Combustion chamber & twin plugs

[Johnny]

When the bore gets over 5 inches it is really hard to get the combustion
taken care of in a timely manner with only one plug. If you could put the
plug in the middle, like what you did, it would be much easier to achieve a
burn, but you just can't fit them big plugs in the middle, and with
airplane engines, they have to have 2 anyway, so they never really worried
about it too much. The GPU version of these engines have only one mag,
harness, and set of plugs partially because they were derated for ground
use (generator or whatever) so they still made the rated horsepower with
only the one plug, and partially because it didn't matter much if a plug
failed (you aren't going to stop flying on your generator), and partially
because it is quite a chore to swap over the parts to make it a 2 plug, 2
mag engine and slip it in an airplane, so even though the GPU's are
essentially the same engine, they sold for a lot less dough even when they
were new compared to there flying counterparts.

What I ended up doing for my experimental aircraft V8 was to go ahead and
add a second plug to each cylinder to achieve true redundancy for the dual
direct ignition. Plus this eliminates that squawk from potential buyers
that come from an aircraft background that require 2 plugs per cylinder.
However, on the SBC, it made absolutely no difference as far as performance
or anything else. Part of this is due to the fact that I use a regular set
of AR heads and just add the second plug to them. this makes the plug
positions fairly close together so you really don't get a different burn.
If you were to design a head from scratch to use 2 plugs, and put one on
one side and one on the other, you might get some different results.

Back when I was drag racing (about a hundred years ago), I found that the
Chrysler Hemi, with it's huge chamber, really did benefit from having a
second plug. on the 2 plug heads, the plugs are still both in the top of
the dome, but they are spaced a ways apart as to shorten the distance
remaining from the plug to the outside edge of the chamber. Especially on
alcohol based fuels, you can get to a point were you just plain can't get
it all lit and burned before the end of the cycle without having to run
ridicules ignition advance. The 2 plug heads shortened the burn time and
aloud running a more normal amount of advance which in turn widened the
power band a bit and also broadened the margin for error as far as
detonation and missed setup for mixture went.

I guess the bottom line would be;
adding more plugs probably never hurts, but you will benefit the most if
you (re)design the chamber for multiple plugs from the start, as opposed to
adding to an existing setup.

-j-

----------
> From: jac at wave.sheridan.wy.us
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Re: Combustion chamber & twin plugs 
> Date: Wednesday, October 02, 1996 5:54 PM
> 
> 
> I have added an automotive type plug to some 
> single plug Continental ground power unit heads. 
> It is possible to install the new plug much 
> closer to the center of the combustion chamber 
> that the standard location of the large aircraft 
> plugs.
> 
> These jugs were used one generators that 
> serviced aircraft.  They were built with a 
> different intake and exhaust orientation but are 
> very similar otherwise.  They bolt up exactly to 
> aircraft crankcases. The Continental GPU jugs 
> were built with a single ignition.  The plug is 
> located in the same place that it is for the 
> aircraft version.  
> 
> With the more centrally located plug in 
> operation, the RPM drop from switching the 
> aircraft plug off is almost negligeable.  The 
> drop from switching the central plug off is 400 
> - 500 RPM.
> 
> I don't think that it is reasonable to expect as 
> anywhere near much power gain from adding a plug 
> to a system designed with one plug, as there is 
> power loss when a plug is removed from a two 
> plug system.  It is important to remember that 
> there is no advance adjustment at all in the 
> aircraft magnetos under discussion.
> 
> 
> 
> john carroll
> jac at wave.sheridan.wy.us
>  
> ---------------Original Message---------------
> > All piston aircraft engines from about 1915 on 
> have twin plugs per 
> > cylinder.  Switch one set off - engine RPM 
> drops about 500 rpm or
> > so.  Must contribute both to power and 
> reliability.  Just part of the
> > background on why I want a bigger flame.  (in 
> the combustion chamber
> > - remember I have asbestos pampers)
> 
> Piston Aircraft have a complete duplicate 
> ignition system in case the
> first one fails.
> 
> Does anyone know what had to be done in order to 
> make a twin plug head
> still work with 1 plug?
> 
> 
> 	RF.
> 
> -------------------------------------------------
> ------------------------
> 83 R100			DoD 749			
> Robert Fridman
> 84 320i						
> fridman at cpsc.ucalgary.ca
> 
> 
> ----------End of Original Message----------
> 
> 
> 
> -------------------------------------
> john carroll
>  jac at wave.sheridan.wy.us
>