392s and 426s, was Why the Early Hemis Were Buried

["D. Dardalis" <dardal@xxxxxxxxxxxxxxx>]

At 10:34 AM 7/13/2002 -0700, you wrote:
>It seems to me pretty obvious that 392 wasn't the limit of
>the hemi, as 426's were designed that could get as much as
>1,000 bhp, and Daimler is still selling crate hemi's.
Roger, sorry for the misunderstanding.  You are right, the 392 was not the 
limit of the hemi as a concept (all large US radial aircraft engines were 
of the "hemi" variety, with the largest, the P&W 4360, a 4,360 cubic inch 4 
row 28 cylinder radial).  However, the 426 was an entirely new engine.  I 
don't think there is any part interchange between the 392 and the 426.  The 
426 (that I think uses virtually the same block as the 413-440s but with 
some reinforcements) required a lot more effort in new tooling than a 
"simple" enlargement of, say the 354 to 392.  So, what I meant was that the 
392 engine could not be properly enlarged any more without a major and 
expensive re-tooling, because there was no space to increase the bore on 
the block casting (referring not to the concept of the hemispherical 
combustion chamber, but to the 392 engine which includes all the tooling 
and castings).  Hope that clarifies my statement.

The Hemi as a combustion chamber has two major advantages over the 
wedge.  1) Larger valves can be stuffed for the given bore.  2) Due to 
central spark plug location, the flame travel is reduced, improving 
detonation performance.  The second advantage became less important for 
street engines as the octane rating of the fuels improved (I think someone 
already mentioned this).  But for a performance-only engine, its importance 
remained.
D^2