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Discussion Starter · #1 ·
did a compression test on all the cylinders.. I just posted because I thought some of you who have superchagers might interested.. Each cylinder read 155 psi and each was within 2-3 lbs of eachother... Is 155 lbs good?

Gerald
 

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The pressure depends on the compression ratio and other things and I assume your static compression was lowered for boost.

The main thing is consistency and to be within 2-3lb after 30,000 miles of 7-800rwhp is freak'n amazing!!! It don't get more reliable than that folks!
 

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no, give the car to me you could hurt yourself. Please this is for YOUR safety.
 

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Stock is 155 - 170 psi per the service manual. That would be for the stock compression ratio of 9.6. I'm assuming you are probably running something lower than that with your big SC which would lower your psi reading, so your number seems like it is probably pretty reasonable to me. Perhaps someone with a similar setup has some numbers to compare against. Usually variation from cylinder to cylinder is the main thing to look for and your 2-3PSI sounds very tight to me.
 

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Discussion Starter · #5 ·
cool!!! I had my plugs out because I was getting a stumble and upon looking at them, they were kind of dirty and glazed. These NGK's had about 5k miles on them. When all banks were empty I borrowed my neighboors compression tester 'fer kicks... New plugs in and feels like a new motor..!
(must make note, change plugs more often)

Thanks for the info. The bottom end is stock with the heads having some minor work and yes the compression has been lowered a little.


Gerald
 

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Jerome said:
Gerald did you make sure to hold the throttle wide open when performing the test? Some people forget or just don't know. Honest question....
Jerome,
What's the purpose of holding the throttle plates open? In case there is pressure loss around the intake valve or something?
 

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KenH said:
Jerome said:
Gerald did you make sure to hold the throttle wide open when performing the test? Some people forget or just don't know. Honest question....
Jerome,
What's the purpose of holding the throttle plates open? In case there is pressure loss around the intake valve or something?
So the throttle plate doesnt restrict airflow to the cylinders giving you lower pressure readings.
 

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FAST? said:
so if you do not hold the throttle open and you get the same reading on all cylinders everything is ok?
I would not say all is ok. It will give you a false, low reading.

You need the throttle plates open to allow unrestricted air to enter the cylinders. Doing the test with the plate/s closed just shows what you can pull past them, not giving an accurate reading of true flow and compression.

Compression test is just the most basic info. It can be done dry, or with a little oil added to each cylinder to give additional info. A leakdown test can give you lots more valuable info.



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Makes a lot of sense in hindsight. I didn't even think about having to pull the air past the closed throttle plates. I haven't done a compression check since the carburator days and I don't remember opening the throttle plates on the carbs for this test. Either the memory is going, or I was doing it wrong all along. :doh:

--- Ken
 

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Jerome said:
Gerald did you make sure to hold the throttle wide open when performing the test? Some people forget or just don't know. Honest question....

If you can do a leakdown test, you will learn a lot more with one.
I'll agree with that.
 

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Discussion Starter · #13 ·
I'll do a compression text next. I noticed that when I held the throttle down or closed it read about 155 lbs instead of about 150 lbs...


Is a leak down test harder? remove all the plugs again? damn, that'll have to wait for my next change of plugs..
 

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Gerald said:
I'll do a compression text next. I noticed that when I held the throttle down or closed it read about 155 lbs instead of about 150 lbs...


Is a leak down test harder? remove all the plugs again? damn, that'll have to wait for my next change of plugs..
Yes it is. :nod: And you get to do it 10 times no less. If you are not sure just have someone who knows how do it. It would take me days to explain it all here. I type with two fingers and I don't have that much time on my hands. :rofl:

Maybe have Doug give her the old once over. I don't know how far he is from you, only been to Fla. once and that was as a kid. Hell I even get directions in my home state from guys who tell me where to turn based on where the nudie bars are located at. :lolcry: I get lost easy.....



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Gerald said:
did a compression test on all the cylinders.. I just posted because I thought some of you who have superchagers might interested.. Each cylinder read 155 psi and each was within 2-3 lbs of eachother... Is 155 lbs good?

Gerald
My stock motored '96 with 25k averages 176 psi on a warm motor with the throttles open. Standard Deviation is about 3.2 psi.
 
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