| Author | Message | ||
     Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Hi guys, My MK VI has 2 coils(luckily) mounted on the firewall. I started running the car with the original coils in place, as the coil that was attached was working fine. About 2 years ago, the coil failed, and through the foresight of Rolls Royce, a quick roadside swap to the other coil found me travelling agin. The symptoms where the car was very hard to start, and after 3 or 4 times where I was unsure if the car would start, the coil actually failed whilst driving. I replaced the failed coil with a brand new coil, leaving the other original coil as the backup. Well, on saturday the new coil packed it in. The car was hard to start in the morning, and after the 3rd start of the day, only just got going. I swapped the coil leads over and presto, instant starting again. The coil has an external capacitor attached to it, whcih is not, in my opinion, necessary, however, not a bad thing. Is this just coincidience, or is something happening to cause my coils to overheat and expire? I know a bad capacitor will burn out a set of points, am I better off disconnecting the capacitor? Will purchase a new coil tomorrow and relegate the original coil to the spare again. Interested in your thoughts. Cheers Martin | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 265 Registered: 4-2003 |
Me again: nothing else to do this lunchtime, with the family in Berlin, I guess. A bad or disconnected condenser (capacitor: same thing) will first burn the points, but will also destroy the coil and plugs in a short time. The condenser is there not for the points or radio. It is to provide a smooth and gentle current waveform in the coil. Otherwise, the waveform will be very spiky and burn everything up like a lightning bolt. Check your condenser. They last about 20 years. Any generic ignition condenser will work fine if it looks the part, or can be used until you find one which does. | ||
| Ashley James Unregistered guest Posted From: 62.252.44.80 |
I believe that R-R moved the ignition coil forward to the thermostat housing on later cars because they got to hot on the bulkhead and were prone to failure. It is also true that the original coils can no longer be considered reliable, I had one fail and am using a modern now. The fuel pump is also a pain in the arse and can give the same symptoms. I'd check it out. | ||
| Bill Vatter Unregistered guest Posted From: 68.215.166.134 |
Martin, What kind of coils are you getting? To my knowledge the original "top hat" coils ar no longer being made, and most of us are lucky to have one good one, and a shot spare keeping up the appearances of originality. The original coil was internally connected for positive ground and it also had internal resistance, which limits current flow. This is a combination that you cannot find in a modern coil. Your best bet is to get a generic coil without internal resistance, connect the positive terminal to the wire going to the points, and put a 2 ohm resistor in between the wire from the switchbox and the minus terminal on the coil. Your parts man should be able to bring up a bunch of different looking resistors. You pick the one that will create the least visual distraction. If you use a coil intended to have additional resistance provided, and you do not have this additional resistance, it seems very reasonable that the coil is going to burn itself up in short order. You will know you have the right coil when it is cheap (less than $20 US) and on it you see words like "for use with primary resistance" which tells you that you need a balast resistor. Balast resistor is nothing new, all of the old cars used them. I ran my car like that for some years without any problem. (A pair of coils on the bulkhead with a balast resistor hiding behind the coils at the bottom) If you got a coil with internal resistance, I am very sure the internal resistance will be between the primary winding and the "+" terminal, which means it is not connected right for your car. It will work, but it will not provide a properly hot spark. Recently I bought a pair of original coils on eBay for $76 US plus shipping. Both coils tested excellent, better than the modern coils I had been using (higher secondary voltage output). Now I have good authentic coils. (Boasting a little here) Car runs great, looks proper under the bonnet, happy driver. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Hi Ashley, Not the fuel pump, definitely can hear the pumps working, giving the twin click, as opposed to the single click in my MG Magnette. Bill, I was thinking I would run a "Non resistor" coil, hadn't thought of installing a resistor in line. I had fun with this on my 1968 Mercedes 280, it had a balast resistor in place, (original) so as to boost the starting voltage when the starter motor was operating. This doesn't seem to be a problem on the Bentley. However, the combination of a 50 year old condensor and no external resistor would mean that my coil is on the recieving end of full current, and thus more likely to fail. How does your car start with a 2 ohm resistor in place? How did you decide on 2 ohms? Interesting that Ashley raises the point of the coil being mounted on the thermostat housing on the R type, and a single coil at that. I would have thought it would have run hotter in that position, but obviously not. The coils on the car at the moment have a screw in lead, modern coils have a push fitting, so looks like I will have to replace both coils, if I am going to run a spare. Will also replace the capacitor/condensor, thanks Richard. Looks like the original one. I wonder why the MG doesn't need a condensor on it's coil, and I have not replaced a coil on that car during my ownership over the last 15 years. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 269 Registered: 4-2003 |
