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Hagerty
Hagerty Employee

Hemi: How Chrysler drew a dome and forged a dynasty | Hagerty Media

Hemi is the Greek word for half. That makes a hemisphere exactly half of a perfectly round bubble. Unfortunately, the engineers who created Chrysler's 1951 FirePower V-8 didn't need that much bubble. In lieu of a full hemisphere, they capped each cylinder with a spherical shape whose height is significantly less than half its diameter.
https://www.hagerty.com/media/magazine-features/hemi-chrysler-drew-dome-forged-dynasty/
23 REPLIES 23
MickHannickle
Intermediate Driver

As a mechanical engineer I applaud you for an outstanding article Mr. Sherman! The detailed and technical descriptions, particularly of the original engine design, aided by the high quality images reminds me of the sorts of articles I'd read in print magazines of three decades ago. If memory serves, your writing contributed to the automotive media of that era also. I'm happy to have rediscovered you.

Stepwilk
Pit Crew

Best Hemi article I've ever read, great graphics, too. And leave it to Dyno Don to note the Grumman connection.
Tinkerah
Technician

Nice thorough, easy-to-read article. Just some minor questions from a novice enthusiast: My understanding of the magic behind hemispherical chamber design has been the significant increase in flow by aiming the valves away from the cylinder walls. Have I been misled or should it have been mentioned? Also wouldn't a flat chamber offer the minimum possible surface area no matter the chamber volume?
RichH
Detailer

I wondered about that minimum surface area comment too. I'm not an engineer and wonder if there is any relationship between surface area and volumetric efficiency.....
Well this sure clears that up 🙂
https://en.wikipedia.org/wiki/Surface-area-to-volume_ratio
JackVan
Intermediate Driver

The hemi chamber allows a flow through design and plug placement that is a significant improvement in flow and burn pattern over wedges. The actual shape of the chamber and piston fit made for more even pressure across the top of the piston and better flame propagation, while placing less stress on the piston itself. I also thought the minimum surface area statement is a crossed wire that slipped past the author. The hemi provides the maximum surface area for the bore size. You can also use higher compression ratios than wedges because there are no sharp edges to hold hot spots and cause preignition. The 426 hemi head flowed 40% more than the 440 on the bench at .500 lift. That is the real difference. Chevy and Ford hemis did not reach the same flow rates as Chrysler because they did not match the same valve angles and flow pattern, just made up for it with wilder cams. The WW2 aircraft engine guys really knew what they were doing.
Tinkerah
Technician

Thanks JackVan - lack of sharp edges allowing higher compression makes perfect sense to me, I'd never considered that! But I question "flow through design": for a good portion of the cycle both valves are closed halting all flow. I understand there's tremendous turbulence and an explosion but there can't be any inertia toward any particular exhaust route. I'm still imagining phenomenal gains by unshrouding the valves, even if that were the only improvement. Not trying to be argumentative, I deeply enjoy these discussions.

win59
Detailer

A great technical explanation with enough detail for it all to make sense, and some history so I now know why.
Don Sherman at his best.
Thank You!
DAW1
Intermediate Driver

Lots of car manufacturers have hemi head engines. Chrysler capitalized by patenting the term.
timktng
New Driver

Excellent and informative article. One minor point however. The 1958 Plymouth had a high performance engine option called the Golden Commando, believe it or not. It was the first of the new wedge shaped combustion chamber engines for Plymouth. It was an extra cost option even on the "58 Fury as I recall. My family special ordered one but before it could be delivered the dealer called my father and told him he had a two door hard-top Belvedere, black with a white top, that had the Golden Commando option and would he like to see it. I was sent to see the car and immediately opened the hood to see the gold painted V8 with two four-barrel carburetors that I had seen in the car magazines. It was advertised as having 350 cubic inchs displacement and 305 horsepower. Motor Trend reported 0 to 60 time of 7.7 seconds. For the 1959 model year, the Golden Commando displacement was increased to 361 CI and the duel four-barrels were gone. Horsepower was said to be the same, however in actual use the '58 was the winner every time. My minor point is the Hemi replacement engines, with the wedge shaped combustion chamber, ran from 350 to 426 cubic inches, not 361 to 426 as reported in the article. Otherwise, the best article on the history of the great Chrysler motor company engines I've ever seen.
gkz
Pit Crew

My brother had a '59 Sport Fury, 361 Golden Commando, Dual Carter 4BBL, Dual Point distributor. Maybe the Sport Fury received the 2 4BBL.
SJ
Advanced Driver

Just the clarification I needed, great article. Ford had to copy the design to beat it with the semi hemi.
JackVan
Intermediate Driver

They did not do a good enough copy. Neither did GM.
Gary_Bechtold
Instructor

Great article and great history here. This was a fun read.  We need more of these!

brb
Advanced Driver

These articles are great! Keep them coming.
Shumifan
New Driver

Great article. I was not very informed about the history of the Hemi. This is the best summary that I have ever read in many magazines. Kudos Mr Sherman
RichH
Detailer

Ho Hum.
My '76 XJ6C is a "hemi", as are all straight 6 Jags from 1949. OH! And DOHC and IRS too! Ditto for my '75 Dino. 

RichH
Detailer

And my 16V, DOHC Miata is sort of a "hemi" too.

DAW1
Intermediate Driver

and my 1960 Jag MK2, but no IRS 

DAW1
Intermediate Driver

Gee whiz my 1976 Toyota Corolla with the 1.6 Litre was a hemi.  Lots of cars have hemis.  Check out the 2.5 litre V8 Jaguar/Daimler engine. It looks just like a Chrysler product but for sure they would not be allowed to say, “Yeah , it’s got a Hemi”.  With the Chrysler engine, it is just big.  

RichH
Detailer

It really is the most efficient head design for combustion, just seems like Detroit has something against the OHCs that complete the package.  Ford's 427 "cammer" (their 616 HP answer to the Mopar 426 - developed in 90 days!) is the notable exception. 🙂

OldCarMan
Instructor

The mid-60s, Pontiac inline six, was a SOHC engine, well thought of at the time, but no further development...
MATTMERICA
Instructor

Thank you for ruining my productivity this afternoon. I just got sucked into this story, and how could you not be?! Now I am going to go bust balls on my buddy who thinks he knows everything about hemis, thanks!
Beestly
Intermediate Driver

Good article, but yeah, no. You’re wrong about the term “hemispherical”. A shape doesn’t have to represent a complete hemisphere or half-sphere to be deemed hemispherical in geometry. Imagine a bowl that is a half sphere. Now pour any amount of water into that bowl. The shape the water now occupies is hemispherical because it takes on the precise geometry consistent with the formula for a perfect sphere. They are not called “hemisphere heads”. They are hemispherical heads. Mathematically representative of a perfect sphere, which in this case, is larger than the diameter of the cylinder and described just like the crater left when the earth is struck by a spherical meteor. As mentioned by others, Toyota, Jaguar, Ford, Mazda etc all had engines with heads they referred to as Hemi heads. Stop the advertising gimmick conspiracy train. The name correctly represents the design. Your argument only detracts from a really fine article and misleads people who don’t know better.