eds240z Posted March 3, 2009 Share Posted March 3, 2009 just had my engine rebuilt about 2k ago. i decided to check compression. this is what i got. #1 135psi #2 132psi #3 140psi #4 135psi #5 140psi #6 135psi. shouldn't it be higher? this is a 81 f54 block with flat top pistons, p90 head stock valves polished ports int & exh. msa stage 3 cam, triple webers. eng runs fine. is this normal or something to worry about. oh, bottom end is stock. bores honed and checked, new rings, bearings and balanced. original pistons. Quote Link to comment Share on other sites More sharing options...
Challenger Posted March 3, 2009 Share Posted March 3, 2009 They seem consistent but low. Thats less than what a stock turbo motor should be at. Whats your calculated compression? Quote Link to comment Share on other sites More sharing options...
z-ya Posted March 3, 2009 Share Posted March 3, 2009 The fact that they are within a few psi of each other is a good thing. What the engine hot when you tested the compression? Quote Link to comment Share on other sites More sharing options...
eds240z Posted March 4, 2009 Author Share Posted March 4, 2009 engine was hot when tested and at wot. don't know calculated compression. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 4, 2009 Share Posted March 4, 2009 The cam is what is throwing off the numbers. Big cam, lower compression test #'s. As Pete said, the fact that they're even is what's important. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted March 4, 2009 Share Posted March 4, 2009 Jon is most likely correct. One thing you do when you get low numbers is squirt some oil in the sparkplug holes and re-do the compression test. If the numbers go up then it's ring seal related. If not then since it is fresh I would assume the cam. If not that then you have leaking valves, but that is not likely considering it is a fresh motor and all are pretty close in numbers. Quote Link to comment Share on other sites More sharing options...
eds240z Posted March 4, 2009 Author Share Posted March 4, 2009 i poured oil in the cylinders and compression did not change. so the cam would throw off my compression? if this is normal then i'm just going to disregard the readings. i have the snap on leak down tester and every cylinder stayed in the green on the gauge. i tested with 100 psi. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted March 4, 2009 Share Posted March 4, 2009 Yes. This is why you hear people say that cams "bleed compression". That's not what they do at all, but that's where that comes from. Really what they do is they make the engine breathe more efficiently at a higher rpm. If you could do a compression test at 6000 rpm you'd find that the big cam improves the situation vs the stock one. Quote Link to comment Share on other sites More sharing options...
Daeron Posted March 4, 2009 Share Posted March 4, 2009 did you pull all the spark plugs for the duration of the testing? Just checking. Don't forget that atmospheric pressure will lower compression numbers artificially as well.. I know you are at sea level like me but the point is that compression testing is not a test where "accurate" numbers are the hallmark; it is consistency as has been said already. Quote Link to comment Share on other sites More sharing options...
ckolander Posted March 4, 2009 Share Posted March 4, 2009 I see all kinds of people talking about doing compression testing, my question is, how do you test your compression??? I need to do so on my cars Quote Link to comment Share on other sites More sharing options...
z-ya Posted March 4, 2009 Share Posted March 4, 2009 I see all kinds of people talking about doing compression testing, my question is, how do you test your compression??? I need to do so on my cars That topic has been discussed at great length here and on other sites. Do some searchin'. Search first, ask second. Pete Quote Link to comment Share on other sites More sharing options...
Challenger Posted March 4, 2009 Share Posted March 4, 2009 I see all kinds of people talking about doing compression testing, my question is, how do you test your compression??? I need to do so on my cars Using a compression tester. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 4, 2009 Share Posted March 4, 2009 So stikcing your tuumb over the plug hole and seeing if they all 'fart' at you and calling is 'good enough' is considered wrong nowadays? What have things come to in the automotive world. Next thing you know you'll tell me ebuillent cooling had been supersceeded, and gum rubber tyres have been improved! Quote Link to comment Share on other sites More sharing options...
Challenger Posted March 4, 2009 Share Posted March 4, 2009 When I said compression tester I was referring to your thumb. Measuring the skin disformation you can find the PSI. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 4, 2009 Share Posted March 4, 2009 PSI: "Peeled Skin Index" The way automotive things were tested when men were men and sheep were sheepishly silent... Quote Link to comment Share on other sites More sharing options...
ckolander Posted March 4, 2009 Share Posted March 4, 2009 Stupid me of course you use your thumb, the louder the fart the higher the compression right. If they all sound the same then it's all good. I do know how to use the search function, but at 4 in the morning after an 11 hour shift at work, it's just too much Chuck Quote Link to comment Share on other sites More sharing options...
