Late S2 Ballast Resistor

[david0078]

Hello

I wondered what your thoughts was on the Ballast Resistor fitted to later S2's. I removed mine years ago.
I have just had the engine rebuilt and I am in the process of connecting everything back up. I have just noticed that I still have the White Blue wire connected from the starter solenoid to the + on the coil. This connection was used to bypass the ballast resistor during starting.
I am confident that as I no longer have the ballast resistor I can remove this connection from the relay / coil, that would leave me with just the 12v coil feed coming via the tachometer as in non-ballast cars.

My question is, what do we prefer. Ballast or Non-Ballast, I have a modern starter and 123 ignition fitted, I'm thinking that the ballast resistor is not required. I have all the parts to put the resister back if required.
I know that the 123 electronic ignition needs a minimum of 1ohm coil resistance, my next job is to check this.

[AshM]

Hi David

Like you I have a late S2 with 123 with Fosseway starter and alternator. Plus sports coil.

I have left the ballast resistor fitted and all works a treat.

Cheers
Ash

[johnetype]

Ballast resistor systems were introduced to improve cold starting and to minimise unburnt fuel whilst trying to pass those all important emissions certifications.

The ballast resistor and coil came as a matched pair so if you remove the ballast resistor you need to change the coil accordingly.

Modern electronic ignition systems give the benefit of good cold starting without needing a ballast resistor setup.

If you've got an electronic ignition system then removing the ballast resistor is the sensible thing to do as long as you change the coil at the same time to match your electronic ignition system.

By removing the ballast resistor you improve reliability by removing two components that could fail (resistor and associated bypass relay) and allow energy otherwise lost in the ballast resistor to be used by the new coil to give you the best possible spark at all times, not just when starting.

Having said that, if you leave the ballast resistor in place as Ash has done everything will still work a treat. Your rev counter should be unaffected either way.

[angelw]

johnetype Wrote:

Ballast resistor systems were introduced to improve cold starting and to minimise unburnt fuel whilst trying to pass those all important emissions certifications.

Ballast Resisters were in use long before emissions certification was an issue. Some systems simply used a Resister Wire.

In quite lay terms, as the starter motor begins to crank the engine, it causes a significant drain on the battery. The ballast resistor allows the ignition system to operate at a lower voltage until the engine starts. After ignition, the alternator begins to recharge the battery, restoring the regular voltage. The ballast resistor then functions to regulate the voltage going to the ignition system to avoid damage to the system.

Many electronic ignition systems take care of the same process the Ballast resister is used for and therefore, the Ballast Resister can be omitted from the circuit. For example, of the two Fireball ignition systems, the XR700 and XR3000, the XR3000 doesn't require a Ballast resister, but the XR700 does, depending on the Primary Coil resistance of the coil being used. A coil with an internal Ballast Resister will have a resistance of circa 3-4ohm, in which case, if used with the XR700 unit, the external Ballast Resister can be omitted (by-passed). The XR3000, takes care of what the external Ballast resister does and therefore, a low ohms coil should be used.

Accordingly, the coil choice and whether a Ballast resister is required with the ignition system being used is an important consideration.

Regards,

Bill

[MarekH]

johnetype Wrote:

Ballast resistor systems were introduced to improve cold starting and to minimise unburnt fuel whilst trying to pass those all important emissions certifications.

The ballast resistor allows the ignition system to operate at a lower voltage until the engine starts.

There are some misconceptions here.

The ballast resistor ignition is not there for emissions purposes, nor to operate the ignition at a lower voltage until the engine starts and it doesn't improve cold starting either.

A car engine is much more difficult to start than it is to be kept running. To provide optimal coil recharge during normal running conditions AND allow for the increased effort needed in starting, the ballasted igntion was invented. This dovetails with the coil and the starter relay to allow full voltage to be applied to the coil during starting but only a reduced voltage to the coil at all other times.

