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Well, after having about 30k supercharged miles on my car, I just

992 Views 15 Replies 10 Participants Last post by  GTS Dean
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?

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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.
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....
What's the purpose of holding the throttle plates open? In case there is pressure loss around the intake valve or something?
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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