Dumb question I'm sure - my 328 has compression readings of 175-180 in all cylinders - I was told by my PPI person (who missed other things like cracked fuel lines (since replaced)) that this is a very good reading in a 328... is that so? What is an 'excellent reading' a 'good reading' and a 'poor reading' for compression? Again, sorry I sound so dumb... don't mean to Jedi
Here's a quote from the 308/328 forum. Doug328 is the owner: "since the compression numbers were all great (all within +/- 5% of 175 I felt there was no need for a leak-down test, as the leak -down just confirms the why of a low compression reading." Your range between the numbers is good. 10% is the high end.
Compression test results can be affected by many factors so there's no super clear good vs bad result: 1. the static compression ratio of the engine (which can vary even on engines with the same nominal spec, or can be different for different versions of the same model), 2. how wild the cams are (since the compression measurement is made a very low RPM) 3. the accuracy of the compression gauge 4. the test method (warm vs cold, # of strokes, charge on the battery affecting cranking RPM, etc.) The attached article from Tony Palladino gives a good description of ballpark results. Your results seem fine -- don't sweat it Image Unavailable, Please Login
Steve's right on, YMMV depending on test conditions. However, those numbers look very good, pretty much identical to my cars cold engine numbers. Most good engines are within 5-7 psi, altho the 10% rule is still a valid rule of thumb. Usually when something's wrong you'll see one or 2 cylinders that are off by 20 PSI or so, or else all of them are down around 120 -130 & a leakdown test reveals that air is blowing past the rings, or thru an intake or exhaust valve.
I beg to disagree: Compression is meaningless. LEAKDOWN is the tell all... "Perfect compression test" results easily can result in a failed smog test... FYI: a 328 with a 26% leak down past only 1 of the exhaust valves on 1 bank will give a septum exhaust reading of 1200ppm...a fail in ANY of the lower 48 Yet, the compression will read nearly "perfect" I've seen it at least 12 to 15 times when an engine overhaul man at a franchised dealer. All using the 328 as a specific sub species as an example. The more the cylinders, the more the valves, the greater the dilution....
BTW and no intention to hijack this thread; but I really regret, that the Shop Talk in Forza doesn't exist any more. These were the most interesting pages for me. Anyway; I still have a subscription. Best Regards from Germany Martin
Same here - I wish Tony P. (and all of the F professionals) would post more on FChat. I had the good fortune of living in Redwood City when Tony had his own shop there, and used him as my F part supplier (before this internet thing got going ). Only later did I realize what a great opportunity it was to be able to look at the other F models in various states of disassembly and be able to talk with Tony about them and their technical issues/quirks.
