What is the big deal with Hemi heads?What makes them so special?I have heard of mopar and ford and olds using versions of these heads.If they were so awesome how come more companies didn't try them.What are the flow rates and specs on the older Hemi heads (smallblock) versus dodges new Hemi small block versus new chevy heads? What promted this question was
a article ina old issue of Super ChevyI read today.august 1996 volume 25 no8 page 22.
The article was titled Smokey andf the small block Mystery Motor.It told about a chevy/smokey project for a dz302 with hemi heads (porcupine heads) for trans am racing.It goes on to state that the program was a flop and it was dropped.They got more power from the double humps than from the Hemis.The article was a over the phone interview with Smokey himself on the subject of the heads.The author was Jeff Burk.

 

Better combustion, I think.

 

In theory Hemi heads will breath better. The down side is that they have large combustion chambers so they need pistons with a bump to get compression. Then if you aren't careful, the piston dome gets in the way of flame propagation. The other down side is that there is more complication which adds more cost. I think the other thing that gives hemi's a big rep is the NHRA AA fuelers and funny cars. The thing is that in those classes with big blowers they have to run low compression so they can use flat top or even dished pistons. finally, remember a true "hemi" is a full half sphere. The "porcupine" head on the Chevy big block is only a "semi hemi." It gives the advantage of a hemi, but without some of the down side.

 

Ever seen a hemi piston? Those things are huge. Don't know what they weigh, but they got to be heavy.

 

Smokey stated that they had to port and port the heads to get the numbers.He said they ported them to the point they were darn near hemispherical.
What about hte new dodge motor that is supposed to have hemi heads.Any info on it.I heard all this talk about it would be awesome and now nothing.

 

Chrysler style hemi heads, you would have to see a cylinder head to really understand why they are the best. The valve location and the combustion chamber are the optimum of the industry. They are also the most efficient style available and why they are the best. No big deal, but that is unless you want to win. The only engine that can beat one is another one. Tremendous horsepower is available with those engines. :D :thumbs:


[Modified by Ironcross, 12:23 AM 6/15/2003]

 

My question is if they are so much better why doesn't other companies use variations of them?

 

It's an ancient architecture that goes back to the early twentieth century. A sphere has the least surface area to volume ratio, which will tend to mininize heat transfer, and it also allows two larger valves than could be fitted with in line valves. It also means fairly high combustion chamber volume, but back in the early days, CRs of four or five to one were about all the available fuels could handle. Even so, hemis have less detonation resistance than a quench type chamber and can't usually run as much CR as a quench chamber on the same octane fuel.

As CRs rose with available fuel octane hemis grew piston domes, which did away with the low surface area to volume ratios and the solution was narrow the valve angle and make them semi-hemi. Then GM developed the modern quench combution chamber in the late forties and was able to run higher CR for better efficiency, but not quite as much power.

Then along came Keith Duckworth in the sixties who rediscovered the four-valve architecture that dated to 1905. He narrowed the vavle angle which yielded a small, shallow chamber with small quench zones on each side and set a new standard for both high flow efficiency, high CR, and low surface to volume ratio.

The fact is that the hemi and semi-hemi are basically obsolete designs. The modern narrow angle four-valve or shallow quench chamber two-valve are generally the best choice in modern engines. Chrysler brought back a modernized version of the "hemi", and they do have a history to exploit from a marketing standpoint. In current tune it has about the same specific output as the LS1, but is larger an heavier.

Duke

 

MANY cars do, almost every DHOC car runs the spark plug in the center of the combustion chamber. Great set of a heads, they were a little tricky to tune, but once set up they were tough to beat.

Dad had a '68 Hemi GTX in the late '60's/ early '70's. :thumbs:

 

There are two major reasons for the hemi head, but only one has been mentioned so far. Yes, the flow is better (when the ports are properly designed) because the air has to turn a much smaller angle, thus reducing flow losses in the port. The other major thing, the one that hasn't been mentioned, is spark plug location. The Hemi heads have a centrally located spark plug which puts the kernel at the center of the cylinder. With this location, the maximum flame travel is only the radius of the cylinder, not the diameter as it is with wedge heads. This reduction in flame travel reduces the amount of time to burn the complete air fuel charge, which means that your optimum power comes at a MUCH shorter spark advance. If you can start the spark later at still make peak pressure at the same point, you'll make more power (the piston is fighting less power at the end of the compression stroke).

