[Diy_efi] Timing table change needed with CR increase?

[cobraman at insightbb.com]

I would like to add that most squish is lost because the piston top is below the deck.  You need ~ .035-.040 from the piston to the head - measure it - or ping could be a problem. TomS 

----- Original Message -----
From: Tom Visel <five10man at commspeed.net>
Date: Monday, January 29, 2007 10:04
Subject: Re: [Diy_efi] Timing table change needed with CR increase?
To: diy_efi at diy-efi.org

> It depends.  Sorry!  Do your cylinder heads have 
> closed chambers? i.e. 
> is most of the combustion chamber's upper side composed of 
> quench or 
> flat area, or is it mostly open to one degree or another?  
> Heads with 
> more quench area - where the piston comes close to the cylinder 
> head and 
> squeezes the mixture into the combustion chamber - tend to be 
> more knock 
> resistant.  Beyond that, heads without exposed sharp edges 
> where thin 
> metal might retain heat and glow and light off pockets of the 
> mixture 
> tend to be more knock resistant.  Pistons that have the 
> minimum of extra 
> cuts in them tend to be more knock resistant, as well as pistons 
> with a 
> thermal barrier.  Spark plugs of an incorrect length may 
> result in sharp 
> edges being exposed inside the combustion chamber, acting like 
> glow 
> plugs and causing preignition.  Find out what plugs you 
> can/will use 
> while the heads are off.
> 
> Did you have any knock retard under acceleration before?  
> If so, and 
> your combustion chambers (including piston tops) were clean, 
> then you 
> don't have much room to increase CR with making improvements in 
> knock 
> resistance and/or retarding your timing.  If not, then you 
> have room to 
> play - it's just a matter of finding out how much.  Either 
> find a person 
> who's built the same engine, or be prepare for continued 
> modifications 
> and programming.
> 
> TomV
> 
> David Allen wrote:
> 
> >  Hey guys, I'm rebuilding a 1989 3800V6 from a 
> LeSabre.  It has the 
> > SFI system using the 1228253 ECM.
> > Anyway.  The engine has 370,000 miles and the intake 
> manifold gaskets 
> > started leaking water into #5 cylinder.  So I tore it 
> down for a 
> > rebuild. Amazingly little wear.  I would like to improve 
> upon this 
> > motor by installing flat-top pistons in it.  The rest of 
> the motor 
> > will repain stock.
> >  What is a good general "rule of thumb" for changing 
> spark timing to 
> > prevent pinging when increasing CR?  Will this be enough 
> of a change 
> > to necessitate spark changes?  What areas of the table 
> will need the 
> > most reduction in advance? These are fairly highly-tuned from 
> the 
> > factory; but it only has 8.5:1 compression. I bet a set of 
> flat top 
> > pistons would wake it up and help on fuel mileage.
> >  Any input would be GREATLY appreciated!
> > Thanks,
> > David
> >
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