Ignition Only

[Bruce Plecan]

From: "Axel Rietschin" <Axel_Rietschin at compuserve.com>

> From: "Shannen Durphey" <shannen at grolen.com>
> > > A wasted spark DIS doubles the rpm limit of the ignition system since
> you
> > > have twice the number of coils.
> > This is not correct.  The max RPM for a distributor based system is
> > higher provided sufficient firing voltage can be maintained.
> Sufficient voltage is precisely the main problem overcomed by a DIS setup.
> >  The
> > waste spark of the DIS system will cause problems in engines with
> > exhaust/intake valve opening overlap as rpm increases.  The waste
> > spark can fire the incoming intake charge at high rpm.  There are
> > posts discussing this in the DIY_EFI archives.
> I was not aware of this potential issue and I don't believe it happens in
> practice.

OK, we're clear that your beleifs can over rule what does happen.

> First, why only at high rpm when you have the most spark advance
> thus the less fresh gases in the cylinder, if any at all?

The worst example of this is a turbo motor where you might be running 17d of
timing.   With too high of overlap cam this can be a major problem.

> If you use such a
> radical cam and highly tuned exhaust manifold as to fill the cylinder with
> enough fresh gases to be ignited, some 30 or so degrees BTDC, consider
> upgrading to a coil-per-cylinder setup. Also, at that particular moment, a
> significant part of the fuel will probably still be in liquid form thus is
> is very unlikely that the super-lean, very-diluted (by burned gases) low
> pressure and low temperature (due to pressure drop and partial fuel
> evaporation) will ignite at all. That said, you never know :)

I've seen it and had problems with it.

> > Distributor systems with 2 coils, which can alternate between coils
> > for firing events can significantly increase available spark voltage
> > at high rpm.
> By far not as much as a coil-per-plug setup, which is also "DIS".

OR DISTRIBUTOR
OR DISTRIBUTOR
OR DISTRIBUTOR
Bruce
>
> --Axel
>
>
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