It is there to limit overcharging the coil during normal running conditions, so the coil specification has to be matched to the ballast resistor being used. During starting the coil is fed full voltage, which it can take for a short time, but this would lead to overheating if persistently overcharged. The plug wires and plugs also don't want to be overstressed needlessly.

Electronic ignitions need to be wired in accordance with their manufacturer's instructions. Their electronics are designed to take this into account and the coil specification needed to be used is part of that.

kind regards
Marek

[mgcjag]

A diagram here to show whats being discussed.....Steve

[Tommd]

I would like to add some more ballast resistor information. Per Jaguar's service information when they introduced the ballast resistor:
"The purpose of the ballast ignition system is to provide maximum spark efficiency at high engine R.P.M and assist engine starting under very cold conditions."
So while it is true that the ballast system does aid in cold starting, and this is what most seem to zero in on, the high rpm benefits are often overlooked. The ballast systems were around before emission system and not all ballast systems even use the bypass relay, on those systems, the ballast is always in the system.
How does it aid high rpm spark? On a point system, there is a fixed dwell, say 35 degrees. That is the time the coil has to "charge." At higher rpms, the dwell remains the same, but the time decreases. Thus there less time to charge the faster the engine runs. There is a point where the coil may not reach the required charge. One way to allow it to charge is lower the resistance of the coil, the 3 ohm coil drops to a 1.5 ohm coil This allows more current to enter the coil. But, keep in mind the coil is not a resistor (even though we test it as if it were.) It is an inductor, thus we did not actually lower the coil resistance, we lowered the coil impedance. And keep in mind, as an inductor, the coil with the higher impedance will also charge slower at higher rpms. Thus by lowering the impedance, the coil will charge quicker at higher rpms, which is what we want. Why the ballast resistor. With the lower impedance, if the key is left on with the engine off, too much current may enter the coil and overheat it. The resistor will keep the current to a safe level. BUT, since the resistor is a resistor, and not an inductor, this increase in resistance will not correspond to a decrease in the current flow at higher rpms, as it would if it were an inductor. Thus, more efficient spark at high rpms.
Some ballast resistor systems also use the resistor as a temperature compensator. As the current flow is higher to the coil, it heats up, increasing the ballast resistor's resistance, thus lowering the current. And of course, all of this is matched to get the best system.
Tom

[MarekH]

...the coil will charge quicker at higher rpms.....

...the high rpm benefits are often overlooked....

There is no high rpm benefit.

It knows nothing about revs and it charges up exactly the same no matter what rpm. The ballast merely stops it from overcharging normally whilst allowing for an extra kick at startup.

A typical modern coilpack ignition has a dwell of about 2 to 3 mSec. Add to that the spark duration and you have a benchmark for ignition turn around time.

For our six cylinder cars, at 6000rpm, there is 10mSec per revolution and three ignition events to deliver, so the coil has 3.3mSec to charge and discharge per ignition event event. The job of the ignition coil and ballast combination is to sit the ignition in the sweet spot whereby it has a low enough resistance to pull enough current to charge up and deliver a strong spark in just 3.3mSec but also not pull substantially more current than that at lower rpm when it has ~15mSec between ignition events. This is possible because the lion's share of the charging current is drawn in the first 1mSec of each time the coil charges, rather than as a constant or linear function spread out over the dwell period.

Because the dwell angle is fixed (and therefore is a longer charging time at low revs), there is risk of persistent overcharging at lower rpm if the ballast is omitted, given the low impedance coil it is supposed to be matched to. Put another way, the coil resistance alone is too low and so it needs protecting from an excessive charging voltage.

kind regards
Marek

[johnetype]

...the coil will charge quicker at higher rpms.....

...the high rpm benefits are often overlooked....

There is no high rpm benefit.

It knows nothing about revs and it charges up exactly the same no matter what rpm. The ballast merely stops it from overcharging normally whilst allowing for an extra kick at startup.

There is a high RPM benefit and it certainly does not "charge up exactly the same no matter what rpm". Likewise there is a cold start benefit to a ballasted ignition system with traditional mechanical points.

