Gents,

I found out something unique................

It is the RPM value when Xentry doing Compression test ( Relative Compression test ).
I have always thought the cranking RPM is what it is............ as per Xentry, example below :


Pressure sensor is at 1st cylinder.


.

Pressure sensor at 2nd cylinder


.
Pressure sensor at 3rd cylinder



Basically, its always been about 200 RPM +- 10 RPM by 3rd time I do compression test, because the oil gets to be spread around a bit by then.



Now.........
I measured the cranking speed based on compression event per second. I get different value and this is a true mechanical value and not based on any CAM or CRANK speed sensor.
Its always 125 RPM +- 3 RPM .


Cylinder 1




.
Cylinder 2



.

Cylinder 3







On M271.820 EVO , W212 E200 or W204 C200/C250 , the Xentry Compression Test cranking speed result is very very FAST like 700 or so RPM, which does not make sense but I never bother to pursue that.


I wonder why would Xentry shows 75-80 RPM faster than actual.... at least for M276 ?


Also take a closer look at the 3 Xentry Compression Test. Anytime I place the digital pressure sensor in that particular cylinder, that cylinder spin faster.
That is because of the extra air-space the spark plug adapter M12 to M14 and the pipe + hose and the sensor itself..... which actually reduces compression value and hence that piston can speed up a bit faster.

I am using this kit :
01. The pressure sensor, 16 BAR type : https://store.rotkee.com/en/ps16-in-...ol-engine.html
02. The M12 to M14 adapter : https://store.rotkee.com/en/ad-m14-m...ransducer.html
03. The pipe. I use the long one only : https://store.rotkee.com/en/ad-m14-r...ransducer.html
04. Finally the hose : https://store.rotkee.com/en/ad-m14-f...ransducer.html

So the air volume of these 4 items can be "read" by Xentry Compression Test.

To note. There is no check valve on this system, which traditional analog compression gauge and hose set will have one.



Compression value & limit for our engines , based on WARM engine 80C coolant.





Me at 30C ambient temperature, I don't bother to warm up my engine.

A guy in Oz which I am sure is during warm weather too, done some test between COLD and HOT on his car. Below :



.

The Rotkee 16 BAR pressure sensor pressure vs voltage value



.

 

Another interesting deep dive... what can be uncovered studying that topic ??

It looks like the relative compression test is super sensitive to your probing pressure with additional volume.

Compression losses by order are from...

1. piston rings stuck
2. cylinder scoring
3. valves leaky seats

As you noted oil distribution improves dynamic seal (**)
I am wondering... can we have piston spray at crankspeed regardless of pump solenoid set for full pressure?
I believe crank speed may well keeps pistons dry, right?


"lower means better" compression vs. RPM

** : perhaps heads oiling does the RPM improvement with 4 camshafts compressing 24 valves + HPFP.

Engine with non-limited pump pressure should have some remaining oil present around the rings shortly after stopping - It may drain backdown more overnight, right?

++++ CAMS lube:
Master Tasos always inspects engine for shaved cam lobes... I wonder if that standard fare could be cancelled by better oiling

 

The relative compression test should be used to determine whether a cylinder is an outlier or not compared to the other cylinders AND to confirm overall adequate compression to support ignition. An outlier would generally identify a problem cylinder. From there further troubleshooting can be conducted. I have found the piston speed to be around 200 rpm on my M276 engines and all within +/- 10 rpm. A faster spinning cylinder indicates lower resistance and lower compression. If all cylinders within a bank are measured at say 400 rpm or even 300 rpm when 200 rpm is expected, but there are still misfires detected, then a physical compression test would be in order. If actual compression is too low, you will not get ignition and therefore misfires will occur.

I see the relative compression test as a tool in the toolkit to identify trouble spots, not necessarily used to measure actual compression pressure. For example, Cylinder 1 is the only cylinder misfiring. Wires are checked, plugs are swapped, and coils are replaced, but the problem remains with Cylinder 1. The relative compression test will tell me if I have a bad cylinder or not. From there, I can look further for the cause of a problem. The next step would be a cylinder leak down test.

 

Last engine ON was during the aux light test, so its been like 5 days ago engine was fully oil bath.

Usually if engine have not been run for 30 days or more the so called RPM of Xentry compression test can be as low as 175 RPM.
But age of oil also matters, aside from mileage.

You can read here : https://mbworld.org/forums/e-class-w...operation.html

Basically if too long engine was not used, cranking RPM per Xentry is always slower at 1st try.
By 2nd and 3rd test, oil has spread around better and RPM speed up a bit to 200 ish RPM. So RPM increase is also about how slippery the engine is and now we prove that having
less compression due to extra air space from the test tool will read as faster RPM too, as expected load wise.
I think for COLD engine ( up to >30C or higher ) the best SOP is to at least do 3 Xentry compression test, and use the 4th test as final and use 205-210 RPM as reference ( for my engine, with battery maintainer )


Yesterday test was for the s-plug adapter + fitting from Rotkee in Ukraine. It is good product, yet low cost. Also of course the 16 BAR pressure sensor is an important sensor to test.
All my other compression test fitting made in China, has minor leak.

