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what does it mean,when someone say you have a hemi under the hood.

A Hemispheric combustion chamber engine has a specially-designed head with a wedge shaped combustion chamber and the spark plug in between the two valves. It was pretty high tech when Chrysler introduced their “hemi” V8 in the 1950’s, which for a time was the engine of choice on race tracks and drag strips, but since then many other manufacturers have used the hemi design and automotive technology has moved on to bigger and better things. At some point a few years ago, some marketing genius over at Chrysler decided start calling their new redesigned truck engine the “Hemi™” to cash in on the reputation of the old engine. The Hemi™ isn’t actually even a true hemi engine, although it does have a novel combustion chamber design and an interesting two spark plug ignition system.

It’s sort of an interesting marketing ploy-- instead of trying to explain to the car buying public the advanced features of your new engine, start claiming that it has a feature that was advanced a half century ago!

Greasy Jack nailed it. “Hemi” originally referred to the shape of the combustion chambers in the cylinder head. So if you have a Hemi under your hood it means your engine has hemispherical combustion chambers, or at least that’s what it originally meant.

Chrysler is not the only automaker to have made engines with “hemi” heads. I was particularly fond of the 1.6 liter 4-cylinder hemi that Toyota put in their Corollas in the early 1970s.

Greasy, you say “Hemis” have a wedge shaped combustion chamber, I thought “Hemis” have a hemispherical shaped combustion chamber.

Didn’t Chrysler have engines that were refered to as “wedge” motors because of their wedged shaped combustion chambers? (I think the 440’s were refered to as wedge motors)

My text book has a illustration of both the Chrysler “wedge” combustion chamber and the Chrysler “Hemi” combustion chamer, not the same.

[W]hat does it mean, when someone say[s] you have a hemi under the hood[?]

It means you might be the punchline of a Jeff Foxworthy “you might be a redneck” joke.

I think the newer Hemi combustion chambers are more similar to the old late 50s/early 60s polyspherical engines (318s, etc.) rather than being a true Hemi. That’s why the newer ones are referred to as Semi-Hemis.
Ford also used a race Hemi and I think? Chevy even had an experimental one.

While most of their models were sidevalves (flatheads) Harley Davidson Motor Co. introduced a hemi combustion chamber in the 1936 model year on their first overhead valve models (the knucklehead motors) and they continue to this day.

OP, here’s what a Hemi combustion chamber looks like. Don’t know the year of the head but it cannot be any later than 1947.

My understanding is that the hemispherical heads were more difficult (more expensive) to make, but they work better.

The hemi heads were much more complex and expensive to manufacture on the assembly line is the reason why Chrysler eliminated the Hemi at one time.
Weight was also an issue; the older Hemi engines could weigh 750-800 pounds.

Know nothing about Dodge but they’re junk? Is this related to your question on the other post about buying a Talon with a jumped or broken timing belt?
If so, you’re barking up the wrong tree. The Eagle is a Mitsubishi and if the car owner had maintained the car properly it wouldn’t be sitting there with a faulty timing belt and they wouldn’t be trying to palm a damaged engine car off on someone.

The big performance advantage of Hemis in the 50s came from being able to locate the spark plug in the middle of the combustion chamber. That allowed the fuel mixture to burn completely in far less time than it would otherwise take.

With the spark plug located at its normal position at the far end of the cylinder, the flame front has to travel all the way across the top of the piston. With the flame front on a Hemi starting in the middle of the cylinder, the flame front only has to travel 1/2 way across the top of the piston.

The shorter time to “complete combustion” meant Chrysler could get more power by further raising the combustion ratios and further advancing the timing (without incurring detonation).

One of the things that killed the hemi back in 1971 was the need to lower N2O (nitrous oxide) for the new EPA regulations. Nitrogen and oxygen more readily combine to form N2O at higher temperatures (which occur with higher compression ratios).

GreasyJack said " A Hemispheric combustion chamber engine has a specially-designed head with a wedge shaped combustion chamber and …"

No, that’s not right.

Back in the bad old days, many manufacturers had what were called “pent roof” shaped heads. Roughly even-sided roofs on the chambers, the peak being in the middle of the chamber. Almost all (noticably excepting Buick) went to the wedge design, simply because it made room for the use of larger valves.

The hemi, on the other hand, was shaped as a half of a sphere, and was pretty close to round. Supposedly this allowed the pressure wave of the burning fuel to focus more efficiently straight down on the piston, and also allowed free flow for the spent gases from the intake side and straight out the exhaust (which is pretty questionable in my opinion since most wedge motors do the same thing).

Bottom line: hemisperical shaped “Hemi” motors do not have wedge shaped combustion chambers.

In addition, it means that the people talking about it don’t know that it means next to nothing. On a car from the earlier days, it might mean a lot of money.

When Chrysler released the first hemis for the race track (Daytona?), one race car driver (RP?) said, after a test drive, that the power kept on increasing, and the rpms increasing, much stronger than the other chamber designs in use up to that time.

I found a history of the hemi, here:

While I agree that Hemi came from the description of hemispherical chamber heads, with today’s powerful design software and manufacturing technologies sophisticated head designs are a normal part of the engine world. Watching fluid dynamics and flame front propogation as well as thermal imaging can be done right on the CRT…and, actually, cathode ray tubes are even obsolete. Modern efficiencies, minimum emissions, and the horsepower we now achieve with 2 litre +/- engines would not be possible without this added ability.

“Hemi” in today’s world is a marketing word more than anything. A good one, but a marketing word.

It’s often good to remember that most of the performance-increasing automotive engine technology that is touted as being “state of the art” or “new” (with the exception of variable valve timing and computer controls, which were beyond the tech of the day), were invented by the early 20th century. Turbos, superchargers, multiple valves/cylinder, fuel injection, hemispherical combustion chambers, most of the purely mechanical goodies were invented by the turn of the century, and pretty much all before WWII. Good ideas by visionaries. Sure, the materials and implementation have improved since then, but the basic ideas have been around for a long time. CAD/CAM and electronics are really the only improvements that ‘our generation’ can truly take credit for.