points gap

[eightofthem (Andy)]

Can some kind sole just clarify the proceedure for gap check and dwell have read the book but i have read myself into a confused state (and i dont have a dwell meter)

[harvey]

As a starting point just to get things running you can initially set the gap to 15thou, but you will need to do the final setting with a dwell meter, so if you're intending keeping the car, either buy a cheap meter, or go electronic.

[Phil Nottingham]

A dwell meter is absolutley essential with points on V8's. It takes account of wear and maximises the spark so its impossible to set properly with feeler gauges

[eightofthem (Andy)]

So all a dwell meter does is adjust the points to take account of the demands put upon them by my right foot! so to speak

[Warwick (Ozcoupe)]

Please excuse if you already know this.

On a standard Kettering ignition system, the spark is produced by the coil which is two coils (windings) of wire in a canister and in essence, a tranformer. It increases the battery voltage of about 12 volts up to many thousands of volts. The points are the switch that applies the 12 volts to the primary winding of the coil. If you switch on a 12 volt supply to the coil, the magnetic field it produces in the primary winding induces another electric current in the secondary winding. This is the current that is fed to the spark plug.

However, if the points were closed whenever a spark was needed, the electric field and resultant magnetic field takes time to build up and nothing would happen in the time required. Therefore the whole thing operates in the reverse manner to what you may expect.

It's the changing magnetic field that induces the second current so it is created when the field is rising when the 12 volts is applied to the primary winding AND when it collapses when the 12 volts is removed from the primary winding. If the field is given time to build up it will reach a higher "strength" and when the primary (12v) current is turned off, the field collapses suddenly and this produces a large pulse in the high voltage secondary winding connected to the spark plug via the rotor button and distributor cap.

So when the points close the field starts to build up and when the engine needs a spark the distributor cam opens the points, the magnetic field in the coil collapses and a big high voltage pulse is generated and sent to the spark plug.

The longer the points remain closed, the more the magnetic field is able to build and the bigger the pulse sent to the spark plug. The dwell is the time that the points dwell in the closed position. Since dwell time is not constant - the lower the engine revs, the long the dwell time - dwell is expressed as an angle. That is, how many degrees out of the 360 available as the distributor spins through one revolution.

Since a V8 has to fire 8 cylinders in one rev of the distributor, then the dwell angle must be less than 360 divided by 8 = 45 degrees. You also have to allow for the points to move open and closed and all this "uses up" available degrees. So in the end we have about 25 to 30 degrees available for the points to remain closed for each spark plug on a V8.

Capacitor discharge ignition (CDI) or transistorized ignition modifies the Kettering system in the following way.

The points are used to operate a remote solid-state switching device (SCR - silicon controlled rectifier) which does all the heavy work. Only enough current is passed through the points to stop them corroding or oxidizing. This is called a wetting current and it keeps them clean without being strong enough to burn them or pit them. Therefore they last a long time without needing adjustment since the contacts don't get "eaten" away.

The 12 volt supply that is normally fed through the points is used to charge a large capacitor. This is a bit like a short-term battery and it holds an electric charge. This charge can accumulate for the entire time that the spark is not required. In other words it's like being able to extend the dwell much closer to 45 degrees. When the points open, the SCR dumps the big charge into the coil quite suddenly so the speed of the rising magnetic field is as fast as the previous collapsing one and a hefty spark is delivered to the plug at a much higher voltage and duration than before. The coil is now operating in reverse, so to speak.

Because of the higher voltage and current produced, it is possible for the pulse to jump through insulation that is marginal therefore if CDI is fitted, spacing between plug leads is important so as to minimize the risk of cross-fire by the "spark" jumping through the lead's insulation and firing the wrong plug. Cross-fire can be a problem on early Range Rovers, even with standard ignition, and you can burn a hole in the top of a piston. Also, if a plug lead becomes disconnected and that spark has nowhere to go, it can jump through the internal coil insulation and burn out the coil.

The next step beyond this is to replace the points themselves with an electronic switch using an optical sensor (Lumenition) or a Hall effect device that sense a rotating metal lobe.

