I'm thinking about a sbd motor and I wanted to know what a good target dynamic ratio would be to shoot for when running 91 octane?

Second question, how much will this number have to be raised when running aluminum heads?

I'm shooting for around 7.8 dcr at 1893ft. elevation on an iron headed smallblock.

thx

I've been hearing that low-mid 8's is about the max you can do on pump gas. But has to depend a lot on your combo and your tuning.

Poking around I've heard mid 7s for open chamber/large bore engines, high sevens for closed chamber heads, maybe a little more with heat control and lowish 8s with aluminum heads and fuel injection. I was hoping to pin it down more than that.

You need to know the whole combo. What pistons, what heads, and what cam? Very generally, under 8:1 will feel soft down low. 8-8.25:1 is about max for open chamber iron heads. 8.25-8.7:1 is about max for closed chamber aluminums.

I was looking at the new iron ram heads with the small combustion chamber. Flattop pistons and either comp's XS268S solid 230/236 or XS276S 236/242 at 0.50". Then again I do not know if the extreme energy line up builds more pressure than usual so that again may through the numbers out.

Does anyone run over 160 psi cranking pressure in a small block and have no gasket issues or ping issues on 91 octane?

I run 165 PSI in my 318 with tight quench and high swirl iron heads (302s), it runs fine on 89 in the summer heat and 87 in the winter and I run a 195 t-stat. More preasure and more octane would be a good thing. I am thinking about going from 9.5 static comp to 10.00 by swapping head gaskets from .039 felpros to .028 mopars that also have a smaller bore. My pistons are .005 below deck. My cam is a factory 273 2bbl cam. Also a thermo-quad carb to help keep the fuel cool. Heat crossover is blocked.

My dads geo metro when new had 220lbs cranking compression when new and never pings on 87

Mine is 11.5 static and has 185-190 cranking with aluminum heads. I can run 93 with zero issues.A lot of your dynamic compression is controlled by the camshaft

The block will be a 360. I was looking at the diamond piston set in at 0 deck. 9.9 cc dish. .040" gasket.

I'm new to the engine building thing so I am trying to err on the cautious side.

P/N 51405 - 21cc dish, 1.457 compression height. With a 62cc chamber is 10.25:1 or so static, and with the 4" crank, you need a cam that will be closing the intake valve by 47°@.050" after Bottom Dead Center on the compresssion stroke. That will give you abour 8.3:1 dynamic, with a .040" quench distance. That's pump 89 range.