Part 2: Classic Cubes from a Modern LS1

To take advantage of the large-bore engine, nothing but the best will do to allow this monster to breathe.

Last month we detailed the metamorphosis of a 346ci aluminum LS1 short-block into a 455ci beast. To recap, Motorsport Technologies of Houston, Texas, bored out the stock block and installed Darton liners that allowed the bore to be taken all the way out to 4.160 inches. Combine this with a custom-made Lunati forged-steel crank with a 4.185 stroke and you have the recipe for a stout 455 combo.

The stock LS1 cylinder heads fitted on the '04 GTO were good for a factory head. After all, not many factory cylinder heads can claim they can flow 233 cfm on the intake and 180 cfm on the exhaust at 0.600 inch lift at 28 inches test pressure.

For comparison purposes, the stock '70 No. 64 Pontiac 455 D-port cylinder heads flow 202 cfm on the intake and 175 cfm on the exhaust under the same testing conditions, and the famed 455 round-port H.O. 7F6 heads flow 231 cfm intake and 188 cfm exhaust. Having established what a factory wonder the LS1 heads are, neither they nor the vaunted LS6 cylinder heads flow enough to support the goal of 600 flywheel horsepower.

Jayson Cohen of MTI pulled out all the stops and had Robert Maenza, lead head porter and machinist, custom port a set of MTI Stage III-R cylinder heads. Beginning with a brand-new set of casting 243 LS6 heads, Robert will take us through the porting, assembly, and testing of these fire-breathing wonders.

From raw casting to the finished works of art, MTI invests more than 40 man-hours of labor to produce flow numbers of 336 cfm intake and 240 cfm exhaust at 0.650 inch lift. These impressive numbers are the result of the hand porting and larger 2.08/1.60 valves that are installed. The heads also receive heavier-duty springs and bronze valveguides. MTI's Stage III-R porting is available for your large-bore LS1; while the Stage II-E and R porting packages are perfectly mated to stock bore applications requiring 300-plus cfm on the intake.

According to Cohen, "Our goal was to run the maximum allowable compression ratio for the application without having to resort to racing gasoline or degrading the overall power of the package by pulling out timing to thwart detonation. Our Stage III-R cylinder heads bump the '04 GTO's compression from an actual 10:1 up to 11.25:1. The aluminum LS6 heads with their 64.5cc combustion chambers are a great basis for our heads."

"We don't CNC our cylinder heads," adds Maenza. "CNC'd heads have repeatable ports, but at MTI, we are continually improving our heads. Subtle porting changes such as bowl shape; short turn radius, or valve angles help us maximize short- and midrange flow and velocity, resulting in a better port. Intake and exhaust flow max numbers are often not the best indicators of power potential. A head that flows 300-plus cfm at 28 inches of mercury but has no low- to midrange flow or low velocity won't run as well as our heads."

Having established how robust the short-block is and the power potential of the Stage III-R heads, it was up to MTI to design the camshaft so that the GTO could be driven daily, pass emissions, and most importantly, create big power. Jayson went to the drawing board and designed a grind specifically for the large- bore GTO. Cut on a wide, 113-degree lobe separation angle and it features 228/232 degrees duration at 0.050 and 0.588/0.574 lift with 1.70 ratio rockers.

This cam would be considered marginally streetable on a 346-cubed application. Add cubic inches, and it's docile enough for daily driving without sacrificing that oh so important top-end pull. According to Cohen, "Camshaft selection was extremely important to the overall bal- ance of the package. The wrong cam could have killed the idle characteristics and made this as finicky as so many other overcammed Gen-III powerplants."

Follow along as we complete the engine build and test this powerhouse on the dyno.