The prinicipal behind the Hemi engine?

[outrun]

I have to many definitions from "experianced" motor heads.

Please explain...so far i understand at as a "bulge top" piston causing some sort of trick compression?

Thanks-

 

[Dan4510]

Outrun,

Its not the piston, its the placing of the valves and ports in the head. They are placed for almost perfect flow. The more flow, the better the hp.

The piston is shaped as it is to make sure there is enough compression.

Maybe the trick compression you are talking about is turbulence forced by shape of chamber and piston. The more turbulence in a cylinder, the more resistance to knock and better for emissions due to better more even burn of mixture.

Dan

 

[AV8R]

Outrun,

The hemi heads were developed in the 60's when the thinking was 'bigger is better', and that thinking was applied to cylinder head design by Chrysler engineers in the 'hemi' engines. The half-bowl combustion chamber design, and angled valves, allows larger intake valves on these heads, which was thought to flow better in those days when relatively little was known about dynamic fluid flow. The hemi design is considered poor by today's standards. There is no 'squish' area, to provide turbulence and therefore enable higher compression ratios without detonation, and smaller valves can give better flow with runners and camshafts designed to better use inertial effects to superimpose pressure pulses with intake valve opening. Modern head designs on production engines (Vortec on GM, for example) and and high-squish, high compression racing heads are far superior to the 'hemi' design.

 

[Gary Allan]

quote:

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The hemi heads were developed in the 60's

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I agree with most of what you said ...except the 392 and the 361 (Firedome) were products of the 50's

 

[greasezerk]

Well lets put it like this. Dan 4510 hit it right. Sure the engines of today are more power packed than the hemi of the 50/60's. But back in the day you couldnt beat a hemi with a chebby or a fjord engine period. Mopar/mb hit it on the head with the reintro of the HEMI. i dont hear chebby saying available with the 427 or ford saying yeah its got a 428. Hemi = Performance. Keep it in perspective!!!!!!!!!

 

[Les]

quote:

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But back in the day you couldnt beat a hemi with a chebby or a fjord engine period.

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Maybe - maybe not...

1. 1966 COBRA 427 Engine / 425 Horsepower 427 2x4 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 12.20 @ 118 Drag Test Published / Car and Driver 11/65

2. 1966 CORVETTE 427 Engine / 425 Horsepower 427 4 Barrel As tested / 4 Speed Transmission and 3.36 rear Performance / 12.80 @ 112 Drag Test Published / Car and Driver 11/65

3. 1969 ROAD RUNNER 440 6BBL Engine / 390 Horsepower 440 3x2 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 12.91 @ 111.8 Drag Test Published / Super Stock Magazine 6/69

4. 1970 HEMI 'CUDA Engine / 425 Horsepower 426 Hemi 2x4 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 13.10 @ 107 Drag Test Published / Car Craft Magazine 11/69

I have a list with a total of the top 50 cars of around 1962 to 1973. The above are the top 4.

Les (Once owned a 1956 Chrysler Imperial with a hemi engine.)

 

[AV8R]

Interesting, les, By the way, any resemblence of the 'new hemi' to the original is ad hype.

 

[JohnBrowning]

Just as a side note DC's 3.7 V6 and 4.7 V8 have heads that are almost Hemi Heads. They have one flat spot along one side. If not for this flat spot they would be considered true hemi head. They have the same general valve setup and the spark plug right through the center.

Other then for emmison purposes I think that the hemi head design still has alot potential. All of the normal tricks used on pent roof, semi pent roof, wedge heaqd etc... like variable tuned length runners and camshafts designed to better use inertial effects to superimpose pressure pulses with intake valve opening etc...... Surely no one here thinks that the use of a Hemi head by DC was purely marketing when they have to compete with Chevy and Ford for HP and still meet emisions laws.

 

[Bob Woods]

When I first went into the USAF I worked on an engine that had Hemi type heads in a radial layout, it had 4360 cid, 28 cylinders, two valves per cylinder, 2 spark plugs per cylinder, in its turbo charged application would produce one hp per cubic inch at 2800 rpm. We were lucky to get 2000 hours between overhauls.

 

[azoner]

can you say marketing? you could make the same power with a pushrod 360 with a few tweaks, if it was power they were after it could have been done alot cheaper.

 

[cryptokid]

its all marketing. the hemi design was great back in the day, but there is much better designs out now.

 

[tweeker43]

one of the things i look at when judging an engine is power density.

that is, how many horsepower do you get out of 1 cubic inch of displacement.

in this case, my subaru gets 165 horsepower out of about 152 cubic inches, or 1.08 hp/ci.

the new hemi gets 335 horsepower out of 350 cubic inches, or .96 hp/ci.

while the hemi obviously has more hp and torque, it's not as efficient in generating horsepower as many smaller cars.

 

[Jimbo]

quote:

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Originally posted by Les:

quote:

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But back in the day you couldnt beat a hemi with a chebby or a fjord engine period.

---

Maybe - maybe not...

