[Diy_efi] Timing table change needed with CR increase?

[Clair Davis]

Be sure to check valve-to-piston clearance as well, as you won't be able to
tune your way out of that situation...

Upping compression is one of the best things you can do to an engine built
to smog specs.  I would guess you could get away with a full point increase,
perhaps even 10:1 if you get the spark tables sorted out and aren't afraid
to run pump premium.

Clair


----- Original Message ----- 
From: "David Allen" <davida1 at hiwaay.net>
To: <diy_efi at diy-efi.org>
Sent: Monday, January 29, 2007 10:31 AM
Subject: Re: [Diy_efi] Timing table change needed with CR increase?


>   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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