Compression ratio is determined by many factors in Imperials as well as
many other vehicle brands (mandatory Imperial content achieved -- wink).
Valve train geometry is fascinating once you get into it. In brief,
factors that influence compression include deck height (where the piston's
top is at maximum extension from the shaft), cylinder bore (width),
combustion chamber size AND the volume of what was being compressed in the
first place. In this case, low-comp pistons simply are shorter or may be
concave on top, resulting in a larger combustion chamber volume = lower
compression ratio.
Combustion chamber shape (open vs. closed) has almost nothing to do with
compression ratio and almost everything to do with combustion efficiency --
that is getting the maximum ponies out of your warhorse. That's one of the
reason hemi engines are such good racing engines -- the hemispherical
combustion chamber dome (the bottom is always a cylinder shape unless the
piston's deck height is flush with the top of the block) gives a more
complete burn.
RV engines are designed for max torque at low revs = towing capacity. In a
passenger vehicle, even a 5,000 lb. one, you will certainly be able to
chirp the tires from a stop sign but you won't necessarily have power above
2500 RPM the same as you would with a stock engine. Overall performance
will depend on your engine's power curve as well as rear end gear ratio.
If you do use the RV engine I would consider using a different cam -- there
are many to choose from. In fact, the interchangeability among 413 and 440
engine parts offers endless hours of contemplating the possible
combinations. An engine rebuilder or performance shop will be able to
advise you, and some people think it's fun to learn about this stuff on
their own.
I recommend you start with
How to Rebuild Big-Block Mopar Engines: Covers All Years and Models of B/Rb
Series Chrysler, Dodge and Plymouth Engines
Author: Don Taylor
ISBN: 1557881901
Retail Price: $18.95
I found it on the shelf at B*rn*s & N*ble -- you may have to order it.
Have fun and get grit under your fingernails,
David
'91 K-Imperial driver
'66 Crown Coupe project
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?
In the meantime, I have purchased a set of flat-top, 10.0 to 1 .030
pistons to use eventually in a new motor. I would either have to obtain
a different block or have mine sleeved. Any thoughts on using sleeves?
Regards,
John Meyer
'65 Crown
Tucson
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