I’ve seen many comercials lately that boast about certain new trucks are equiped with Hemi engines. What exactly is a hemi engine and how is it different from a standard, run of the mill 4-stroke engine?
There is a really technical way of explaining it, or, you can say that the engine has hemispherical combustion chambers. Searching. Searching. Damn.
I’m sure someone will find the techie explanation - that is the shorthand version.
The supposed advantage has to do with how the gas burns, the efficiency of combustion.
They have hemispherical combustion chambers. This is supposed to allow for better volumetric efficiency, and optimum placement for the valves. The valves in a hemi are across from each other rather than side by side, which helps the gases to flow smoothly in and out of the cylinder. The spark plug is also located closer to the center of the combustion chamber, aiding ignition.
I don’t know just how much better thay are supposed to be. I am guessing that the new hemis are meant to capitalize on the reputation of the older ones, as much as anything else.
The original Chrysler corp hemi engine of the 50’s and 60’s referred to the shape of the combustion chamber. Most engines of the day had a wedge shaped chamber. Looking at the combustion chamber from the side you would have a triangle shape with the top of the piston being the base, and the chamber in the cylinder head the top, angled relative to the piston.
The hemi, short for hemispherical, chamber more resembled half of a spere, with the intake and exhaust valves staggered. Intake valves were located towards the intake manifold, and exhaust towards the exhaust. This involved a very complex valvetrain with pushrods at odd angles.
Chevy answered with their own design, often called the “semi-hemi”. this was the Mark IV big-block. It still used a wedge shape, but with the valves slightly staggered. This gave them most of the advantages of the hemi, without the complex valvetrain.
The demise of the Chrysler corp hemi was that the hemispherical chamber produced very poor emissions values.
As for the new hemi, I think it is mostly a marketing ploy taking advantage of a well-known classic design.
/Markus
The hemi, as others have said, had a half sphere combustion chamber. The spark pug was at the very top, and when it fired, the flame front burt evenly down to the piston head.
A non-hemi had a flat head (combustion chamber). No mater where the spark plug was placed, when it fired, the flame front hit the piston before it reached the corners of the head. Less power.
A wedge shaped head is OK. The spark plug is at the wide part of the wedge. The flame front does NOT hit the piston evenly but does burn all of the compressed gases.
The bad thing about the hemi is that it is a more complicated layout. The intake and exhaust valves have to be on the round hemi head instead of a flat surface.
Also the spark plug has to be on top so it comes out the top of the valve cover, giving another oil leak path. On a non-hemi head, the spark pug is on the side and does not go through the valve cover.
I just got a Dodge Ram 1500 with the 5.7 litre hemi…it has 100 more horsepower than the 5.9 magnum it’s replacing.
My very limited knowledge of the hemi was that the configurations already mentioned did increase it’s relative power but that it was difficult to keep well tuned, a requirement to actually maintain those power increases.
The “Hemi” head was designed to minimize engine knock (when fuel-air mix pre-ignites prior to the combustion front reaching that area of the cylinder) The spark plug was located at the top of the cylinder head and the smaller volumes near the outer edges of the hemispherical combustion chamber reduced the chances of pre-ignition away from the combustion front.