1.4 CGA3 N/A Build - High compression psi

Hi,

Just wondering if anyone can explain how to convert between the compression ratio of an engine to the psi I should get from a compression test.

In other words, I know a stock healthy Micra engine is 9.5:1 and produces 180psi per cylinder.
I'm getting around 190psi on my high comp 1.4 (with some 272 degree rally cams), how do I translate this into a XX : 1 compression ratio?

I'd really appreciate any help, thank you!
 
I'm getting around 190psi on my high comp 1.4 (with some 272 degree rally cams), how do I translate this into a XX : 1 compression ratio?
You don't, it wouldn't make any sense, a compression ratio is derived from the geometry of the engine, the engine has the same compression ratio irrespective of what cam is in it, the compression test results would depend on the ring sealing, cranking speed, cams, compression ratio, valve sealing, cam timing etc I'd consider a compression test as a health check, and probably only really useful compared against a previous result or a engine of the same spec.
That said you have a higher pressure on a longer duration more aggressive, more overlap cam so I'd say it means good things as cranking speed would be a worst case for a cammed engine, whether the pressure is too high further up in the rev range where the cam begins to work is another question.

You can work out the compression ratio based on the components you used to build the engine as well as how much material may have been removed from the head and or block.
 

SuperUno

Buy & Sell Member
It is also about the balance across all 4 cylinders, if they are all roughly around the 190psi mark then the rings etc... are all healthy.
 
There's online calculators, it depends on your cams and the timing they are at

Enviado do meu MI 8 através do Tapatalk

You don't, it wouldn't make any sense, a compression ratio is derived from the geometry of the engine, the engine has the same compression ratio irrespective of what cam is in it, the compression test results would depend on the ring sealing, cranking speed, cams, compression ratio, valve sealing, cam timing etc I'd consider a compression test as a health check, and probably only really useful compared against a previous result or a engine of the same spec.
That said you have a higher pressure on a longer duration more aggressive, more overlap cam so I'd say it means good things as cranking speed would be a worst case for a cammed engine, whether the pressure is too high further up in the rev range where the cam begins to work is another question.

You can work out the compression ratio based on the components you used to build the engine as well as how much material may have been removed from the head and or block.

It is also about the balance across all 4 cylinders, if they are all roughly around the 190psi mark then the rings etc... are all healthy.

Thank you to all of you for your help, it makes sense to me now.

Yes the readings were very similar. I tested each cylinder three times and kept getting all readings around the range of 185-195psi.

Very glad I asked on this forum, can always count on some some support :)

Thanks guys.
 
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