Xentry (relative ) Compression Test is similar to Relative Compression Test using starter cranking amperage, but is a more pleasant and easy way to see the result.

Still, I am wondering why Xentry reported that bogus 200 ish RPM when it is only 125 ish RPM ?
What is the reasoning behind this ?

To think that a flywheel is so big in diameter and so many teeth +- 140 and starter motor is so small with about 12 teeth count... that means the starter motor is doing like 1,400 ish RPM...yikeees !!
140 / 12 = 11.6 ratio
125 RPM x 11.6 = 1,450 RPM

M276 flywheel and starter

 

I count 34 teeth. Maybe a proper drawing tool is in your future! (I happen to use Snagit.)

 

Oke.... Bank 2.
Cylinder 4 to 6.

More experiment for fun and knowledge............


Short compression test kit vs longer compression test kit.


.



.





I would not speculate that longer test kit with extra hose gains 0.5 RPM faster , but pressure drop is massive at 1.15 BAR ...Dang !!!



As per Xentry, supposedly 10 RPM faster due to reduced compression. Sensitive Xentry is... not bad.




Cylinder 5 and 6 with pipes only, no hose.


.



.




.

 

its very interesting to witness how you are able to affect your compression tests !

 

Bloody -hell, I AM AN IDIOT !!!
How could I forgot about this.......

Each combustion/compression cycle in any cylinder is based on 720 degrees or twice the full 360 degrees spin of crankshaft....for a 4 stoke engine.
So that is why Pico scope showed as only 125 RPM between compression pulses which is a 720 degrees of flywheel spin , while Xentry is showing 220 ish RPM which is correct based on a 360 degrees per 1 flywheel spin.

 

.... it's just a "senior moment" when the brain doesn't make total sense of the eyesight

Above we see you can affect compression RPM and we see relative compression vary widely... to say the least.
I think you've found another smoking-gun evidence.

We know.... :

• pistons are dry at that crank RPM
• any crank oiling improvement from heads
• what is No1 straining factor in the heads?
• HPFP single/double!

When oil has reached the back of Bank1 we have smoother crank RPM not to be confused with better compression... true?

Let's see how this is played out.... entirely different graph with less HPFP jerking....


oberserve Cyl. 1234 only


oberserve Cyl. 1234 only


oberserve Cyl. 1234 only
(Cyl. 56 have your pressure probe affecting compression)

We see uneven Cyl.1234 evolving.

I think the differences we observe are interesting to help give meaning to cranking RPM being only partially related to cyl. compression.

The least we can say is to consider this test with "a grain of salt" knowing relative cyl. crank RPM is easily affected by... dry heads.

The workaround is to repeat test long enough to lube the HPFP.
Then we can get the most meaningful test results.

In the end we can apreciate meaningful cylinder differences - The fix being to clean stuck piston rings with oil at temps when it stops burning into carbon.
Black oil is the telltale sign of what's going on inside. Don't just ignore burning conditions.
Drafty high blow-by engine may only need R&R if it was not abused under weak conditions.


+++ Now... ADDITIONAL INTERESTING MTR....

IMPROVING HPFP No1 JERK

On stock lube the pump(s) jerkiness have a high impact on timings jitter in adition to stuck dry rings.

MEANING... what Master Surya shows here with crank RPM test is directly transposed at run time near idle: jerky HPFP significantly impacts timing jitter.

The HPFP cause additional randomness to crank rotation picked up by high resolution sensor.

I think this poor lube condition is helped by API-SP chemistry. I believe my lubed pump piston has become smoother despite having steep 120°/90° lobes!
We can easily guess the pump proportional valve timings are helped by predictability, right?

> SECRET MOJO RULES :
With this GDI injection, nothing is more essential than evenly predictable engine timings:

1. Cylinders lube wet/dry
2. Piston rings sealed/stuck
3. CAN-C CGW solderless chaos
4. Jerky HPFP wet/dry
5. Cam. chain tensioners
6. (Inbalanced engine: valves/bores)

-- Jittery timings earn de-tuned lean fuel maps.
vs.
-- Predictable timings get GDI torque from neutral maps.

Luckily there are easy ways to cancel these issues one by one.

 

Ha ha ha....My Senior moment indeed Cali

Battery charge state also matters, while my battery CCA is 800, with 25 amps battery maintainer, it is minor deficit each and every 6 seconds of cranking at 500is peak amps and constant about 200ish amps.
I keep 3 minutes between test per the same cylinder and about 15-20 minutes apart between different cylinders.

Anyhow the goal was to test actual cylinder pressure while using Xentry cranking feature, because W212 has no clear flood mode to do such compression test, without DTC.

Overall Xentry relative compression is good enough to detect obvious anomaly.

One thing I don't like about Xentry Compression Test when used as means of engine cranking without engine starting, similar to clear flood mode is : The throttle opening must be FULL, otherwise Xentry wont allow us to crank.
While full throttle opening is good for peak pressure, it is not good for suction between peak pressure which can be read for valves activity and it does not represent normal engine cranking ( crank to start )

 

MS! it's always interesting when you research topics your way - We get to understand new things and sometimes find nuggets.