[Diy_efi] Spark Plug Projection into Combustion Chamber

[David Gallant, 12 Point Racing]

> This question is only peripherally related to EFI but I know that there is
> a lot of engine building expertise on this list so I hope you don't mind.
>
> I am machining my motorcycle cylinder heads to add a second spark plug to
> the combustion chamber.   I am wondering if anyone has any recommendation
> as to how far the plug should project into the combustion chamber.    I
> have seen some installations which recess the plug in a shallow pocket
> (~0.1" deep....really not much more than a chamfered edge of the spark plug
> hole) in the roof of the combustion chamber.  This is supposedly to allow
> the ignition spark to start the flame in a protected area.
>
> Any comments from anyone?
>
> Obligatory DIY_EFI content:   I will be using my DIY_EFI WB O2 meter to
> fine tune my AFR once I have completed the dual plugging project.
> Theoretically the engine should run on a leaner mixture than before.  This
> should be a good example of not tuning to a particular AFR but instead to
> the AFR that produces the best power.

Having gone down this road with multiple motors now, I can speak directly to
the dual plugging of Ducati "two valver" engines. Dual plugging Duc motors
(in my experience) has not shown any huge power gains. We tried this from
with everything from aftermarket "super coils", to the stock set up. There
were slight increases throughout the range on some motors, however, the
increase in power is not why most engine builders dual plug the heads. With
the really big bores (+96mm mind you, thus leading to ever increasing piston
rock values), the dual plugging seems to help evenly set off the charge in
the chamber, and (as wise men attempt to sell us) helps crank and bearing
life on high strung road race engines.

As far as the o2 meter is concerned, I would wager you will see little to no
change IF this is your only modification, and this is still a relatively
tame motor. I would be interested in your findings on your particular
motorcycle.

-daveG
-dave at 12pointracing.com

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