Please educate me about the Hemi engines

[P20565R16]

Hello! I wanted to ask the opinion of those that would know about the appeal and benefit of Chrysler's Hemi engine. I know it means Hemispherical, beyond that I don't! I know, or think, this style of engine debuted in the 70s and has made a return in the 2000s. So there is one for a truck and one for the cars. Well then, what is the appeal of the Hemi?

Wanting to get more into fast cars, I sometimes people talk about how the Hemi isn't all that great, or was and is no longer. I have an analytical mind and I have to wonder, if the Hemi is so good then why is it not in ALL of Chrysler's engines? Compact? Why are people not clamoring to get one? As I said, please educate me.

Considering buying a muscle car of some sort, something to toss around on weekends. Unsure where to start in the whole deal. Thank you

 

[OVERKILL]

The new HEMI doesn't have hemispherical chambers, they more resemble a pent-roof because a truly hemispherical chamber is an emissions disaster.

That being said, the new engines branded as "HEMI" are a strong powerplant that makes a great deal of power. You can't go wrong with a Challenger for example with any of the available V8 powerplants.

 

[antonmnster]

It's marketing. That is all.

 

[mechanicx]

I'm not a Chrysler guy but I'll add what I know off the top of my head. The Chrysler Hemi head V8 was developed in the 50's (the idea of hemispherical combustion chambers is much older). It was a rival to the wedge combustion chamber that was being used and developed for the other V8s in the early 50's.

The benefit of the hemi is it shrouds the valves less, which means biggger valves can be used, better breathing and more power potential. Also I think it gives you less combustion chamber surface area for a given volume, which is good for thermal efficiency, anti-knock, so more compression and timing and thereforer more power.

The drawbacks are more expensive, more complex valvetrain, bulkier, heavier. Also the hemi has less quench and is usually not as fuel efficient as the wedge. This is why the wedge head pretty much dominated and was used more even by Chrysler. Besides, canted or splayed valves, a variation on the wedge chamber, can give you less shrouding, bigger valves, and higher power potential. Modern wedge head engines, and really back in the day, make about as much power as hemis. Look at the the GM LS, it is at 430 HP out of a 6.2L, and when they go to the canted valve they'll probably be at 450hp.

 

[jimbrewer]

These days the world is awash in muscle cars. The horsepower has outstripped your ability to enjoy a muscle car and still keep your license. So getting wrapped up on one technical feature or another probably isn't useful.

The Mustang for example, starts at 305 hp--adjusting for more conservative rating techniques, its probably more horsepower than the Mustang Steve McQueen drove in the movie "Bullit" Similar power to weight ratios can be found all up and down the various car lines.

 

[OVERKILL]

Originally Posted By: mechanicx
I'm not a Chrysler guy but I'll add what I know off the top of my head. The Chrysler Hemi head V8 was developed in the 50's (the idea of hemispherical combustion chambers is much older). It was a rival to the wedge combustion chamber that was being used and developed for the other V8s in the early 50's.

The benefit of the hemi is it shrouds the valves less, which means biggger valves can be used, better breathing and more power potential. Also I think it gives you less combustion chamber surface area for a given volume, which is good for thermal efficiency, anti-knock, so more compression and timing and thereforer more power.

The drawbacks are more expensive, more complex valvetrain, bulkier, heavier. Also the hemi has less quench and is usually not as fuel efficient as the wedge. This is why the wedge head pretty much dominated and was used more even by Chrysler. Besides, canted or splayed valves, a variation on the wedge chamber, can give you less shrouding, bigger valves, and higher power potential. Modern wedge head engines, and really back in the day, make about as much power as hemis. Look at the the GM LS, it is at 430 HP out of a 6.2L, and when they go to the canted valve they'll probably be at 450hp.


Honestly, I think the pent-roof is the most efficient design. In good fun here, but to counter your 6.2L example, I could mention the BOSS 302, which makes 444HP out of its 5.0L of displacement, or BMW's 5.0L V10, which makes 550HP

 

[mechanicx]

Originally Posted By: OVERKILL
Originally Posted By: mechanicx
I'm not a Chrysler guy but I'll add what I know off the top of my head. The Chrysler Hemi head V8 was developed in the 50's (the idea of hemispherical combustion chambers is much older). It was a rival to the wedge combustion chamber that was being used and developed for the other V8s in the early 50's.

The benefit of the hemi is it shrouds the valves less, which means biggger valves can be used, better breathing and more power potential. Also I think it gives you less combustion chamber surface area for a given volume, which is good for thermal efficiency, anti-knock, so more compression and timing and thereforer more power.

