Best way to achieve a desired compression ratio?

[travis]

Say your trying to achieve a 10:1 compression ratio, and determine that with a certain head gasket, deck height, etc that you need 73cc between the head and piston volume. Which would be better (and why)...72cc chamber and 1cc effective dish volume (small dome with valve reliefs), or 58cc head and a 15cc dish? Or 68 and 5? Or 60 and 13? Does it matter?

I know that with OEM sbc stuff, the 64cc heads and a dish piston are usually a lot stronger on the bottom end and more detonation resistant, but how does this play out with a heart shaped, more modern cylinder head?

[mag2555]

The basic rule of Thumb for air flow and hence power is that the shallower the chamber the faster the valves become unshrouded as they open, this generally infere's that a chamber on the small side and a dish piston is the way to go.

In regards to 73 CC chamber and then needing a 1 CC dish I would just grind it into the chamber in such a way that at the max Intake valve lift you are running your grinding out that needed 1 CC where the edge of the Intake valve comes closest to the cylinder wall.

[randy331]

Say your trying to achieve a 10:1 compression ratio, and determine that with a certain head gasket, deck height, etc that you need 73cc between the head and piston volume. Which would be better (and why)...72cc chamber and 1cc effective dish volume (small dome with valve reliefs), or 58cc head and a 15cc dish? Or 68 and 5? Or 60 and 13? Does it matter?

I know that with OEM sbc stuff, the 64cc heads and a dish piston are usually a lot stronger on the bottom end and more detonation resistant, but how does this play out with a heart shaped, more modern cylinder head?

I would bet on the method that puts the most volume closest to the spark plug when the piston is at TDC. That's most likely to produce the best BSFC numbers and therefore the most power.

As an example, I recently dynoed 2 pulling truck engines. One a BBC the other our 421. The BBC is pulling 5-6 gallon an hour more fuel through it than our 421 does, but makes less power out of that fuel. It's very apparent BY the BSFC difference between them. I know it's not a exact comparison but it shows how important BSFC is to making power.


The last thing I'd do is add volume in the chamber far from the spark plug and think I came out ahead on power cause I gained a few cfm.

There's more to making power than chasing flowZ !!!!


Randy

[Zmechanic]

The basic rule of Thumb for air flow and hence power is that the shallower the chamber the faster the valves become unshrouded as they open, this generally infere's that a chamber on the small side and a dish piston is the way to go.

If you can manipulate the valve angles to get a favorable shallow chamber it generally tends to enhance energy recovery/pressure recovery too. As long as you don't get closed minded on the idea of a small chamber (think old school BBC closed chamber heads) and end up hurting something else.

[user-17438]

Say your trying to achieve a 10:1 compression ratio, and determine that with a certain head gasket, deck height, etc that you need 73cc between the head and piston volume. Which would be better (and why)...72cc chamber and 1cc effective dish volume (small dome with valve reliefs), or 58cc head and a 15cc dish? Or 68 and 5? Or 60 and 13? Does it matter?

I know that with OEM sbc stuff, the 64cc heads and a dish piston are usually a lot stronger on the bottom end and more detonation resistant, but how does this play out with a heart shaped, more modern cylinder head?

Most times i try not using head gasket to determine compression ratio,it is normally chamber volume and piston displacement. Off the shelf valve reliefs can sometimes ruin what you are aiming for. As a tuning aid i may use different head gasket thicknesses, but thats a different story.

If naturally aspirated i prefer the smallest chamber period, if needed, a slight dish in the piston is what i like.

As far as detonation goes, use the right fuel with a properly calibrated fuel system. A homogenized AFR is always the best.