When we are talking about setting the compression for a certain build, are you using the mechanical compression or calculated compression based on a certain cam? For instance it seems to be a common opinion to set compression for S&S 570's at 10:1. Is that mechanical or calculated? If it is mechanical, isn't the calculated somewhere around 9.2:1? I know the cam discussions are all day long on here, I am just wondering.

 

that would be static (mechanical)

 

OK, so static is what you are looking for in a certain combination?

 

well,its a start.you want a cam to match your complete build,so corrected compression & ccp is also very important.but we`ll start with a static ratio and fine tune the desired compression from there to get the corrected we`re looking for

 

A lot of the Wood and Lieneweber are ground in a fashion that they like/thrive with more than your average bear.
And then the squish/quench area, or lack of in certain applications, comes into play, along with fuel quality, etc, etc, etc.
Scott

 

Scott and Kirby are the professionals and I am but a lowly shade tree DIY guy. I use a target of 9.3 corrected (mechanical/dynamic) compression and 185-190psi for CCP which, IMHO, I believe a good starting place for a "street" build or even a touring motor. Once I narrow down the field at that point, I look at cam profile data like LSA, TDC lift, etc. to "fine tune" the selection to the intended purpose of the motor. Static CR will be what it will be once I have made the selection but does not drive the selection process. JMHO.

 

Standard (or static as Kirby calls it) compression ratio is calculated by the formula.


Chamber Volume + Displacement
CR = ------------------------------------------
Chamber Volume

and is generally expressed as XX.X to 1 (reduced ratio)

"Corrected" (which you call "calculated") is sort of the same but the displacement is corrected for intake valve close since pressure cannot trapped in the cylinder until the valve is closed.. There are exceptions to this but it still provides insight into how the motor will perform.. This calculation is a little harder as the position of the piston needs to be calculated which requires some trigonometry to figure out.. Big Boys has a calculator that does this for you..

What does corrected compression do? If gives you an idea of what the dynamic compression ration will be at lower speeds. Dynamic compression definition is the actual volume trapped under dynamic conditions.. Some call corrected and dynamic the same but I prefer to call them different as do the better simulators. Anyway since setting the corrected compression can give you an idea of what kind of cylinder pressures you can generate when the motor is turning slower speeds.. Typically corrected compression on a HD type motor with squish seems to work best at about 9.0 to 9.6 to 1.. Since this number correct CR based on and intake close, it shows that cams with later intake close do better at higher compression.. Most seem to that longer cams always move the power band up. While they can, the amount cam be reduced by increasing the compression so that the corrected compression is the same.. Failing to to so is a typical novice mistake.

An example would be a Andrews 26 (35 deg intake close) and Andrews 60 (56 deg intake close) will have roughly the same corrected compression ratio if the 26 build has a 9.8 to 1 CR and the 60 build has an 11.2 to 1 cr..

 

In a 110"er the school-bus 255 cam will hammer out 205-207 psi ccp.........and those engines are 9.2 cr from the Moco.
On the flip side, you'll have to set that same engine at 11.8-12.0 cr to carry 200 psi ccp with an S&S .640.
That last, is a combination I'm only using for reference.
We ran 17.25 cr in our AHDRA 108" Mod Class bike........
Scott

 

17.25? What fuel?

 

Either VP, C-14 Plus, or C-16.
Almost like alcohol.
Scott