What Your C20let Compression?

[Bodster]

Jus checked mine and its bout 130 in all 4 spot on

Engine is jus been rebuilt and dunno if this is good

Ps i used an old tested and after 5 turns it was firing out of plug hole so maybe there more to come??????????

Also see this thread

http://www.migweb.co.uk/forums/showt...68#post2018068

[SimonT]

You should hope those readings go up when the rings have bedded in properly.

[Rs1]

Aye 130's poor,get it ran in and check it again.

[Adam]

should be 170 no?
i shall check mine monday when get timing belt done

[chip]

depeonds on what CR, what cams, and what cam timing.

150-170 is more normal, but thatdoesnt mean 130 is wrong, might be perfect for his setup.

[will kirby]

Sorry to come in on this thread but does anyone know what the C20XE should be?

[SimonT]

About 200psi +/_ 30 psi depending on how worn the rings are.

Most 100K+ mile engines do about 170, a freshly built one should do about 220-230.

[Nemesis]

Mine were all between 140 and 160...

[Kruger]

On my xe last week I got 220psi across all 4, not bad for about 120k.

[rodgerq]

both my LETs are sitting about 170. both standard cams and pistons and timing.

rodgerq

[TurboAde]

Am I wrong in thinking that the maximum compression you can achieve is simply the CR x 14.5psi (personally I can't see how it could be higher than this). If this is the case then a "standard" 9.2CR LET will be around 135psi.

[Rs1]

Isnt comp ratio and cylinder comp refering to 2 different things though??

[Rs1]

OK maybe im wrong but your cr stays the same no matter what yeah?? Lets your valves aint opening properly and you get a lack of compression, that doesnt change the cr,does it?? Infact.................... no it doesnt,it cant, i'm confusing myself now,lol.

[TurboAde]

Isnt comp ratio and cylinder comp refering to 2 different things though??

Well, the way I understand it is that the compression ratio is a measure of the comparative volume in the cylinder, when the piston is at the bottom versus when it is at the top. Thus assuming there is atmospheric pressure (14.5psi) in the cylinder when it is at the bottom of its stroke then the maximum pressure achievable in the cylinder will be atmospheric pressure x the compression ratio, when the piston is at the top??

[TurboAde]

Thinking about it, there is one way that the cylinder pressure could exceed the above simple calculation. That is when the cylinder has not totally exhausted all the gas pressure from the previous stroke. In other words if the starting pressure is, say, 1.1atm, then the cylinder pressure will be increased by 10% etc. So in this example the LET might be closer to 150psi. I expect this is not a significant effect at cranking speeds, though.

[Rs1]

Isnt cr more about area/cc than actual pressure??

[Rs1]

Like how many time the area/cc of the combustion chamber fits into the area when the piston is at bdc??

[Rs1]

You divide the cylinder capactiy by the combustion chamber and gasket/piston volume to get the cr.
Bollox this is confusing,i have a nvq in motor vehicle mechanics too,lol,explaning it is much harder than knowing it.

[TurboAde]

Isnt cr more about area/cc than actual pressure??

It might be defined various ways, I'm not sure, but the way that makes most sense is simply the ratio of the cylinder volumes at TDC and BDC (or alternatively (combustion chamber volume + swept volume)/combustion chamber volume)

[TurboAde]

You divide the cylinder capactiy by the combustion chamber and gasket/piston volume to get the cr.
Bollox this is confusing,i have a nvq in motor vehicle mechanics too,lol,explaning it is much harder than knowing it.

Imagine you have a tube, sealed at one end, with a piston in it. If you push the piston half way down the tube the pressure will build up to 2 atmospheres (if there's no leakage). If you push it up three-quarters of the way you'll get 4 atm. If you push it nine-tenths you'll get 10atm etc etc. This last example is fairly close to a typical engine!

Strictly speaking when you compress a gas, the gas will heat up and cause a slight variation in expected pressure (can't work out if this is up or down) so the cylinder pressure calculated in this way is only strictly correct when the heat of compression has dissipated. Heat created by compression is, of course, how diesel engines work.

[rodgerq]

the compression in an engine is a different thing to the compression ratio - although they are related. the compression is basically the pressure inside the cylinder with the piston at TDC compression stroke and its given as a psi reading.

compression ratio is calculated as (swept + unswept) / unswept and is more a calculation of the engines compression regardless of cylinder leakage(whereas the compression in psi will take into account cylinder leakage)

heres a bit more in depth look

C/R ratio

rodgerq

[Bodster]

,ight try mine again but like i said i couldnt hold the pressure tester in above 130 as its a big bulky item and doesnet quite fit down head, plus this was a cold engine so thinking it makes more/less when warm

[artfulmatt]

Well if I go by the sum of 8.6(my CR) X 14.5psi then my comprssion should read (once run in ) 124 psi ??

Which sounds very low to me ??

[Lee H]

Mine was 170 across all 4, standard 90k engine.

[chip]

YOU CANNOT MULTIPLY THE CR BY 14.7 TO GET THE EXPECTED COMPRESSION

So stop all doing it!

It would rely on the engine being 100% volumetric efficient and the leak down being zero, neither of these two things are ever correct so its a pointless exercise.

Cam timing effects the compression test reading because it effects the engines ability to fill with air, and in some cases (if the vlaves are still open after BDC) allows it to partially empty again.

There is no easy formula for working out what it should be.