Can we talk..Compression ratios..?

[Mick]

Hi i'm wanting to know more about compression ratios' but really need some more knowledgeable help hence why i've posted it here as some of you are quite well clued up on many 2 stroke aspects..

How do we know a compression ratio of 6:8 is around 100 psi if we assume atmospheric pressure is 14.7..?? 14.7 x 6:8 = 99.96psi..


I have a TDR125 and recently fitted a rebuilt engine with rebored barrel(+1mm) and got a reading just over 80psi(manual states 6:8 comp ratio),at first i thought this was too low as i had read on "other" forums compression needed to be around 150psi on newish engines but wondered where does the 150 figure come from.?.It was only when i started to look into compression ratios that i realised this is impossible due to the varying ratios manufacturers give..
Also a question someone has asked  is where is the corrected ratio taken from..??see below

To get the corrected compression ratio, you substitute the stroke with the effective stroke. Where the effective stroke is the measurement taken from the top of the exhaust port to the top of the cylinder, minus the distance from the top of the cylinder to the top of the piston at tdc. But, where to you take the measurement from with a powervalve? Do you measure with the valve closed, or valve open? With the valve closed the measurement will be greater than with it open as the blade varies the exhaust port height.

[EEKNOWS]

PSI readings from cylinder pressure gauges can be changed by many things so much so that the readings become irrelevant.
aonly thing that really matters is if you run high comp does the fuel, ignition and cylinder head match?

[Mick]

Ok then the TDR125 i have is  Belgarda one(italian) it comes as a full power version,original barrel(they always stamps theres' up with a "P" between 3MB00 and 124 cc,head,ignition standard but there are no specs or manauls for the Belgarda model so how could i find out it's true compression ratio..??

[Neal]

Is you cylinder upright on the motor ? If it is then you remove the head , use a little grease to seal the rings then replace head . Remove spark plug , pour in some premix from a pipette or burette and measure the volume of fuel poured in . It will be around 11- 13cc . From there you will know what your head volume is with the piston at TDC . You can work out your ratio from there