Part 2: Reading Plugs, pistons, and valves (with Pics)

[David Papworth]

One full turn on a 14mm sparkplug represents
a change in combustion chamber volume of
0.7*0.7*3.14*0.12 = 0.18 cc
For a 58cc nominal chamber this is a net
change of 0.31%.
Are the ports matched to 0.31%?
Intake manifold?
The head temperatures?
The injectors?
What about the leakdown of each individual cylinder?

A variation of deck height between cylinders of more than
0.0009 inch has more effect on combustion chamber volume than one
turn of the sparkplug.

Are the rod center lengths (on a 5.7 inch rod!) accurate to
this tolerance? What about the pistons and the crank throws?
Is the block decked to this kind of tolerance? Is the crank
that straight?

With very, very careful technique you might be able to measure
a difference of 0.18 cc in combustion chamber volume, but I think
it is pointless -- other components that drive combustion efficiency
vary much more than this.

I agree with Bruce -- index your plugs for optimum placement
of the ground electrode and initial flame-front travel.
Don't worry about the variation in chamber volume of
moving the sparkplug up or down..

----- Original Message -----
From: "Bruce Plecan" <nacelp at bright.net>
To: <gmecm at diy-efi.org>
Sent: Sunday, December 03, 2000 9:46 AM
Subject: Re: Part 2: Reading Plugs, pistons, and valves (with Pics)


>
> I'm talking about a street car, not many are running cc'd heads.
> Heck the we can start worring about the specific gravity of the fuel.  The
> actual O2 content of the immediate air supply, etc.
> There is a practical point to me, where you just take your lumps and go
on.
> If really worried about the volume of the heads then need to run water
> injection about full time to keep any carbon deposits from collecting.
> The thick spacer is what .020", racing a plug that much (or less) isn't an
> amount I'd worry about in a street car.
> In a spec. class or when the budget, time, money allows, but so many tune
> ups are so far off that seeing that difference is not a big issue in my
> book, IMO
> Bruce
>
>
>
>
> From: "Len Sabatine" <sabatine at epix.net>
> >   The seat washers that make indexing  proceed faster also increases
> > chamber cc's
> >   in the specific Cyl, versus  stepping Thru numerous plugs to get the
> > index on the $. The Nitty gritty.
> >   The seat washers don't cut it, after investing hours to get the
chamber
> > cc's balanced. Ok if having
> >   the chambers vary doesn't matter , It would to me!
> >   Len
>
> > >Ahh, got what ya mean.
> > >think it's Jegs has a plate with a plug tapped hole in it.
> > >Then you can scribe a line to show where cylinder lines up per plug,
> > >supposedly for faster indexing.
> > >Me, I gota couple 3 packs of shims, a mike, and a magic marker (ie
cheap)
> > >Bruce
> > >
> > >
> > > > Oh definetly not the place for indexing.. the serious drag guys
obsess
> > >over
> > > > that level of detail (I've read).
> > > >
> > > > I thought the detail would be interesting. While the heads are off
> though
> > > > its good to screw the plugs in and see where they end up, depthwise.
> > > >
> > > >
> > > > ----- Original Message -----
> > > > From: Bruce Plecan <nacelp at bright.net>
> > > > To: <gmecm at diy-efi.org>
> > > > Sent: Saturday, December 02, 2000 7:19 PM
> > > > Subject: Re: Part 2: Reading Plugs, pistons, and valves (with Pics)
> > > >
> > > >
> > > > >
> > > > > In anything on the street indexing really isn't a HP issue, IMO.
> > > > > I does give ALOT more uniform plug readings.
> > > > > The side electrode when positioned just right in a SBC can mask
the
> burn
> > > > > pattern for 1/2 the plug.
> > > > > Plugs should be in and out often enough that there is no carbon
> **build
> > > > up**
> > > > > on the exposed threads.  Also, if the head sees and anti-seize
from
> time
> > > > to
> > > > > time treats the plug enough for it not to stick with AL heads.
I've
> > >even
> > > > > been known to use a little AS on iron headed engines where the
plugs
> are
> > > > > changed alot
> > > > >
> > > > > Also, be advised, AS is an insulator, and when used in excess can
> cause
> > >a
> > > > > miss
> > > > > Bruce
> > > > >
> > > > >
> > > > >
> > > > > > The only thing I have to contribute is that I've read that the
> plug
> > >base
> > > > > > should be exactly flush with the lowest point of the comb
chamber
> > >entry.
> > > > > ie
> > > > > > there should be no 'thread pit' for carbon to collect in
> regardless of
> > > > how
> > > > > > much male plug threading is exposed on the 'thin side' of the
plug
> > >hole.
> > > > > > Indexing with spacers without considering this leaves
compeditors
> who
> > > > have
> > > > > > indexed thru plug selection (buy 200, find the ones that thread
in
> > > > PERFECT
> > > > > > indexed position, sell the rest) with an advantage, however
small.
> > > > > >
> > > > > >
> > > > > > ----- Original Message -----
> > > > > > From: Shannen Durphey <shannen at grolen.com>
> > > > > > To: <gmecm at diy-efi.org>
> > > > > > Sent: Saturday, December 02, 2000 5:51 PM
> > > > > > Subject: Re: Part 2: Reading Plugs, pistons, and valves (with
> Pics)
> > > > > >
> > > > > >
> > > > > > > I see a problem.  Piston 2 is in the wrong spot!  It should
> never be
> > > > > > > between 2 others in a sb chebbie... : )
> > > > > > > Shannen
> > > > > > >
> > > > > >
> > > > > >
> > > > >
> > > >
> > >
>
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