Attention: TDR Forum Junkies 
Hemis..
There is actually an advantage to not having a "true Hemi" chamber.
The domed piston used on earlier Hemis was a necessary evil. The chamber volume was so large that a domed piston was necessary to get any kind of reasonable compression out of it. While a regular 440 chamber runs about 88ccs, a (426) Hemi chamber could top 175ccs easily. With only a 3. 75" stroke, you're not going to get any reasonable compression ratio without a huge dome.
The domed piston is bad, though. It causes the combustion chamber at TDC to resemble an orange peel. This hurts flame propagation and results in high emissions and poor economy, and (believe it or not) less power due to incomplete burn.
Moreover, you can't design a "true hemi" (meaning a sphere intersected horizontally by a plane running through its center) to have effective piston quench without anemic cam specs.
All original Chysler Hemis do not have perfectly hemispherical chambers, as was mentioned. It just doesn't work well.
The new 5. 7L Hemi is even less of a perfect hemisphere. Instead of the plane intersecting the sphere at its center, the current combustion chamber looks as if the horizontal plane intersects the sphere about 1/3rd the way from center to the top of the sphere. Why?
First, this allows the benefits of the Hemi design with further refinement. Now, the chamber maintains a much smaller volume (for better emissions and more power) without the use of a huge dome. A mildly domed piston can be used, which enhances flame propagation and allows for some semblance of quench.
It's interesting to note that the rated power of the current 345hp 5. 7L Hemi is roughly comparable to the rated power of the great 426 Hemi of 425hp, when you account for the difference between SAE gross (old Hemi) and SAE Net (new Hemi) horsepower. While many feel that the 426 street hemi was grossly underrated at 425hp, it's probably an urban legend. While it probably was underrated, it would have been slight. Quarter mile times from magazine tests in 1970 and 1971 often showed a near draw between the 425hp Hemi and the 390hp 440 six pack-- in some cases the six-pack cars were faster.
Now the Hemis certainly had much more potential horsepower than the 440 (given the stock heads), as the 426 was basically a homologation engine. But in factory trim, the 426 Hemi was often not much faster than than a hot 440 car.
Justin
There is actually an advantage to not having a "true Hemi" chamber.
The domed piston used on earlier Hemis was a necessary evil. The chamber volume was so large that a domed piston was necessary to get any kind of reasonable compression out of it. While a regular 440 chamber runs about 88ccs, a (426) Hemi chamber could top 175ccs easily. With only a 3. 75" stroke, you're not going to get any reasonable compression ratio without a huge dome.
The domed piston is bad, though. It causes the combustion chamber at TDC to resemble an orange peel. This hurts flame propagation and results in high emissions and poor economy, and (believe it or not) less power due to incomplete burn.
Moreover, you can't design a "true hemi" (meaning a sphere intersected horizontally by a plane running through its center) to have effective piston quench without anemic cam specs.
All original Chysler Hemis do not have perfectly hemispherical chambers, as was mentioned. It just doesn't work well.
The new 5. 7L Hemi is even less of a perfect hemisphere. Instead of the plane intersecting the sphere at its center, the current combustion chamber looks as if the horizontal plane intersects the sphere about 1/3rd the way from center to the top of the sphere. Why?
First, this allows the benefits of the Hemi design with further refinement. Now, the chamber maintains a much smaller volume (for better emissions and more power) without the use of a huge dome. A mildly domed piston can be used, which enhances flame propagation and allows for some semblance of quench.
It's interesting to note that the rated power of the current 345hp 5. 7L Hemi is roughly comparable to the rated power of the great 426 Hemi of 425hp, when you account for the difference between SAE gross (old Hemi) and SAE Net (new Hemi) horsepower. While many feel that the 426 street hemi was grossly underrated at 425hp, it's probably an urban legend. While it probably was underrated, it would have been slight. Quarter mile times from magazine tests in 1970 and 1971 often showed a near draw between the 425hp Hemi and the 390hp 440 six pack-- in some cases the six-pack cars were faster.
Now the Hemis certainly had much more potential horsepower than the 440 (given the stock heads), as the 426 was basically a homologation engine. But in factory trim, the 426 Hemi was often not much faster than than a hot 440 car.
Justin
Can anyone confirm this?
) instead of the 429 Cobra Jet or Super Cobra Jet when my wife and I bought our first new car after college - a new 1970 Mercury Cyclone GT. We both wish we still had that car!! 
