The Importance of Dynamic Compression Ratio

The Dynamic Compression Ratio (DCR) is not to be confused with the Static Compression Ratio (SCR).  The SCR is the compression ratio number that’s most often referred to when talking compression ratios but it is not the final say in determining if a particular engine is going to be pump gas friendly or not.  While the SCR value may get you in the ball park, it is not accurate enough on those engine combinations that are built on the ragged edge for pump gasoline use versus needing a blend of higher octane fuel and/or racing fuel.  The SCR value is simply the calculation that is the result of the difference in values of the piston being at bottom dead center versus the piston being at top dead center.  Camshaft events and piston connecting rod lengths do not play a part in the SCR calculation which is where the DCR comes into play.

To calculate the DCR, all the values used to calculate the SCR are required but in addition to those the connecting rod lengths and the intake valve closing events are also required.  The DCR value essentially calculates the volume of the combustion area with the piston at TDC versus the volume of cylinder where the piston is sitting in the bore at that point where the intake valve has just closed.  The connecting rod length can vary this value so the length of the rod is also required in the DCR calculation.  Because the intake valve closes after bottom dead center and the piston is on its way back up in the bore, the DCR value will always be lower in numerical value than the SCR.  When engines are built on the borderline of being pump gasoline capable, then the DCR value becomes much more important than the SCR value.

A case in point is a freshly built 312 Y-Block that came into the shop to be checked over along with a change in plans to install a set of Mummert aluminum cylinder heads in place of the iron heads that were originally going to be used.  Already installed in that short block is an Isky E4 camshaft that is installed with 4° of advance.  The combustion chambers on the aluminum heads were measured and found to be averaging 56.8cc.  With the aluminum head chamber values thrown into the numbers mix, the static compression ratio is 9.9:1.  When the 6.245” rod length and the 51½° ABDC intake valve closing events are added to the calculation values, the dynamic compression ratio is 8.6:1.  That becomes a problem as the DCR limit for an aluminum headed Ford Y is 8.2:1 when using 91-93 octane premium grade gasoline.  That 8.2:1 value is at the top side of the limit and is where the fuel octane, ignition timing, and carb jetting do not have much latitude for any variance.  If putting together an aluminum headed engine so it has a margin of safety built in, then 8.0:1 DCR is a good target regardless of the SCR number.

The short block on that particular 312 engine was already assembled and the customer is not very excited about changing out the pistons to make it more pump gasoline capable.  To use the Isky E4 camshaft with this particular set of aluminum heads, either custom dish pistons would be required or extensive work performed to the cylinder heads to increase the combustion chamber volumes.  At this point his pocket book is already smarting from the amount of expense already put into the engine.  The 312 engine is bored 0.040” over and has cast flat top pistons that are ~0.034” in the hole.  With the factory iron heads having the larger combustion chambers, the DCR using the Isky E4 camshaft would have been okay with 91-93 or even a lower octane fuel.  Here are the numbers for the iron head combination with the ‘supplied’ camshaft.

Small cam / Iron heads

Using the smaller combustion chambered aluminum heads does create a problem with this particular engine combination in that the DCR is increased substantially to the point that pump gasoline cannot be used without causing some form of future engine damage.  Attempting to run the ignition timing at that point where aluminum heads are known to run at their best will cause detonation issues to come to the forefront.  Backing up the ignition timing so that the engine will not have detonation issues simply takes away from the power.  In addition to that, the retarded ignition timing will also bring forth heating issues where there may not have been any before.  Here are the numbers when using the aluminum heads instead of the iron heads and leaving the original camshaft in the engine.

Small cam / aluminum heads

Here is the short list of some basic things that can be done to lower the dynamic compression ratio (DCR).

  1. Lower the static compression ratio (SCR)
  2. Install longer connecting rods
  3. Increase the advertised intake duration of the camshaft.
  4. Increase the lobe centerline of the camshaft (more distance between intake and exhaust lobes)
  5. Retard the camshaft
  6. Tighten up the valve lash
  7. Any combination of the above

In lieu of taking the engine apart and changing the pistons to lower the static compression ratio or doing extensive work on the cylinder head combustion chambers, a different camshaft grind is looked at to determine what can be done to reduce the dynamic compression ratio.  In doing it this way, the SCR would remain the same but the DCR can be lowered making the engine pump gas suitable even with the higher SCR.

Lowering the SCR by working on the pistons or heads is out of the question so it looks like a camshaft change is the direction to pursue.  Now the question is “Which camshaft to use?”  By inserting various combinations of intake closing event numbers, lobe centerline values, and degrees of camshaft advance into the DCR calculation, it can be determined what the camshaft must look like to get that DCR value back down to 8.2:1 or less and still have a reasonably streetable and drivable engine.  In playing with the numbers, the Isky 301-S-598 grind will lower the DCR enough that pump grade premium can be used and still be able to run the ignition timing where it’s optimal for both performance and economy.  That particular camshaft has an advertised duration of 272°, 238° duration at 0.050”, and is ground on 110° lobe centers.  Here are the calculated dynamic compression numbers when substituting that particular camshaft into the equation.

Larger camshaft / aluminum heads

So there you have it.  The short block did not have to be reconfigured as a later intake closing event by lieu of additional intake duration and an increased lobe centerline value lowered the dynamic compression ratio enough so that the engine would still be happy on pump grade premium gasoline.  In running the engine on the dyno, it would still idle at 850 rpm but it was definitely happier at a 950 idle rpm.  Until next time, happy Y motoring.  Ted Eaton.

Previously published in The Y-Block Magazine, Issue #162, Jan-Feb 2021