How much compression should someone expect to see if they are running a 12:5:1 piston at Sea-level? I know mine with the old Samcoe kit was at 195 P.S.I. at 5000 ft. in Utah. does anyone know the ratios? I am just trying to figure out what my exact compression ratio is from the test numbers I am going to get and also what is the standard for fuel Octane? I was running 101 octane with that compression no problems. How high would the compression P.S.I. have to be to need to run 110 octane?

 

Great question Punisher, I was wondering the same thing. Also, what is the exact method for getting an accurate comp. reading on the DS having two plugs and all?

 

Static compression and cylinder pressure are two different things. Static compression is determined be things like cylinder volume, combustion chamber cc's piston dome or dish, etc. Cylinder pressure is greatly affected by cam overlap. My 14:1 engine with large cams has less cylinder pressure than a stock bike! But at 9000rpm it has way more cylinder pressure because of the high static compression.

Cylinder pressure is more of a checking tool for things like valve seal, cylinder seal (blow by) etc.

You can get away with more compression on pump gas with big cams, because they bleed off cylinder pressure down low in the rpm range, which stops detonation.


MrHorsepower

 

I've been using this excel spreadsheet. It seems accurate to me and has correlated well to cranking measurements made on the bike:

http://users.rcn.com/alokaeng/swfiles/dcomp.xls

If you don't have excel there is a free excel viewer that MIGHT work, but I don't think you'd be able to save your changes.

I have a pic shwing the excel spreadsheet in my image station album.

 

the excel viewer doesn't allow you to edit the values in the spreadsheet.

you need to know about how much leakdown the cylinder has and the cam timing (intake valve closing) to use the spreadsheet.

you can enter the barometric pressure and static compression ratio.

 

From the calculations:

12.5SCR = 277psi cranking pressure with stock cams (I.C. = 45 ABDC), 2% leakdown, 29.3" Hg
= 234psi @ 24.8" Hg (5000 feet?)

12.5SCR = 255psi cranking pressure with Web 109 cams (I.C. = 54 ABDC), 2% leakdown, 29.3" Hg
= 216psi @ 24.8" Hg (5000 feet?)

 

I'd say your pretty much dead on with those numbers Scooby, at 4500 ft with my 109's and 12:1 piston I was right about 210-215psi - sea level was about 235-240 psi.

 

http://smokemup.com/

check out their utilities.


jbt

 

SMOKEmUP!...[img]i/expressions/face-icon-small-happy.gif[/img]

 

So if I need 100 Octane at sea level do I only need 92 Octane at 5000'?

Doesn't the pressure determine the pre-detonation?