[DeTomaso] 351 C Compression Question

Wed Dec 10 20:19:42 EST 2008

Bill, Jack pointed the cam timing issue out and I'm trying to contact the engine builder to see if he can tell me where he set it. I also gather that you can have the different compression results with changes in duration/overlap. I have to look at my cam specs. The cam I'm running is 489 intake lift with 510 exhaust lift...not sure about the overlap.

Tom Borcich

From: bwerner at live.com
To: tborcich at msn.com
Subject: RE: [DeTomaso] 351 C Compression Question
Date: Wed, 10 Dec 2008 16:24:26 -0800

Tom:  Great question.  Let me throw something else in there that will also make a hugh differance in HP. I haven't been following this thread and maybe someone else has already spoken of it.  Your cam will make a hugh difference in power, but it is related more towards cylinder pressure, not so much in static compression ratio.  Correct me if I am wrong.  I believe you can adjust for that power you need by picking the right cam and building the cylinder pressure you need with a lower static compression of oper chambered heads.  Just my two cents worth.

> From: tborcich at msn.com
> To: JDeRyke at aol.com; detomaso at realbig.com
> Date: Wed, 10 Dec 2008 16:02:19 -0800
> Subject: Re: [DeTomaso] 351 C Compression Question
> 
> 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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