Compression Ratio

[tha_SwEdE]

I know its a noob thing to ask but I have forgotten how..
Plz help me...

Edit: I'm tierd too so I even forget the Q. hahaha

I wounder how to calculate the Compression Ratio..

[RacerX11]

RE: Compression Ratio

What is the question?

[X11_STE]

The Q is how to find the compression ratio of a given engine. I would answer this except my auto tech (aka school) books are in the garage in another town and the formula they give is grossly inaccurate.

[RacerX11]

You take the volume of the combustion chamber/cylinder with the piston at BDC, and divide it by the volume of the combustion chamber with the piston at TDC.

Marty

[tha_SwEdE]

http://users.spec.net/home/emxjc/compression_ratio.html

But whats described there??

[DEI37]

The other think that you can do is go a compression test, and then figure from there. If you have a PSI reading of 150, then you have approximately 10:1 compression, given that 14.7psi is atmospheric pressure, and you would divide your PSI number by 14.7.

[geoffinbc]

The other think that you can do is go a compression test, and then figure from there. If you have a PSI reading of 150, then you have approximately 10:1 compression, given that 14.7psi is atmospheric pressure, and you would divide your PSI number by 14.7.

Well no because if you have worn rings or a cam with huge overlap then you will have way different compression readings and that formula makes no sense.

[TurboZ24]

Yeah, there is no formula for calculating the "Static" compression ratio based on the cranking PSI of an engine. For example, I run 185 cranking PSI on mine, but I'm only 8.0:1 static compression ratio.

[Guest]

Yeah, there is no formula for calculating the "Static" compression ratio based on the cranking PSI of an engine. For example, I run 185 cranking PSI on mine, but I'm only 8.0:1 static compression ratio.

There is a formula for calculating SCR, but I have never seen it to be accurate, usually off by at least 2 full points.

Besides that 14.7 ratio is absurd, Considering that is a measurment as sealevel, under perfect atmoshpering conditions. So you'd have to know your exact barametric pressure at the TB inlet, recalculate for venturi effect, through the TB, and then again through the runners, for key crank time, down to the nanosecond, the fact that your girlfriend is calling from the apartment telling you that dinner is ready and that she feels that you love your car more than her, since you stick more things in it in more places than you do to her, wind speed and angle of incedence to the TB, among many other varibles and then you could be within about 1.8 points of true SCR.

[TurboGTU]

The only good way is with liquids and mesurements.