[DeTomaso] 351 C Compression Question

[Tom Borcich]

OK so help me understand...a compression ratio is
calculated on the volume that is in the cylinder at BDC
including the head chamber combined vs. what that total
volume ends up being compressed to when the piston is
at TDC, hence the 9 to 1 or 10 to 1 CR.  When I look at
the 4V open chamber head in pictures, it appears to be
shallower than the 4V Closed/Quenched Chamber Head, but
maybe that an optical illusion...I have not compared
the two heads side by side. The Open chamber is
76-78cc's and the Closed/Quenched chamber is
62cc's...does that 15cc's+/- really account for that
much drop in the ratio? I know that a lower piston
crown will account for a drop in CR and definitely a
dished piston would account for drop in CR.  
 
With Open Chamber heads and a flat top piston is there
a way to read psi from a compression test to determine
what the actual, or approximation on compression ratio
is? I frankly don't want to pull a running motor apart
and spend time changing heads when I could be focusing
on so many other areas. But I have the disease...the
motor is sitting on the stand and I know it can make
more horsepower.

Regards,

Tom Borcich


 

 

 

  _____  

From: JDeRyke at aol.com [mailto:JDeRyke at aol.com] 
Sent: Tuesday, December 09, 2008 12:42 AM
To: tborcich at msn.com; detomaso at realbig.com
Subject: Re: [DeTomaso] 351 C Compression Question


In a message dated 12/8/08 12:30:48 PM,
tborcich at msn.com writes:



I have read conflicting reports that the Open Chamber
heads had better flame travel because the plugs were
not shrouded the way the quenched chamber plug was, but
I have also read that the open chamber head was more
susceptible to detonation.


         
Engine builders know a lot more about flame travel etc
now than they did 40 years ago, thats forsure. The open
chamber is in fact more suceptible to detonation, so
you must back off ignition advance to compensate, and
that ireduces the power output. But the main power
builder is the compression ratio, and with an open
chamber, its not physically possible to get much
compression unless you use domed or lumpy pistons. And
once thats done, the dome or lumps changes the way the
flame front propagates- usually for the worse. Its all
quite exotic and for our old motors, what it mostly
means is, one just can't get enough power outa open
chambers without a whole lot of trouble, dyno-tuning
and expense (like adding a blower). But even an amateur
like me can get cheap power out of a closed chamber
head. 
As for the dished piston, my August build '72 engine
had cast flat-tops, which I still have around in my
shop, along with the original open chamber heads. My
understanding is, dished pistons weren't used until the
smog laws got really restrictive in late '73 thru '74,
for the last run of Clevelands in the U.S. Cheers- J
Deryke



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