I read here somewhere that the advertised compression ratio on a stock 1970 350 HC is really like 9.4 vs 10.25 advertised? Is this correct? Thank you!
Hard to say. Not sure it would be that low, but it is possible. depends on how much the pistons are in the hole.
Yes, but more of a range of around 9.2 to around 9.8:1 than a exact number depending on how much or how little extra deck height a block had. Going by the factory spec deck height number the math makes the advertised what it is suppose to be, but unfortunately the deck height was usually +.030" or more for a sbb 350.
everyone is different unfortunately. the deck height was rarely cut to spec out of the factory. i believe they erred on the side of caution and would rather have them further in the hole than sticking out. that being said there is some chance yours is built close to spec. a teardown is the only true way to know.
Just be aware, to bring it back to factory fresh you'll have to use a set of steel shim head gaskets or the compression will go even lower than it is now under the advertised. The replacement steel head gaskets are $99 vs $45 for the Fel-Pro blue that are twice as thick; http://www.taperformance.com/products.asp?cat=62 Not sure if those prices are for one gasket or for a set?
Roughly 9.55 with steel shim gasket. 9.2 with aftermarket gasket. .040 thick. .058 below deck for piston is about what I have found an a few engines.
Man, I wish I would've measured mine more, exact # down the hole, CC the chambers, etc. I have .042 gaskets on mine and it runs great on 91, so my guess is around 9.5:1 or less with the thicker gaskets.
Last one I did, we used off the shelf .030 hypereutectic pistons (Sealed Power? - been a long time), reconditioned cap screw rods, steel shim gasket and cut the heads accordingly (orig. SP block so didn't deck it) and came in right @ 10.25 SCR and seemed to perform fine on pump gas. IIRC, if we hadn't milled the heads, it would've wound up around 9.6, but that'd be assuming the rod/piston combo. would've put the piston in the hole the same as OE. If I ever run across an original again (just missed a 40k mi. 4-dr '70 Skylark with a HC 350 - see pic) I'll have to check it upon tear down.
Having a static compression ratio between 9-9.5:1 is actually the ideal place for it on today's ethanol infused pump gasoline. Typical aftermarket 'performance' camshafts seem to want to have the intake valve closing between 66-70* (OEM was 71-75*), which puts dynamic compression in a safe zone for use with questionable quality 'premium' fuels ranging from 91-93 octane. So many factors can determine your detonation threshold, but it's much better to err on the conservative side than ending up with a pinging street engine that requires racing fuel to operate properly and safely. Remember these engines don't have all the modern computerized accouterments to control fuel delivery and ignition, so you have to make sure it's in a safe zone under a wide range of conditions (for street/daily use). You can still get plenty of performance out of a slightly reduced static compression with the right components and tune. In fact, it'll perform better with lower compression and advanced timing than it would with higher compression and retarded timing on the same fuel octane.