And why are they supposed to be so bad ass to have?
An engine having a combustion chamber that is (part of) a hemisphere, with the spark plug in the centre.
Other designs of the day, e.g. the Chev small block, had kidney shaped combustion chambers, with somewhat less predictable turbulence and combustion propagation. The Chev big block (and others) came in “semi-hemi” configuration, with poly-spheric heads and canted valves.
A true hemi is instantly recognisable because the spark plugs pass through the rocker covers. A semi-hemi such as the Chev big block or the Ford Cleveland small block is also recognisable because of the sheer width of the heads.
It’s not such a big deal now, with quad-cam V8s doing the job much better. However, back in the sixties, the Mopar hemi was big, strong and mean.
Here’s a nice photo.
Are they ordinarily exposed like that? (Or is there a covering I cannot see in that particular photo?)
This probably comes from that commercial - who is it, Dodge? - about the two cars at a stop light. That thing got a hemi?
Just say " 426 dual quad hemi" what else comes close these days?
It’s a rod, dantheman, not some candy-ass stocker. And that particular engine has never been anywhere near a Dodge factory, it’s a Keith Black.
I meant that the OP comes from that commercial, not your photo. Sorry about that. You might not have seen the commercial Down Under, but its on here ad (no pun intended) infinitum.
No sweat. Anyhow, here’s another photo, this time the stock unit in a 71 Challenger.
The stock version was rated at 425 HP, but really produced something like 480 HP.
The engine’s also an over-square design (stroke is shorted than the bore), so it has less trouble going to high rpms. Really quite an amazing drag racing engine for its time.
The Duke boys had fun with it too. (Although not all of the 220 or so cars they wrote off had the hemi installed).
What?! No mention of huffers? A hemi without a huffer is like a morning without orange juice. I don’t know what a huffer is, but all the car geeks I’ve ever been around always talk about hemis with huffers. And a Cleveland was always thrown in there, too. Don’t know what that is either, but any time I wanted to join in the conversation I’d alway say something like, “Whee, boy. I got me a 418 with a 325. It’s got a hemi with a huffer, a four quad Cleveland dual cam manifold and 30 meg of RAM. I tell you wha’, that sucker’ll burn rice in a corn field.”
That would always buy me a couple of moments of dumbfounded silence.
Well, I’ve posted a photo of one, and the history story I linked to discusses them at length, without actually using the word “huffer”. 
Wow, I know what a hemi is and I didn’t understand half of what was said above. Since you’re not familiar with a hemi, I can only assume that you aren’t familiar with “predictable turbulence and combustion propagation”, etc. so I’ll explain the hemi in lay terms like it was explained on one of those TNT hod rod shows.
(Now I’m sure someone will come in and nitpick, for in order to keep this simple I’ll have to exaggerate. Also it’s easier to explain in 2 deminsions rather than 3)
The normal cylinder top is flat, so the area between the top of the piston and the top of the cylinder is basically a square. Now you have your spark plug stuck through the top of the cylinder down into the square shaped empty space. Draw a square with a spark plug entering from the top. When you have your little explosion (from the plug igniting the fuel and air put in by the valves) a little fire ball is created. The graphic on the hot rod show really looked like a tiny mushroom cloud. Now draw a mushroom cloud in your square. You’ll notice that the upper right and left corners are still empty…a mushroom shaped fire ball in a square shaped space. Well that empty space has to fill with fire before the pressure pushes the piston down. Here comes the Hemi. In order to make the engine more efficient, they hemi makers domed the top of the cylinder to more closely match the shape of the fireball. That’s where the name comes in, hemispherical shaped engine. Now when that little fireball explodes it has already filled all empty space and pushes the piston at a fraction of a second faster than the standard engine. Sure it’s a better design, so why did it not catch on and stick around? Well the engine is much more complicated than I explained and when you dome the top of the cylinder is causes some engineering problems with the valves, etc that have to be fitted to the dome shape. They were very hard to keep tuned up properly. Maybe that’s why Bo and Luke were always working on the general?
