Why distributors are not good enough
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Author:  Fred [ Sat Feb 09, 2008 12:34 am ]
Post subject:  Why distributors are not good enough

A few quick lessons on ignition theory :

Dwell time:

  • 1ms very weak spark
  • 2ms usable spark in SOME applications with decent modern coils (maybe yours)
  • 3ms usually enough, coils stays cold, spark is ok
  • 4ms enough for most, coils may get warm, spark is good
  • 5ms max, full dwell to saturation, coils will get warm, spark is best
  • >5ms, part of the time is spent purely as a DC load, this solely heats the coil and does not benefit spark energy at all

Obviously this depends on both the coil design and how how they are switched with some approaches producing full saturation in a shorter time, and with a higher energy level, than others.

Spark duration:

  • We'll assume that its 1ms long for point of discussion.

Formula to show time available per cylinder:

Time per cylinder = 1000 ms / ((((X RPM / 2 RPEC) / 60 s) * Y cyls) / Z coils)

Where RPEC = Revs Per Engine Cycle

At 7000 RPM with 6 cyls and 1 coil = 2.85ms

If you subtract a spark time of 1ms then dwell = 1.85ms which is WAY too short. Even assuming the spark is instant, it's still too short really...

So, if you want boost and a dizzy, wheres your effective good ignition zone?

With a 6 cylinder at 7k RPM you can have:

  • 3ms dwell to 5krpm
  • 4ms dwell to 4krpm
  • 5ms dwell to 3350rpm

Now, go wasted spark and things are looking a lot better:

  • 7k 4.7ms dwell
  • 7500 4.33ms dwell
  • 8000 4ms dwell

You are instantly OK.

COP just means you allow the coils to stay totally cool working for 1/4 the time or so.

For example, my 4 cylinder with 4 coils : 16ms per engine cycle to do as required, 5ms dwell leaves 11ms for the spark and a nice coffee break :-)

That is why distributors are no good in a high performance application. There are many other reasons such as long leads, caps, rotors, firewall issues with RWD setups.

Here is the formula loaded into google calc ready for you to play with for your application. ... tnG=Search


Author:  Fred [ Mon May 26, 2008 6:43 pm ]
Post subject:  Re: Why distributors are not good enough

Comments here please :


Author:  Fred [ Thu Oct 20, 2011 8:11 pm ]
Post subject:  Re: Why distributors are not good enough

A thread popped up recently and it has some pertinent information in it, so I'm linking it here:


The first post has just been updated slightly to be more clear with some formatting and wording changes etc.


Author:  Fred [ Mon Dec 31, 2012 6:21 pm ]
Post subject:  Re: Why distributors are not good enough

Additional notes about this:

Distributor: energy jumps two air gaps, one at low pressure, one at high pressure, energy is lost to both jumps, moisture can cause the energy to be dissipated into wrong places too.

Wasted spark: energy jumps two gaps, but one is full of high pressure air, and the other highly ionised exhaust gas. This latter jump is effectively a dead short, so the in-cylinder gap receives almost all of the spark energy.

COP/CNP: energy only jumps one gap, all energy is delivered to the plug gap itself, and short of part failure, the spark WILL jump in the cylinder, even if at a sub-optimal time due to insufficient voltage when desired.

This is on top of the dwell issues.

Note, both wasted and COP/CNP are also much lower maintenance than a distributor. Neither have wear and tear parts (except the plugs) and typically only the leads (and/or carbon rods) themselves degrade slowly with time and heat cycles. A distributor has multiple parts to wear out and fail, even if it's a modern electronic one.


Author:  Fred [ Sun Mar 10, 2013 7:09 pm ]
Post subject:  Re: Why distributors are not good enough

This is data from the user "russian" who measured his OEM setup in some way. It clearly shows two things, more dwell at low RPM where duty is low, and voltage potentially low, and less dwell at high RPM when it's running out of time.


At the top end you can see the ECU pulling dwell back to allow sufficient spark time:

6000 / 60 = 100 RPS
100 / 2 = 50 cycles per second
50 * 4 = 200 ignitions per second
1000 / 200 = 5ms per ignition

Of which 3ms is dwell and 2ms is available time for the spark to jump.

Original thread is here: viewtopic.php?f=5&t=2071

Thanks to russian for this data! :-)


Author:  Fred [ Thu Jun 20, 2013 4:01 pm ]
Post subject:  Re: Why distributors are not good enough

Another piece of information worth adding to this thread:

Wasted spark is OK up to the output of this: 60000 / (dwell + spark time allowance)

If we use 3ms dwell and 0.5ms spark, then we're good up to a little over 17000 RPM, yes, that's 17k or seventeen thousand RPM. Effectively wasted spark is OK well beyond what most engines can spin to.


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