[DeTomaso] Compression ratio estimate

[Tomas Gunnarsson]

Opening and closing at BDC and TDC would only work well at very low
rpms. At higher rpms it's more beneficial to open e.g. intake a little
earlier than TDC and close after BDC. With all other things equal a
higher mechanical CR results in a higher real life peak pressure and
that's why it's interesting to know it.
 
Tomas

<-----Ursprungligt Meddelande----->

 	  	 From: Pantdino [pantdino at aol.com]
Sent: 14/8/2013 6:22:49 AM
To: JDeRyke at aol.com;boyd411 at gmail.com;detomaso at poca.com
Subject: Re: [DeTomaso] Compression ratio estimate 

Given that both valves are not closed from bottom dead center to top
dead center, of what significance is the mechanical CR anyway? 
Seems like what matters is the psi at the time of ignition. 

I know I must be wrong or they wouldn't tear down engines to determine
it. 

Jim 


-----Original Message----- 
From: JDeRyke <JDeRyke at aol.com> 
To: boyd411 <boyd411 at gmail.com>; detomaso <detomaso at poca.com> 
Sent: Mon, Aug 12, 2013 11:45 am 
Subject: Re: [DeTomaso] Compression ratio estimate 


Boyd, in a word- no. That's why in spec-racing classes that require a
max. 
compression ratio, they still do tear-downs. If there was a dependable
way 
around it, those guys would have found it. 
The clearances inside the motor must be physically checked; there are 
simply too many possibilities for changes- both by the factory and
'other 
people' 
to assume or infer anything. Cam lift & lobe center spacing, piston
design, 
block & head work including valve jobs & gasket changes- even wear- all 
will alter any compression tests. 
Unless you are the original owner and know for sure that NO machine work
has ever been done (which is either sad or scary), the way that always
works 
is to pull the heads and measure the void volume using Archimedes'
Principle, 
and ratio that against the measured bore & stroke. There are no short- 
cuts. 
Gasket sets are available and you'll learn a lot from a winter
tear-down. 
Enough others on this Forum have done this that you can't get into much 
trouble. An engine stand and an overhaul manual are invaluable for this.
You may 

even save some money and will certainly gain some power! Good luck- J
Deryke 
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