chris240 Posted March 24, 2009 Posted March 24, 2009 guys , Ive got an L28 (which was/is a runner) up on my engine stand..I want to do some sort of compression test on it. I spoke to a friend yesterday (retired mechanic) and he came up with this... firstly gut the inards of a sparkplug and carefully tap or weld a tyre valve onto the end of the plug, then you get a hold of a good quality tyre inflater with a PSI guage, hook it upto the valve , connect to your air compressor , next ensure cylinder is at TDC , lock off the crank pulley, and give it a blast to 300-ish PSI ,taking note on how long and the speed it takes the reading to drop (if it drops at all)...repeat for all cylinders, and cross reference the results.. anyone heard of this approach or is there another method...appart from hooking up a battery to the starter (I dont feel confident with this last one..) Quote
Zeddophile Posted March 24, 2009 Posted March 24, 2009 That's a leakdown test, not technically a compression test. However, for an idea of the condition of a motor you know bugger all about, its a pretty good method, as it tests the seal of all the valves and rings, whereas a compression tester simply records the highest pressure seen in the cylinder. Doesn't mean the cylinder is holding compression well, just means that it managed to reach that pressure before it all leaked out. The other problem with traditional compression tests is consistency in results - the battery gets lower as you work through the cylinders, so when you get to the end, the engine doesn't spin as fast, which can affect the readings. 300psi sounds a bit extreme though, you'd want to make sure your fabrication skills are good with the adaptor (actually, you may be able to buy one through normal tool suppliers?). You don't want bits of metal flying off with nearly half a litre of compressed air behind them. A standard L series engine is unlikely to exceed 150psi on a comp test I'd imagine, with flattops it'd probably just go over 150. 300psi is approximately what you'd expect on a 20:1 CR engine like a diesel. So in reality testing at 200psi would likely give just as good an indication - especially since you are testing a cold engine, meaning that piston/ring clearances are much bigger than they would be once at operating temp. Quote
chris240 Posted March 24, 2009 Author Posted March 24, 2009 I was going off memory for the PSI !! your right , 150-200 max. Quote
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