[Diy_efi] Timing table change needed with CR increase?

[David Allen]

  It has lots of quench area!  I do oplan to polish the chambers lightly 
with a scotch-brite "cookie" grinder as there are a few rough areas.  It had 
zero KR on cheap gas with low CR.  I'm betting it will be quite knock 
resistant with a little rounding sharp edges and good plugs.
  Thanks!
David


----- Original Message ----- 
From: "Tom Visel" <five10man at commspeed.net>
To: <diy_efi at diy-efi.org>
Sent: Monday, January 29, 2007 10:03 AM
Subject: Re: [Diy_efi] Timing table change needed with CR increase?


> 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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