How is figure out your compression on your own

[GoFast.....]

This has been a great site to help me figure out my compression. What is diffrent about this site is you also plug in your Alitutude which affects the compression

http://bigboyzheadporting.com/TwinCamComp.htm

Thanks BigBoyz, I have learn a lot from your site

[tireater]

How much does opening the valve intake pockets in the pistons affect compression...or putting in grooves...?

[mayor]

How much does opening the valve intake pockets in the pistons affect compression...or putting in grooves...?

grooves and valve pockets reduces the compression.  General rule of thumb would be 1cc for grooves (two) and an additional 1/2cc for the valve pockets (-2cc piston volume). 

[Faast Ed]

Hard to be accurate without actually measuring, but I bet subbing -2cc like Mayor said would be darn close enough for most applications.

[Sonny S.]

tireater,

do a compression test then you'll know whatcha got

Sonny

[mayor]

tireater,

do a compression test then you'll know whatcha got

Sonny

not if he has a junk compression tester like I have.   

[Sonny S.]

I don't think he minds spending money...lol

[help-u2]

CR= CV + CCV
     ________
    CCV
CR= compression ratio
CV=cylinder volume
CCV=combustion chamber volume


do some measuring...

altitude?   compression ratio is the difference in the voulme of air/fluid when the piston is at bottom dead center compared to the volume of air/fluid trapped when the piston is at top dead center. 

[Sonny S.]

CR= CV + CCV
     ________
    CCV
CR= compression ratio
CV=cylinder volume
CCV=combustion chamber volume


do some measuring...

altitude?   compression ratio is the difference in the voulme of air/fluid when the piston is at bottom dead center compared to the volume of air/fluid trapped when the piston is at top dead center. 

Yes altitude.  It has an effect on compression.
At sea level you have 14.70 psi of atmospheric pressure filling your CV+CCV. Also at sea level you can attain 29.921 In HG of vacuum.
At 6000ft you only have 11.77 psi of atmospheric pressure to fill the same area in the same amount of time. At this altitude only 24.0 In HG is attainable. So that also changes tuning. Your MAP will have no effect in the 26-30 IN HG range.

That's why IMHO there is no SET CR for a build. A guy that stays at 6000ft. can run much higher STATIC COMPRESSION than a guy at sea level or one that tours.

Sonny

[mayor]

Yes altitude.  It has an effect on compression.
At sea level you have 14.70 psi of atmospheric pressure filling your CV+CCV. Also at sea level you can attain 29.921 In HG of vacuum.
At 6000ft you only have 11.77 psi of atmospheric pressure to fill the same area in the same amount of time. At this altitude only 24.0 In HG is attainable. So that also changes tuning. Your MAP will have no effect in the 26-30 IN HG range.

That's why IMHO there is no SET CR for a build. A guy that stays at 6000ft. can run much higher STATIC COMPRESSION than a guy at sea level or one that tours.

Sonny

I agree with Sonny on everything (although I did just learn something about the affect of elevation on timing).  Just to further clearify, Sonny's essentually talking about corrected compression.  The static compression "is what it is" regardless of elevation. The corrected takes into account when the intake valve closes and the pressure builds.  Atmospheric pressure has an affect on that.     

[help-u2]

thanks mayer for telling difference in cranking compression and real compression.
i was about to throw away my physics books and even the one i wrote for MMI..
lol.
even more fun is figguring out compression for a 2 stroke.  what with the exhaust port being open most of stroke. 
i dont use 14.7 p.s.i. to fill my volume when measuring.  i use a fluid...
so much for math..

[Sonny S.]

help-u2,

Of course everything you were saying about measuring volume is correct, I use fluid too.
Was just the altitude thing I was trying to clarify, and why it was important when planning a build.

Sonny

[GoFast.....]

CR= CV + CCV
     ________
    CCV
CR= compression ratio
CV=cylinder volume
CCV=combustion chamber volume


do some measuring...

altitude?   compression ratio is the difference in the voulme of air/fluid when the piston is at bottom dead center compared to the volume of air/fluid trapped when the piston is at top dead center. 

Yes altitude.  It has an effect on compression.
At sea level you have 14.70 psi of atmospheric pressure filling your CV+CCV. Also at sea level you can attain 29.921 In HG of vacuum.
At 6000ft you only have 11.77 psi of atmospheric pressure to fill the same area in the same amount of time. At this altitude only 24.0 In HG is attainable. So that also changes tuning. Your MAP will have no effect in the 26-30 IN HG range.

That's why IMHO there is no SET CR for a build. A guy that stays at 6000ft. can run much higher STATIC COMPRESSION than a guy at sea level or one that tours.

Sonny

What Plant did you guys say you were from wanted to use this guy for a long time and could not fine the moment untill now.

[Sonny S.]

Barry,

That's it ?   No great words of wisdom ?

[help-u2]

LOL....  and Sonny is also right....  we from the same planet, sonny and i...
a lot to figgure on a build.  including the weight of bike and rider and a bunch of variables.  shift points, riding style, altitude, relative humidity... but i just answered the question as i read it...
also valve over lap.  lets not forget cranking compression is different than running compression.  all sort of fun stuff and math...
yeah.  lots of questions to ask of build so customer is happy.  as a old ex-racer my parameters were narrowed.  but as a sorta fast street rider now my needs have changed...

but math is math...  and generally is repeatable...  is kinda why i stopped posting in numbers type  threads years ago.   and i forget the tech/formula stuff.  sometimes have to open and check the book i wrote on theory of internal combustion engines...
(go ahead and put on those straight pipes...lol.... i wont argue ... not worth my typing time)