: November 19, 2011, 12:20:14 PM »

I'm not a mechanic by any means, just a parts changer.  But I remember long ago that when you did a compression test and it came out low you were supposed to squirt a little oil in each spark plug hole and then see if it came up.  If it did, the rings were bad.  In your case it might mean it's not broken in yet.
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68Z28
myty
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« Reply #4 on: November 20, 2011, 08:39:46 AM »

 I have seen that 190 psi figure too but it just seem s that a short stroker with a huge duration cam and lots of "bleed off" could develop such high Numbers. The only thing that is not stock are the pro magnum rockers. I guess I'll try some stock rockers next. Hasn't someone out there ever done a compression test on a 302?  Thanks, Gary
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TODD
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« Reply #5 on: November 20, 2011, 09:42:37 AM »

I guess I 'd ask the basic question, why are you performing a compression test? Having rebuilt my 302, I have never checked compression. I've been busy reading plugs and lashing for best tune vacuum.
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myty
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« Reply #6 on: November 20, 2011, 02:12:53 PM »

 Although it may seem unrelated, my z starts hard, runs very rough and spits and sputters for the first several minutes of warm up. Been thru several carbs and now have the  original back on after having it restored with slightly larger primary jets and squirters. After removing the intake we found the intake runners soaked with oil and found where the surfaces mate with the head weren't right (to tight at the bottom as compared with the top edge). Still sucking oil but better as my vacuum is up to a mere 7.5 ".
 Well it seems the manifold has been over worked so I just received another 472 in great shape to try. When I do this I will ck. the head angles to see if someone angle  milled them  like they used to
 Any way, thru the constant pondering we checked the cyl compression and I thought it to be low.
Bottom line... Low vacuum, sucking oil and starts very hard maybe because of oil on the plugs?
This car just finished a frame off resto. We changed the cam out and degreed thinking it was  the vac, issue
  Thanks Gary
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TODD
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« Reply #7 on: November 20, 2011, 02:45:43 PM »

Gary;
 My car had the same issue, intake vacuum leak at idle and sucking oil bottom of runner, had the machinist take some more off the intake runners and rails .
I used the Felpro 1256 intake gasket it was the correct size for my 472 intake runners. I also used the Intake Manifold Gasket Alignment Tool posted over at Camaros.net
and used only silicone at front and rear runners. Leak free and vacuum now is up to 8~9 in hg. at idle. That is with the rockers at .030" lash.   
My idle is a lot better now I have it set at 1050 rpm. When it gets summer again I'll adjust it lower to see if it will hold a lower rpm. Plug check now shows no fouling and a good tune.
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myty
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« Reply #8 on: November 20, 2011, 03:14:55 PM »

 Todd, that is good info on that felpro gasket #. The last 2 gaskets seemed to sit to low on the intake openings which worsened my chances for a good seal with that imperfect intake angle.  Thanks again, Gary. I will let u know!
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TODD
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« Reply #9 on: November 20, 2011, 04:31:41 PM »

Gary;
 I used these for reference:http://www.camaros.net/forums/showthread.php?t=121095&highlight=Intake+Manifold+Gasket+Alignment+Tool
http://home.teleport.com/~cosa/vap/tech/inmanifold.htm
Best posts yet I've found on the subject. Following these provided a good installation with no oil leaks.
For machining the intake (which by the way after all this effort I'd true any original intake before I'd use it) I used the Supertuning Your Z28 Camaro book by John Olham.
Goes as follows; Table 5-2 Cylinder head .005", Side .006", Bottom .009". head .010", side .012", Bottom .017".
My machininst took about Side .012", Bottom .017" and that barely cleaned up the corrosion at the water crossovers, It left enough gap at the rails for a good seal.
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JohnZ
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« Reply #10 on: November 21, 2011, 11:33:27 AM »

Here's the Chevrolet-recommended machining for maintaining intake port and bolt hole alignment when heads are milled.  Smiley
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'69 Z/28
Fathom Green
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tmodel66
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« Reply #11 on: November 21, 2011, 01:37:22 PM »

John, Is this chart for the aluminum intakes only or does it also apply to Iron intakes?
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Daniel  
'69 SS 350/4 speed  Fathom Green--POP
JohnZ
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« Reply #12 on: November 22, 2011, 11:59:54 AM »

John, Is this chart for the aluminum intakes only or does it also apply to Iron intakes?

That's for any intake - iron or aluminum.
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'69 Z/28
Fathom Green
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