PLUG GAP

[Lee Dutton]

Hi all just a quick question, if you have electronic ignition fitted to your distributor

should the plug gap be increased?

Cheers Lee

 

 

[RogerH]

Hi Lee,

generally, yes.   Up to 0.030" usually.    What system are you using.

 

Roger

[Lee Dutton]

Hi Roger, thanks for the quick reply

I am using the Accuspark system (which was recommended,) seems ok so far, with NGK BP6HS plugs.

 

Cheers

Lee

[Lebro]

In my opinion, the plug gap should only be increased if the energy available is increased from standard.

This could be due to fitting a high output coil, or a high output electronic ignition system such as capacitor discharge, & inductive discharge etc.

Simply replacing the points with an electronic switch (Accuspark, Powerspark, Pertronix etc.) does not boost the energy, it just makes for less maintenance. (which is good)

Bob.

[pfenlon]

6 hours ago, Lebro said:

In my opinion, the plug gap should only be increased if the energy available is increased from standard.

This could be due to fitting a high output coil, or a high output electronic ignition system such as capacitor discharge, & inductive discharge etc.

Simply replacing the points with an electronic switch (Accuspark, Powerspark, Pertronix etc.) does not boost the energy, it just makes for less maintenance. (which is good)

Bob.

Not wishing to hijack Lee's post, talk of coils is a bit of a puzzle for me, I have a petronix system on another car and bought the "Flamethrower" coil that they recommended.

MY TR4 which I'm renovating had a petronix system in the dizzy, but with a normal coil, Are these super coils really necessary? (I have got the TR on point at present)

[Lebro]

A very good question, which I would like to know the answer to.

Perhaps someone can answer it !

Bob.

 

[RobH]

The 'Flamethrower' is not just one particular coil - it is a family of different ones. 

Some are oil filled, some are resin filled and they are available with different max HT outputs. 

https://pertronixbrands.com/search?q=&hPP=12&idx=shopify_products&p=3&dFR[vendor][0]=Pertronix Ignition&hFR[named_tags.level_1][0]=Plug Wires and Coils > Ignition Coils&is_v=1
 

 

The limitation of a standard coil ignition system is the shortness of the dwell time as engine revs increase.  At low revs there is plenty of time for the coil to become saturated (i.e. fully charged) during the dwell but at high revs the dwell becomes short and the coil may not fully saturate. (That is particularly so for 6 cylinder engines which fire 3 times per engine rev as opposed to only twice for a 4 pot). This happens whether the switching is done by points or by electronics - it makes no difference with a simple system*. The effect is that the coil achievable HT drops off at high revs.  For a standard engine with standard spark plug gap the required HT to reliably fire the plug is around 10 to 15kV so even a normal 30kV coil has plenty in hand up to reasonable top engine speed, as there is no problem reaching the  breakdown voltage even when the coil output has dropped a bit.  

If the engine is tuned to be used at very high revs however, the dwell time may well become just too short for the standard coil to work reliably and one which can produce higher HT initially may be necessary so that it still gives enough voltage at peak revs.   

The required breakdown voltage increases with the plug gap so using a higher voltage coil on a standard engine allows the plug gap to be opened a bit and still have reliable firing at peak revs.  Whether that gives any benefit in engine performance with no other changes to the engine is a moot point. 

 

* Some electronic systems have circuitry which standardises the dwell time irrespective of engine speed. That will help prevent coil heating by shortening the dwell at low revs but it cannot extend dwell time at high engine revs. 

Edited March 6, 2021 by RobH

[pfenlon]

Thanks for the info Rob, My "Flamethrower" coil is

fitted along with the Petronix system on a 5.8Litre V8 engine, so maybe money well spent.

I shall try the TR4 petronix system with the standard coil, and see what happens.