The V8 Owners Forum -

Trying to decide if it's wothwhile fitting one to my 3.9 V8 Disco. It's on petrol no lpg.

Seen some good and bad reports on them. Early types seemed unreliable but later ones maybe ok. Some claim the extra spark output can burn the coil out.

Has anyone fitted one of these and if so can you recommend buying one.

Can't see much point. I'm sure Rover made sure that the OEM ignition system set fire to the fuel properly.

Chris.

Why dont you try such like MSD 6AL? I think it will give mutc bigger spark than any of RPI power amp..

The amp mod on here is good, I had Tony's old setup - noticeably better starting, it was just instant. No probs with the coil. Didn't notice any other difference from the factory one though.

Robrover wrote:Seen some good and bad reports on them. Early types seemed unreliable but later ones maybe ok. Some claim the extra spark output can burn the coil out.

Hello Rob, In reality one would expect the peak voltage of the HT system to be wholly dependent upon the ratio of Coil Windings in the primary (LT) and secondary (HT) circuits of what is essentially a transformer.

So claims that an amplifier has the ability to generate some "extra spark output" might be considered fanciful.

On the other hand if the amplifier is designed to have sharper leading and trailing edges to the output (HT) pulse, one could then see how the spark would be more efficient .

Therefor one might also conclude that with no increase in voltage, then there is little reason to believe that the coil would burn out for that reason.

On the other hand (that'll be my third hand), if the amplifier also has a voltage boosting circuit on the low tension side then with greater than +12 volts input to the coil the output voltage would be proportionally higher.

Now that would have the propensity to burn out coils that may not be expecting such a shocking event.

I dont know the answer to the original postulation but the science is basic.

ramon alban wrote:

Robrover wrote:Seen some good and bad reports on them. Early types seemed unreliable but later ones maybe ok. Some claim the extra spark output can burn the coil out.

Hello Rob, In reality one would expect the peak voltage of the HT system to be wholly dependent upon the ratio of Coil Windings in the primary (LT) and secondary (HT) circuits of what is essentially a transformer.

So claims that an amplifier has the ability to generate some "extra spark output" might be considered fanciful.

On the other hand if the amplifier is designed to have sharper leading and trailing edges to the output (HT) pulse, one could then see how the spark would be more efficient .

Therefor one might also conclude that with no increase in voltage, then there is little reason to believe that the coil would burn out for that reason.

On the other hand (that'll be my third hand), if the amplifier also has a voltage boosting circuit on the low tension side then with greater than +12 volts input to the coil the output voltage would be proportionally higher.

Now that would have the propensity to burn out coils that may not be expecting such a shocking event.

I dont know the answer to the original postulation but the science is basic.

What you say seems to make sense but I believe that the rate of change of the voltage at the LT has a big effect on the HT voltage, it's not just down to the windings ratio. (The ratio only gives you the theoretical output voltage).

Most people think that the condenser on a points system is to reduce the arcing of the points but really its there to suck out the LT out of the coil when the points open. If you run a points system without the condenser it usually won't even produce a HT spark.

From what I've read on the Cobra forum on the RPI box of tricks, it's not worth bothering with.

Pete

I'll stick my neck out and disagree with the physics stated above (accepting that I could be wrong).

Basically, you 'charge' the magnetic field by passing a current through the LT side. When you interrupt the current (i.e. points opening, or ign amp switching off), the magnetic field collapses, and induces a voltage across both coils. Because of the turns ratio, it induces a much bigger voltage across the HT side.

Now, the larger the magnetic field before collapse, the larger the spark. To get a larger magnetic field, one keeps the current flowing into the coil for longer. However, there is a limit, called saturation. Once saturation is reached, the impedance of the LT coil drops drastically, and as a result the current in the LT side rises drastically. This extra current will be dissipated as heat, and will burn the coil out in time.

Rover will have designed their matching amp & coil to be as close to saturation as possible without ever becoming saturated.

