Trying to make sense of a compression test done on a 355 All numbers are in MPa. The Maseraterati PicoDiagnostics gave the following results as "Passed" Cyl. MPa PSI #1 1.298 188 #2 1.000 145 #3 1.038 151 #4 1.273 185 #5 1.115 162 #6 1.300 189 #7 1.302 189 #8 1.302 189 A couple notes. The test was done with the PicoDiagnostics so I was told to convert 1.0 to 145PSI. so 1.25 would be 181.3. The MPa (MegaPascal) was "estimated" by me by the graph and so it could be a little off in the thousanths or possible by 1 in the hundredths but unlikley. For sure, #1 was above 1.25 and #2 was at 1.00. #3 was a smidge higher than #2 and #5 was right between the 1.00 and the 1.25 hash marks. I have been told that on a 355, all cylinders should be over 170 AND not farther than 10% from each other. The last major was a bit over a year ago. What is reality?
The reality is we don't know how to interprete the results unless test conditions are defined: 1. was it done on a hot or cold engine 2. what altitude was the test conducted. Mile high of Colorado or sea level of Las Angeles 3. Was battery fully charged, throttle fully open
I"m not 100% sure what Pico Diagnostics are unless they mean the picoscope In that case of picoscope they are merely reading amp spikes in the cranking voltage not a compression gauge in hole. I use a picoscope all the time and nice tool Food for thought [ame]http://www.youtube.com/watch?v=spkpkRvQPHY[/ame]
That test needs to be rerun on a warm engine, both before and after a dollop of oil is placed in each cylinder under test (separate readings.) If the numbers continue as above, that engine needs a full leakdown test. If the leakdown test confirms the compression test, that engine needs to come apart.
I just posted this in another thread: [ame]http://www.youtube.com/watch?v=oY0jqb8Ld-4[/ame] Add some BG 44K, drive the crap out of the car for 100-200 miles, check again. If the values go up, carbon is your problem. I've done compression checks on warm (not hot) vs cold engine and saw no difference - can someone confirm they have actual experience with a difference? I've also done a WOT vs closed and saw no difference although I do WOT as a standard (just cause) Battery must be charged fully and charger to stay on thru testing. Number of cranks must be the same on all cylinders.
The PICO testing is not what we are familiar with. The OP continues to ask for opinions on something we ... have never heard of.
I'm very familiar with it, been around forever Pico get you to say, yes now I need a real compression test leakdown If they were all the same value then your in the ballpark. With that one bad cyl he needs to look deeper with a compression and leakdown gauge. Image Unavailable, Please Login
First, thanks all for the quick comments. Allegedly this is a Ferrari Maserati test system that measures in megaPascals which is a relative test. After chatting a bit more I found that while this test shows per Maserati spec the engine passed, it doesn't directly relate to the mechanical compression test. That is coming. It will be interesting to see how they correlate. My understanding now is that the pico test is not comparable by converting to PSI and that the mechanical readouts from the compression test are the only way. I'll get more info tomorrow and then be able to compare.
+1. In the service history of my previous 1997 F355 spider, there was a compression and leakdown test done pre-sale before it was listed (just prior to my purchase). The numbers were a little off (about 11 or 12%) on two cylinders, and the FNA dealership used GM Top Cleaner, run through the intake system to remove carbon they suspected on the intake valves. They then drove the car "spiritedly" for a weekend by the service manager and the numbers came right back within spec. I subsequently owned the car for 10 years and drove it almost 40k miles to a grand total of 64k miles. It never had a compression or leakdown issue through two major services.
Mechanical compression results 172 psi 155 psi 160 psi 165 psi 160 psi 170 psi 168 psi 165 psi Had a Major 2 years ago. I forgot to ask if hot or cold. Maybe I don't have enough info that yelcab was suggesting but any thoughts would be appreciated.
How many starter cranks? What year 355? I don't see an issue with those values as they are all pretty close...
Here are my issues with these results: 1) Cylinders 2, 3, 5 look off. 2) This PicoDiagnositcs technique seems to be a short cut, and I wouldn't trust it. 3) A leak-down test really should be done also. There is a methodology to a compression test. There are many different variables which, if handled incorrectly, could lead to spurious results. A qualified mechanic knows how to do this test, but you yourself should be aware of the methodology because just about everyone thinks he is a "qualified" mechanic. Everyone is the best. Here's my methodology for a compression test: 1) Make sure vehicle battery is sound. Load test it or replace it. 2) Put battery on low-amperage charge or battery tender float overnight. 3) Prepare to perform compression test. 4) Warm up engine to full operating temperature. This means either 20 minutes of driving or about 25 minutes at idle. Oil temperature should increase all the way to about 200 degrees Fahrenheit. 5) Switch off engine inside the shop and let the car sit for 1 hour. 6) After an hour, remove spark plug covers and spark plugs. 7) Disconnect relays for fuel pump(s) for added safety. 8) Connect 10amps of charge safely onto battery positive and negative terminals. 9) Perform compression test as follows: use 10 compression strokes for each cylinder. Take a video of pressure gauge so that later on you will be able read the numbers at different stroke intervals. 10) After the mechanical portion, transcribe results from the video as follows: for each cylinder, record the pressure at 5 strokes, 8 strokes, and 10 strokes. This is important because it's also important to see how quickly the pistons build up pressure. Cranking the engine 12-20 times in order to get the pressure increases to cease is not the way to observe the compression, even though you'll find many people who think cranking the thing to death to in order to observe the highest number possible is the correct method. 11) Share results with us or with qualified people. Next: Perform leakdown test. There's really only one way to do this. Cylinder must be at top dead center, and the proper equipment must be used.
