Guest occrj Posted August 18, 2013 Share Posted August 18, 2013 Hi all,I finally got around to doing a compression test on the DB's engine today.The engine was cold (not ideal granted), but with the throttle pedal down while turning the engine over, the four readings averaged 55psi, +/- 3psi. So they're consistent, but on the low side. What readings should I be looking to see on an engine of this type? I haven't tried a drop of oil down the bores and re-testing, to see if it makes a difference as yet. My hunch is that it would benefit from a de-coke and valve grind at the very least.thanks, RJ Link to comment Share on other sites More sharing options...
Guest 1930 Posted August 18, 2013 Share Posted August 18, 2013 Not positive about the early 4s but 90-120 pounds ideally is where you would want to be. I would say that the early 4s should have the same readings Link to comment Share on other sites More sharing options...
robert b Posted August 18, 2013 Share Posted August 18, 2013 The Dodge Brothers Mechanics Instruction Manual page 187 states: Compression 50 to 55 lbs at cranking speed, 65 lbs at 470 rpm. Link to comment Share on other sites More sharing options...
Guest occrj Posted August 19, 2013 Share Posted August 19, 2013 Very interesting, thanks Robert, I think my (incomplete) instruction book finishes at about page 170 So maybe my engine's figures aren't too bad then, mmmmmmmRJ Link to comment Share on other sites More sharing options...
robert b Posted August 19, 2013 Share Posted August 19, 2013 Very interesting, thanks Robert, I think my (incomplete) instruction book finishes at about page 170 So maybe my engine's figures aren't too bad then, mmmmmmmRJRJ read also pages 103 and 104 ,gives a bit more on compression. bob Link to comment Share on other sites More sharing options...
32bizcoupe Posted August 19, 2013 Share Posted August 19, 2013 My 32 DL 6 was 56-58 and that is on the high side. This is because the engine was just rebuilt prior to the car being stored. I still have a hand written 6 page letter from the mechanic ( late 1970's) that says to run the car for 45 min at high idle because its still real tight. 120 is way high! Unless its a newer ohv motor or a 1920 futuristic hot rod! Link to comment Share on other sites More sharing options...
R.White Posted August 19, 2013 Share Posted August 19, 2013 (edited) My compressions are all 52 psi (cranking speed) which is correct for this engine. You have equal compressions so that is O.K. It may sound low but it is right for your engine. You could try skimming the head if you want but remember there is no oil pressure at the crankshaft big end or main journals. Ray. Edited August 19, 2013 by R.White (see edit history) Link to comment Share on other sites More sharing options...
MikeC5 Posted August 20, 2013 Share Posted August 20, 2013 That's what mine reads and what the manual says... Like they say in music, if it ain't baroque... Link to comment Share on other sites More sharing options...
R.White Posted August 20, 2013 Share Posted August 20, 2013 Like they say in music, if it ain't baroque...Sorry Mike, you've lost me....? Link to comment Share on other sites More sharing options...
robert b Posted August 20, 2013 Share Posted August 20, 2013 Sorry Mike, you've lost me....?Ray I read Mikes comment as "if it ant broke don't fix it " good humour . Bob Link to comment Share on other sites More sharing options...
MikeC5 Posted August 20, 2013 Share Posted August 20, 2013 Bob's got it. Just a little play on words... Link to comment Share on other sites More sharing options...
R.White Posted August 20, 2013 Share Posted August 20, 2013 Thanks guys. I would go along with that. These "spit and hope" engines are indeed of their time. Hindsight can be a wonderful thing! Ray. Link to comment Share on other sites More sharing options...
Guest occrj Posted August 20, 2013 Share Posted August 20, 2013 Glad to hear that mine's about right, time perhaps to look at tappet adjustment as I've read elsewhere that incorrect gaps can cause power to tail off once the engine is up to temp.thanks everyone for the input, RJ Link to comment Share on other sites More sharing options...
R.White Posted August 20, 2013 Share Posted August 20, 2013 When I first checked my tappet clearances there was no clearance AT ALL!! They recommend you adjust the valve clearances with the engine hot. My argument against this is that it is such a time consuming process that the temperature will have fallen considerably by the time you have finished. Another recommendation is that you do it with the engine running. I have never found this to be a very pleasant experience. I have always done mine cold and I suggest 0.010 for inlet and 0.012 for exhaust. Obviously you do not want the tappets to clatter and too big a clearance will also lead to inefficient running but on the other hand, too tight an exhaust clearance can lead to burning of the exhaust valves and a drop off in power because when they get hot, as the valves stretch, they are held open slightly. That's my take on it anyway.Ray. Link to comment Share on other sites More sharing options...
MikeC5 Posted August 20, 2013 Share Posted August 20, 2013 I don't think mine had ever been adjusted over the life of the car... they were in the 2 mm range. I was surprised it would even start like that. Link to comment Share on other sites More sharing options...
cahartley Posted August 21, 2013 Share Posted August 21, 2013 When I first checked my tappet clearances there was no clearance AT ALL!! They recommend you adjust the valve clearances with the engine hot. My argument against this is that it is such a time consuming process that the temperature will have fallen considerably by the time you have finished. Another recommendation is that you do it with the engine running. I have never found this to be a very pleasant experience. I have always done mine cold and I suggest 0.010 for inlet and 0.012 for exhaust.I've always added .002" setting them cold too.Who the heck wants to burn themselves to a crisp adjusting valves....... :mad: Link to comment Share on other sites More sharing options...
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