The drawbacks are more expensive, more complex valvetrain, bulkier, heavier. Also the hemi has less quench and is usually not as fuel efficient as the wedge. This is why the wedge head pretty much dominated and was used more even by Chrysler. Besides, canted or splayed valves, a variation on the wedge chamber, can give you less shrouding, bigger valves, and higher power potential. Modern wedge head engines, and really back in the day, make about as much power as hemis. Look at the the GM LS, it is at 430 HP out of a 6.2L, and when they go to the canted valve they'll probably be at 450hp.


Honestly, I think the pent-roof is the most efficient design. In good fun here, but to counter your 6.2L example, I could mention the BOSS 302, which makes 444HP out of its 5.0L of displacement, or BMW's 5.0L V10, which makes 550HP

Apples and oranges because those are 4-valve DOHC engines, and the pent roof is not what is really responsible for the power density. Of course they make more power/L, but I wouldn't call them more efficient. I'm comparing 2-V OHV designs. There's really no difference between pent-roof and hemispherical combustion chamber

.

 

[mechanicx]

Well I'll agree there is some difference between a pent-roof and hemi. A true hemi has about a 45 degree valves angle and a larger combustion chamber and usually a domed piston. A pentroof has shallower valve angle and a smaller combustion chamber. I consider them variations of basically the same thing. But you can't really compare 4v DOHC with 2V OHV power density.

 

[OVERKILL]

Originally Posted By: mechanicx
Well I'll agree there is some difference between a pent-roof and hemi. A true hemi has about a 45 degree valves angle and a larger combustion chamber and usually a domed piston. A pentroof has shallower valve angle and a smaller combustion chamber. I consider them variations of basically the same thing. But you can't really compare 4v DOHC with 2V OHV power density.


Well, there are variances on the theme

I mean the current "HEMI" is pushrod with chambers that more resemble a pent-roof than a hemisphere. The BOSS 429 had semi-hemi heads, essentially a hemisphere with a small quench area on the side of the chamber. THEN there was the 427 SOHC which was a 2V OHC engine, but had hemispherical chambers

It was the most powerful production engine of the 60's, making 657HP (factory) with a pair of quads on it.

That all being said, I didn't realize we were being specific to 2V pushrod engines, I was just responding to your point about higher power density

Oh, and with respect to the pent-roof and the HEMI, in order to operate properly, a hemispherical chamber has to have swirl. This requires some pretty good air movement in the chamber to happen. This means you need RPM. At low engine speeds, the burn is incomplete and the design, particularly in large-bore applications, is horrendous in terms of emissions and low-speed power. Adding quench areas does a fair bit for the low speed power (BOSS 429) but ultimately the engine suffers from poor burn characteristics until proper swirl is achieved.

With a pent-roof, you have two good sized quench areas on each side of the chamber, a significant lack of valve shrouding, and the triangle shape in the middle of the chamber allows plenty of room for the valves to open. It isn't considered to have anything in common with the hemispherical chamber. FWIW

 

[OVERKILL]

Originally Posted By: mechanicx
Well I'll agree there is some difference between a pent-roof and hemi. A true hemi has about a 45 degree valves angle and a larger combustion chamber and usually a domed piston. A pentroof has shallower valve angle and a smaller combustion chamber. I consider them variations of basically the same thing. But you can't really compare 4v DOHC with 2V OHV power density.


Canted-valve pushrod (BOSS 302) head:

Hemispherical chambered, SOHC (427 SOHC) head:

Pent-roof chambered, 4-valve DOHC (BMW S62) head:

Pent-roof chambered, 4-valve DOHC (Ford 4.6L) head:

Chrysler's current "HEMI" heads:

GM's current LS6 heads:

Ford's current 2V SOHC head:

I think the difference between all of them are pretty interesting

 

[mechanicx]

Yeah good point on the hemi and low speed power and combustion. The hemi basically has the most peak power potential for a 2-valve, but has various other drawbacks. The pentroof offers more turburlence but it is similar in concept to the hemi. The wedge gives good turburlence and has lot of squish.

Good pics of the different examples

. Just need a classic hemi pic.

 

[NHGUY]

In the 1960s Hemi,a pack of cigarettes would fit thru the valve.

 

[OVERKILL]

Originally Posted By: NHGUY
In the 1960s Hemi,a pack of cigarettes would fit thru the valve.


Reminds me of the BOSS 429 with the tennis-ball sized intake ports.

 

[carwreck]

A V16 hemi was tested in a XP-47 in 1945. More hemi info at http://www.allpar.com/mopar/hemi/chrysler-hemi.html
1951 331 hemi cutaway. Hemi combustion chamber, spark plug in center.