One can increase the LT 'on' time to get more energy into the coil, however one should really get a coil that can withstand a higher magnetic field to go with it.

E&OE.

Chris.

ChrisJC wrote:I'll stick my neck out and disagree with the physics stated above (accepting that I could be wrong).

Basically, you 'charge' the magnetic field by passing a current through the LT side. When you interrupt the current (i.e. points opening, or ign amp switching off), the magnetic field collapses, and induces a voltage across both coils. Because of the turns ratio, it induces a much bigger voltage across the HT side.

Now, the larger the magnetic field before collapse, the larger the spark. To get a larger magnetic field, one keeps the current flowing into the coil for longer. However, there is a limit, called saturation. Once saturation is reached, the impedance of the LT coil drops drastically, and as a result the current in the LT side rises drastically. This extra current will be dissipated as heat, and will burn the coil out in time.

Rover will have designed their matching amp & coil to be as close to saturation as possible without ever becoming saturated.

One can increase the LT 'on' time to get more energy into the coil, however one should really get a coil that can withstand a higher magnetic field to go with it.

E&OE.

Chris.

Chris,

I don't see that what you said really dis-agrees with anything that I've said. You are right in that the coil needs to be virtually saturated to get the maxium HT, the problem is that as the revs rise there is less and less time for this to happen, the result is that the HT voltage starts to drop. Note that the number of winding in the coil has not changed yet the voltage is dropping.

You are right with regards to the saturation etc but the rate of colapse of the LT and therefore the magnetic field still has a direct effect on the HT voltage. Think about it this way, what would the output be if the LT was a nice low frequency sin wave say at 1Hz. I think that the HT output would be zero because the rate of change of the magnetic field would be too slow to induce any HT voltage. (Sharp square waves are what is required).

Some systems do not colapse the LT voltage but many do as the voltage can be colapsed quicker than it can be created. CDI systems do quite the opposite, they send a huge (like 300V) spike up the LT side of the coil which then creates the HT. That's why even the LT side of a CDI system can send you flying across the garage.

Anyway it's good to hear other points of view.

Regards,

Pete

Chris, Pete, I thinks you are both correct and the speed of Magnetic Field Collapse is the crucial factor.

However I would just like to highlight two points in my original observation.

In reality one would expect the peak voltage of the HT system to be wholly dependent upon the ratio of Coil Windings in the primary (LT) and secondary (HT) circuits of what is essentially a transformer.

I have highlighted the word "peak", meaning "maximum possible" but perhaps I should have said "ultimately" instead of "wholly"?

On the other hand if the amplifier is designed to have sharper leading and trailing edges to the output (HT) pulse, one could then see how the spark would be more efficient .

So the speed of the magnet collapse is enhanced but I think the the words "output (HT)" should be removed.

What do you think?

Ramon

ramon alban wrote:Chris, Pete, I thinks you are both correct and the speed of Magnetic Field Collapse is the crucial factor.

However I would just like to highlight two points in my original observation.

In reality one would expect the peak voltage of the HT system to be wholly dependent upon the ratio of Coil Windings in the primary (LT) and secondary (HT) circuits of what is essentially a transformer.

I have highlighted the word "peak", meaning "maximum possible" but perhaps I should have said "ultimately" instead of "wholly"?

On the other hand if the amplifier is designed to have sharper leading and trailing edges to the output (HT) pulse, one could then see how the spark would be more efficient .

So the speed of the magnet collapse is enhanced but I think the the words "output (HT)" should be removed.

What do you think?

Ramon

fair enough!

Anyway the guy that started this post only wanted to know whether the RPI box of tricks is any good!

If it sharpens up the wave then it could help but who knows! (I doubt that RPI will tell you, or they will just tell you what you they think that you want to hear ).

I think that it does have a retard function so that you can switch from LPG to petrol and still fire the mixture at the right time. (or the other way round, I know nowt about LPG as it's not really a 'Cobra' fuel!)

