[Diy_efi] Timing table change needed with CR increase?

Tue Jan 30 04:12:54 UTC 2007

Although absolutely dead-on advice (on ANY rebuild check ALL clearances)
I kind of doubt a smog 8.5:1 motor with the cam likely to have shipped
with that motor will have any issues with valve pocketless (flat top)
pistons of stock deck height.

My Camaro is an example of what can be done with engines in the high
compression realm - it is dead stock at 10.25:1 compression ratio. Cam
grind keeps effective compression ratio almost linear from low RPM to
5500 RPM - giving it an almost linear torque/HP curve. And it runs on
anything from 80-something octane up - the key factor being a knock
sensor which eliminates the bad moments on cheap gas. I have a
supercharger kit for that car - never put it on as the car was hot
enough for me without it - but think of it - a 10.25:1 car with 6 lbs of
boost - Vortech spent a LOT of dyno time on that product. Bottom line
doing 10:1 on that 3800 should be easy - do flat top pistons with
thermal barrier coating, smooth the hell out of the chambers on the
heads, choose a cam with a broad range (2000-3000 RPM sweet spot for
street driving) - well I'm sure you can find folks expert in your
specific engine and vehicle. And my personal experience - Cam grinds
will kill you if not chosen wisely - on a high CR motor putting in a cam
designed for low RPM power means nasty 'static'/low RPM chamber
pressures and ping city, the opposite is a high RPM cam - very low
static pressure at low RPM's. Or in serious old school words don't mix
high CR pistons and an RV cam :-) unless you want to burn av-gas or VP
or Sunoco 260 :-)

On Mon, 2007-01-29 at 20:18 -0600, Clair Davis wrote:
> 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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