Whats Up With Hemi's?

[Prius]

i'm talking about in the Muscle cars, 425 HP, 130 MPH, doesn't match now does it?

[johnnynumfiv]

i'm talking about in the Muscle cars, 425 HP, 130 MPH, doesn't match now does it?

When moving a 5,000 lb car, it does match. You have to think back to the day, not many cars were doing those speeds back in the day.

[Prius]

yeah, but if you can crank out THAT much HP you should be able to make it go at least 150

(my openion)

[KnifeEdge_2K1]

dude ur comparing speed with the engine design, thats like the dumbest thing you can do

so many other factors affect speed

the only things you can compare are stats related to the engine tiself which is the only time crank hp and crank tq ever matter

and hemis even the new ones dont come to the 100hp/L mark

[johnnynumfiv]

yeah, but if you can crank out THAT much HP you should be able to make it go at least 150

(my openion)

What would be the point going that fast when you have drum brakes all around......

Aerodynamics play a big part in thier top speed, for example, the cobra hit an aerodynamic "wall" at 165mph.

[Matra et Alpine]

but its also impossible to orient more then 2 valves per cylinder in a true hemi design,

it was Toyota IIRC that had a 5 valve design in a true HEMI shape many years ago

I think you mean you can't have hemi and multiple valves and PUSHROD.

Which ??

[culver]

Just a few points:

Hemispherical combustion chambers didn’t start with Chrysler. Aircraft engines used them well before Chrysler made the name famous.

The current Hemi name is used as a successful brand name rather than a true technical claim. Say what you want about the technical validity of the name, it is attached to a good motor that delivers on it’s promises (to bad it’s also quite heavy). I can’t think of any engine moniker that is more successful.

The Chrysler Hemi’s were actually semi-hemispherical. A true hemispherical combustion chamber was so large that it was difficult to get the needed compression without a very domed piston. A large domed head and piston are less efficient because it increases the surface area of the chamber relative to the volume of the chamber (more heat loss to the walls). The big benefit to the Hemi heads was they could have larger valves when compared to a wedge head motor. While a wedge motor places the valves side by side on an effectively flat surface. The Hemi head places them on opposite sides of a domed surface. This allowed for larger valves and, like a 4 valve head, for better breathing compared to GM and Ford’s wedge motors. The drawback was a more complex rocker arrangement to the valves and a physically larger head. Ford’s answer (which NASCAR effectively killed) was the SOHC 427 Cammer.

DOHC motors use penta-roof combustion chambers. They are very similar to hemi-type heads but because the intake and exhaust valve pairs must be in plane (ie the intake valve stems are parallel to each other) the heads are not hemispherical. It can be done but it is complex (see below).

The new HEMI is not actually a hemi head at all. Its design started (and evolved away from) the combustion chamber of the last generation air cooled 911 head. Among other difference between a true hemispherical head and the new HEMI motor, the new motor’s combustion chamber is much flatter. The big similarity between the new and old motors is the more complex valve train. Unlike GM and Ford motors who’s pushrods and rockers are all in plane and in line, the Hemi motors actuate rockers that are in two separate planes/line. In this regard they are distinctly different from the other guys and distinctly similar to the motors from the 1960’s. This layout is more expensive to manufacture than the wedge heads because head machining work is more complex.

BMW actually did produce a 4 valve true hemi racing motor (1967 BMW M10 2L I4, Hemispherical head, diametrical valve arrangement designed by Ludwig Apfelbeck). It was a very interesting I4. The valves were arrange such that the center lines of the sparkplug and 4 valves intersected in the combustion chamber. The intake valves were on opposite each other and roughly inline with the length of the block. Air was feed into the cylinders from between the cam shafts and exhausted on BOTH sides of the head. The motor had two exhaust manifolds. Very interesting design. The valve train looked every bit as complex as the Valvetronic stuff. It had two rocker arms per valve.

[Fleet 500]

When moving a 5,000 lb car, it does match. You have to think back to the day, not many cars were doing those speeds back in the day.

The Hemi-powered muscle cars didn't weigh anywhere near 5,000 lbs. They weighed around 3,800-4,200 lbs. Cars like Hemicudas, Chargers, Challengers, Super Bees, etc.

Even the full-sized, luxury Hemi cars of the '50s, like the 300 series and the DeSoto, weighed about 4,300-4,600 lbs.

[Fleet 500]

yeah, but if you can crank out THAT much HP you should be able to make it go at least 150

(my openion)

Car and Driver (Nov., 1967) got a '68 Hemi-Charger up to 139 mph @ 5800 rpm and estimated top speed as 156 mph @ 6500 rpm.

Car and Driver (Nov., 1969) got a '70 Hemi Challenger to 146 mph.

Playboy magazine listed the top speed of a '68 Hemi Coronet R/T as 155 mph.

Remember, aerodynamics was practically nonexistant by today's standards, so when they went that fast, they were doing it by brute power alone.

When the aerodynamics was better, like for the '69 Dodge Charger Daytona, which had a .29 drag, they could go much faster (a Hemi-powered '69 Daytona could do at least 170 mph and more like 180 mph from the factory).

[Fleet 500]

The Chrysler Hemi’s were actually semi-hemispherical. A true hemispherical combustion chamber was so large that it was difficult to get the needed compression without a very domed piston. A large domed head and piston are less efficient because it increases the surface area of the chamber relative to the volume of the chamber (more heat loss to the walls).

That's interesting... I've read a lot about Hemis, being a big Mopar fan. And I've never heard of the '50s Hemi not actually being a true Hemi. I have read that the '66-'71 Hemi is not a true Hemi because the combustion chamber and valve positioning had to be tilted about 10 degrees toward the center of the engine to reduce overall width enough so that the complete engine assembly could be dropped down into the front compartment on the assembly line. This means that the exhaust port has a little sharper curve in it than necessary, the intake valve diameter is restricted somewhat and there's a little more shrouding around the lower edge.

The book which dicusses this goes further:
"The early Hemi doesn't have this compromise. It's a true Hemi, with symmetrical chamber design, minimum curve in the ports, centerline location of the spark and minimum shrouding around the valve edges. It's said to breathe measurably better, other factors being equal."

In fact, many of the dragsters back in the '60s and '70s used the early '57-'58 392 Hemi rather than the '66-'71 426 Hemi because the earlier ones breathed a little better.

[culver]

That's interesting... I've read a lot about Hemis, being a big Mopar fan. And I've never heard of the '50s Hemi not actually being a true Hemi.

Here’s a picture of some Hemi head from the 60’s (don’t know which one since I can read French). Anyway, it’s not a true hemispherical combustion chamber because the dome is slightly squished from the top. A true hemi has a combustion chamber that is literally half a sphere. It’s only a small difference but SEMIHEMI doesn’t have as nice a ring.

http://dbusso.typepad.com/photos/unc...d/cylinder.jpg

I believe this even applied to the 50's vintage motors.

Automobile article on the current motor:
http://www.automobilemag.com/news/0407_hemi/

[Fleet 500]

Here is the 1951 Hemi: