Frank Massey’s Top Ten Tests with PicoScope: #9 Compression testing – Howtoshtab – how to, lifehacks, tips and tricks

The next test I have chosen is one with a lot of controversy in my opinion But we are going to go for it Compression testing now I do agree that we have adopted And have adopted now for quite some time dynamic compression testing Now what I mean by dynamic compression testing is Rather than using a compression gauge which has a one way valve Which captures and retains the pressure in the gauge whilst cranking the engine Is no longer appropriate We want to see real time evaluation of compression in engine Whilst the engine is actually running under acceleration and deceleration And also while the valve lifters which are run with hydraulic Have the full oil pressure in them at the right viscosity So it is real time dynamic measurement using the scope With the fabulous little WPS we love it to bits But you know what I mentioned the thing about accuracy speed Process simplicity as accurate and incredible this tool is There are times I would not want to pick it up There are times I would just use a gauge and that is the controversy bit Of some of what I am going to say because I have seen of late Some incredibly gifted technicians use fabulous processes of applying the scope To make measurements which quite frankly You can do in a matter of seconds with a gauge So I do not buy it I am going to be a little controversial I have also seen a lot of compression testing predicting where Bottom dead centre is well you know what I spent 15 years as a professional engine builder that was my first profession Building anything from production to race engines You cannot predict where valve timing is electronically Let me explain why In the past I would agree that with fixed valve timing you could Because the valve timing did not change My CAR Cupra R uses AVS Audi Valvelift where not only does the valve lift change But the angle of opening and closing is also changed Dependant on the duty cycle of the actuator So where is bottom dead centre You actually do not know there are too many parameters now changed You have got variable valve timing on both inlet and exhaust cams on some engines You have got AVS lift change where the base lift and the actual lift of the cam Not only changes in terms of lift by 0.

6 it also changes the angle by 13 degrees Where is bottom dead centre I am going to show you simplistically So we are going to do a bit of compression testing So there is a bit of the controversy from my point of view You know what guys if you really want to know where your valve timing is Take it apart and pin it period that is it for me But I do agree with using the scope to predict that there is a problem So I would like to put my interpretation on compression testing How I would view it without over egging it basically Right WPS first thing we need to select is range It is charged power on I think range 1 should be up to 34 bar I am only expecting 10 or 15 bar on this engine Well within range And lets connect the device We have adapted this device to our personal fittings Everything we measure water air fluid diesel petrol all uses the same fittings So we are adaptive We now need to select timebase and range First of all the device now I am using channel 4 Black is a nice picture it is a nice bold colour Channel 4 input I am going to change it from And I am going to break a habit of a lifetime now And change from voltage which I can actually keep You could use voltage but I am not going to WPS range 1 has been selected – 4 to 34 bar Is now displayed as actual pressure And how long do we want the timebase Lets leave it as it is and see how it goes We could switch every other channel off A off B off C off D channel only Full screen deflection Lets start the engine and see what we have got I think I will shift the scaling up a little bit We are not going to use all of the scaling I am going to increase the throttle I have terminated the test as promptly as possible Because it is incredibly difficult to get to the injectors I have left it delivering fuel to the cylinder for a Few seconds I was running the test I am interested in air pressure And providing we do not suffer bore wash Which we are not going to in that short space of time We are not going to affect pressure Right so here we have real time pressure measurement in the cylinder Lets simplify some of the conclusions As I said you can become awfully over complicated in your assumptions Lets first of all set a baseline that is ground zero so everything below that line is Negative pressure suction I do not like the word suction We live on this planet at 1 bar Outer space is zero and we normally measure pressure Between there and plus 1 bar of atmosphere 2 bar absolute So I like to think of this still as a pressure but it is less than atmospheric So as the piston comes down it is going to create a negative pressure in the cylinder Compression of course is fairly easy to look at positive pressure And we are measuring here 2.

