compression - Pontiac GTO Forum
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post #1 of 34 (permalink) Old 08-15-2016, 06:06 PM Thread Starter
 
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compression

Wondering what compression i should be looking for in a 400 with a 68cam. i will be taking cold readings as the carb has been sent off for rebuild. any info will be greatly appreciated.
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post #2 of 34 (permalink) Old 08-15-2016, 08:46 PM Thread Starter
 
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Readings

1-150
2-150
3-160
4-180
5-150
6-150
7-155
8-60

Looks like #8 is bad. 3and 4 slightly higher.
any comments or observation? need all the help I can get.
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post #3 of 34 (permalink) Old 08-15-2016, 09:01 PM
 
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Originally Posted by deanhickey View Post
Wondering what compression i should be looking for in a 400 with a 68cam. i will be taking cold readings as the carb has been sent off for rebuild. any info will be greatly appreciated.
From the 1968 Service Manual.

Compression test needs to be done warm engine and all spark plugs removed. Spin engine over through 5 compression cycles before doing test.

8.2 - 9.2 compression engines - regular fuel. 150-170 PSI @ 155-165 RPM's
10.5 - 10.75 compression engines - premium fuel. 185-210 PSI @ 155-165 RPM's

Note: Low compression pressures on HO 400, HO 428, & Ram Air engines are not a valid indication of engine condition. Due to the long valve overlap period with camshafts used in these engines, compression readings at cranking speeds as low as 120 PSI are considered normal.

No cylinder should be less than 80% of the highest cylinder reading.

Number 8 does look bad.

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post #4 of 34 (permalink) Old 08-15-2016, 09:06 PM Thread Starter
 
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Thanks Jim, Just sent the Carb out to Cliff to be redone I guess i will be pulling the engine now, I hope its not the block.
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post #5 of 34 (permalink) Old 08-15-2016, 09:14 PM
 
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Thanks Jim, Just sent the Carb out to Cliff to be redone I guess i will be pulling the engine now, I hope its not the block.


Na, don't get so negative. Probably just worn out rings or even a bad valve. Good excuse to freshen up the engine and get it running like new again. Even IF you had a bad cylinder, they can be sleeved so it can be saved. Stay positive.
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post #6 of 34 (permalink) Old 08-15-2016, 09:18 PM Thread Starter
 
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Thanks, I needed some positive news. having it refreshed is on the list, it just moved to the top.
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post #7 of 34 (permalink) Old 08-15-2016, 09:43 PM
 
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Thanks, I needed some positive news. having it refreshed is on the list, it just moved to the top.
You did not say how many miles on the engine or why you decided to do a compression check.

Was it burning oil or smoking out a tail pipe? Running poorly/missing?

You may want to also pull a valve cover to make sure the rockers have not loosened up on that bad cylinder. I am thinking if a valve is not opening fully, your readings may be affected. Once purchased a '68 GTO that the guy thought the engine was bad. It ran, but poorly. Pulled a valve cover and one of the rocker arms has loosened up and gone sideways off the valve. No damage, just put it back on and tightened it up and that car would catch rubber on a hard 1st to 2nd shift of the TH-400. Next time I saw him I told him about it and I could see the regret in his eyes 'cause he thought he was selling a car with a bad engine - cheap.

So, you may want to check a few things just to be sure, could even be a worn cam not opening the valves fully. Otherwise, you need to do the rebuild.
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post #8 of 34 (permalink) Old 08-15-2016, 09:47 PM Thread Starter
 
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She has been running good, getting second gear rubber on Saturday. Just had the time to do a test on her after i pulled the carb off. exhaust has been clean only smoke was from the tires.
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post #9 of 34 (permalink) Old 08-15-2016, 10:06 PM Thread Starter
 
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the milage is a bit of a mystery i would guess around 150,000. I will pull valve cover tomorrow and take a look and see what ti find. Have started to look for a good machine shop in Maine. I am actually looking forward to getting the running gear back up to spec. I will have to make my mid up on weather to put dished pistons in the engine so it will run on pump gas. Have you done this? How big of a reduction of power would there be?
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post #10 of 34 (permalink) Old 08-15-2016, 10:34 PM
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Originally Posted by deanhickey View Post
the milage is a bit of a mystery i would guess around 150,000. I will pull valve cover tomorrow and take a look and see what ti find. Have started to look for a good machine shop in Maine. I am actually looking forward to getting the running gear back up to spec. I will have to make my mid up on weather to put dished pistons in the engine so it will run on pump gas. Have you done this? How big of a reduction of power would there be?
I just did this, dished forged pistons and forged rods 30 over on the bore.
Compression ratio is 9.5-1 and the goat runs like a scalded ape on pump gas.
The old 335 HP was tired and now it is around 370 with the cam.
It will smoke the tire going into second with the TH-400 no problem in fact I really have to baby it so it doesn't chirp on the shift.
My experience with different engines is if you just do the top end it puts pressure on the old tired bottom end and you end up doing the whole engine anyway.
Go big or go home.
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