I'm spec'ing out my short block. For multiple reasons I'm probably going to stick w/ my #5 heads, but want to leave open the possibility of changing to the Edelbrock SB heads at some point. Because of the difference in chamber sizes I will need different head gasket thicknesses in order to keep the compression ratio manageable around 9.8, which means w/ the iron heads I would end up w/ a quench of 0.060" and w/ the aluminum heads it would end up being 0.036". What does "optimal" quench depend on, specifically for these Olds heads?

 

The usual number is .040". It can be risky once you start going below that number. More of a chance of parts hitting, especially with high lift cams and higher rpm. Go way above, I had pinging with .065" on a 9.6 to 1 motor with a small cam with 91.

 

Not sure what your current heads are, but 5, 6, and 7 heads are typically 68-70cc’s, not the 64 you see stated in most places. New Edelbrocks are 68cc.

 

Thanks - I should be able to verify the actual volume soon. What is the ideal range for quench using Olds heads?

 

With steel connecting rods recommend 0.040" to 0.045" quench. Factory used a steel shim head gasket of about 0.017" compressed and the deck clearance was generally around 0.025", provides total quench of about 0.042"!

 

The reference to steel rods makes it sound like quench is just a valve clearance consideration, which makes sense that that would be the primary consideration. Beyond that, though, what is optimal? Is the goal to have the piston as close as possible to the head without interfering with the valve in order to get peak power and efficiency?

 

The goal is to get the head-to-piston clearance near 40 thousandths.

Closer than that risks clash of piston and head under very high RPM.

Wider than that risks ignition of the mixture in that clearance volume. That would create knock and piston damage.

 

olds70supreme, the issue is not about piston to valve clearance but piston to cylinder head clearance as mentioned above by VC455. Piston to valve clearance in a second generation Oldsmobile motor is not an issue unless you will be incorporating a radical camshaft grind. You need to consider the dynamic of the piston rocking in the cylinder, clearances being taken up during acceleration and deceleration, heat expansion of the metal, etc. We state that 0.040" quench is the goal, more if using aluminum rods; and yes, too much quench invites detonation! No doubt there are some folks out there that run a tighter quench than 0.040" but I would not want to say to you that 0.036" is safe and then have you crying because you had a piston strike the cylinder head and maybe break a ring land! Try to stay within that 0.040" to 0.045" quench regardless which cylinder head you are using.

 

How much does it matter? Not much, really. But 40 thou is the general rule of thumb for pump gas engines.
The easiest thing to do is use a 0.040" head gasket and zero deck the block. Poof! A perfect 40 thou quench.

 

^^^^^^