8 bar or there about It does not sound an awful lot is it accurate Well the gauge is accurate so we have to assume in real time That is what we are getting I the cylinder It is even for one The controversy about where bottom dead centre is I will show you guys how easy it is I am not going to use mathematics or any form of complication I am going to measure peak compression 140 ms between the two events So 7 and 7 is 14 I am going to move one to 70 70.61 near enough so that is where bottom dead centre is That is how simple it needs to be Now we can look at the events So we know top dead centre is there we know that is bottom dead centre So this is the point where induction takes place This is obviously the exhaust stroke You can see there is a slight increase in pressure on the exhaust stroke This is where the cylinder is sweeping down where it comes up on compression So this is the induction stroke So how best are we going to interpret this in proving what is going on in the cylinder Well if all the valves were operating correctly they will create the same amount of pressure Does that mean the valve timing is correct No of course it could still be out So how are we going to try and prove valve timing Well the way I might suggest is on over run with a closed throttle In a perfect world we should create absolute zero Though we are not going to do that as that is minus 1 bar We are not going to do that but we should come a good way towards it Lets have a look at the event when we lifted off Because what happens here is a closed throttle at high speed Now because the throttle is closed you cannot pull in the same amount of air So your compression value is going to drop But we do create a hell of a lot more suction So by looking at the negative value That gives us an idea of how efficient suction is And whether there are any valves stuck open Because if any valves were stuck open valve overlap If the valve timing were out or anything were wrong with the valve seats Then we are not going to get good suction properties Now we are producing minus 642 so it is 0.

6 of a bar under atmosphere So that is just one cylinder of course If you did this measurement in a manifold that would improve as per rotor Depending how many cylinders you have got you create more vacuum That is not a bad figure to be honest and they are all even So it suggests there is no untoward leakage of suction on the induction stroke Obviously there is very little compression to speak of And so really that is the most accurate indication of valve performance But it is just a pointer guys I would still want to do real time examination of valve timing because We have got hydraulic actuators we have keyless mechanical systems now Where the sprocket and the cam do not have a mechanical relationship any more It is normally pined and then assembled We have got variable valve timing And we have got variable valve lift with phasing shift as well All of which is electronic controlled with infinitely complex software My car is a 2 litre car producing 380 horse power from 2 litres 35 or 40 years ago as an engine builder I would dream of producing that sort of power in a full race engine from 2 litre Now you go into a show room and buy one with a cup of coffee in your hand That is what technology and electronic control has brought us So honestly guys I am really quite nervous of trying to predict too much I am also being cautious with the scope I love a scope I am enthusiastic I am sold But I am also tempered by the fact that you should be careful what you try to prove with a scope Lets take a look on acceleration Because we had an issue here that did prove the value of this test The great thing with this test is we are not actually saving pressure in the tool We have removed the one way valve We have one if we want to put one back in But we have removed the one way valve so this is real time measurement Of air pressure during acceleration And we had a BMW that had a misfire only on acceleration only when cold And when conduction this test what we found remarkably Was that each of these events was a singular compression Remarkably between each singular compression event There was a dramatic difference in the pressure attained in the engine So it had to be losing pressure on a single event Now we would not have found that with a normal compression gauge Because the one way valve would have captured that pressure And added to it so any drop in pressure would not have been noted Obviously the increase of pressure on compression would have been a factor but not in real time We could not have run the engine So there is a very good value of this test of which I fully support But please when trying to establish valve timing from this test With the technology I have just presented to you Please be very cautious because I do not really buy it Now of course on acceleration you do not get much suction You tend to get an increase in pressure And we can measure that directly off We are reading just over 7 bar of pressure in the cylinders So I love it as a test we have adopted it as a standard test for compression testing In every single case we do not use a gauge any more We do use the scope So I do support the use of this test And I love the WPS it really is great Other uses would I use it on fuel pressure and turbo testing I am tempted to say not guys I would go for a gauge in that case Simplicity speed accuracy always is the equation If you want to make an accurate recording electronically go WPS If you have not got one you really need one guys The sort of things we measure these days in real time demands these tools On an almost every day basis.

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