American auto makers have stayed away from the Hemi, and I honestly don't know why. The major drawback to the hemi-style head is the space required for the valvetrain. If you notice, the original Chrysler Hemis were WIDE engines, about the same width as a DOHC motor (valve angles are the same). IMO, a 2-valve hemi head is a waste of space unless you are running a twin plug setup (plugs aren't centrally located, but one on either side of the valve centerlines), which burns even faster than a single plug Hemi (2 kernels instead of one). Porsche has been using that setup for years, however (2 valve hemi, single and twin plug setups).

As far as comparisons between the current Dodge Hemi and the original, the major difference is cam location. The following info is from memory based on a few discussions I've had with the engineer I report to at Dodge for my research who worked on this motor...but it's from memory, so take it for what it's worth. The new motor has the cam WAY up in the block, such that the puhrods are not even CLOSE to acting in the same plane as the cylinders. This allowed them to really reduce the width of the motor because (if I understand it correctly) it made the exhaust valve rocker geometry much easier to work out. I do not know how the two heads compare in flow, but I don't think you really CAN make a comparison because memory tells me all of the original Hemis that anyone used in racing were all big blocks (I know of the 383 and 426, but I think there were others....) I do know that the specific output of the motors are similar.

 

Unless the valves are very small you can't locate the spark plug along the axis of the cylinder as you can with a four-valve pentroof chamber. Most large valve hemis have the plug offset quite a bit from the cylinder axis, and two plugs are common. Wedge chambers can have fairly short flame travel (relative to bore size) with large quench area, but large quench area tends to have high HC emissions, so it's a tradeoff.

The ultimate hemis are probably the radial aircraft engines of the mid-20th century with two widely splayed pushrod operated valves. As do all reciprocating aircraft engines, they have two spark plugs per cylinder (symmetrically disposed on either side of the cylinder axis), and by starting two flame kernels total combustion distance/time is shortened and detonation resistance is increased. With up to 80" of manifold pressure at takeoff these engines could produce about 3000 HP and with turbo-compounding (exhaust gases routed through a turbine that was geared to the crankshaft) they produced up to about 3800 HP.

The contemporaneous "inline" engines of the era were V-12s, which had four valves per cylinder driven by a single OHC and pentroof chambers with two centrally located spark plugs. Probably the most famous of these is the Rolls Royce Merlin, which, built under license by Packard, was known as the Packard Merlin V-1650 [CID] and powered the P-51.

Duke

 

I disagree with this. If wedge chambers had such short flame travels, they'd combust the air fuel mixture very quickly. However, they have some of the worst burn times of any combustion chamber shape in use. For example, a good race head with a wedge chamber may have it's 10-90% burn time at about 28-29 degrees. I've heard rumors of as fast as 26, but never seen it. A twin plug hemi chamber, however, will achieve the same mass fraction burn time in roughly 15 degrees of crank rotation. That right there is why you can start the spark so much later in a motor with hemispherical heads. While I have only done combustion analysis on wedge headed motors, I have seen combustion data from hemi motors, and the mass fraction burn rate difference is clear. Contrary to popular belief, the large squish area does not reduce the distance the flame has to travel, as the flame is still burning after the piston begins it's descent. With the piston at TDC, less than 50% of the mass of the air charge has burned (you hope...if more has burned, you're in a WORLD of hurt) It does, however, significantly affect turbulence in the chamber.

 

I thought everyone knew the plug was in the center between the intake and exhaust valve. The only problem with the early Chryslers was the weight. The 392 Hemi was unique to it`s self. They look similar but only the cam was interchangable on the larger engines {354-392}. The heads can be changed, but a spacer is required for the intake to match. Dodge and Desoto engines were also available as hemis but way lower in cubic inches. Good for selected racing classes. As the 92`s got used up, 426`s took over with a much stronger bottom end. As both became scarce aluminum copies with even stronger botton ends were produced by Donavon for 392`s as a 417 and Milodon and KB as 426 copies. Stronger, lighter and able to handle even more HP and 100% nitro loads, now restricted to about 90% because of the speeds recorded in an attempt to slow them down. It really didn`t work, as there back to mid 4`s. I believe Kalitta just ran a 4.54. However only the 426`s really won class events because of the development of the A990 426 race hemi in the NHRA stock classes. The 392`s only were good when completely modified with injections and superchargers, because of the HEMI HEADS. Aside from these unique things about hemis, a good L88 or even a LS7 is its best competition in stock form. :thumbs: :cheers:

 

Ah, yes, but the key is twin plugs. The old Chysler hemis from the fifties and sixties had single plugs. The current generation of Mercendes three-valve SOHC engines can be roughly described as "hemis" and they have dual plugs that are independently timed.

Your points on wedge chamber mass fraction burned vesus crank angle are well taken.

Duke