The final evolution is to use a coil for each spark plug and have the blackbox fire the appropriate coil and its plug thereby eliminating the distributor.

[johnwp5bcoupe]

A very clear description Warwick and I am sure it will help some of the guys with the timing and how spark is derived

[enigmas]

Excellent description Warwick. Definitely worthy of an elephant stamp.

As an aside I just purchased one of these 'Silicon Chip' Kits from Jaycar.
www.jaycar.com.au/productView.asp?ID=KC5442&CATID=25&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=347

Hopefully I won't screw up the soldering!

~ Vince

[Warwick (Ozcoupe)]

Thanks John,

Perhaps you could jump in and expand on it or add anything I've overlooked. This might be a good place to add other ignition stuff for the electrically-challenged among us.

Or maybe Phil could throw in the odd link to another thread where it's already covered.

So here's a bit more ...

You may notice that some points have a hole through the middle of the contact. (The contacts of the points, or any switch, are the little pieces of metal that come together to conduct the electricity. By using contacts with holes, some degree of air-cooling is achieved giving greater points life.

Some distributors are designed to cause the contacts to move sideways slightly as they touch. This gives a whiping action that keeps them clean and maintains optimum electrically conductivity.

The points gap is important for a number of reasons. If it is set too close (small gap) the opening points will draw an arc that will continue to flow thereby preventing the total interruption of the current and the necessary collapse of the magnetic field in the coil. (And it burns the points prematurely).
But if the gap is set too wide, when the cam lobe passes and they close they are travelling fast enough when they meet to bounce before closing firmly. This robs dwell and disturbs the field in the coil.

Many distrubutors have the dwell adjusted internally with the distributor cap off. Lucas designed a very clever method of allowing it to be adjusted externally while the engine was running. Very handy. BUT - in typical Prince of Darkness fashion, the execution of the idea was bloody awful and the whole thing wears badly and causes all sorts of points and timing problems.

[Warwick (Ozcoupe)]

May 7, 2008 7:04:29 GMT enigmas said:

Excellent description Warwick. Definitely worthy of an elephant stamp.

As an aside I just purchased one of these 'Silicon Chip' Kits from Jaycar.
www.jaycar.com.au/productView.asp?ID=KC5442&CATID=25&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=347

Hopefully I won't screw up the soldering!

~ Vince

Thanks Vince,

I didn't realize Jaycar had Elephant stamp kits.

Looks interesting. I'm planning to fit the CDI that I built for the FJ in 1970 when I was 20 and my Rover was built. So it's period equipment. It came from an Electronics Aust mag. I built one for me, one for Dad and most of one for my boss in East Boundary Rd. I now have all of them plus a handful of unused spare power transistors so longevity shouldn't be a problem.

Anyone know if knock-sensors are available as an add-on?

[enigmas]

This is their full listing on the automotive stuff including the knock sensor. The Programmable Ignition Kit is meant to be used with the coil driver and the (optional) knock sensor. Take a look. Some good stuff if you can use a soldering iron.

PS. I might need to contact you Warwick on some clarification re: how it connects to the optical pickup of my Pirhana ignition unit if thats ok.

www.jaycar.com.au/productResults.asp?FORM=CAT

You can search by selecting the kits and modules option in the top left hand box (under BROWSE PRODUCTS) then select automotive in the lower box.

[Warwick (Ozcoupe)]

Thanks Vince,

I thought knock sensors were sort of built into the cylinder head. I didn't realize you could simple take one from another car and adapt it. Must look into this. I'll have a closer look at the Jaycar site in detail later. Have to take a day off work tomorrow to nip up to Baw Baw to let an electrician into a lodge. (Hope it doesn't snow - 20mm of rain up there today according to the weather report).

As for the connection to the Pirhana system - I think that's probably a bit too modern for me. I might be able to guess if I read up on the two units. I'll see if I can suss it out later but might be beyond me. I've got a couple of people I could ask too.

[norvin]

Go electronic, fit it and forget it.

