Hemi engines

[roperz111]

I've seen a re-emergence on the Dodge/Chrysler commercials of the hemispherical engine, better known as the "Hemi". Richard Petty raced this engine design in his heyday in the early 1970's when he was Plymouth's driver. I'm curious about a few things here.

First, how much dome radius protruded from the top of the piston on those old hemi's?

I know my Dad's old D Alky Mercury motor (700ccH) was referred to as a "looper". The intake ports were angled such that the gas/air vapor entered the chamber at an upward angle, then the path of the vapor "looped" in a circular direction upon hitting the top of the cylinder. This would allow more vapor material into the cylinder, thus increasing the power (I think?). But, I also remember the piston was rounded at the top, which I believe not only helped facilitate the looping action, but also reduced the squish area as well.

I'd love some feedback on whether anyone believes this "hemi" principle works, or if not, why?

Why is Chrysler bringing this out again? lol

Also, the Yamato 102 pistons have a slight rounded dome on top as well. Is this there for the squish area reduction only?

Thanks,

Chris Johnson
111-Z

[David_L6]

It appears to me that you are confusing two different things.

Hemi (hemispherical) is the shape of the combustion chamber. Two stroke or four stroke doesn't enter into this.

Looper (loop charged) is two stroke stuff that I just got interested in the last couple of years. Fascinating. Cross flow is another two stroke stuff term (think old Mercury).

[pro350hydro]

Ask Anthony, He is a tech GURU here and builds HEMI heads.

Hope you get an answer

:D

[Bill Huson]

I've been wrenching on Mopars since `62 when I aquired a `58 Plymouth ragtop (think "Christine" with a soft top). That beast had a 318 - not the famed Hemi which I believe came in 361ci, 413ci, and 427ci displacements, the 427 being the NASCAR mill.

The piston dome was a simple way to adust compression and the amount of "pop-top" varied depending on how much compression one wanted. Us shade tree kids could also aquire pop-tops for the non-Hemis, but I was always spending too much money chaing girls so I relied on milling the heads and decking the block to get more squeeze while still having $$$ to play with my squeeze.

I haven't examined the new Hemi, but if Chrysler really wanted to kick some butt the new engine will have dual overhead cams (instead of those anchient pushrods/rocker) and 4 valves per cylinder. The new Hemi is 5.7 litre/360ci. I'm thinking new Dakota p/u down the line and the Hemi is a bit much for that truck.

[Bill Huson]

Yamatos are schnuerle (sp?) ported and engines of that design have a domed piston. In 2-smokes the piston top design is what works best for induction/exhaust flow for a particular porting design. Compresion ratio adustments are usually handled by the head/combustion chamber design.

[modifiedoutboard]

Dodge 426 Hemi was the legend at the track.
New dodge engine may or may not have hemi head chambers
but it certantly is not a 426 Hemi of old.

Hemishperical shape gives max volume with min distance flame has to travel, makes more complete combustion.

Ford made a 427 overhead cam hemi but Nascar would not allow it in.

[roperz111]

Actually David, the term "hemi" refers to the hemisphetical shape of the top of the piston, not the chamber itself. Or, at least that was what was explained on a do***entary show. I was comparing that shape and its capabilities to the dome-shaped piston tops we are running now. I didn't mention anything about 2-stroke vs. 4-stroke.

CJ
111-Z

[Composite Specialties]

.... they are this far (holding fingers close enough to pinch a dime flat ways) from fraud!


Heyyyyyyyyyyyy, I work for DaimlerChrysler, I did not know we were that close to fraud! LOL

[Bill Huson]

Those doco boys and girls need to check their facts. The Hemi had a hemispherical combustion chamber and domed pistons to emulate the combustion chamber shape. Most engines of that era of two valves per cylinder had wedge shaped combustion chambers with both vales on the same side. The Hemi had the valves on opposite sides. Dead giveaway on sight was the real wide valve cover with depressions/holes in the center for the centrally mounted spark plug.

