A cutaway of the front of...
A cutaway of the front of the LS6 intake manifold plenum. Note that the floor of the intake is near flush with the top of the valley cover. This increase in volume, makes the new manifold 10 hp to the better. Photo: GMPT Communications
Ron Sperry, one of GM's top motorsports cylinder head guys during the '80s, lead the team that did the 1997 LS1 head. One of the restrictions put on Sperry was whatever he did with the intake port, injector targeting, which affects idle quality and exhaust emissions, was to be the prime concern. Ideally, port-injected engines should have injectors squirting fuel straight down the port, directly on the back of the hot intake valve. The temperature helps vaporize the fuel and the turbulence of the charge air blowing around the valve does the rest. For least emissions we want really good vaporization.
While Sperry did a lot of cool stuff with the original LS1 intake port, a compromise he was forced into was port walls that didn't interfere with injector targeting. The fear was if fuel contacted walls, it would end up as droplets or pool on the port floor. Anything other than a fine spray burns poorly and causes exhaust emissions to go up. To keep injector spray off port walls, LS1's port floor was flat, low, and had a short-turn radius tighter than was ideal for optimum performance.
For the LS6 intake port development, the port floor was raised to recontour and soften the short-turn radius. That improved flow as it transitioned into the combustion chamber. When air flows though a port of varying cross-section, as was that of the LS1, there are localized fluctuations in flow velocity which reduce efficiency. The mid-section of the LS6 port was widened and the roof was raised in the interest of better consistency in cross-sectional area. Effects of these changes on injector targeting and emissions were addressed with improved fueling software and calibration.
At high-valve lift, flow improved. At .550-inch lift, just slightly more than the LS6's maximum, flow jumped 10 percent. That GM validated the flow increase at .550 lift means the port has additional potential given more camshaft. Is this a clue to the future? Perhaps.
The LS6 combustion chamber is quite different than that in LS1 heads. The compression ratio (CR) gains four-tenths of a point to 10.5:1, but getting there wasn't easy. A CR increase has a hydrocarbon (HC) exhaust emissions penalty, but GM wanted the payoff: more power, increased thermal efficiency and better fuel economy. The addition of the small, auxiliary catalytic converters engineers call "pup cats" required for the Corvette to meet the LEV standard also allowed the engine the small increase in HC from the higher compression.
The engineer's challenge with the chamber was to increase compression by reducing its size but without shrouding the valves which would hinder air flow and reduce performance. Compared to the LS1, the roof of the chamber in the new head was lowered. This not only decreased chamber size, which increased CR, but it improved air flow over the short-turn radius. Additionally, lowering the roof slightly unshrouded the valves, which also improves flow into the chamber. The exhaust port was also revised with the same goals: improve short-turn radius and make cross-sectional area more consistent. The port exits were given the pronounced D-shape that many racing cylinder heads use. The changes netted about a 10 percent improvement in high-lift flow.
There were some other changes to the cylinder heads that were driven by the revised ports and combustion chambers. Obviously, coolant passages had to be revised. Also, the oil drain back holes were altered to clear the different ports.