Black Cat Posted August 31, 2022 Report Share Posted August 31, 2022 Thought I would undertake a compression test on my 1500 1977 Spitfire, the results were as follows: Cylinder 1 104 psi Cylinder 2 110 psi Cylinder 3 103 psi Cylinder 4 105 psi I used a gauge which I brought from eBay so it might not be calibrated correctly but all four readings are well within 10% of each other so on that front I’m OK, my concern is they seem a little low, was expecting 120-150. The engine has been complete striped down and rebuilt and has done under 1000 miles since the rebuild. Should I be concerned? Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 31, 2022 Report Share Posted August 31, 2022 was this tested with all plugs out and throttles open ???? Pete Link to comment Share on other sites More sharing options...
NonMember Posted August 31, 2022 Report Share Posted August 31, 2022 Compression readings are sensitive to your technique as well as the completely uncalibrated eBay gauge. The absolute values are essentially meaningless, it's only the "being all about the same" that matters, especially if the gauge is new to you. You could repeat the test with a drop of oil down the bores, to see how much difference that makes (it shouldn't be much). Does the engine run OK? Was there a reason for doing a compression test, other than it was "a thing to do"? Link to comment Share on other sites More sharing options...
johny Posted August 31, 2022 Report Share Posted August 31, 2022 Yes I dont think technique is going to explain those readings so its either the gauge or all four cylinders have a problem. The rings might not have bedded in? so a repeat test with oil will be very interesting... Link to comment Share on other sites More sharing options...
Black Cat Posted August 31, 2022 Author Report Share Posted August 31, 2022 All plugs were out when the test was undertaken and throttle was open, the engine was warm and when the engine was rebuilt the bores seemed OK and new ring were added. Will try again with a little oil in each cylinder and see what readings I get then. Link to comment Share on other sites More sharing options...
dannyb Posted August 31, 2022 Report Share Posted August 31, 2022 We're the bores honed before new rings fitted. Danny Link to comment Share on other sites More sharing options...
Black Cat Posted August 31, 2022 Author Report Share Posted August 31, 2022 The bores were supposed to have been honed before the new rings were fitted. Now completed the wet test First Cylinder Pressure test (Dry) Cylinder 1 104 psi Cylinder 2 110 psi Cylinder 3 103 psi Cylinder 4 105 psi Second Cylinder Pressure test (Wet) Cylinder 1 115 psi Cylinder 2 114 psi Cylinder 3 110 psi Cylinder 4 115psi Slight improovement, oviously a slight leak in each cylinder. The engine runs and sounds OK so unless somebody has any other ideas I'm going to leave it as it is for now. May get a friend who has a garage to check it with his pressure gauge. Link to comment Share on other sites More sharing options...
NonMember Posted August 31, 2022 Report Share Posted August 31, 2022 That's a pretty small change for the wet test, so I don't think it indicates anything amiss. Looks like your gauge is way out of calibration, so just drive the car and have fun. Link to comment Share on other sites More sharing options...
johny Posted August 31, 2022 Report Share Posted August 31, 2022 I take it the valves were lapped while the head was off? So its either the gauge or something strange has happened with a replacement cylinder head or low compression pistons etc Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 31, 2022 Report Share Posted August 31, 2022 with nice even figures i agree with Rob ...drive the smile sleep easy so many things can affect the readings from the gauge to the state of the battery but even results indicate ....very little is wrong Pete Link to comment Share on other sites More sharing options...
Black Cat Posted August 31, 2022 Author Report Share Posted August 31, 2022 Thank you gent's, will sleep easy tonight. Link to comment Share on other sites More sharing options...
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