I'm rebuilding this engine I pulled from my camaro (going back in something else... but that's another story).

I have most of my build figured out, but I'm kinda stuck at my compression ratio combination. Not so much what compression ration to run, but exactly how to attain the compression ration that I want.

Some Background...
I've never run the car. Got it as a parts car for $400 bucks. Pulled the engine out and tore her down, and guess what I found, but an old 400 Crank ground down to fit the 350! SWEET!! Instant 383 Stroker!
I was thrilled! Although not exactly a 383, since the 350 block was still on standard bore (surprising for a 37 yr old engine... originally came out of 76 Vette). But I dropped off the block for prep work, clylinders had rust pitting and needed bored out .030 so there... now I got my 383... (as well as cam bearings ,new freeze plugs, magnafluxed... the whole bit).

Also came with an Isky 280 Megacam, standard rods, junk heads and GM intake. So at least I have the Crank and Cam to work with...

SO... I need new pistons due to boring.
Heads need rework anyway, so rather than rebuild, I'm going to put the money towards a set of new heads.


So the plan...
I'll have a 280 Cam, 400 Crank, also new intake (edelbroke performer or similar), new high flow oil system (pump, pan, etc), headers (since I have none anyway), forged pistons and rods and new Aluminum heads. Running mildly moded/enhanced GM HEI, with decent wires.

But here's the kicker... I'm going to run 100% dedicated propane, and bump the compression to about 12 to 1. Give or take a little.
Will break in with a quadrajet running race gas, then go single or dual impco LPG carbs. (eventually plan to go full LPG liquid injection).

I'll still have to figure out the effect of cam timing on my Dynamic CR, but I figure 12:1 Static is a good number to aim for. Plus or minus. Could stand to go slightly higher based on engine dynamics, but I don't want to push my luck.
But, There are many different combinations of heads and pistons that will get me there.

Standard deck height. Variable from .5 to 1.0 CR depending on gasket thickness.
59cc w/ -2cc will yeild about 11.8:1
64cc w/ 6.3cc dome yeilds about 12.3:1
68cc w/ 10cc dome yeilds about 12.3:1
68cc w/ 11.1cc dome yeilds about 12.4:1
68cc w/ 12cc dome yeilds about 12.76:1
72cc w/ 12cc dome yeilds about 12.0:1
72cc w/ 13cc dome yeilds about 12.11:1
72cc w/ 15cc dome yeilds about 12.44:1

There are many more options to yeild similar results...
What would be the pro/cons of going with smaller chambers and near-flattop pistons vs Larger chanmbers and larger domes?

Been reading everything I can find so I'm still learning about performance theory, so I know I have a lot to learn. But haven't found as much on chamber vs piston size. Flame prop better with flattop. a little more surface area with domes increase lbs of force (same with dish, but lower CR). flattop lighter... squish& quench tradoff...


This is going in my "play" truck.
Will drive regularly (but probably not daily).
Will also be used for a little bit of hauling/towing.
But mostly some 4 wheeling, Cruising, Mudding (maybe), and anything else I can thrown at it.

Probably won't rev it up to 7500 or beyond regularly (rarely actually), but 5-6000 might be common if I'm hot rodding a little. So favoring low to mid range a bit. Probably won't ever boost it.

What are your thoughts?
If you can point me in the right direction I would be grateful.

 

I would mill the deck flush with the pistons, use a .035" headgasket, flat tops and 64cc heads. Not sure what cr that would yield.

 

Yeah almost 12:1 depending on how much cc the valve pockets take.

 

Actually, it would "almost" be 12:1 if there were no valve reliefs.
typical valve reliefs for SBC flat tops are about 5cc.
On a 64CC head the CR drop is more considerable than it would be on a 76cc head.

That combo with 5cc valve reliefs in the piston would net about 11.15:1 Static.

This issue isn't really about how to get to 12 to 1.
But why use a particular combo.

Why use small chamber and flat top pistons?
Or Why use larger chamber and Dome pistons?

Both can get you to 12 to 1. But what are the pros/cons to each?

 

I was told by a late model dirt car engine builder (whom I've know 20+ years) to always try to use flat tops. I think it has something to do with better quench.

 

Flat tops promote better fuller equal burn in the cylinders, domes will get hot spots and the cylinders may not burn as complete and equal throughout the cylinder.

 

Yes, always run flattops whenever possible. In a naturally aspirated build with a tight quench, you get the best mixture motion and unobstructed burn.
The shorter burn time pays off in two ways:
1. less ignition lead required for best power, therefore less prone to detonation.
2. less ignition lead also results in more power because you're doing less negative work on the piston as it's still rising in the bore.