Daeron Posted March 4, 2009 Share Posted March 4, 2009 Stupid me of course you use your thumb, the louder the fart the higher the compression right. If they all sound the same then it's all good. I do know how to use the search function, but at 4 in the morning after an 11 hour shift at work, it's just too much Chuck No, the volume of the fart is actually a test of the battery and starting system. The compression test is either run based on the frequency that the fart noise makes (IF you have the fancy equipment for that) or, as Tony mentioned, we shadetrees are forced using the Peeled Skin Index http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92720 The short version: Run engine, warm it up, pull all spark plugs. Plug compression checker into each cylinder one at a time, and run the starter for a few moments with your foot flooring the gas pedal. Your assistant will tell you when the reading is no longer increasing, and that is your compression reading for that cylinder. The objective number doesn't mean much; generally speaking, if all the cylinders are around 100 PSI an internal combustion will still run.. but 100 is sorta low. 200 is pretty bloody high, and most motors tend to be around the 150-170 PSI range when tested brand new.... but there are 230964972356459782398697234 different things that can effect the actual numerical readout of the compression test. The IMPORTANT things about a compression check are "high enough?" and above all EVEN. The cylinders should not deviate more than 10% from each other. By that token, take two identical 4-banger engines which SHOULD read say, 170-170-170-170. One reads say, 125-128-130-122, and the other 165-165-140-165. I would bet money that the low PSI motor runs like a top, and MAYBE has lost half a drop of efficiency and power. The higher PSI motor might run... but probably extremely rough, it may overheat.... who knows? Theres my compression test 101. Quote Link to comment Share on other sites More sharing options...
z-ya Posted March 4, 2009 Share Posted March 4, 2009 So stikcing your tuumb over the plug hole and seeing if they all 'fart' at you and calling is 'good enough' is considered wrong nowadays? What have things come to in the automotive world. Next thing you know you'll tell me ebuillent cooling had been supersceeded, and gum rubber tyres have been improved! I think that the fart sound from each cylinder needs to be analyzed in the frequency domain using a spectrum analyzer for consistency across all cylinders. Really, the thought of your thumb being sucked through the plug hole and into the cylinder is not a good one.... Ouch!! Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 4, 2009 Share Posted March 4, 2009 In general, the compression test is how the cylinder pumps up as much as what it pumps up to as a final number. Generally accepted is the 3 to 5 pump rule. The first pump is what is referred to as 'cranking compression' and should always be near 100psi. This is a general number that allows the engines to fire reliably and self-sustain the combusion process. You will notice people at higher altitudes with low compression cars have an issue starting, and they also may have a cranking compression around 65psi! The second-to-fifth pump will determine what condition the rings are in, as well as the valves. If you jump 95/125/155 and then maintain 155 through the next to pumps in the cylinder, this would indicate a generally well balanced performance. If you had the same engine, but went 135,155,155 you may deduce maybe carbon deposits (though likely the number would b e higher, like 175)---without a baseline test to know what it was before, a 'grab sample' of numbers is useless. This is the reason the compression test should be performed every 30K miles when the valves are adjusted. Do one before and one after and see how the performance changes. If you haven't done this before, you will be surprised! If you see an engine that goes 95, 120, 135, 145, 155...it's having issues pumping up to pressure. The rings may be a little worn. It should never take more than 3 to 5 pumps to bring the compression to it's highest point. But then you will get guys that use 'held in' gauges, which are useless. Or others with cheap gauges or bad dials you can't really read. Which puts you back on saying the number it'self is about meaningless. It's how the number is reached, and that the differential between them is minimal that is the most important. I have had people 'condemn' and engine because of 'low compression' of 100psi. Car would start reliably, and didn't consume oil, but their 'mechanic' condemned the engine to 'needing an overhaul' after doing his compression test. Thing would bang at 75, 85, 100psi and stay there on the gauge. Put my gauge on it, and it was 110, 145, 175! Remember "Thread in Connections"... (I'm sure that guy justified a hearty engine overhaul business based on his 'trusty old gauge set'...) Now, about this ebuillent cooling and wood alcohol gassification project I've been hearing so much about... Quote Link to comment Share on other sites More sharing options...
ckolander Posted March 4, 2009 Share Posted March 4, 2009 So in knowing that you get what you pay for, would the $100 compression tester from napa be better to have than the $10 harbor freight?(More accurate and easier to use)??? Quote Link to comment Share on other sites More sharing options...
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