RE: 383 compression

["Earl H. " <earlh@xxxxxxxxxxxxxx>]

In my opinion the best way to set up a big block Mopar with flat top pistons
is 
to always zero deck the motor. Of course it is important to pick a flat top
piston 
with a pin height that lets you do the minimum amount of decking. Then, you
adjust 
your compression with chamber size and gasket thickness. Unless you do this
you 
will never be able to take advantage of having a proper quench area which
gives you 
the ability to control detonation.

I just got thru convincing a friend to tear his motor apart and do this. He
had a machine 
shop that knew nothing about Mopars set his motor up. The detonation on his
motor was 
so bad by the time he retarded the timing enough to get rid of it, no power.
It was a 
440 with Eddy heads. The pistons were down in the hole, and he had a Fel Pro
gasket. 
At our altitude, this spells detonation. He tried everything under the sun
to no avail. Finally 
I convinced him to break down the engine, put zero deck flat tops, with a
.045 Comek, put 
the thing back together. Now it is a ground pounding 440. With open chamber
heads, you 
don't get as much quench effect, but it will make the difference in a good
engine via a 
mediocre engine. If you have a head that gives you a quench area, the rule
of thumb is 
to have about .035 to .055 quench clearance. The lower the number without
any interference 
the better. .035 is about as close as you can come with steel rods, I would
suggest .040 just 
to be safe. Over .055 you start to loose the sonic boom effect very quickly.

Earl


The low compresion pistons are different and lower in the bore.  or have a
lower compression height if you want the technical term. Now it gets dirty.
too lazy to build a proper piston for both engines  piston maunfactures
fudged the specs for both motors and produced a so called 9 to 1
replacement. piston. Ever seen a 383 redone and it seemed to be ok but
lacking just that something? Well that is why. Even Momma mopars are fudged
them. 
the only real 10 to 1 pistons are the TRW forged replacement pistons. I have
been unable here to find any others. TRW number is L2315F and they like the
original magnms were only "close to 10 to 1 if the heads were in spec. (79.5
to 80 cc. ) Most heads are acually 86 to 91 ccs so even with everything
going your way you could still wind up with 9 to 1 or less even with the
good pistons if you didnt take care of the head thing. Add to that  the
extra 020" for felpros over the original steel shim gasket and your back to
an 8.65 or something to 1 engine with all 10 to 1 parts. if you have the
wrong pistons to start with for example it is possible that even 8 to 1
would be unreachable. 
This is BTW why cc the heads is so important. it doesnt have to be a "super
critical gotta pass tech" job but you need to know how much extra meat you
heads have on them above factory specs.  I use and have for years used a
large cattle siringe for checking them .(In another life (during my career
Sabatical) i was a cattle trucker and feedlot operater "Dolmetsch Beef
Farm")  TSC should have that in stock. You can get a 50cc or 50 ml siringe
and you will have to fill it twice (or get two. ) a dab of dishwashing soap
in your water will keep it from forming a surface tension curve. Then just
screw in a spark plug and level the head and gently squeeze away . Keeping
track. Not fancy but very practical.  The few minutes spent here and the few
dollars spent correcting this alone will yield mountains of burnt rubber and
tire smoke if you are so inclined. 
Don

Author of
Return to Deutschland (True Adventure)
Old Reliable (Mopar)
http://stores.ebay.ca/Don-Dulmage-Enterprises
http://seniordragster.bravehost.com/index.html
[Non-text portions of this message have been removed]


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----
Please address private mail -- mail of interest to only one person -- directly to that person.  I.e., send parts/car transactions and negotiations as well as other personal messages only to the intended recipient, not to the Clubhouse public address. This practice will protect your privacy, reduce the total volume of mail and fine tune the content signal to Mopar topic.  Thanks!

'62 to '65 Mopar Clubhouse Discussion Guidelines:
http://www.1962to1965mopar.ornocar.org/mletiq.html. 

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