low-comp pistons was 413 motorhome/truck block

[William Parker <parkerw@xxxxxxxxxxxxxxxxxxxxxx>]

John Meyer wrote:
> 
> On Thursday, Jul 15, 2004, at 11:10 US/Pacific, William Parker wrote:
> 
> >  I don't know exactly your answers, but the replacement
> > pistons for this motor, such as Silvolite #1272D, are deep
> > dished and rated at  7.5(!) to 1 nominal static compression
> > ratio.  I've heard that the heads have additional water
> > passages, which I don't see would be of any harm in your
> > Imperial, but you may need some early closed-chamber heads
> > to get any decent compression.
> 
> This is interesting to me because my 413 was rebuilt with (I think)
> these pistons AND bored .060 over. It runs well and has OK power but I
> know it should be snappier especially at highway speed. Your answer
> suggests that I might be happier if I replaced the heads. From what
> years/engines do these closed-chamber heads come from? How much benefit
> would that provide in terms of compression?

      Making no other changes to a Mopar big block except
exchanging heads between typical closed chamber and typical
open chamber  changes your static compression ratio about 1
point.  As another poster pointed out, there are
considerations with detonation, flame front travel, and
quench when you change heads, but you certainly do change
the static compression ratio going from open chamber heads,
such as '906 or '452 heads, to closed chamber such as '915
heads.  With your engine, using closed chambered heads off
of a mid '60's standard big block will reduce your port
volume and valve sizes, but raise your static compression
ratio.  You'll want 6 valve cover bolts, so don't use an
early '60's head.  The best would probably be '915 heads
from '67, which were the first year of the "magnum" sized
ports and valves but still had closed chambers.  You
probably wouldn't do it anyway, but don't use max wedge
heads either, since these have pretty large semi-open
chambers, and also would require you to come up with a
matching intake manifold, the cheapest being the Weiend
single four intake for max wedge ports made in the mid-60's,
and sometimes popping up on eBay.  
        Since you have a pretty low ratio right now, I don't think
the new higher ratio, which even with small closed chambered
heads would be in the 8.5:1 range, will cause you any
problems.  Another consideration in putting something
together is the camshaft specs, as the cranking compression
is greatly affected by this, but again, if you don't change
anything else, I can't see you having any problem with this
given those low compression pistons.
        In my experience a higher static compression ratio
does result in better throttle response, other things being
equal and other parts, especially the cam, being compatible. 
        With regard to sleeves, I'm getting ready to put my 413 max
wedge motor back together, having had the block sleeved.  It
had been bored out to 426 + .030 size in its last race
configuration, and while 413 blocks were said to have very
thick cylinder walls, I think it would have been too risky
to bore another .010, at least without sonic checking.  My
machinist talked to Indy Cylinder Head and they said that
they would be glad to sonic check the block for us, but they
certainly would recommend sleeving instead.  One
disadvantage of sleeving is said to be possible heat
transfer problems, but my machine shop says they've sleeved
many blocks and had no problems, and they build many of the
race motors for the local circle track guys.  So I don't
know yet, but I'll shortly be finding out how well its going
to work.  Of course, the other disadvantage of sleeving is
the expense, around $600 for the full set installed.  I did
it to save the original max wedge block, but you might want
to consider finding a used core engine instead for that
reason.


-- 
Bill Parker, South Central Indiana
'56 Chrysler Windsor; '60 Chrysler Saratoga; '62 Plymouth
Max Wedge; '64 Dart Convertible (Kathi's car); '65 Imperial;
'65 Barracuda \6 (Kathi's other car);'68 Barracuda Fastback
340-S; '69 Barracuda Fastback now 360 (20 y.o. son's car);
'72 Cuda 340