Monday, September 11, 2006

what is a hemi, anyway?

i'm sure most of us by now are tired of hearing all of the hoosier-speak like "dat thing gotta hemi". but, have any of you gear heads asked what a hemi is?

the original hemi idea was bounced around in the 50's by chrysler. it reached full tilt in the 60's with the 426 cubic inch "elephant" motor. the whole deal about this engine was the head design. the combustion chambers in the moster sized heads were literally shaped like half of a grapefruit...hence, hemi-spherical became hemi. this MAJIC shape yeilded lots of power and torque, but at the cost of major compromises. the valvetrain was uber-complicated, still being a cam-in-block, or pushrod overhead-valve, design. to have more quench and create a high enough compression ratio, the pistons had to be domed. there were cooling issues along with much valve clatter, horrible milage, and completely uncontrollable emissions. but when tuned properly, they made enormous amounts of torque and power. the basic hemi design is still used in drag racing circles around the world.

so how does chrysler get away with that sort of engine today? simple. it doesn't.

the modern "hemi" is hemi in name only. it's actually better than the famed hemi of old. to make monster power from such a lightweight, compact design, chrysler engineers have resorted to tricks beyond those that basic hotrodders can play. the cam, still in the block, is mounted so high that it pushes more OUT on the pushrods than up. this keeps the pushrods short and valvetrain inertia low. the rocker arms are shaft-mounted, very low in the compact heads. the pushrods enter the heads at such an angle and at such a postion that they intrude much less on the intake and exhaust ports than any other pushrod engine. the kicker? the combustion chamber is not even remotely hemi shaped. it has two spark plugs in each chamber, firing at different points to create both more power and lower emissions.

the result is an engine that is both lighter and physically smaller than overhead cam engines 3/4 of the size. if not for it's length, it would conceivably fit into areas that would normally house dohc V6's.

hmmm. the old, much maligned detroit V8 turns out to be more advanced than plenty of new-age wonder-motors. i just wish they didn't call it a hemi.

2 comments:

Daniel B. said...

Thanks for the lesson. Now when somebody asks me, "Do you even know what a HEMI is?" I can provide a thoughtful answer. BOO-YAW!!

come to think of it... I'd be willing to bet that people that work in a Chrysler showroom don't even know that stuff, TK.

tk said...

ah, but marketing is a powerful tool, young daniel-son!