1. 1966 COBRA 427 Engine / 425 Horsepower 427 2x4 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 12.20 @ 118 Drag Test Published / Car and Driver 11/65

2. 1966 CORVETTE 427 Engine / 425 Horsepower 427 4 Barrel As tested / 4 Speed Transmission and 3.36 rear Performance / 12.80 @ 112 Drag Test Published / Car and Driver 11/65

3. 1969 ROAD RUNNER 440 6BBL Engine / 390 Horsepower 440 3x2 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 12.91 @ 111.8 Drag Test Published / Super Stock Magazine 6/69

4. 1970 HEMI 'CUDA Engine / 425 Horsepower 426 Hemi 2x4 Barrel As tested / 4 Speed Transmission and 3.54 rear Performance / 13.10 @ 107 Drag Test Published / Car Craft Magazine 11/69

I have a list with a total of the top 50 cars of around 1962 to 1973. The above are the top 4.

Les (Once owned a 1956 Chrysler Imperial with a hemi engine.)

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All of those horsepower ratings were "hype" compared to today. They are SAE "gross" ratings, much more generous than DIN or the SAE "net" that started in passenger cars in 1971. Example: 1969 Dodge 318ci 2bbl, 230hp gross, 177hp net. The above also requred 100+ octane leaded gasoline.

 

[gudmund]

Nastaljic retro = hemi. mopar's "pt cruizer"idea put to work building engines. All hemi is is a word they(mopar) owns the rights too! There are many engines out there that are "hemi's" they just can't call them that without mopar's ok. Just about every 4valve cyl engine is a hemi being the spark plug ends up in the direct center of the cyl because of the placement of the valves.

 

[needtoknow]

quote:

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Originally posted by tweeker43:
one of the things i look at when judging an engine is power density.

that is, how many horsepower do you get out of 1 cubic inch of displacement.

in this case, my subaru gets 165 horsepower out of about 152 cubic inches, or 1.08 hp/ci.

the new hemi gets 335 horsepower out of 350 cubic inches, or .96 hp/ci.

while the hemi obviously has more hp and torque, it's not as efficient in generating horsepower as many smaller cars.

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How about power to weight ratio and torque to weight ratio as well.

 

[tweeker43]

ntk - yup, that too. that's where my subaru suffers. they're kind of heavy for small cars. the awd system adds a bit of weight. plus, subarus seem to have kind an industrial / agricultural design. it's fairly simple, but overbuilt & heavy.

if i didn't want awd, i'd go with an hp to weight ratio of at least .065 (hp/lb). my sube is about .053 or so. i like the wrx, but i'm too old to be constantly flogging a gear shift.

my motorcycle makes about 50 hp, and with me on it weighs about 650 lb. this gets me .077.

 

[XS650]

quote:

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Originally posted by Bob Woods:
When I first went into the USAF I worked on an engine that had Hemi type heads in a radial layout, it had 4360 cid, 28 cylinders, two valves per cylinder, 2 spark plugs per cylinder, in its turbo charged application would produce one hp per cubic inch at 2800 rpm. We were lucky to get 2000 hours between overhauls.

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I thought the were "only" 3,500 hp. And more impressively, 3,500 lb so at least 1 hp/lb. Very impressive for an engine designed during WWII.

How about 336 spark plugs on a B-36!

Every gearhead owes it to himself to go see a P&W R-4360 engine. They are on display at several museums. If you are lucky you will find a museum that has a cut away version that was used for training.

 

[JohnnyO]

Basically better airflow through the combustion chamber. A crossflow kind of thing instead of side-to-side valves like a small-block Chevy or Ford. AFAIK, any 4 valve DOHC and most SOHC engines have hemispherical combustion chambers. For the record, back in the day the Ford 427 SOHC and Boss 429 engines were also hemis. Street performance was not spectacular since all 3 made their power at high rpms and were limited by the tire technology of the day (yes I know the 427 SOHC was never released as a street engine). Unless you freed up the exhaust with some headers and put on slicks, a 440 Six-Pack would spank a 426 Hemi, a 428 Cobra Jet would walk on a Boss '9, and an SS 454 would put serious hurt on all of them when in street trim. The barely disguised race motors had no low-end. Also back in the 60's and 70's it was common for the automakers to supply the magazines with secretly modified cars (see the Brock Yates book "Sunday Driver"). Further, the Mopar Hemi was a pretty finicky engine and most people couldn't get them to run right, at least not for long.

 

[VaderSS]

quote:

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Basically better airflow through the combustion chamber. A crossflow kind of thing instead of side-to-side valves like a small-block Chevy or Ford.

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It has nothing to do with the "crossflow" or the airflow through the chamber. It's about valve angles, shrouding(or lack thereof), and port shape/size. The hemispherical head allows for close to optimal design of all of the above.

 

[MillerMan]

cobra and corvette vs the mopars..one thing to remember is weight and wind resistance.

how much less does the cobra and corvette weight?
1000 pounds at least 600 pounds

and yes tp make the 426 hemi run like it really could it needed tuning , open headers, slicks and 410 or 456 gears. the 426 hemi had 475 to 550 gross HP not 425.

lets not forget the 426 max wedge in the 64 mopars, those were really the fast of the fast, but they weren't really daily street drivers

[ March 10, 2004, 01:58 PM: Message edited by: MillerMan ]