Compression/ Leak Down

[Javajoe]

So I am in the market for a Magic Deck and when I find one I want to get it checked out but someone may already have done a check on it so I am wondering what are safe numbers I should be looking for regarding Leak Down and Compression. Are there any items related to Magic’s I should be aware of as weak links that I should look out for?

 

[Shlbyntro]

Different engines will have different compression and numbers can also be swayed by battery and starter health.

Generally when looking at cylinders in a compression test:
Within 10% of eachother is good
Within 15% is acceptable
Within 20% you are pushing the lines of healthy

In a leak down test, you want to see less than 20% leakage to be considered healthy, less than 10% leakage is excellent.


Every mechanic will have a slightly different answer but this is my personal rule book and has done me and my customers well for years. These numbers apply to your standard production engines. Once I get above 500hp, I pretty much cut those numbers in half for my acceptable limits.

 

[Javajoe]

Thanks for the info

 

[Lavey 29]

both test in conjuction are good tests but neither one is 100 percent full proof. example. I have seen a broken valve spring show great leak down also a broken roller rocker the same great leak down results. Compression tests I have seen scorn piston skirts and or cyc walls with great compression numbers and so on for a small jist of examples.

Next both tests basically are outdated tests especially in the current auto world. In cylinder pressure transducer testing is the best way for testing. Also I like running compression testing and vacuum testing with a vacuum gauge. At idle rpms you will have lower vacuum readings on MPI engines because of the IAC operation from the old standard set in stone readings of 17 to 21 readings. Current reading as low as 14 could still mean a good sealed engine with an MPI / IAC set up. Just saying