The hemi is a dated design that worked in the day when there was very little known about how an engine uses fuel efficiently. The issue with a hemi is that it requires a domed piston to achieve any kind of compression ratio because the chambers are so big. Great right? Domes pistons are high performance stuff! Well, sure, in the '60s, but not so much anymore. There are several problems with the design. First, it creates a tremendous amount of surface area between the piston dome and the chamber. There's literally no way to get more surface area, which is not a advantage for two reasons. First, its a lot of area for a flame front to travel when trying the burn the mixture, so the engine needs a ton of spark lead to burn all the fuel before the exhaust valve opens. Second, its a whole lot of area where a hot spot can develop and lead to preignition. This is one reason why the fuel cars all run dual spark plugs. This allows them to introduce two flame fronts simultaneously to ensure complete combustion, plus even if one plug doesn't fire it ensures at least partial combustion because as close as those engines are to hydrolocking with fuel if a cylinder didn't fire it would probably shear the crank in half and destroy the engine.
From an efficiency standpoint, for obvious reasons, the hemi needs more fuel to make the same power as a conventional modern style chamber and piston design. More of that fuel is also wasted due to the flame front issues. Sure, the most powerful engines on Earth are still hemis, but they run nitromethane and operate under completely different combustion characteristics than gasoline.
What engineers have discovered over the years is that squish makes a combination efficient. In fact, the best design is a dished piston with as little chamber in the head as practical. This creates maximum squish area and maximum efficiency, as well as maximum power. These modern engines look a lot like a Diesel with obviously much lower compression. Joe Sherman built an engine like this for one of the Engine Master's Challenges some years back, which he handily won. He used a dished piston and welded up the chambers in a set of aluminum heads to get them down to something like 30ccs.
What's that got to do with valve shrouding though? Well, when the chamber gets smaller it can be much shallower, which allows the valves to open up into the cylinder without a wall of a combustion chamber to shroud them. Take a look at a modular Ford some time. This is how they are done.
The modern Dodge "hemis" really aren't. They run squish pads and a much shallower chamber to still get some efficiency from the design. Though prior to cylinder deactivation they got truly terrible mileage, like 9-11mpg in the trucks, indicating that its still a compromised design.
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6sally6 wrote:
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