Originally Posted by dmx6
I have a set of 993 heads and plan on putting larger intake and exhaust valves--any hot tips on what to do to them other wise or should I not use them. Building a 350 bored .030 over and some kind of domed piston to reach 10.5 CR as close as I can. Using the 350 ci 350 hp style hyd.cam.
Bad choice, not just the heads but overall configuration. The hot tip would be to use the 993s for a boat anchor.
The problem is your trying to fix a smog era engine with smog era thinking. This is no way to build power and efficiency (power received for money spent).
The open chamber heads are lazy in that they don't develop a good swirl nor maintain it well. They don't have an effective squish/quench function unless run with a flat top piston. Even then, their spark plug is way off to the exhaust side and poorly placed by the intake valve, this makes for a long burn time which drives you into excessive advance to try and force some efficiency and power out of the engine. Domed pistons only compound the problem because the dome gets in the way of the flame front slowing it further. This combination makes for an inefficient engine that has a tendency to detonate.
The best use of todays fuels is found with the Vortec style head. Against the best prepared 993 head, the Vortecs are at least a 40 horsepower improvement when bolted to any bottom end. They can be had as cheaply on ebay as the cost your spending on the 993s. And "BAM" they pump it up.
The Vortec has a very efficient combustion chamber and excellent porting out of the box. The chamber is a high swirl, high squish/quench design which really extracts all the energy from the fuel that went in there and is very detonation tolerant. It places the spark plug as close to the center of the chamber as possible and favors it toward the exhaust valve side. This gets the burn done quickly requiring less advance for optimal burn. This reduces early pressure trying to push the piston backwards thus producing more power on the crank, and the burn gets over and done with before detonation enters into the picture. This head is essentially the LT4 done in cast iron and built for standard cooling, it is also the cast iron cousin to the Fastburn aluminum head. So it is a huge bang for the buck.
It works best with what's called a D dish, D cup (I love that term) or inverted D dome piston. See the Keith Black web site, and there are others. The total compression with today's fuels and a fast burn chamber needs to be held around 9.2 to 9.5 with a cast iron head and 10 to 10.5 with an aluminum head. Your vision of 10.5 on an open chamber cast iron head just won't work without a lot of octane booster and race fuel. The D shaped dish locates all the volume necessary to trim the SCR (static compression ratio) into what's tolerated by the fuel at its maximum value under the valve pocket. This leaves a flat surface on the piston to counter the matching surface of the head to maximize squish and quench at the optimal values of .040 inch as best to .060 at the max. The squish and quench functions occur from the same parts in different times.
Squish occurs first, as the name suggests it squishes the mixture from the far side of the chamber toward the valve and spark plug pocket. Two things happen; first, the mixture gets one hell of a final stir before the plug lights it. Second, all the mixture is shoved toward the plug increasing the density in front of the plug which makes for fewer miss and hang fires, and speeds the burn so it occurs early in the power stroke but not so early as with an extreme amount of advance that wants to push the piston backwards. But it makes this pressure available from slightly after TDC to about 110-120 degrees after which is where maximum effort of piston on crank occurs.
Quench comes next. Typically as cylinder temperature and pressure increases, the unburnt mixture on the far side of the cylinder reaches a flash over point and explodes before the flame front gets there. The close approach of the flat part of the piston crown to the deck portion of the head creates an area of high surface area to volume, this sinks the excess heat building on that side eliminating the spontaneous combustion of the mixture ahead of the flame front. The limit of .040 inch as a minimum is a mechanical one dictated by the distance the piston needs to avoid colliding with the head. This dimension is arrived at from a combination of the possible build ups of part expansion, bearing clearance, and piston rock in its bore. This is good for a steel connecting rod, aluminum rods need twice the minimum because of their greater thermal expansion. Getting a clearance with a steel rod of more than .060 inch looses much of the effect very quickly. The factory uses circular dish pistons because the same piston can be fitted to either side of the engine, that doubles the production from a set of tools reducing their cost. You get to pay for this in two ways; one, the real compression ratio is always about one full point under the advertised so the engine just doesn't work the fuel you're paying for all that hard. Two, they just let you buy higher octane fuel if the engine happens to ping a little. Then with modern fuel injected engines they use a detonation sensor. When it hears a ping, it just pulls some advance out. The effect is the same as lowering the compression ratio or using higher than necessary octane fuel, you just get less power from the fuel you're paying for.
Anyway, a long way around your question, which hopefully explanes why the 993s and circular dish or in your case domed pistons aren't the best way to get 350 hp out of a 350, which these days is a no brainer to do with the modern parts available to us as used, rebuilt or new. Take the 993s and use 'em to stun cat fish on the weekend.