High Compression Head / Compression Test Questions

[xonix_digital]

Hi all,

 

So I I either have some compression issues or some issues understanding compression.

 

I have been troubleshooting my DCOE Weber setup and in doing so just tested my compression on all cylinders with the following results:

 

1: 140

2: 130

3: 130

4: 130

5: 126

6: 128

 

This is with the carbs at WOT or really close to it. I also double checked numbers with a second pass.

 

These numbers seem low compared to some that I have seen posted recently.

Should I be concerned about these numbers?

 

My second question is related to my choice in cylinder head. I have the Maxima N47 head on an N42 L28 block with the stock pistons (75 280z) I think they are 10cc.

 

Now supposedly this head on the L28 creates "Higher Compression".

Shouldn't this be reflected in my pressure test results? Shouldn't the numbers be higher?

 

Thanks guys.

Edited August 13, 2013 by xonix_digital

[Reverend]

As far as i know, you should have higher compression. I've also heard stories about faulty pressure gauges (el cheapos), maybe you should try another and verify results that way? 

 

You could also enlighten us the method you measure your compression, plugs off/on, how many times you let it crank etc. All of those affect the final result.

[Mack]

has this engine ever ran?   did you rebuild it?  with numbers fairly close but low, maybe check your cam timing>?

[Xnke]

Compression numbers mean little to nothing, as far as 140 vs 180, ect.

 

All a compression test is really good for is finding a bad cylinder in the group. The engine should be at operating temperature, all the plugs out, throttles wide open, when you do the test. The lowest compression cylinder should be within 10% of the highest cylinder. If it isn't, then dose it with a little oil and do it again...that picks out bad rings vs, bad valve lash or poorly sealing valves.

 

Valve lash, pin bore clearances, little differences in stock vs. aftermarket pistons and rings, ect will all vary compression numbers a little.

[johnc]

Engine seems tired from those numbers and how low 5 & 6 are compared to the rest.

[xonix_digital]

The engine was cold and the plugs were in. I let it crank until the gauge stopped increasing in pressure (10 or so revolutions)

The motor runs but it only runs on three cylinders at idle which I believe to be a carburetor problem.

 

I will try again with the engine warm and the spark plugs out.

 

The cam gear is set to the #1 position.

I read last night during my search that setting the gear to the #2 or #3 position will increase compression and should be configured as such with an old timing chain. Is this true? If so, what criteria do you use to decide which number setting to use? 

I did not read anything about this in my L series engine rebuild book.

 

JohnC what do you mean when you say tired?

 

I rebuilt the motor and have roughly 50 miles on it. Just drove it to the ZCon car show just this past week.

[Chris83zxt]

The engine was cold and the plugs were in. I let it crank until the gauge stopped increasing in pressure (10 or so revolutions)

The motor runs but it only runs on three cylinders at idle which I believe to be a carburetor problem.

 

I will try again with the engine warm and the spark plugs out.

 

When I did my first compression test I did mine with a cold-ish engine and other plugs in as well, and my numbers were around 120 on 1-5 if I'm remembering correctly.  When doing it the proper way, the compression tested 145-150 on 1-5, low (115) on 6 due to a ring issue.  Your numbers should look better if testing a warm motor with the plugs out.

 

I also remember reading somewhere that your compression might read lower for the first couple thousand miles on a freshly rebuilt motor.

[Xnke]

With only 50 miles on it, you may not even have the rings seated yet. You don't advance the cam gear to pick up compression...that is to correct cam timing from a stretched timing chain. If you just rebuilt the engine, then you should not have a stretched chain!

 

Set the valve lash cold, then run the engine up to temperature, and set it hot. Then do your compression test. A freshly rebuilt head will need the valve lash checked several times over several thousand miles before it will "break in". Some valve/seat/cam/rocker combinations will take longer than others. Most people say I'm full of shit and they set lash once and never check it again...I usually ask them to pull the valve cover and check it, and it's normally very wrong.

[johnc]

^ what he said...

[xonix_digital]

Great info guys. 

will post my results.

 

Thanks!