Hey, that porsh gotta hemi in it?
06-01-2015, 05:10 PM
#16
Rennlist Member
2valve is not hemi like, because the exhaust and intake are side by side.
i was talking about the 4 valve.
the hemi was thought to meant Hemispherical combustion. air enters on top and is pushed out the bottom in a straight line.
do we have a side cut away view of the S4 engine??
i was talking about the 4 valve.
the hemi was thought to meant Hemispherical combustion. air enters on top and is pushed out the bottom in a straight line.
do we have a side cut away view of the S4 engine??
06-01-2015, 10:30 PM
#18
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06-01-2015, 11:00 PM
#19
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About 10 years ago I took one of the mechanics from my shop for a ride in my 86.5 5 speed. He was about 60 years old at the time and was into 60's era Fords. I got on it to show off a little and he said "wow, is this a turbo?" I said "no, it's a V8." He turned around in his seat to look into the back of the car, and said "it must be tiny!"
06-02-2015, 05:00 AM
#20
Rennlist Member
About 10 years ago I took one of the mechanics from my shop for a ride in my 86.5 5 speed. He was about 60 years old at the time and was into 60's era Fords. I got on it to show off a little and he said "wow, is this a turbo?" I said "no, it's a V8." He turned around in his seat to look into the back of the car, and said "it must be tiny!"
06-02-2015, 12:03 PM
#22
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About 10 years ago I took one of the mechanics from my shop for a ride in my 86.5 5 speed. He was about 60 years old at the time and was into 60's era Fords. I got on it to show off a little and he said "wow, is this a turbo?" I said "no, it's a V8." He turned around in his seat to look into the back of the car, and said "it must be tiny!"
What makes this even better, the hood was open.
06-02-2015, 12:07 PM
#23
Rennlist Member
1st pic: From the Mopar Engine book itself, showing the HEMI cross section in detail... not really a hemisphere.
2nd pic: Did you guys forget the ration of $hit I got for having this on my car?
2nd pic: Did you guys forget the ration of $hit I got for having this on my car?
06-02-2015, 06:38 PM
#24
Rennlist Member
so, we have a hemi?...... looks like it.. sure, not a deep of a hemi as a dodge, (no domed piston), but still the same concept... air/fuel enters and exits in a straight line, and head combustion chamber is hemispherical.
06-02-2015, 06:43 PM
#25
Rennlist Member
sounds like we get the best of both worlds........ from the web:
HEMI Pros and Cons
There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:
You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
HEMI Advantages
The last item in the above list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved.
HEMI Disadvantages
If HEMI engines have all these advantages, why aren't all engines using hemispherical heads? It's because there are even better configurations available today.
One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breathe easier than two large valves. Modern engines use a pentroof design to accommodate four valves.
Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well.
HEMI Pros and Cons
There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:
You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
HEMI Advantages
The last item in the above list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved.
HEMI Disadvantages
If HEMI engines have all these advantages, why aren't all engines using hemispherical heads? It's because there are even better configurations available today.
One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breathe easier than two large valves. Modern engines use a pentroof design to accommodate four valves.
Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well.
06-03-2015, 01:31 AM
#26
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So the last true production "hemi" I had my hands on was a Daimler V8. An early 50's Ferrari 250 4 cylinder had the most unique arrangement, with the head and cylinders cast as one. Challenge was cutting and grinding the seats for the valves up inside the cylinders. Was an interesting tooling challenge.
The Chrysler/Dodge/DeSoto hemis have a flatter chamber, flat enough I think to make the 928 (and virtually all other 4-valve heads) a 'hemi'. A true hemi is tough with 2 valves, virtually impossible with four valves without some valve gear gymnastics.
The Chrysler/Dodge/DeSoto hemis have a flatter chamber, flat enough I think to make the 928 (and virtually all other 4-valve heads) a 'hemi'. A true hemi is tough with 2 valves, virtually impossible with four valves without some valve gear gymnastics.