I have two different ways at looking at dynamic compression ratio on a solid lifter cammed engine.  One is to simply look at the intake closing as it relates only to the ramp on the camshaft and the other is to look at it from the valve actually closing at the cylinder.  With the later, the dynamic compression ratio is typically about a ½ point higher when taking the rocker arm ratio into account.  When looking at the camshaft closing event only, 8.0:1 is my own limit for 91-93 octane fuel.  But when looking at the dcr with the rocker arm ratio taken into acount and what is actually happening at the intake valve, the dynamic ratio limit is 8.5:1 on the same engine without changing anything else.  It’s simply easier on my end to use the camshaft only value and use the lower dcr value limit than it is to go through the math with the rocker arm ratio and get the ABDC closing number at the valve in which to use the higher dcr limit value.  I hope that made sense.

As far as quench goes, I don’t see a major difference in performance attributes where the piston is 0.065” away from the head versus the piston being 0.035” away from the head if the compression ratio is being maintained the same in both instances.  Where the quench area is being reduced from say 0.065” to 0.035” by raising the piston (or lowering the deck) and the compression ratio increases as a result, there will be a power increase but this will be a from the compression ratio increase rather than from the change in quench.

If the piston is more than 0.065” from the head, then the propensity for detonation is increased and that’s where a loss in power will come into play.  The reduction in quench area by keeping the piston to head clearance nominally close is what helps to push the fuel air mixture back towards the spark plug and at the same time keeps the mixture lively in what would be normally dead or stagnant areas of the combustion chamber.  That’s the simple explanation.

Rather than altitude, knowing where the top of the piston is exactly within the bore at a given point ABDC is what I use for calculating the dynamic compression ratio.  As the DCR calculation relies on the intake valve closing event, then the connecting rod length and its subsequent angle within the bore becomes a more accurate determination for the DCR.  Because the rod angle changes as the rod length changes, the length of the connecting rod becomes a necessary part of the equation.  As the rod length gets longer without changing anything else, the DCR does decrease even if only slightly.

Regarding head gasket volumes, you must keep in mind that Y-Block head gaskets are not circular at the fire ring and are subsequently much larger than the bore size as a result.  I use 10.12cc for the Best Gasket big bore head gaskets.

Not knowing what your rod length was, I just used an available & generic aftermarket scrub length in the following calculation.  This calculation is simply done on an Excel spreadsheet.

grovedawg (7/14/2010)

---

.....My question is this?  If I want a durable, STRONG street fighter what should I do?  Leave everything as it is?  Or cheat a little on the pistons with a dome?  What would you reccomend?  I'm not a genius at timing, tuning, or what have you, but I am patient and would get it right!

Here’s the workup with the 6.125” long rods.

As was brought up earlier, subtle changes in rod length do not make big changes in the DCR but the information is still necessary for the calculation simply due to the Trig math involved to get the angle which in turn determines exactly where the piston is at in the bore at 70° ABDC.

Both the static and dynamic compression ratios are simply guidelines.  There are no absolutes.  If there were some definitive or set in stone numbers for either the SCR or DCR that worked in all cases, engine combinations could be nailed down right at the beginning of the build.  Unfortunately, there are a multitude of other variables involved which would also include the timing curve, fuel mixture, actual combustion chamber burn characteristics, engine temperature, camshaft overlap, spark plug heat range, variability in rated pump octane ratings, etc. that all end up affecting what takes place when the fuel mixture is ignited.

The general rule about maintaining a maximum of 9½:1 static compression with iron heads is a very generalized rule but can be manipulated or increased by varying any of the above.  Where your particular combination is at 10:1 SCR but the DCR is 7.7:1, then just put 91 or better fuel in the tank and go forward.  After you get the engine running, perform a cranking compression check and if you are at 180psi or less, then pump premium should be adequate when used with a realistic ignition curve.  At 7.7:1 DCR, the cranking compression is expected to be less than 180psi.  If you end up with some clatter (detonation) then revisit the ignition timing curve or some of the other variables that were mentioned.  Be forewarned that any detonation that’s experienced can be a problem with head gaskets and especially when using milled unposted heads.