1.4 Twincharger Cylinder Compression - what should it read

[Richf]

Dont pay too much attention to those cheap gauges either , as above a balance across the pots of what you are looking for

[Breezy_Pete]

Compression ratio x atmospheric pressure isn't the right calculation in this context. There are a few factors that complicate matters like temperature rise during compression.

Irrelevantly to the OP's specific engine type but as an example of how far adrift that calculation can be, when I last compression tested our humble 1.2 AZQ engine it achieved 16 bar on cyl 1, and 15 bar on 2 & 3.  Compression ratio of that engine is quoted as 10.5 : 1; so the 'simple' calculation would suggest theoretical maximum of 10.5 bar.

[Jabozuma]

Hmmm, not sure about that one.

Physics 1-0-1 simply tells you ( Boyle's Law discovered in 1660 by our countryman Robert Boyle ) that unless there is a combustion event  taking place inside a sealed volume it is not possible to obtain a higher pressure that the compression ratio calculates or in other words pressure is inversely proportional to volume. Decrease the volume and pressure increases. Decrease the volume 10 times and your pressure will raise 10 times. So it will always be lower than theoretical max due to leakage.

So the measurements higher than 147PSi (depending on your altitude of course) are simply wrong or there is some other restriction down the line not allowing the air to be evacuated from the cylinder fully when the exhaust valves open.

 

EDIT:
Perhaps  you were thinking about the combined gas law where (P*V)/T=k where P=>Pressure, V=> Volume, T=> Temperature ans k=> constant. So with the same amount of gas if start decreasing the volume the pressure will have to raise and temp will stay the same

Edited 30 March, 2015 by Jabozuma

[walterwilson85]

When I have searched I have read that a vw polo gti 1.4 tsi was 180Psi standard engine.. The test was done as it should be so I'm.unsure why the figure is wrong.. All plugs removed along with fuse 30. Started cylinder 4 turned ignition for 5 seconds or so and recorded the number. Repeated this. Unless the gauge is broke but it looked new..

[Jabozuma]

Maybe you confused 1.4 tsi 180 PS (Pferde Stracke - a German for Break Horses, their engine output unit, slightly lower than bhp). if it was 180psi then the compression ration would be 12.5:1 - highly strung N/A motor territory.

[walterwilson85]

Here's the link bud. Half way down a little http://m.vagoc.co.uk/vb/showthread.php?t=12993&showall=1&perpage=999999

[Breezy_Pete]

Pressure and volume are only inversely proportional IF the temperature is kept constant; which it isn't in this situation.

[walterwilson85]

I'm.still baffled by my figures then. Everything was done the way it should of.. Anyone else any compression figures?

[Breezy_Pete]

Your figures are fine.  It's the theory that maximum compression pressures can be predicted solely by compression ratio that is wrong.

[newbie69]

Straight from ELSAWin:

 

New engine: 10-15 bar

Wear limit: 7 bar

Difference between cylinders: 3 bar

[walterwilson85]

So my 170 fits in between that ok. Let me know how you get on if you do yours bud

[newbie69]

Too busy and too bad weather atm. Will be doing this around Thursday probably, cheers.

[walterwilson85]

I just bought one ignition coil to replace on.number 4 just because I put a small crack around the bottom of it when I removed it. The new one I am putting in its place just missfires as soon as I put my foot down. I take it its ok to replace just the one.. This one I just bought must it be nackered

[walterwilson85]

Is it because I just changed one and should all be done together.

[Jabozuma]

Straight from ELSAWin:

 

New engine: 10-15 bar

Wear limit: 7 bar

Difference between cylinders: 3 bar

 

Cheers mate, most helpful but makes me rather confused.

Is it specifically for twincharger or for VW engines in general?

My only explanation would be increasing back pressure on the exhaust valve side ...

 

 

Pressure and volume are only inversely proportional IF the temperature is kept constant; which it isn't in this situation.

 

Your figures are fine.  It's the theory that maximum compression pressures can be predicted solely by compression ratio that is wrong.

 

Temperature is constant, nobody is heating anything up in the cylinder and increase of temperature due to higher pressure still fulfills the requirements of the unified gas law.

 

It is easy to say sth is wrong without actually offering any alternative/explanation - care to elaborate?

[Breezy_Pete]

Yep. Compressing gases raises their temperature. Ever pumped up a bike tyre and felt the end of the pump?

That's why turbos benefit from intercoolers, to bring the temperature back down after the compressing has happened.