[Phil Nottingham]

Till it fails and you are stranded in the middle of nowhere - that's the trouble they either work or do not work, points setup give long warning of failure

[norvin]

In the unlikely event of trouble i keep all the bits in the glove box ready to swap back, i have had an electronic ignition on my p4 for over ten years, so far no trouble, just think of the bounce on those points, timing is different on every cylinder.

[Phil Nottingham]

That's what I thought till I had 2 Lumenition units fail in the same year and had to be recovered as did not have spare dizzy to fit and was in my suit and it was raining!

[Warwick (Ozcoupe)]

My old home-made CDI unit, soon to be resurrected after lying in a box for 25 years, is going on the P5B now that I have thrown away (figuratively) the worn distributor with Lumenition and reverted to standard with a local Bosch distributor.

I have a small hole in the case, plugged by a rubber grommet, through which I can insert an Allen key and operate a 3-position rotary switch. CDI or standard ignition, with the 3rd position operating the horn if the ignition is turned on. The unit plugged into the car using a trailer plug and socket. I also carried another trailer plug in the glove box. This was wired with jumpers so I could unplug the CDI and obtain standard ignition by inserting the jumper plug. Never needed either.

[enigmas]

The programmable ignition kit that I'll be using can be triggered by any of the 'electronic' triggers available. I'll use my Pirhana optical switch. It can also be triggered by points (which puts a small 'wetting current through the contact to keep them clean and ensure effective triggering...as Warwick explained earlier). In the event of failure a jumper lead is used to bypass the module. My vehicle has used the Pirhana Ignition kit for the last 16 years and prior to that used a Silicon chip designed unit sold through thingy Smith, Jaycar and Tandy Electronics. These kits can still be found and were priced at approx $45 AUD but are now around the $80 mark. I've found that the only time I've had trouble with a kit is when I tried to use a 'sports' coil...which tends to eventually blow the power transistor. (Simply unsolder, remove & replace).

The biggest downside of any conventional Kettering points system is that most are never rebuilt properly and allow 6-8 degrees timing variation. On a V8 this is critical and causes mediocre performance unless the ignition is severly retarded (leading to even more mediocre performance). On a petrol engine at idle set at 6 degrees BTDC the variation could also fire it at 2 degrees ATDC and all the variations allowed by the erratic timing. Each cyl can fire at different times and if you're running points, probably does. Performance, economy and smoothness of operation will never be realized. Of course some electronic units are pigs.

Since I run a classic car I always like to refute the argument that these vehicles are unreliable and inefficient, pollute and waste resources.

There is host of add on technology that can be utilized by these vehicles to make them efficient in a contemporary context.
For example Jaycar Electronics
www.jaycar.com.au/productView.asp?ID=KC5374&CATID=25&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=347
provide a simple (cheap) Air/Fuel ratio meter that can be used to monitor and adjust precisely the A/F ratios (on your SUs or anything else) but you may need to weld a bung into the engine pipe near the manifold to fit an oxygen sensor. This can then be used to feed information to a stepper motor for a precisely controlled LPG system.

Old cars mainly break down because the mechanical parts are worn out. Fitting a new set of points or spark plugs is only superficial if the components housing them have been around since Methusilah was a boy. Just my 2 cents guys. ~ Vince

PS * This board will not allow me to use the four letter Christian name (D_ _ _ word) for Richard Smith Electronics and likes to substitute thingy. Perhaps this is taking euphemisms for words or propriety too far?

[Phil Nottingham]

I agress the bin is the best place for Lumenition not your car

[norvin]

Thats bad luck Phil having two go down, i have fitted Pertronix on the p5B and i have had no trouble so far, that was about thee years ago, i know other guys in the club have fitted Pertronix as we all bought them at the same time, would be nice to hear if anyone else has had trouble with electroic ignition and what make it was.

[Phil Nottingham]

I have many good reports of Pertronics from a number of sources and may consider it in the future but have spent hundreds of £'s on 2 top of the range systems I expect a longer life that 6 years or so as that would hahe bought a lot points/condensors for as long as I would have nereded them!