As Tom points out Jaguar themselves list cold starting as one of the benefits when they introduced the ballasted system.

On the high RPM benefit, in your justification you haven't allowed for the dwell angle of ~ 35 degrees which means in practice you haven't got 3.3mSec to charge the coil, you've only got ~ 1.5mSec or less at 6000RPM once you've allowed for the time the points take to open and close so having a coil with a lower impedance as you do in a ballasted system allows it to store more energy in the limited time and give a higher energy spark at high RPM over a system without a ballast. This becomes all the more important as the points wear and degrade between replacement.

These limitations only apply to ignition systems with mechanical points. Electronic systems without points can vary the dwell angle to maintain a long enough charging time irrespective of RPM as for instance, the Lucas Constant Energy system does which was introduced on the Series 3 4.2 XJ6s.

[Tommd]

Marek, obviously I disagree with you on the "no high rpm benefit." I firmly believe that is the reason Jaguar added it, not for the cold starting benefit. The cold starting is only a side benefit. Keep in mind, some ballast resistor systems are used without the bypass relay, the resistor is in play even during starting. I understand Jaguar added the ballast system for better ignition of the extra lean mixtures that the emission engine was burning, thus the addition in this time frame.

Keep in mind, Jaguar says it is a benefit at high rpm, and they certainly did not get that idea from me!
Tom

[MarekH]

John,
I think the difference here is perspective.

The benefit in the ballasted system is that you can run either with or without the ballast in the circuit, so it better suits both startup and normal running. By comparison, the alternative is EITHER running just a coil which provides a weaker spark at cranking but adequate spark during normal running OR a strong spark at cranking and too hot a coil during normal running.

It's an improvement on both of those two alternative non-ballasted outcomes. Hopefully that reconciles what you think a ballast brings to the table and what Jaguar claimed for it.

I don't see it as being "better", but rather that you are not forced to chose one of two copromised setups.

Clearly someone will have to explain to me the incremental high rpm banefit.

kind regards
Marek

[MarekH]

I've had a read-up on the topic and "the benefit of ballasted ignition at high rpm" is related to temperature.

The material chosen for the ballast resistor is such that the temperature of the ballast resistor affects its resistance. Its temperature is dependent on the current flowing through it. More current equals more heat and this leads to a higher ballast resistance.

At high rpm, the coil risks not being fully charged as there may not be enough time between ignition events. If so, the average cumulative amount of current falls, the coil runs cooler AND the ballast resistor resistance drops. That means the coil now gets a higher charging current at high rpm to compensate for the fact that it wasn't getting fully charged, so it's a self adjusting system which tries to keep the amount of charge put into the coil constant whether the rpm was high or low. (Conversely, the low rpm, the coil gets fully charged, the ballast gets hotter, its resistance goes up and that means the coil gets less charging current.)

Essentially, it lessens the effect of variation in dwell time on how much charge is put into the coil.

It's your choice as to whether you say it is helping add charge at high rpm or whether you think it is helping limit overcharging at low rpm.

kind regards
Marek

[Tommd]

Marek, I am glad you found some more information. I did mention in my post about the temperature compensation, but not all ballast systems use that. Some use a resistor wire- and I do not believe those have any temperature compensation. The meat of the issue is the shortened dwell time at speed. I believe the ballast resistors first appeared with the higher speed V8 engines. The V8 with with its short dwell time caused issues. Lowering the coil impedance and increasing external resistance was one solution. I believe you are missing the point of the internal coil impedance and the external resistor's normal resistance as being key.
Tom

[MarekH]

Tom,
the only relevant point I was missing was that the material chosen for ballast resistor was one which had a very strong positive temperature coeficient, but thanks for trying to explain how it works.

My car doesn't have any of this - I just built my own and then programmed in the dwell I wanted. The OP wants to use the ballast designed for the coil he has fitted.

kind regards
Marek