Dave, When using a remote starter switch (not the key), the relays can stay in place as nothing will energize them. Personally, I'm only in favor of leak down tests for the purpose of diagnosing the source of a problem if poor compression.
Even a perfect compression test can yield spurious results which otherwise appear to be excellent results. The leak-down test cannot possibly lie.
Oh yes it can A LD is a static test. Compression is dynamic. Perform a LD once, rotate the engine a few times (to allow the valves to seat differently) and repeat - results can vary
Quiz time: When doing a compression test it is possible that the for any given cylinder the piston is already on the compression stroke or partially through the intake stroke. Thus it makes sense that on the first compression stroke the pressure doesn't rise to the max value. However, after the 1st stroke, a full intake, compression, expansion and exhaust cycle is performed. So why isn't the pressure at a max? All the compressed air in the cylinder escapes on the exhaust stroke. The next air charge, and everyone there after, must be the same. If some air at elevated pressure remained in the cylinder it would only impede the ingestion of air on the following intake stroke. Think about it, when the car is running doesn't it draw the same amount of air/fuel mixture on each cycle? It better! And since the rpm is higher it has less time to do so. So why do compression numbers take 5-10 cycles to "pump up"? It's even more baffling when you consider the results from a picoscope. The figure below shows the current wave form from a picoscope test of a hypothetically perfect engine. Note that the current spikes are all the same amplitude. Thus all cylinders have the same pressure on every stroke. (The large initial spike is due to the rush current when starting engine rotation from a dead stop.) Also note that the area under each current spike from the low point to the max is proportional to the work done in compressing the air in each cylinder. Since it is constant it indicates that them max pressure in each cylinder is the same on all strokes. Or, more simply, the same magnitude peak indicates the same max pressure. Image Unavailable, Please Login
The y axis of the pico needs to be expanded and set to show absolute values. That graph is so scrunched up of course the peaks look the same. The grid on the x axis is .135 so a slight variance can easily exceed 10%. If you zoom in you can see that the peaks are not identical in amplitude at all. On your original question as to why it would vary with repetitive cycles, I would guess that when starting from still as each cylinder is done, that there is some better sealing of rings on successive cycles?
I belive on these high performance NA engines it tends to climb a bit do the dynamic effect and the valve overlap. I believe this is why battery must be fully charged and not go down much during test.
But the current trace from the picoscope clearly shows that the magnitude of the current spike, once the initial transient associated with overcoming static friction and the inertia of the moving mass of the crank and pistons and bringing up to cranking RPM, is constant, indicating that the cylinder pressure is the same on every stroke. Hint: We have all pumped up a football or basketball with a small hand pump. If you take one of those pump and place you finger over the outlet and compress the plunger it takes significant effort to push it all the way in. If you connect the pump to a deflated ball you find initially it take very little effort to push the plunger in, but as the ball inflates it gets harder and harder. Image Unavailable, Please Login
FWIW, here is an image of a picoscop test of an engine with a bad cylinder. Again, notice the consistency of each cylinder on every stroke, even the bad one. Image Unavailable, Please Login
No body? Here is how I view it. When the cylinder is sealed with a spark plug we know the total volume is about 0.44L. The compression ratio is 11:1 which means the combustion volume (volume at TDC) is 0.04L. From experience we know that a good cylinder will have about 200 psi compression. Now when a compression tester is connected to the cylinder it is necessary to consider that when the piston is at TDC the volume of the compressed air is the combustion volume plus the volume of the tube connecting the cylinder to the gage plus the volume of the Bourdon tube in the gage, assuming a Bourdon tube gage is used. Assume that extra volume is 0.008L (about 1/2 a cubic inch). ( For reference, a 1 Ft length of tubing with a 1/4" inside diameter has a volume of about 0.0096L). So the total volume of the air compressed on the first stroke is about 0.048L. If you assume that air compressed into 0.04L at a pressure of 200 psi then expands into a volume of 0.048, with no losses, the maximum resulting pressure would be 151 psi. So this is an estimate of the maximum pressure the gage would read after one full stroke. It will actually be less because of losses. There is a check valve, either at the base of the gage or at the end of the connecting tube, or both. So there must be a pressure difference across the check valve to open it, and there will be additional losses associated with friction as the air flows through the check valve(s) and connecting tube. After the first stroke, the check valve retains the air in the gage and on subsequent strokes the cylinder pressure will have to rise to slightly above the pressure in the gage before the check valve will open, allowing more air into the tube/gage, raising the presser somewhat. Since the air in the gage is at higher pressure, it is as if the extra volume of the gage and tube has been reduced. This "effective volume" of the gage and connecting tube gets reduced more and more on each additional stroke and less and less air passes through the check valve until the amount of air passing through the check valve becomes very small and the pressure in the gage changes very little as it approaches the true cylinder pressure. This is like driving 1/2 the distance to a destination on day one, and on each following day you drive 1/2 the remaining distance. You eventually get very close to being there but you never actually get. You always have 1/2 of the previous days distance to go. The figure below illustrates. Image Unavailable, Please Login
Absolutely, you can crank until the cows come home, the max pressure will not increase. We watch the needle as we crank, the first couple should see a rapid rise to max on a good cylinder, generally 4-5 is plenty enough.
I've tasked John with the calculations to determine how many cranks should yield the max pressure and theoretically, what should the max pressure be. It's been months and he's let me down Come on John, I'm still awaiting that as you are much better at those calcs than I am