There is another spanner that can be thrown into the works...Basically if the LT voltage colapses too quickly then the duration of the HT spark is very short, this can mean that the ignition of the mixture is poor. (Its not just about the voltage, its about the duration. in fact its about the number of milli-joules of energy).

I think that some CDI systems use a shunt resistor to broaden the duration of the LT spike. Other systems have a multiple spark setup.


Pete

I aint a clue about the science and posts above but when i had my V8i Discovery on LPG i was toying with buying one. I bought the magnecore leads which did make a difference but TBH i was sick of RPi pushing and pushing for me to buy a amp. When i asked question about them on various forums the main reply i got, and alot where from people who had bought them, was it made no difference to the Petrol side. LPG it might make a diference to as it relies on a good spark. If your just running petrol i wouldnt bother with it TBH.

sidecar wrote:
I don't see that what you said really dis-agrees with anything that I've said. You are right in that the coil needs to be virtually saturated to get the maxium HT, the problem is that as the revs rise there is less and less time for this to happen, the result is that the HT voltage starts to drop. Note that the number of winding in the coil has not changed yet the voltage is dropping.

Pete

Ah, that might have been true for points ignition (as the dwell time got shorter as the engine revs increased), but it's not true for an electronic ignition. If you look at the low tension side with an oscilloscope, you can see that the charging time of the coil is constant. In other words, the clever electronics works out when the spark is needed, and turns the coil on a precise amount of time beforehand to ensure the coil is fully charged. This is the meaning of 'constant energy' ignition.

I think CDI ignitions use the coil as a conventional transformer, simply to step up the voltage from the capacitor discharge to HT. In this case, it's the buildup of the magnetic field that triggers the spark in the HT side, rather than the collapse.

I wonder if the condensor on a points ignition somehow controls the field collapse.

Other points accepted.

Chris.

ChrisJC wrote:

sidecar wrote:
I don't see that what you said really dis-agrees with anything that I've said. You are right in that the coil needs to be virtually saturated to get the maxium HT, the problem is that as the revs rise there is less and less time for this to happen, the result is that the HT voltage starts to drop. Note that the number of winding in the coil has not changed yet the voltage is dropping.

Pete

Ah, that might have been true for points ignition (as the dwell time got shorter as the engine revs increased), but it's not true for an electronic ignition. If you look at the low tension side with an oscilloscope, you can see that the charging time of the coil is constant. In other words, the clever electronics works out when the spark is needed, and turns the coil on a precise amount of time beforehand to ensure the coil is fully charged. This is the meaning of 'constant energy' ignition.

I think CDI ignitions use the coil as a conventional transformer, simply to step up the voltage from the capacitor discharge to HT. In this case, it's the buildup of the magnetic field that triggers the spark in the HT side, rather than the collapse.

I wonder if the condensor on a points ignition somehow controls the field collapse.

Other points accepted.

Chris.

Are you saying even a low-tech Rover amp that's stuck on the side of the dizzy can carry out the task that you described above? If so you can knock me down with a feather! (and I stand corrected!)

Yes I do think that the condensor controls the field colapse in a points system, I've worked on some points systems that will not even create enough HT to make a spark without the points being fitted.


I still think that the RPI box is a load of 'snake oil' though!

Pete

sidecar wrote: Are you saying even a low-tech Rover amp that's stuck on the side of the dizzy can carry out the task that you described above? If so you can knock me down with a feather! (and I stand corrected!)

Pete

I hereby knock you down with a feather!

If you prise one open, you'll find it's not as low-tech as you imagine.

Which is one reason I can't understand what the attraction of some 'polished billet super-duper' yet points based aftermarket distributors are.

Chris.

Having tried 3 of them over the years I wouldn't touch them with a barge pole.

Stick with the Rover kit unless your using LPG then consider a CD ignition kit.