[Keaka26]

I have heard the term "hemi" used in relation to a motor when talking about cars like a Charger or barracuda....but then i heard it also used to describe our own 4g63....so tell me, what IS a hemi, by definition?

 

[jdbTSi]

http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=hemi+definition

 

[cOiL]

a lot of people are calling the new dodge hemi a "semi-hemi" because it is lacking an important portion of what a true hemi had. i dont remember what that difference is, but i read it in an old automotive news. :dunno:

-j

 

[NickSimcheck]

New schoolers... I bet you don't know about "Rocket fuel" either?

 

[JayHäss]

Hemi is short for Hemisphere, as in half a sphere.

Back in the 60's where high technology was having more than one carburator, Chrysler made a head with the chamber shaped like half a sphere and spark plug in the middle. It's better for combustion and makes more power. It was the big technological "oooooooo" of the time.

Basically all DOHC cars now are hemi's but with 4 valves.

 

[Keaka26]

"rocket fuel" is that old stuff they used to use in Formula cars (i think) which was composed mostly of xylene and toluene....

 

[itdood]

Hemi is short for Hemisphere, as in half a sphere.

Back in the 60's where high technology was having more than one carburator, Chrysler made a head with the chamber shaped like half a sphere and spark plug in the middle. It's better for combustion and makes more power. It was the big technological "oooooooo" of the time.

Basically all DOHC cars now are hemi's but with 4 valves.

Just to add to the definition...

The big performance gain for a hemi design are the "canted" valves. A hemispherical combustion chamber allows for the valves to be tilted toward the air flow and allows for the intake / exhaust ports to be more straight going into the cylinder, which of course helps with air flow. In a none hemi design, the ports need to take harsh turns to lead into the cylinder.

Making HP is all about moving more air through the motor, so there are huge gains to be had with head design, so a hemi head w canted valves is the ultimate in head design.

Big block chevies were refered to as semi-hemis because the exhaust valves were canted, the intake valves were not canted as much to save space. Because of the canting, the head is much larger than a none-canted setup. That's the only draw back to a canted (hemi) design is the amount of space taken up by the head and it's increased weight.

I'm not sure if the center plug placement is necessitated by the larger head or just because it helps with performance. The plug location may just be needed because there is no other way to get them in the combustion chanber due to increased head size.

the 4g63 looks like a true hemi design to me. It's awesome to know I drive a turbo hemi!! even though it's only a 4 banger. When I grew up, it was the shit to own aything with a chrysler hemi. those were / are bad ass.

One thing I've noticed about the 420a in NA 2gs, the head is small, so I assume those valves are not canted (if so, not much), so the head probably doesn't flow as well as a 4g63.

 

[16G TSi]

simply put, its a head design that has the valves directly inline with the airflow.

Intake valve directly in front of the intake port, and exhaust valve in front of the exh port.


In most SOHV vehicles, the I/E valves are inline from front of engine to rear, where as Hemisphereical heads are rotated 90 degrees.



std SOHV:

I/E I/E I/E I/E



Std hemi:

I I I I

E E E E