Here's the poop on the fast burn heads from the GM Performance parts site. I can't seem to find the specs on the ZZ4 heads right now either.
12464298 Cylinder Head "Aluminum Fast Burn"
The all new 23° Fast Burn small block cylinder head has the highest performance potential of any 23° small block head developed by GM. The Fast Burn head combines new technologies with the best of GM motorsports and production cylinder head technologies. The design creates tremendous power on engines from 350 to over 400ci. The name "Fast Burn" refers to the head's ability to quickly and completely burn the air fuel mixture, resulting in higher cylinder pressures and more power. The shape of the combustion chamber is designed to accomplish this "Fast Burn" with flat top pistons, so flat top pistons are recommended with this cylinder head. It is not recommended that the combustion chamber be modified or reshaped, as this could decrease the efficiency of the chamber. This head is designed with a .400" deck. This new technology removes material from other portions of the head, allowing for considerably larger ports and water jacketing. The deck also provides unsurpassed clamping force for cylinder head gasket retention. The super rigid .400" deck thickness can be machined down to .340" safely for all-out performance applications and higher compression. Other ways to adjust compression ratio with the Fast Burn head include top of piston design and piston installed height. Unlike the GM Performance Parts Bow Tie heads and most aftermarket performance heads, this head requires no additional porting for maximum performance. In the past, the industry has added material to heads to allow substantial porting, which can result in poor "out-of-box" performance and additional cost. The Fast Burn head utilizes GM Performance Parts' Cast-Ported technology, which means that improvements in flow, combustion and cylinder fill were incorporated into the machining tooling, achieving maximum performance "out-of-box". While additional porting is not recommended, light sanding to remove minor casting imperfections and polishing of combustion chambers and exhaust ports is acceptable. All Fast Burn heads are CNC machined to exacting tolerances, thus eliminating the need for "blue-printing" of machined tolerances, resulting in a cost savings and unsurpassed "out-of-box" performance. This head has taller than typical rocker cover rails, providing exceptional clearance for rocker arms and valve train supports typically used in all out performance applications. The rocker rails are CNC machined for superior rocker cover gasket sealing. Front head faces are drilled and tapped for typical accessory drive bracketry. The Fast Burn head accepts both center bolt and early style four bolt flange mount valve covers. Signature etched with GM Performance Parts logo. Intake manifold mating surfaces are drilled and tapped for both Vortec and conventional raised port style manifolds. "D" shaped 78cc exhaust port and runners provide adequate flow for applications well in excess of 500 hp. Raised runner (.240" higher than conventional 23? head) intake ports with 210cc ports and runners provide adequate flow for applications well in excess of 500 hp. Raising the top of the intake valves provides a better "line-of-sight" through the port and onto the back side of the intake valves. The 62cc fast burn combustion chambers - the most efficient ever to be incorporated on a GM Performance Parts cylinder head - produce higher cylinder pressures by burning more of the available fuel before the piston starts its power stroke downward. By more completely using available fuel, the engine produces more power per quantity of fuel. The 2.00" hollow stem lightweight intake valves are utilized to reduce loads on valve train systems at high rpm's. The 1.55" sodium filled lightweight exhaust valves have all the same benefits of the hollow stem intake valves, and additionally they are able to perform under extremely high-temperature performance applications. This head has specially designed "deep" valve seats which can accommodate up to 2.02" intake valves / 1.600" exhaust valves. Lightweight valve spring retainers combine with the lightweight valves to help ensure long-term high rpm durability. Screw-in 3/8" rocker studs are used, and accept most available roller rocker arms. Uses all conventional "low cost" readily available 23° rocker arms and valve train supports and hardware. This head is a bolt-on 30 hp increase when used on our ZZ4 crate engine. When tested on a 383 ci small block engine with 9.5 to 1 compression ratio and .540" lift roller cam, single plane intake manifold and 750 cfm Holley carburetor, the Fast Burn heads produced 497 hp at an incredibly low 5800 rpm. Total ignition timing to be used on a Fast Burn head will vary based on a number of factors, but most configurations made the most power with 34 to 36° of total timing.
Technical Notes: Use intake manifold P/N 12496820, P/N 12366573, 12496822, or 10051103 with the Fast Burn cylinder head. Use intake manifold gasket P/N 12529094 and eight attaching bolts P/N 12550027. Use Fel-Pro exhaust gasket P/N 1470 for these Fast Burn heads (some trimming may be required for your application. This head includes intake valves P/N 12555331, exhaust valves P/N 12551313, valve spring cap P/N 10212808, and valve springs P/N 12551483.
Notice that the fast burn heads are worth 30 hp over the ZZ4. If I'm remembering right the ZZ4 and L31 iron Vortec heads are the same except for material. The iron Vortec is the best as cast iron head GM has ever offered. The fast Burn 385 with the GM H.O.T. cam installed is supposed to make 425 HP. I rode in a '68 with this combo at Sharkfest a few years ago. It was very fast but the owner complained that it didn't sound hot. The idle was quite smooth. A product of modern hyd. roller cam designs.
I think that the Fast Burn 385 has even more potential and I'd like to try one with an aftermarket cam from Crane or Comp that would have noticable lope but not loose it's low rpm responce.
Tom
