Look closely at this Triumph engine and you see a small tuning adjustment to scavenge more power from the twin. The spark plug has been repositioned to be directly over the center of the piston. What is the reason for this? Well the valve positions relative to the chamber and domed piston head provides the most efficient and powerful bang from the cross flow pattern within the chamber. Of course carb tuning and ignition timing are essential to attain these extra ponies!
Found online, here is a good summary of a Hemi engine…..
The term Hemi which is short for Hemispherical which simply means half sphere which is what the combustion chamber looks like, with one valve on each side.
The Hemi has been around for a long, long time in various forms. Early Triumph motorcycles used this design in 1937 and continued using it up until 1988. Chrysler borrowed ideas for their design from Weslake which built speed parts for Bonneville salt flat racers. Chrysler really utilized it best in automobiles in the early fifties and it’s still the all time power king in racing. A drag racing blown hemi produces in excess of 6000 horsepower which is many times what anything else is capable of producing.
The hemi has four main advantages:
1) The spark plug(s) reside directly in the middle and top of the combustion chamber. This makes for a nice, even firing pattern. This is why Hemi engines have spark plug holes right in the middle of the valve covers. Other “wedge” engines like Chevy and Ford V-8’s have their spark plugs off-center which is cheaper to build but less efficient.
2) Larger valve size. Draw a circle on a piece of paper. Now draw a line through the middle of it. Now draw two circles on ONE side of that line and you’ll see how most ordinary V-8 heads are designed. To draw a Hemi, place one big circle on one side of the line and another big circle on the other side of the line. Bigger valves mean more efficient and rapid airflow through the engine.
3) Larger combustion chamber surface area. The chamber is of the pentroof design (used by Ricardo in the 1910’s) which means it’s kinda funnel shaped. More area means more even heat transfer and distribution. More fuel and air can be crammed into the combustion chamber thus producing more power.
4) By placing the valves across from each other, the intake and exhaust flows across the combustion chamber. On a standard wedge head, they enter on one side, turn and cross and turn again to exit the cylinder. That’s inefficient. The Hemi is a true flow-through design.
Today, there are numerous manufacturer’s who make more efficient designs than the 2-valve Hemi. Most of the DOHC 4-valve designs will easily outflow a Hemi to a point. They have more valve area, however the size of the ports is usually very small increasing intake and exhaust velocity. That’s great if you want maximum combustion efficiency but not great if your goal is to move massive amounts of fuel and air through the engine.
For today, the modern Dodge street Hemi is primarily a sales tool. The name, reputation and mystique alone will sell cars. It does produce great power & torque and is definitely an option worth considering.
………
Basically big lungs, Big Bang, big power!
