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I mapped this out for my own notes and figured it would be useful for others who are learning. Easily derivable from Fred's notes above. But I physically validated the TDC at each of cylinder and crank position
The impact of potential cam timing changes on compression test results is not relevant when it comes to the leak down testing procedure. Both the inlet valves and the exhaust valves are closed for approx 180 crank degrees -i.e. 90 degrees either side of TDC on the firing stroke.
Here are the results for the first 360 revolutions of all eight cylinders. Again, this is a stone-cold engine, so I expect the leak-down numbers to be higher. These results are pretty frightening! I am hoping this is the exhaust stroke and not the compression stroke for these high-leaking cylinders. I hope to finish the second revolution later tonight. Also, note the crank position for cylinders 1 & 6. I wasn't able to get the lock to be perfect at 0-degrees.
Michael,
There is something odd about about your crank position values- they should increase by 90 degrees increments as you move from cylinder to cylinder following the firing order which you have correct so something not quite right in that regard.
Remember No1 cylinder is firing when the finger of both rotor arms is pointing to the 3 O' clock direction not that you need to see both to determine such
Fred, I am referencing to the numbers in degrees on the ATI damper. When I start with cylinder 1 and the ATI damper displays TDC (Zero) as I turn the engine 90 degrees (clockwise), the numbers on the ATI damper count down form 360. So the first 90-degree turn goes to 270-degrees on the ATI damper, and so forth.
I realize you may not be familiar with the ATI damper markings. Here is a closeup
The OEM damper markings are organized a little different so you are making with a chaulk as you turn it.
OK- with you now. Whether hot or cold you should be getting more consistent results. Is there any reason why you cannot conduct the test with the motor warm? Presumably you checked the crank position had not moved during the test?
Agreed, I should be getting more consistent results. I can re-assemble the motor and do the tests warm but I would need a helper since doing the tests alone takes a long time and the engine would cool down significantly during the test. Also, I am making sure the engine is locked with the flywheel lock for each test and ensuring the engine does not move. I figured if I am off +/- a degree using the flywheel lock, the valves will still be closed +/-180-degrees. So I should be getting good readings.
The test results look very inconsistent. What do you recommend my next move is ?
Agreed, I should be getting more consistent results. I can re-assemble the motor and do the tests warm but I would need a helper since doing the tests alone takes a long time and the engine would cool down significantly during the test. Also, I am making sure the engine is locked with the flywheel lock for each test and ensuring the engine does not move. I figured if I am off +/- a degree using the flywheel lock, the valves will still be closed +/-180-degrees. So I should be getting good readings.
The test results look very inconsistent. What do you recommend my next move is ?
Test results on a cold engine are less reliable and as your results to date show they are not consistent. Now you have some base line readings as it were I suggest you get the motor up to normal running temperature and repeat the tests- the motor will not cool down too quickly. You can check the engine temperature halfway through and warm it back up if you feel it is warranted. I am confident you will see lower leakage rates and more consistent values.
Update - I performed another leak-down test while the engine was still hot. Here are the summary results of the compression test and both leak-down tests. You can see there is a direct correlation between the cylinders with the lowest compression values and the highest leak-down values. The information I read indicates that the numbers below 20% are good. Is that a correct assumption? For the cylinders with the highest losses, adding oil improved compression, and in all cases, the leakage could be heard and felt through the dipstick. Subsequently, I am concluding the losses are coming through the rings. Any additional thoughts from you all?
I should note that what prompted this work was to determine if the rings were allowing oil into the combustion chamber on high RPM and load situations. I am seeing blue smoke under hard accelerations, and the car is quite stinky. The above numbers are leading me to belive it is time to rebuild.
Do you think it could be stuck (or coked) oil rings or something similar on those cylinders with bad results? I think the point of Porsche not drilling the oil return holes was to preserve the bores etc and thus excess oil is always present. Although that may be Rennlist Lore
Have you dynoed the GTS yet? What power is it putting down?
Can you run a "cleaning oil" like mobil1 15-50 with seafoam for a few changes to clean up the crankcase?
Not to say that a rebuild is due, but if you don't have to just yet then don't.
But as always, if you do, you can do all sorts of good things when it's out of the car including the slippery slope of larger bore, new crank, etc etc etc. It's only money right?
The primary reason I am doing this test is to gain insights into the sealing performance of the position rings, valves/seats, and valve guides/seals. This GTS consumes a lot of oil (1 quart per 400 miles). I can see blue smoke under accelerations over 3500 RPM. The car is stinky; I can smell oil being consumed (The fuel mixture is good). For reference, this car has a vacuum pump on the crankcase and does not return evacuated crankcase air to the intake (see graphic).
The motor seems to be running well and has strong power delivery.
Just saw your note (red underlined) that the fuel mixture is good. Keep in mind that the O2 sensor sees burned oil as "fuel" so if there's excess oil in the exhaust it'll tell the O2 that the mix is rich and the LH will lean out potentially causing it to run hotter.
Thanks for the comments, Tom. I agree with your assumption that the rings are stuck. Your idea of seafoam and a high-detergent oil like M1 is a good recommendation. No dyno data on this GTS yet, however, it feels peppy enough and keeps up with Ken's supercharged S4 just fine. This series of tests helped confirm my assumption that the oil smoke was resulting from the rings. This GTS motor is due for a rebuild, and now it looks like it will be this summer. I may just start that now. I really can't drive it too much with all the oil smoke.
Your latest round of test results shows a degree of convergence as would be expected. However you have two cylinders that show an increase when tested warm and that is nonsensical. The test rig you are using will probably account for this in that top quality Bourdon gauges can exhibit sticky behaviour never mind bargain basement Chinese sets if by any chance that is what you were using.
Next time you try this test try flicking the gauge with the back of your index finger before taking the readings- it is very normal for gauges to stick slightly due to the linkage and especially so for cheapo gauges- that alone could explain some of the inconsistent numbers you are seeing. A professional level quality gauge set would probably cost US$250 as opposed to the harbour freight/Amazon clobber costing US$30 a set.
Regarding your compression test numbers I use 185 psig as a datum point on my S4 based motor and a 5% tolerance range gives values from 176psig to 194psig which pretty much fits what you report.
Beyond that 928 motors in general perform pretty poorly in terms of oil lifting at higher rpms and commensurate oil consumption and the GTS with oil scraper ring lands that are not drilled together with a stock breather system that is pretty poor all add to the issue. That you have a modified system and a vacuum pump on the crankcase should improve matters but..? On the stock system GTS oil consumption was so bad they issued a technical bulletin about the issue advising how to handle customers complaints about it!
As a data point, a while back I found an S4 engine with 60k miles on it, and had its heads rebuilt.
Once in the car, I did a leakdown and a compression test with GT cams - the engine was never started, no extra oil in the cylinders, and occurred during the summer.
For the leakdown we used 100 psi input pressure at TDC, and 6 cylinders had 0% leakage, and 2 cylinders were at 2%.
For the compression test, nearly all cylinders were 180 psi, and a couple were within 5 psi. Besides the good numbers, they also told me that the cam timing was good bank-to-bank.
So in my experience, there was no need to warm-up the engine, nor add oil to the cylinders to get a good test.
One good reason to use 100 psi for the leakdown is that you'll automatically know the leakdown percentage. So if the gauge is showing 98 psi, you'll immediately know that you have 2% leakage.
How many miles are on the engine? At the very least you can pull the heads to see if the exhaust guides are shot which is common. But since it is a GTS, you should pull the pistons and have the oil-control ring-lands drilled if they're not.
05-31-2024, 09:32 PM
Which cylinders are at TDC and which degrees
