RSR BMW Advanced Dynamic Compression Ratio Calculator
BMW k1200RS: Intake closes 40 deg ABDC measured at .050" lift. Bore 2.775". Stroke 2.953". Static Compression 11.5:1. Rod Length 5.118".
It's the System...Not the Parts!
see what the effect of bore, stroke, rod length, cam timing,
compression ratio, boost pressure and altitude is on your dynamic
compression ratio, simply enter the seven variables and the calculator
will display the results in a new pop-up window. This will give you an
idea of what happens before you assemble
your engine and also how you should plan your motor for your target
altitude whether it is for racing or day to day operation. Of the
variables, the most important is cam timing which has a dramatic effect
on your "dynamic" as opposed to your static compression ratio. The more
"overlap" your cams have, the lower your "actual" as opposed to your
static compression ratio will be. Assuming your motor is somewhat
"fixed" then this is the one area you can easily change.
Make Your Motor Work For You...Not Against Itself
you are designing a new motor another consideration is rod length and
it's effect on the engine's output. Changing the rod length will affect
the piston location in the bore relative to cam timing everywhere
except at TDC and BDC thus changing the dynamics of the engine. Piston
design cam selection, and static compression can planned in a more
thoughtful manner with the use of this calculator. The calculator will
generate your "chamber volume" for you as a target for your "cc'ing"
Cheat, Cheat, Never Get Beat
you play around with this you may see why RB Racing has run "high"
compression ratios on certain turbo bikes when we were running at
Bonneville which is at an altitude of 4500 feet. Corrections on a
particular day at Bonneville may render this to be a "much higher
altitude" due to temperature and other conditions...hot thin air may
actually look like 7000 feet! Building a motor that sacrifices torque
for peak power by running too low a compression ratio can be a mistake
in some cases, especially if you live at higher elevations like in
Denver, Colorado. Secret camshafts have have big role in this...but
secrets are secrets!
in mind that it is airflow that we are concerned about and that RB
Racing Turbochargers are designed to move large volumes of air at low
boost levels. Do not confuse boost with airflow...it is CFM that makes
the torque and power, not pressure alone.
final point to consider in this scenario are the mechanical issues of
bore size and combustion chamber efficiency...you have to make leeway
for these. On less efficient combustion chambers you may elect to keep
your static compression lower and compensate with boost. There are a
myriad of considerations and experience will beat unproven theorems
time after time. A bit of forethought and knowledge never hurt your