 

The only way you'd see pv=constant in the context of a compression test is if you cranked the engine over mind-bendingly slowly, allowing the heat of compression to dissipate away into the metalwork. Trouble with that is, all your compression would have time to leak away through the ring gaps

 

See under the heading Entropy Increase here.

[newbie69]

Cheers mate, most helpful but makes me rather confused.

Is it specifically for twincharger or for VW engines in general?

My only explanation would be increasing back pressure on the exhaust valve side ...

 

This was with Skoda Fabia 2011 CAVE 132 kw as selected vehicle.

[Ootohere]

As simple as this basically, but on a 4 cylinder engine obviously.

Edited 31 March, 2015 by goneoffSKi

[newbie69]

Did mine today at last. All cylinders very consistent as you can see. I was testing them from 1 to 4. The only difference between them, cylinder No.1 needed 6-8 cranks to reach top pressure while cyl. 2-4 rose up to max pressure after 3-4 cranks.

 

I think it looks good, what do you think?

 

Edited 3 April, 2015 by newbie69

[Jabozuma]

Yep. Compressing gases raises their temperature. Ever pumped up a bike tyre and felt the end of the pump?

That's why turbos benefit from intercoolers, to bring the temperature back down after the compressing has happened.

 

The only way you'd see pv=constant in the context of a compression test is if you cranked the engine over mind-bendingly slowly, allowing the heat of compression to dissipate away into the metalwork. Trouble with that is, all your compression would have time to leak away through the ring gaps

 

See under the heading Entropy Increase here.

 

OK, got you, had a senior moment and it did not compute that during compression test lots of kinetic energy is introduced into the system (cranking action i.e. piston is forced up) and this energy is then transferred into the pressure and makes the manometers read higher than the nominal compression ration would suggest - cheers for clearing my head . BTW, I liked that relativistic take on system's entropy, one thinks one is able to describe the reality when there is another layer underneath, and then yet another layer under that one - deliciously frustrating!

 

@ newbie69 Good readings mate! No1 taking longer to go up to the max pressure means the rings are not sealing as well as the other three, that's the usual symptom. Nothing to worry though as test results are splendid imho BTW, plugs in or out?

 

I just realized that when I did my test I did not depress the throttle pedal lol - bit embarrassing he he he. I will buy Denso plugs (just to try them on) and while changing I will do another, proper this time, compression test. I will also do one with plugs in and plugs out just to see if there is any difference for the hell of it

[Breezy_Pete]

It'll be interesting to see how much difference it makes with throttle well open, and useful to others to know about if there is a big difference, which seems likely to me.

Likewise with all plugs out versus only the one; make sure your battery's fully charged before starting too.

The other variable it would be useful to compare is with engine cold versus warm, on the same engine, something I could maybe do on my Polo over the long weekend, as I haven't tested it since purchase a couple of months back.

[newbie69]

Car was properly warmed up before I started. Oil up to 80C when I switched off, then I started unmounting stuff, pipes, coils, plugs (all 4 went out). After some 20 min (adding a bit of oil along the way) and while the engine bay was still unsafe to touch on metallic surfaces I started testing from 1 to 4. Oh, I didn't depress the accelerator pedal at all either. Why is this necessary in these engines? Didn't read you had to do it anywhere.  I should have maybe gone back and test cylinder no.1 when I was done with no.4 to see if it would max out quicker this time after I had been through each cylinder once.

Edited 3 April, 2015 by newbie69

[Jabozuma]

My motor was barely warm to touch when I started and stone cold by the time I tested the last pot, was rudely interrupted by SWMBO

 

If the throttle is closed the piston going down on intake stroke pulls against closed intake tract so against partial vacuum. So if you open the throttle it sucks in atmospheric pressure and should get better cylinder filling and better compression values.

On hot engine piston rings and valves seal the cylinder better so again better compression readings than on cold one.

 

To establish whether piston rings are at fault when low compression pour a teaspoon of engine oil into the cylinder through the spark plug opening before doing compression test, if improved then rings are gone, if not you have a hole in your piston or your valve

[Wakey]

Did mine today at last. All cylinders very consistent as you can see. I was testing them from 1 to 4. The only difference between them, cylinder No.1 needed 6-8 cranks to reach top pressure while cyl. 2-4 rose up to max pressure after 3-4 cranks.

 

I think it looks good, what do you think?

 

Although the values are ok i would be concerned about the cranking requirement on cylinder 1. You need to perform a leakdown test to ensure you don't have an upcoming problem on #1

[Richf]

Although the values are ok i would be concerned about the cranking requirement on cylinder 1. You need to perform a leakdown test to ensure you don't have an upcoming problem on #1

Absolutely! Leakdown is far more useful than compression