compression ratio vs PSI

Steamboat Aka J. R. Stevens /

Anyone, is there a direct relationship between compression ratio and PSI when measured with a meter? If atmospheric pressure is 14.7 psi and compression ratio is 10:1, should my meter measure 147 psi at the spark plug hole? If not ,why not. Thanks Jim.

Re: compression ratio vs PSI

Absolute pressure (PSIA) is one thing and gauge pressure is another. Gauge pressure starts off at zero psi, not 14.7 psi.

Tangible pressure is "zero" or balloons and tires would blow themselves up to 14.7 psi, but forces are balanced so the balloons just lay there flat..

Gauge pressure is always 14.7 psi less than absolute pressure.

A perfect, theoretical 10:1 compression ratio under ideal gas laws would have a gauge pressure of somewhere around:

(10 x 14.7) - 14.7 = 132.3 PSIG (G=Gauge)

Re: compression ratio vs PSI

Holy crap! Then "the old mopeds" have some high compression ratio numbers!

Re: compression ratio vs PSI

Don't forget that 14.7psi is at sea level. Our pressure is 14.45 here in S.Ohio.Don't know about Denver's and other mountainous regions. don

Re: compression ratio vs PSI

well.. its close enough for govt work unless you're gonna get technical about it .. ;)

but i did say "ideal gas laws" and only mentioned compression.. so only Boyles' law would apply. You got temperature and barometric pressure (and altitude) other stuff to consider to get it perfect in the real world.

Re: compression ratio vs PSI

Jason Luther /

joe, what do you do for a living? you are smart and articulate. i hope youre a teacher.

Re: compression ratio vs PSI

Smart? Like everyone else, I'm ok with some things and stoopid as shit with other things..

I've run a small business since accidentally discovering a new product. Poverty, a lack of education and other reasons forced me to study and learn a mixture of things so i could build a few custom machines and manufacture the product myself.

Any communication ability you might detect is just normal, mandatory customer-relations skills, mostly learned through trial and painful error.

Re: compression ratio vs PSI

Jeff Strahle /

what mopeds do you have

Re: compression ratio vs PSI

i have just one type of bike .. '78 Hobbits.

Re: compression ratio vs PSI

The other problem with compression ratios in two strokes is the difference between theoretical and actual ratios.

Theoretical is the swept volumn of the cylinder from bottom dead center compared to the total volumn of the cylinder and the head. Actual is the swept volumn taken from where the exhaust port is closed.

When the manufacturers state a compression you need to find out if it is theoretical or actual.

Since loop scavenged two strokes supercharge the cylinder with the primary compression of the crankcase it is almost impossible to make any comparison of the stated compression ratio and the reading from a compression meter.

Re: compression ratio vs PSI

heh.. you're right of course, but nevermind CR .. who cares about that?

They sell you a bike that is supposedly 50cc displacement, but where does displacement actually start? The piston might displace 50ccs if it were encased in a closed cylinder, but until the exhaust port closes on a 2-stroke cylinder, no useful displacement happens. So we are all riding around on 36.9ccs (wild guess) bikes or thereabouts.

Re: compression ratio vs PSI

Steamboat Aka J. R. Stevens /

Well said,Silverfox. Jim.

Re: compression ratio vs PSI

Joew you are right. I never thought about it.

I guess that they measure the total swept volumn. If all the two stroke makers use this method then they have a common

point of reference for comparison.

Re: compression ratio vs PSI

They should use corrected compression ratio but they don't. I think it's competitive selling technique and mistrust of competitors.

To be consistant, if a bike manufacturer sell a 250cc 2-stroke bike that uses corrected Compresion Ratio, it can't be called a 250cc .. it would be like 190cc. The power stroke and compression strokes are in reality shorter than swept volume..

The bike mfg down the corner doesn't follow suit and advertises the same size engine / bike as a "250cc" .. People wanna buy the bigger engine. Size matters even if its just a meaningless number.

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