I need to settle on a target static compression.  Does this dynamic cranking compression sound right?  154 psi.  Here are the variables that I plugged in giving the calculated 154 figure..  Alt. 1330, bore 3.86, stroke 3.44, rod length 6.25, intake .020 before closing is at 58 degrees ABDC, static compression 9.7:1.  The 154 psi seems lower than expected so I would like someone else to have a go at the numbers. 

I edited the first post.  Maybe it is clearer now. 

Also, I dont recall seeing the ceiling of cranking compression for running regular.  What is it, and would it be raised a little with E10?  Iron heads in use for all of this.

With Mummert's rods, popup pistons and their nice pin location, shooting for prescribed CR sure is a lot easier.

Since you seem to be playing with a computer program to predict the compression PSI rather that taking an actual real world measurement what would running a 10.5 to one compression ratio and using a much longer duration cam have on cranking PSI? Can you set up the computer program to forecast  PSI by changing all the variables? If not, which ones can you change?

I take it you already have the cam? If your worried about the cost of premium gas have you looked into a knock sensor  or just restrict ignition timing as gas mileage is really most effected by vacuum advance with the engine under almost no load while mechanical advance can occur under extreme load. Pete

With the cam advanced 4 degrees, and using the resulting 54 deg. ABDC in the calculator, and bumping up the target static CR to 9.8, a dynamic cranking pressure of 162 psi, with Dyn. comp ratio of 8.13 is the answer.  It looks to be a good objective.

With a comb. chamber of 70cc, piston dome vol. of 5cc, and piston to deck clearance of .004, the static CR works out to 9.82.  Cool.

I used the calculator you linked to and I did not come close to what you posted either the first or second time. I think your cam specs are not accurate as a cam you describe as mild should not bleed off as much compression as shown. Something ain`t right. Pete

It just occured to me that the calculator does not compensate for solid lifters vs hydraulic lifters. Edit

You should ask your cam maker. He is the expert. Pete

Mike.  A little bit of math shows that the 58°ABDC value you state is a good intake closing value for the 260° camshaft when installed straight up (108° intake lobe centerline).  This is assuming that the cam manufacturer is using the standard 0.020” measurement for the advertised duration measurement as is typically done for solid lifter cams.  Hydraulic camshafts will typically use 0.006” as the value for calculating the advertised duration.  Using your values I do get the same 9.7:1 static compression ratio (SCR) and an 8.1 dynamic compression ratio (DCR).

If advancing the camshaft 4°, here’s what the DCR looks like using the same combination that targets a 9.7:1 SCR.

I’ll add that my calculations for DCR do not involve using an altitude value.  Because the cranking speed among other variables influences the actual cranking compression, I use the DCR value rather than the calculated cranking compression value as a more accurate determination of what octane fuel should be used.  Here’s the chart I use for octane rating versus DCR.  Keep in mind that these are for the octane ratings stated on the pump and are not MON or RON octane rating specific.  There’s a safety cushion built into the chart due to the variability found in pump gasoline.

I am going to print off that graph and tack it up.  The target is 8.2-8.25 DCR, at least for now.  Have reread on the topic and quench is a biggy.  Hoping to get a zero deck for quench of .043 to help keep away unwanted explosions.

Edit:  This is worrisome.  I looked at the "mechanical cam specs"  sheet that I ordered from.  Right there it says duration at .020 260 degrees, duration at .050 225 degrees.  Tomorrow will check duration at .050 and .020 to see what is going on.  But if it is only 258 at .006, that doesnt sound right. 

Mike.  Did you check to insure that the camshaft is degreed in?  Be sure to also check the #6 cylinder and to be on the safe side, also check the exhaust lobe centerlines.

The chart I’m using is specific for measurements at the camshaft and not at the valve.  But because the chart is biased in this regard, I don’t worry about the actual lift at the valve.  If I was to actually take into account what’s happening at the valve itself and if using the 0.020” value at the ramp of the camshaft, then valve lift at the valve with 0.020” lash and 1.5:1 rockers will be 0.010” instead of 0.000”.  That means the actual intake closing event would be later by whatever number of degrees that 0.010” takes to close up.

But to give you something to think about, here’s a set of values from a custom Isky camshaft using various potential lash settings in which to determine the intake valve closing events at the cam lobe.

As far as quench goes, I don’t consider it a problem until it’s over 0.065”.  That means the piston can be as much as 0.020” in the hole with a 0.045” thick head gasket before needing serious consideration.  The distance that the top ring is down from the top of the piston should really get more consideration but typically doesn’t.  Do the math on the diameter at the top of the piston and how far down the top ring resides and see what kind of volumes start popping up.  These volumes can be a serious detriment during the actual fuel expansion process.

Thank you for more great information. Can you confirm that you octane vs dynamic compression ratio is based on a fixed amount of mechanical distributor advance timing? If so, what are you using, 38degrees or something else.

The reason I ask is my first car was a Model A Ford that had the spark advance/retard on the steering wheel. So that technology is out there as are knock sensors to automatically retard or limit spark advance. Do you think they are worth looking into? For a street driven car not a dragstrip car.

Thanks again. Pete

Pete.  The chart is simply a guideline.  The timing is whatever is optimal for the engine and not octane based.  For the Y engines, the optimal total timing at WOT typically falls in the 36°-40° range depending upon the compression ratio if you take fuel octane out of the equation.  Unfortunately it’s not as simple as using a single chart in selecting a compression ratio.  As it is, different engines have different efficiency factors and as such, there isn’t a set in stone rule.  Selecting a camshaft falls in the same ambiguous category as differences in head design will dictate different rules from one engine design to the next.

Knock sensors are definitely nice parts to have but if the engine is having to use them constantly to keep detonation at bay, then the overall combination needs some adjustments.  Compression ratio, fuel octane, engine temperature, ignition curve attributes, air/fuel mixture, cylinder head burn characteristics, and intake manifold sizing among other factors all have a part in this.  Consider knock sensors a safety cushion in the event something out of the ordinary puts the engine in a position to experience detonation.  In general, the ignition timing needs to be optimum for the engine combination.  That test is performed with a fuel high enough in octane that detonation will not occur if optimum timing is exceeded but the power loss comes from cylinder expansion pressure occurring too early (or too late) in the cycle.  If the ignition timing has to be backed up from optimum because of fuel quality or some other outside factor, then the ignition timing curve or even the engine combination as a whole needs to be re-evaluated.  For the Y engines, ideal total timing at WOT wants to be around 36° total timing with high compression and 40° total with low compression.  For the GM Vortec heads, 24°-26° total timing is the norm.  Cruising ignition timing is another scenario that is tuned separately for.