318 Build.. Zero Deck or Stock Deck Clearance?

318willrun said
He asked about compression, so I answered that he is at the compression already that will be challenged by todays gas. In other words, he doesn't need to "deck" the block to gain compression, IF all he is after is compression. His compression with those heads and pistons will be more than adequate for a street engine, no matter if his pistons end up .010 in the hole, or .040 in the hole.
AND...... AND..... the compression will be more than sufficient to perform.

The problem is that you're making a whole bunch of assumptions to get to the compression he's going to end up at. You're assuming the pistons will be .010" in the hole, but they may not be. The deck heights on these engines vary, the factory was not super accurate with them. My 340 was dead on factory specs for deck height and the same bank-to-bank even without a cut, my machinist couldn't have improved it. My 318 wasn't even the same from one bank to the other. So if I had built my 318 the way it was without decking it the compression wouldn't have even been the same on each bank. You're assuming the head volume on the magnum heads too. Are they really 59cc? Probably not, they're usually a couple cc's bigger. And of course that's all with a .026" gasket.

I mean, will it run? Sure. The 318 in my Dart went at least 80k miles the way it was and the couple times I drove it before I tore it down it was fine. But the OP's engine is at the machine shop. For a few hundred bucks more he can make sure that the compression is the same on all the holes. He can set the pistons up at zero deck so he can easily control the quench distance with the head gasket thickness using off the shelf gaskets. And if he does that now his engine will perform better, even if he sets the compression at 8.5:1. Now it would be better if he set it at 9.5:1, but regardless the engine will perform better. Maybe only a little bit, but better.

As far as "today's gas", the 340 in my Duster has a static compression ratio of 9.8:1 and a calculated dynamic compression of 8.2. It runs just fine on 91 octane, and that's basically at sea level with California gas and factory open chamber iron heads. It's piston's are .018" over the deck, so its quench is by no means ideal. It's pretty much at the limit, if I go much past 34* of mechanical timing I can get it to detonate, but it's not a "challenge". I've put over 12k miles of street driving on the engine just pumping 91 octane into it, using the cheapest gas I can find with no additives or anything special. The 318 is way easier to deal with than the early 340, set up for zero deck it's a piece of cake to control the compression ratio just by swapping the head gasket up or down for thickness.