RE:Coil question

["John Fielding" <johnf@domain.elided>]

Zamani asked:

Date: Wed, 24 Jul 2002 00:35:25 -0700 (PDT)
From: Zamani Zambri <zzambrimail@domain.elided>
Subject: Coil question

Hi All,

Is there any benefit of using a coil from say Accel or MSD over the
stock Alfa coil?

Thanks.

Zamani


Well that sort of depends on what you are trying to achieve!

The original OEM coil fitted will differ from an aftermarket part for some very good
reasons.

1.    The OEM part was specifically designed to work in unison with the rest of the
ignition package.

2.    The OEM designers made the coil to give acceptable performance at an economic cost.

3.    Aftermarket coils can never fit into every system perfectly because they are
designed to cover a broad range of applications.

I am not saying that brand X coils are bad or unsuitable for your application, rather that
it is unlikely it will interface with the hoped for improvement - and that is a hard
thing(y) to define.

I have spent many hours testing coils whilst developing my CDI ignition systems.  Ranging
from the $5 Autozone el-cheapo through to the exotic types such as MSD and Accel.  In most
cases the expensive aftermarket coils are no better, and often worse than the original OEM
part.  One particular exception to this was the Lucas coil fitted to the Ford Sapphire,
this has a far superior performance than even a Bosch Gold coil, which rather surprised me
as the cost difference was about 3:1 in favour of the "Prince of Darkness" coil!

Before you can compare coils you need to do some math's.  Energy storage is what coil
technology is all about.  Being able to firstly accept the energy, store it for a short
time period and then release as much of the energy stored as quickly as possible is what
determines if a coil is good.  There are many tricks to enhance the performance of a coil.
Some are somewhat dubious when the physics are examined.

A MSD coil I examined closely had a poor performance when measured under critical
conditions compared to an OEM part, but the manufacturers spec sheet stated quite the
opposite.  I contacted MSD and enquired how they made the measurements, to which I
received a stony silence.  On further prodding I was eventually sent a totally different
set of test measurements, which compared closely with what I had measured.

The "hang-up" which seems to dominate coil advertising is the so called "turn-in" ratio, a
poor
explanation for the ratio of primary to secondary turns.  In a perfect inductor - which is
what a coil is in reality and not a transformer as many people think, even some coil
manufacturers! - the dominant feature is the primary inductance.  Without knowing what the
inductance is the spec sheet becomes meaningless.  Nearly all manufacturers quote the
primary resistance, but that is a pointless exercise.  You need to know what the primary
inductance is to do the math's for energy storage!

Ask your favourite coil manufacturer what the primary inductance is for their coil and I
bet they won't or can't tell you!  It is mostly advertising hype, like the colour of the
coil makes a difference!  Fortunately I have equipment which can measure the coil primary
inductance and knowing this factor I can tailor an ignition system to the coil to extract
the maximum energy from it.  You can't simply bolt on a brand X coil and expect the
ignition system to improve, you have to match each item to the next to get the best
working performance, sort of like selecting bearing shells to suit a known crank journal
diameter!

my 2cents worth.  If you need more technical information please contact me off-list.

John
Durban
South Africa
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