For comparison, cutaway of 1966 Ford 289

 

[morris]

i use it run 50s 392 hemis. with every thing out of the head, i could pass a small welding rod through the intake, chamber, exhaust with out it bending. put one in 1959 ply wagon. dad tryed to race GTOs, but by giving way 1,000 lbs he didnt have a chance. dont for get there are two 426 hemis, the 1964 race hemi, 1966 street hemi. on the out side they look alike but inside the cam ect was much different

 

[mechtech2]

Originally Posted By: OVERKILL
The new HEMI doesn't have hemispherical chambers, they more resemble a pent-roof because a truly hemispherical chamber is an emissions disaster.

That being said, the new engines branded as "HEMI" are a strong powerplant that makes a great deal of power. You can't go wrong with a Challenger for example with any of the available V8 powerplants.


Right.
The only true hemi that I've seen are the old 392s, with the spark plug dead in the middle.
Many many cars have 'hemi' head engines - kinda hemishperical.
It is a sales name now for Chrysler - living on past glory.

 

[P20565R16]

Excellent information thus far. I figure it turned into more or less of a meaningless namesake, akin to what happened to the moniker 442 on the Oldsmobile Cutlass in the 80s. Not worthy of its predecessors, instead some of the first badge jobs. I do not know much about those cars, either.

And those pictures are spectacular. Amazing how BMW engine makes 550HP out of the same setup? 10 best engines list appear deserved there, you always see BMW on that list.

Now if I may, I would like to direct the conversation to another concept of engine I am wholly unfamiliar with, please forgive me if I get this wrong but I believe they call it rotational mass. I recall on some forum, I do forget where, a Pontiac Firebird WS6 (?) and a 2000 Audi S4 1.8T owner were going at it, and the WS6 guy said quote While my engine is pulling and getting all that horsepower to rocket the car into the triple digits, yours will be frying its rings and lucky if it doesn't blow apart from all that boost you need to run and hope it doesnt basically blow on you because you have a lack of rotational mass in that engine yes you have turbos but there is a limit as to what you can do vs a large engine like the WS6 unquote.. discuss and please educate me there too? I am unsure what I want to do yet, the more understanding I have the more I can determine what to do on car number two, I will call it the Weekender.

 

[Rhymingmechanic]

If by "muscle car" you mean something from the pre-computer, pre-smog era, a Hemi is about as costly as you can get. An original Hemi car is the kind of thing you see on TV auctions, and a Mopar Performance Hemi crate engine starts around $15,000.

The Hemi mystique made it the holy grail of engines, at least for Mopar guys. They were always used in drag racing, and the top fuel engines still use the design. They also did pretty well in NASCAR for the year or two they were allowed.

Anecdotally, I have been told that you would rather race against a Hemi than a 440 on the street, because Hemis were harder to tune right and, as noted above, they needed RPMs to make power.

I think the 426 Hemi wins the prize for biggest intake valve at 2.25" diameter.

 

[mechanicx]

Originally Posted By: P20565R16
Excellent information thus far. I figure it turned into more or less of a meaningless namesake, akin to what happened to the moniker 442 on the Oldsmobile Cutlass in the 80s. Not worthy of its predecessors, instead some of the first badge jobs. I do not know much about those cars, either.


Well don't get the impression that hemi, semi-hemi or pentroof or whatever you want to call it is totally meaningless. it does have high power potential. Just power isn't the be all end all in a street car engine design. Also improvements to the wedge chamber has made the power potential very close.

Quote:

Now if I may, I would like to direct the conversation to another concept of engine I am wholly unfamiliar with, please forgive me if I get this wrong but I believe they call it rotational mass. I recall on some forum, I do forget where, a Pontiac Firebird WS6 (?) and a 2000 Audi S4 1.8T owner were going at it, and the WS6 guy said quote While my engine is pulling and getting all that horsepower to rocket the car into the triple digits, yours will be frying its rings and lucky if it doesn't blow apart from all that boost you need to run and hope it doesnt basically blow on you because you have a lack of rotational mass in that engine yes you have turbos but there is a limit as to what you can do vs a large engine like the WS6 unquote.. discuss and please educate me there too? I am unsure what I want to do yet, the more understanding I have the more I can determine what to do on car number two, I will call it the Weekender.


This is mostly nonsense. For performance you want rotational mass and reciprocating mass as low as possible. You can overboost an engine, but you can also overrev an engine. High RPMs can be just as hard on an engine as boost is. What they are probably getting at, but not explaining right, is that a small high reving and boosted engine can only safely produce so much power per liter of displacement. And a bigger engine can make power without as much boost or RPM, so lower stress. But the higher rotational mass doesn't help performance.

 

[morris]

" living on past glory." you need to check out the class SS/AH at NHRA. its THE 1968 Hemi Cuda. this is the RACE HEMI. the FASTEST stock car PERIOD. when new they turned 10.80s off the trailer today they run 8.80s STILL NO challengers. i got to see them new, talked to Ronnie Sox in person.