The ET Performance assembled, ready-to-bolt-on heads we chose retail for $2,399.95 (plus any options). Carrying PN 215A, these heads feature a 215cc intake runner, 62cc combustion chamber (though ETP can make them to any size you want) and, like all heads in the ETP G III series, an 11-degree valve angle.
Cylinder Heads: The Heart of Horsepower
The single part of the engine that has the most influence on horsepower production is the cylinder head. After all, it's where the air/fuel mixture flows in, gets compressed, burns, and flows out. The shape of the intake and exhaust ports, the angle and sizing of the valves, and the contours of the combustion chamber all have a huge effect on the way in which the mix travels and is burned and, hence, the kind of cylinder pressures you end up with.
There are a whole lot of cylinder heads on the market for the Gen III (see our "Gen III/Gen IV Cylinder Head Buyers' Guide," January 2006) and a lot of them are very good. The choice wasn't easy, but we went with a company that has a reputation for very high quality and exceptional power production: ET Performance.
ETP offers several different styles of cylinder head for LS1-based engines--a line it refers to as ETP G III--with intake port sizes ranging from 215 cc all the way up to 265 cc. As we're putting together what is essentially a street motor that we'll be having some fun with on racetracks, we needed something that would be very driving-friendly. This put us in the lower range of ETP's offerings, and the guys at the company recommended their 215cc heads, which they said work well for "bolt-on 346 to 383 cubic-inch combinations for daily driving." While we had an eye on the 225cc versions, designed for more radical 346-383-cube engines as well as even higher displacements, we were told that for our streetable application the 215s would give us better torque and horsepower across the rpm range. "There is a lot that comes into play when picking a cylinder head, such as what the engine will be used for and the personal opinion of what is streetable and what is not," says ETP's Craig Thibeau. "Camshaft choice and compression ratio also play a big part in selecting the correct head. About the worst thing to do is to have no 'plan of attack' and just buy a product because you are told it is the best on the market. This is a common thing that people don't think of; many overlook the overall combination and how every piece falls together." That said, we trusted ETP was giving us the right head for our ride: an assembled, ready-to-bolt-on head featuring 2.04-inch intake and 1.57 exhaust valves. This particular head is advertised as flowing 320 cfm on the intake and 211 on the exhaust at 0.600-inch lift (3.900-inch test bore, pressure of 28 inches of water). Flow numbers, while always fun to quote, don't always tell the whole story, and there's a whole lot going on with these heads that sets them apart from others on the market.
Looking at the ETP combustion chamber, we note its fast-burn design for increased flame travel, eliminating hot spots and decreasing the chance of detonation. The 2.04-inch intake valve is well suited to our 3.903 bore: its size and placement help unshroud the valve, allowing air to flow into the cylinder with less interference from the cylinder wall than would occur with a larger valve. Interlocking valve seats eliminate the possibility of metal erosion between typical intake and exhaust valve seats.
As these heads are a proprietary casting, ETP was able to fine-tune many areas that couldn't otherwise be altered on a factory or factory-style head casting. The most major change is that the valve angle has been changed from 15 degrees to 11 degrees, which helps promote better low- to mid-lift flow. It also improves piston-to-valve clearance, allowing the use of more aggressive camshafts. The intake valve is moved toward the center of the cylinder bore, decreasing valve spacing to 1.880 inches, and the spark plug relocated. The cylinder head deck is a full 0.800-inch thick, double that of the factory, spreading the load for better head gasket retention. In addition, the head's coolant passages are C5R-style, decreasing the chance of blowing a head gasket into a water passage. The exhaust ports are raised 0.100-inch to improve flow; as with the intake ports, they are an extremely efficient, high-velocity design with increased wall thickness. A raised valve cover rail provides more space for aftermarket rocker arms.
Even with all these changes, the great part is that these heads still bolt right up to factory intake manifolds, valve covers, and headers. Many options are available including various valvespring types, titanium valves, copper seats, Inconel exhaust valves, titanium retainers and locks, and much more.