Car Tech 101: Hemi engines explainedA Hemi. What is it? Well, it's an engine technology, a trade name, and something of an anachronism today. CNET's Brian Cooley visits Keith Black Racing Engines in Los Angeles to see perhaps the most famous shop to ever build Hemis.
Four little letters, one big burly word synonymous with high-power engines in Mopar power vehicles for decades and resurgently lately, but what does that mean? Hemi is short for hemispherical. Okay. Why that's a big deal to an engine makes for an interesting story. What better place to tell the story of the hemi than the most famous place that ever builds them up, Keith Black Racing Engines here in LA. All right, Ken, so here we are, legendary Keith Black Racing Engines. How long has this place been here? -Since 1966. -Keith Black, Ken's father, began building hot engines for his and his friends race boats in the late '50s. Chrysler came calling in 1965, asking Keith to work over their 426 Hemis, for dragsters, funny cars, and boats. Soon, the Hemi engine was almost a household word. By the late 60s and early 70s, Dodge Hemi Chargers and Plymouth Hemi Cudas were becoming among the most fabled American muscle cars, and today, among the most valuable. What do these engines do that almost nothing else can do? -I'll say it's the amount of power you can obtain from them because of the design of the hemispherical combustion chamber. -And Hemi comes from what? What does it refer to? -For hemispherical, would be the terminology. -So, what are we looking at here? This is what kind of a head? -This is a wedge head. This is basically a conventional wedge-type design head. -Notice how the intake and exhaust valves line up, but side by side across the airflow in the combustion chamber. -In the Hemi head, they're opposite each other, and you can actually get larger valve sizes in there. The advantage of this is the cross-flow of the intake to the exhaust puts a better swirl on the mixture, and with the spark plug being in the center, it fires off in the center of the mixture, which is right over the top of the piston, that gives it a better push down in the cylinder board. -Yeah. So, disadvantage? -Well, the downside, looking from a manufacturer's viewpoint, is it takes a lot more to produce a Hemi head and make it work than it would a wedge head. This is the layout for the valve setup. Your intake valves across here and exhaust valves across here. -Okay. -And because they are opposing each other this way, you have to now be able to open and close the valves. So in order to accomplish that, it has been pretty tough to have one shaft in here. So what they have, and Chrysler did all this stuff and they engineered it all, is they have a rocker stand setup. -And all that apparatus makes for a wide head design and a wider engine at a time when every millimeter under the hood is valuable real estate. -Is it an efficient-- a fuel efficient design or is it more of a power design? -It's-- Boy. If you talk about, you know, solenoids and everything else, I think it'd probably be, in my opinion, more of a power deal, you know, efficiency. -This is making power the old way, right? -Yeah, yeah. -The old school. -Pretty much, yeah. -And they don't use the same kind of pistons that other engines do. -No, they're unique to their own application because of the hemispherical design of the chamber for a high-compression 12.5:1, you need a piston that has more of a dome on it to be able to compress it and will also form in the shape of the chamber along with that. -There's a little downside to a domed piston, right? -Yes. They're a little bit heavier, quite a bit heavier. -Yeah. Let me see this guy. That's a big chunk amount. -It's big. Yeah. -Yeah. And the problem being, when you have that, if you're trying to make a high-revving engine, you've got to pull the same back. Every time it comes to the top of the stroke, it's gotta be pulled back down by the rod again. And there's a lot of way to keep pulling back. -So, for the modern engine maker who's trying to do a high RPM, small, narrow light engine, there are a number of things about a traditional Hemi that doesn't really go there. -It's not a desirable feature. You want a lighter weight piston. You do want to rev them a lot higher-- -Yeah. -smaller board, and stuff; so, that would not be conducive for what they're trying to do now. -Yeah. This remains a very specialized red-blooded-- -Yeah. -American engine. -Horse power.