Analyze my compression test - Team Camaro Tech
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post #1 of 11 (permalink) Old Jan 17th, 01, 04:25 PM Thread Starter
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Dave
 
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I went ahead and got a set of compression readings from my 396 325hp engine. It has been real sluggish and just doesn't pull like it should.

I replaced all the plugs recently. #3 & #2 plugs had a lot of carbon on them. The other plugs seemed okay at the tip.

Here is what I got;

1 124 2 115
3 128 4 134
5 120 6 138
7 120 8 125

What can you guys say about those readings?

Thanks.

Dave
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post #2 of 11 (permalink) Old Jan 18th, 01, 02:32 AM
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Not knowing exactly what a 325hp 396 is supposed to pull, it looks to me like your readings are too low...I would think something closer to 150-160 pounds would be right on. However, the important thing is that all the readings are within 10 pounds of each other. Does the engine have a stock cam in it? It isnt unusual for a stock motor with a bigger cam in it to make lower numbers. If the engine is bone stock, it sounds like it might be time for rings and a valve job. Your best bet would be to do a leakdown test on the engine.

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post #3 of 11 (permalink) Old Jan 18th, 01, 03:45 AM
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Did you remove all the plugs and have the carb at WOT? That would affect the readings somewhat.

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #4 of 11 (permalink) Old Jan 18th, 01, 12:40 PM Thread Starter
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Dave
 
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I'm supposed to have ALL the plugs out when I do this???? I pulled out a plug at a time.

No the throttle was at the idle position. When I bought the car, I was told the engine was rebuilt about 3000 miles ago. I suspect that IF indeed it was rebuilt that it was built very mild. I was told the engine had a valve job and a cam that is one up from stock. Whatever that means. Never could get the paperwork on it though.

I basically have an engine that is an unknown but I am trying to see what I can do with it.

Dave

[This message has been edited by Scooby Doo (edited 01-18-2001).]
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post #5 of 11 (permalink) Old Jan 18th, 01, 01:12 PM
Patrick
 
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Pipeman is right. You should pull all plugs and then WOT before you do a measurement. You can check a Chilton or other manual to get an idea of what the compression should be. If they vary more than 10-15 lbs you may need to check valves or rings.

Gutz
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post #6 of 11 (permalink) Old Jan 18th, 01, 04:15 PM Thread Starter
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Dave
 
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I followed your advice on the compression test with all the plugs out and the throttle open and I did get higher numbers. Here are my new numbers:

#1 138 #2 127
#3 137 #4 134
#5 129 #6 145
#7 130 #8 144

A little better compression this time. Comments?

Dave
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post #7 of 11 (permalink) Old Jan 19th, 01, 03:40 AM
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Still seems low for the 396, but maybe ok with a cam with a bunch of overlap, not sure. anyone? Bill?

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #8 of 11 (permalink) Old Jan 21st, 01, 04:16 AM Thread Starter
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Dave
 
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That's what I was thinking. For a big block with a 10.25:1 compression ratio, I should be getting numbers higher than that.

Could this be that I just have low compression pistons in the engine or does this lie in the heads?

Dave
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post #9 of 11 (permalink) Old Jan 21st, 01, 05:07 AM
 
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it looks like you might have a leaking head gasket between #5 and#7 and #2 has also got a problem. I would get a leakdown tester to check the cylinders. they should be about 160 to 180 somewhere in there may also have blowby the valves. can you hear any hissing through the carb when you are starting?
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post #10 of 11 (permalink) Old Jan 24th, 01, 01:56 PM Thread Starter
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So...... what kind of compressions are all the other 396 owners getting?

Dave
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post #11 of 11 (permalink) Old Jan 24th, 01, 06:21 PM
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I just bought my 69. It has a 68 396 325hp in it (at least that's what the engine block ID says it is). It has been modified to ?. I checked the compression (warm) and I had 155-165 across the board. I tested with the throttle closed. I have not checked it cold; my thinking was to check it in its normal operating mode.

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