compression figures

[stampy_54]

what kind of compression figures would people expect from a 1300 8v nova engine with a head that has been skimmed to pretty much the limit?

[Andy]

way,way too vague.You need to work out your new combustion chamber cc
in Stuart's sig there is a site that when you enter your cc's,works out your compression ratio.

[Will F]

Why do you need to know? If you need the figures then as long as they are within 5% of each other don't worry. If its the ratio you're after then it's very hard to tell without stripping the engine down...

[Martin.p]

As said, as long as there within a % of each other that's what maters! You should do a wet and dry test. If you had 2 pots that where a fair bit lower and right next to each other then it would be fair to say gasket is leaking across top. If one is lots lower on its own then it would point to ring fail! If there was a big difrence between wet and dry test then could be oil seal ring/ ring. Iirc I had 130psi within +\- 2 psi on my. C20LET if that's any indecation. But it's more a comparison thing!

[Andy]

Nothing to do with a compression test.He wants to know his new CR after a hefty skim

[Martin.p]

Ok. Sorry :)

[Andy]

Again,i dont think its a "should i?" thread,the kids probably had a big skim and probably wants to know if he needs a vernier or not,but without figures its pisssing in the proverbial.

[Martin.p]

Well the cc of his combustion chambers will be oh so slightly smaller and the compression oh so slightly higher. I've tested mine befor and after re build and it's hardly measurable.

[Andy]

likewise but it depends what its used for to be fair.

[Martin.p]

We will have to wait for OP to repost befor we know. :)

[Stuart]

I've recently got the hang of some CAD software so its 'possible' to estimate the chamber CC from the model and then remove Xmm of it and re-estimate for a better idea without all the measuring faff.

[Andy]

That sounds good.
Its a bit involved measuring combustion chambers.But then especially if its a used head,you have no idea whether its been refaced in the past or not so the measurements are needed.

[mowgli]

unless this engine has had new rings fitted properly, then any gains in compression ratio will be minimal

[philip]

The engine is a class 2 grasser motor, so can't do much modding wise to them tbh besides skim head to death and seat profiling and a 40thou overbore

[stampy_54]

As Phil said its used for autograss. Spec is again as Phil has said, 1mm overbore with head skimmed to 94mm, three angle valve seats. I'm not interested in compression ratio, just a rough guide as to what the cylinder compression pressure should be when I test it to decide wether it's worth stripping and re-ringing/rebuilding or if its still reasonably good

Andy thanks for the replies but it's not the compression ratio I'm interested in its as Martin said its more the compression test results I should be expecting to get from the motor

[Stuart]

There is no real figure... Over 100psi is good, 150 is an ideal (but obviously can be less).

Compression Test Actual figures vary depending on:
Air Temperature
Engine Temperature
Static Compression Ratio
Battery condition
Starter Condition
Oil grade and condition
Atmospheric pressure
Humidity
Engine Stroke.
And then the rest of the engine parts you are trying to test against.

[stampy_54]

I know every engine is different and will never produce the same but I'm looking for an estimate to see wether the engine is still pretty good, I know it's never going to be the same as a fresh motor but want to try and wait until the engine isn't performing as it should before stripping it down. The figures you have just given are a pretty good tell tale. It was producing 170-180 psi last time I tested it approx 9 months ago