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What exactly is a "Hemi"?

I’ve hear this term all the time. I think I heard the teenagers when I was a kid talk about “hemis” too. “You gotta get a hemi George!”. But I have – and had – little idea what this term means, or why I’d want it. I’ve always assumed – well really guessed – it is short for “hemisphere”, and that it must have something to do with the shape of something. The shape of the combustion chamber maybe? But if hemispherical was the best shape for the combustion chamber, wouldn’t all engines use that shape, so the term would be redundant?

'lil help … what exactly is a “Hemi”?

Yes, Hemi refers to the domed shape of the engine’s cylinder head. Half spherical. The thing about hemis is that the nature of the shape of the chamber forces you to splay the valves out so they fall on opposite sides of the cylinder, producing a very large cylinder head. The shape of other cylinder heads are known as “wedge heads” because both valves are aligned in the same direction producing a wedge shape.

Chrysler developed it back in the early fifties, and they produced quite a bit more power than the extant engines of the time, due to the efficient nature of the Hemi. The tradeoff of course is that hemis are (usually) much heavier than wedge heads because of the huge cylinder head arrangement. (think of those characteristic wide valve covers with the plug wire emerging from the middle.

But, hot rodders discovered quickly where hemis REALLY shine. Supercharging. Hemis took quite well to forcing large amounts of fuel/air mixture into the cylinders without detonating. Plus the engines themselves were quite stout and stood up to the abuse quite nicely. For a while, at least.

You’re correct; the combustion chamber is hemispherical in shape. An advantage is that engines can breathe a little better in the upper RPMs and they’re less prone to spark knock.
Higher production costs led to the demise of the Hemi at one point in Chrysler history.

Hemi heads have been used on overhead valve Harleys since the mid 1930s. Here’s a pic of some reproduction Harley heads which illustrate the combustion chamber pretty well. Note the right side pic of the three.

True hemispherical combustion chambers are actually going out of vogue, even in racing engines, especially with the advent of 4-valve heads. They require a high domed piston to obtain a decent compression ratio. I believe those new Chrysler “hemis” are actually pentroof combustion chambers.
They used to be used in a lot of performance engines. Look at a Pratt and Whitney R2800 for an excellent example. The old Offenhauser fours were also hemis. A lot of old motorcycle engines used hemi heads also.

Holy cow! $1204.99!!

A friend of mine asked me this question a while back. After giving essentially the same answer as those above during a three minute dissertation on the term ‘hemisphere’ and a bit of internal combustion engine theory, he said “I just wanted to see if you knew”.

Yeah right.

You guessed correctly, it originated as a description of the combustion chamber top (under the head) as being hemispherical. The first time it was done it improved flame propagation and created better breathing ( a valve tilted can make a larger hole in a given sized cylinder when opened than a valve flat), but that was a very, very long time ago (1964) and has been transcended by other technologies such as more than two valves per cylinder.

Well, actually, what I wrote is only a myth. Hemispherical combustion chambers were around and their benefits known long, long before the myth that Chrysler created that it was their improvement. They deserve credit only for exploitation of the idea by their marketing department and probably copywriting of the term “hemi engine” by their lawyers.

In short, it’s a common cylinder design that incorporates a hemispherical combustion chamber that Chrysler marketing had the genius to tag one of their engines with.

Most WW2 radial aircraft engines were hemis, to take advantage of turbo and supercharging.

Chrysler’s first unit was the 1951 “Firedome” hemi, used in the Chrysler 300 Sedan.

The Brits were even in on it with motorcycles dating back to the 30s.

Belgian car maker Pipe used a hemi engine in 1905.

You have to hand it to Chrysler though. They managed to take credit for the invention of hemi engines, the invention of the minivan (a concept that goes way, way back), the invention of the “cab forward” design, the invention of the idea of pushing the wheels way out to the car’s corners for better ride, handling, and interior space, and a few other things that I’ve forgotten. They never actually originated any of these ideas, but they claimed them all as their own.

And look what some of those old Hemi engines are worth now; well into the 5 and 6 digit range.

I remember walking into a local salvage yard back in about 1971 and they had a low miles, complete 426 Hemi with dual carbs and long tube headers sitting there for sale. At the time my thought was cool, but who in their right mind would pay 450 bucks for that rig.

Who would have thought that engine would essentially be an oily 401k sitting there… :frowning:

I dunno… sounds pretty sweet to me. If I were building a '60s muscle car, I’d want to look at it.
Heck, what’s a new Holley 4-barrel go for? $1,200?

@Moutain bike; I think you are confusing marketing with inventing. Chrysler has marketed their engines as Hemis, their styling as cab forward and their vehicles as minivans. This is a method of promoting a product.

McDonalds didn’t invent the hamburger but this is what they are known for.

Nevada, Chrysler has claimed in countless ads over a period of years that they were the inventers of the minivan… and created the impression that they invented the other things I mentioned.

I’m crediting them here for taking credit in their marketing for these things even though they weren’t original ideas. And yes, I gave credit specifically to their marketing rather than their engineering. I specifically pointed out that they did not invent these things.

I’m not confusing anything at all. You perhaps need to read my posts more thoroughly.

Your amusing.

I’m glad you’re amused.

What about hemi engines in minivans? Is that an invention or just ugly marketing?

The current Chrysler/Dodge ‘Hemi’ isn’t. While a good engine, it doesn’t have the hemispherical layout, not that it would be better if it did. It’s sort of a ‘Semi-Hemi’…

I think the proper term for the current generation of engines is polyspherical because the combustion chamber is not a true single dome. But I doubt they would have sold as many Magnums with the word “POLY” on the side.

All DOHC engines are hemispherical/pented/domed. Dodge just registered “Hemi” as a trade name.

Well, these newer engines use a portion of a hemisphere, not a true hemisphere (half of a sphere). So that’s why they’re not a ‘real’ hemi…