[Diy_efi] Timing table change needed with CR increase?

[Tom Visel]

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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