I have the new 5.7 liter Hemi Dodge. Pretty nice.
What is the purpose for having 2 spark plugs for each cylinder on a hemi? I assume it’s to more completely burn the fuel, but what is the mechanism? Do they both fire at the same time or what?
In the drag race engines, having two plugs is essential due to the RPM and the fuel.
Top Fuel cars run Nitromethane, which is damn near an explosive, sometimes cut a few percent with methanol.
At 6,000 to 8,000 RPM, there’s only fractions of a millisecond to get the fuel burning, and you need it to burn in the cylinder where it can make power, not in the exhaust where it just makes pretty flames.
Lighting the fuel charge in two spots reduces the distance the flame front has to go to burn completely, which means it burns the charge more completely.
Also, nitro is death on spark plugs- they’re usually considerably eroded by the end of the run- as in ruined. I recall one estimate that the loss in spark plug gap can mean the engine is down something like eight hundred hosepower by the end of the run. Two plugs reduces this loss somewhat.
-Yeah, but did you put in a set of chrome piston return springs? That and a set of “Mooneyes” stickers are worth two tenths in the quarter.
Huffer = blower or supercharger
lets not forget the Ford 427 SOHC from the 60’s. AKA cammer.
I never really thought of the 351 Cleveland (or the Boss 302) as a hemi, there is still a quench area.
Top Fuel will use 2 plugs/cylinder because the pressure in the cylinder will try to blow out the spark.
I once saw Carol Doda wearing a “Mooneye” tee shirt at the Fremont, CA dragstrip. Betcha don’t know who she is/was—don’t Google, now.
Second (third?) hand information here, but I asked my step-dad (a car guy) this same question because of those obnoxious Dodge ads. From what I can remember he said that when Hemis first came out they were a big deal, but not so much anymore. There are other hemispherical engines (?) or other powerful engines that are better, but Hemi is the most recognizable. He said Hemis have been associated with racing, which is why they have a bit of a connection with the … stereotypical lower class white crowd (see Joe Dirt ).
How I envy you!
Where is a moderator when you need one:o
Apparently the legendary Vincent Black Shadow V-Twin Motorcycle was a true “hemi” engine as well. The engine was designed by Phil Irving - a self taught engineering genius who left Australia’s shores in the mid 1930’s. Lovers of historic racing will be aware how famous the Vincent Black Shadow is.
During the mid 1960’s Phil Irving was employed by the Repco people in Melbourne to build Jack Brabham’s 3 litre V8 Formula One engines. The block was an Oldsmobile alloy pushrod V8 block which was reconfigured to use Phil Irving’s specially designed alloy DOHC heads - and they were “hemi” heads as well. But they were only two valve per cylinder heads for some reason, and in even though the Repco V8 won the World Championship in both 1966 and 1967, by 1968 Ford had signed off on a little known project in England known as the Coswoth V8 - which featured 4 valves per cylinder - and 168 Grand Prix wins later, the rest is history. The Repco struggled to rev above 9,500 RPM while the Cosworth instantaneously raised the bar up to 11,000 RPM - which was a major horsepower increase. By 1971 Ferrari were getting close again with their 3 litre falt boxer 12, but the Cosworth really held the high ground all through the 1970’s due to it’s superior power to weight ratio. It was a very, very light and robust engine - and in motor racing that’s the Holy Grail. Indeed, it’s lineage continues to this day in Champ Car racing with the Ford Cosworth DFX 2.6 litre turbo V8 - an engine which is ostensibly unchanged since 1968 save for some internal improvements with metallurgy etc. It still runs springs in the valves. No pneumatics involved.
For some odd reason, the word “Hemi” is most often ascribed to the Chrysler people but my research indicates that the concept was largely accepted as the optimal cylinder head configuration for quite some time before the Chrylser folks adopted it. One of the quirks of history I rather think.