I know of the calculators. I'm really more interested in how it is calculated. Thanks, Ted.


Lawrenceville, GA

heres another article with a calculator at the bottom.

http://members.uia.net/pkelley2/DynamicCR.html

Kenny  Onalaska, Texas

Charlie,

Try this:

www.wallaceracing.com/dynamic-cr.php

Butch Lawson

Manchester,  TN

I’m performing these calculations in an excel spread sheet.  Here’s the formula for dynamic compression ratio.

 

=((B6*B6*(B7-((((SQRT((B5*B5)-((0.5*B7*SIN(RADIANS(B12)))*(0.5*B7*SIN(RADIANS(B12))))))-(0.5*B7*COS(RADIANS(B12))))+(0.5*B7))-B5))*12.87036997)+B8+B9+(B6*B6*B10*12.87036997)+B11)/(B8+B9+(B6*B6*B10*12.87036997)+B11)

 

B5   = rod length

B6   = bore

B7   = stroke

B8   = head cc’s

B9   = piston dish cc’s (use a negative value for dome volume)

B10 = deck height

B11 = head gasket cc’s

B12 = degrees ABDC that the intake valve actually closes.

Lorena, Texas (South of Waco)

Thanks Ted,



Shortblock disassembly not an option at this time. While previous posts would indicate running wider than Isky recommended lash might produce smoother idle, It's reassuring to see my original target chamber volume should be OK with their specs. Can always experiment with increased lash later on, as other setup factors might provide additional latitude. Wish I knew for sure how much iron I can safely remove around the intakes? Will have to study sectioned head photos.

6 VOLTS/POS. GRD. NW INDIANA

Ted, What is the formula? I'd like to figure mine.


Lawrenceville, GA

For a solid lifter camshaft and using the closing event as measured at the camshaft and not at the valve (using the actual valve event will numerically increase the dcr value), I lean towards 8.0:1 dynamic cr being the safe limit for 91-93 octane gasoline so with that in mind, 70cc combustion chambers are required without altering the piston dome further.  The 70cc value is based on the assumption that the intake valve is closing at 60° ABDC which appears to be a valid number if the E4 camshaft is installed 2° advanced (106° intake lobe centerline) and the lash is set at 0.018”.  Advancing/retarding the camshaft as well as altering the valve lash makes the cr value flexible.  Here are the numbers using the same 60° ABDC for the intake closing event.

 

If the camshaft is indeed installed straight up (108° intake lobe centerline), then the dynamic compression ratio reduces slightly from 8.35:1 to 8.23:1 with the same 0.018” lash adjustment.  Decreasing the lash further to 0.015” while staying at 108° intake lobe centerline drops the dc to 7.72:1 so you can see where lash adjusment is a big player.

 

If remachining your pistons into flattops and leaving the heads as they are, then here are the numbers using the same 60° ABDC intake closing value.

If the rotating assembly is already balanced, then this may not be an option unless ‘overbalance’ has not already been figured into the bobweight calculation for your particular rotating assembly.  If only 2-4 grams of oil was used for the bobweight calculation, then machining the pistons is an option which would automatically add ~27 grams of overbalance to the bobweight calculation and would not require rebalancing of the rotating assembly.

Lorena, Texas (South of Waco)

Ted,



Assuming dome volume estimation is in the ballpark, can you calculate what target chamber CCs would bring dynamic CR into the 93 octane safety zone using this formula? It did also occur to me that much improved flow that might result from extensive intake unshrouding on only a few cylinders could be just as detrimental to smooth running as existing CR variations?

6 VOLTS/POS. GRD. NW INDIANA

Not knowing the dome volume for the pistons, I simply estimated it at half of normal and used 5cc’s.  The problem with using assumed values is the overall accuracy is compromised and the final compression ratio numbers still end up being fuzzy.  Here’s what the calculations look like based on what’s supplied.

Lorena, Texas (South of Waco)

The critical part of the dynamic compression ratio calculation is the value used for the intake valve closing.  The connecting rod length is a minor player but does make a subtle difference in the dynamic compression ratio calculation.  For a solid lifter camshaft, the intake closing value is easily obtained when degreeing in the camshaft but it’s typically not stated as such on the spec card supplied by the manufacturer.  Here are the numbers for the 314 combination using out of the box cc’s for the G heads but using an estimate for the intake valve closing.  This combination will run well on 89 octane and could possibly get away with using 87 octane fuel if not being flogged heavily.

Lorena, Texas (South of Waco)