The piston dome on the Hemis was a bit taller than the one on the Yamatos. Even taller in the hotter versions that boasted compresion ratios of 11.5:1 and 12.1:1 (figures from my foggy memory and may not be accurate)

The 318 in my `58 was a semi-hemi, cambers almost hemispherical but with the spark plug opposite the valves. That head design dissapeared shortly after the introduction of the 273ci engine. Chrysler liked that cheaper to build and lighter design and punched it out to 318ci, 340ci, and 360ci. Yes, the heads on my 5.7litrel/360ci van engine can be mounted on a 273 block. One must niotch the top of the cylinder wall to clear the 2" exhaust valve to make that switch. The 318ci and 340ci heads are a no hassle swap.

The downside of the new head design was that it didn't produce the max horsepower of my old 318ci engine. That beast with a 10.5:1 compression ratio and a few mods to the ignition system made 425 horsepower at 5,600 PRM. The solid lifter valves floated around 7,200 RPM, never got around to beefing up the springs. Probably a good thing since the way the car was I got myself banned from driving in the fine state of North Carolina for two years (unless I filed SR-22 insurance which I did) after literally blasting a speed trap into weeds on I-40. I saw the timer trip-strips and locked the brakes up, ripped the strips up and sent them spiining into the bushes. I think my registered speed was 84 and I took the fifth when the trooper asked how fast I was really going :-)

[Merc55H]

Chrysler originally built the Hemi engines for their circa 1952/53 model lines. Dodge and DeSoto had small Hemi engines. Early inboard hydros ran the small Hemi's before the advent of the small block Chevy. Chrysler Hemi's started out as 331 cubes then 356 and finally 392 in 1957. They stopped making the Hemi I believe in 58 coming out with the wedge motors 318 and up, 361, 383 , 413 and 426 motors were all wedge engines. In 1962 or early 63 Chrysler came out with a "new" Hemi design which is the motor that kicked ass for years in drag racing. The Hemispherical heads have valves opposite one another rather than side by side in the wedge type head configuration.

[Geezeracer]

In 1962 or early 63 Chrysler came out with a "new" Hemi design

Those were pretty good, but the initial design had a better valve angle that produced better breathing and power and were the choice for racers over the later Hemi, and that layout was incorporated by Donovan and other builders and copiers.

Something similar showed in the move to the GM 215 aluminum engine in 225 inboard. On the surface the three engines should have been equal, but the Buick outshone the Olds/Pontiac by virtue of a better breathing valve layout.

My favorite Hemis on water were Roger Murphey's 7-Liter Galloping Gael...The first limited I ever break 150, and the 266 "Aquaholic", powered by a 241 Dodge so loud that each firing pulse hurt your ears and that pushed the boat fast enough to leave you gasping...This was back around 1960.

John

[Bob Dunlap]

They are called Hemis because of the shape of the combustion chamber. They still use basically the same design in all the current drag cars. Most all the top fuel dragsters and funny cars use the Chrysler designed hemis, even though the engines are all aluminum and made by after market manufacturers, they are still copied from the older design hemis.
I went to the shop tonight and dug out an old hemi piston. It's a flat top but it's not out of a stock hemi. It's out of a top fuel dragster, an old 392 Chrysler hemi from back in the old days. Unlike our outboards you can still get pistons for a hemi in any configuration, domed, flat or dished depending on the compression ratio you want. Higher compression for a carberated engine and lower compression for a blown or supercharged engines and it depends on the fuel mixture. I could write a book about it, but I not a big writer so I'll just show you the picture. You can't compare a 4 stroke engine to our 2 stroke engines but here's a picture of the hemi piston beside my A HotRod piston. "Man" we could sure use some rods like that in the HotRods.:D :D :D

[Fast Jack]

Dont forget the v shape araingement of the Hemi's valves allow the valve to be unshrouded when it is open, it has a better flow than say a chevy, their small block valves are very close to the cyl wall and part of the flow is deverted when the valve is open.