Martin, If you look inside the distributor of your MG you will see it has a single point set. Like on most cars, the condenser is either inside the distributor or screwed to the outside. They all have one somewhere. There is no room for one inside the Bentley's distributor due to the dual points. Hence the need for an external condenser. Doesn't your car have a spare external condenser ? In support of Ashley's comment on coil locations, the coil on my R-Type has never failed. I last replaced it in 1970 for fun !! It is sitting in a relatively cool direct air stream from the radiator fan, ie maximum temperature around 50 degrees. At the bulkhead there is far less air flow and 80 degrees is common. I find the Lucas autoelectrician in Falcon St, near the old Pacific Highway at Crow's Nest, exceptionally good in all these matters. They will provide you with a suitable coil, resistor and condenser, and can provide all brushes for generators and starter motors, fuel pumps etc. | ||
| Bill Vatter Unregistered guest Posted From: 68.217.42.5 |
Martin, I don't remember for sure where I got the 2 ohm spec. I think when I was thumbing through the resistors at the parts store, they were mostly all 2 ohm. Look at the modern coil you are using. Does it have words printed concerning resistance? Anyway long ago, pre-war cars all had ballast resistors because the coils were without resistance. Regarding starting, no difference. Car starts instantly all conditions. Oh yeah, I have a Stromberg carburetor on my Silver Wraith, not those finicky SUs that take a few cranks to get fuel into the engine. Let me be clear on this point: If your car is hard to start but runs well once you get it going, it is not a problem with your ignition system. Regarding capacitors, they are supposed to be everywhere on my car. Early cars had one on the distributor laying horizontal, later cars two in vertical orientation. The coil moved up front at some time, perhaps about the introduction of the R-type. Originally two on the bulkhead like you and I have. The parts book also shows a capacitor there, connected to the "SW" terminal, and the service book says it is for radio static control. Even though my original radio works, the sound is far from good quality, and it gets few stations, so I do not use it. I do not have that condenser on the bulkhead connected to the coils, but I suppose I could put one on there. Seems to me it is just something else to fail. If the capacitor shorts out you are going to stop, and probably blow a fuse as well. My MG also has no capacitor on the coil, but like the RR there is one at the distributor. That capacitor is needed for proper operation of the ignition system. Note the capacitor for proper function connects between the coil and the points. The capacitor for static suppression in the radio connects between the coil and the switch box. Can't remember any other car that has a capacitor on the switch terminal of the coil either. | ||
| Ashley James Unregistered guest Posted From: 62.255.0.5 |
I get Lucas standard coils (they are patterns as Lucas no longer exists) that are available from any UK classic car part specialist. The I get them painted with two pack in black. The original terminal screws fit and they are the same size, all they lack is the flange on the top. They are about 1 Ohm less primary resistance (2.5 as opposed to 3.5 ohms) and about 2000 ohms more secondary. This applies to almost any modern general purpose coil. They take about an amp more and have a higher output. I have dismantled and rewound the originals and found no resistor in them. They are potted in bitumastic and have soft iron cladding which cannot have helped to keep them cool. I have know quite a few people have them fail lately so I think it is reasonable to suggest that they are a potential cause of breadown. If you are suspicious of capacitors, test them with a multimeter on resistance. Set it to high resistance (megohms) put it accross the terminals, the needle should swing accross to zero and slowly return to infinity, if it doesn't then the capacitor is "leaking" and should be replaced. Hope this helps. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 270 Registered: 4-2003 |
Cold starting: as I live on the other side of the world, unfortunately my R-Type is driven infrequently. To start it after a while, turn on the ignition and flood one carburettor using the tickler until fuel comes out the overflow. Then lift the throttle, cock the automatic choke and release the throttle. Press the button and it starts first time. Only flood one carburettor to give it a chance if you overdo it. It works with a manual choke too. I use the same trick on my friend's Silver Dawn sans tickler in -10 degree temperatures. Turn the ignition on and pump the throttle a number of times instead, but the rest is the same. The single carb motors have a useful manifold drain tube with a one-way valve which closes as soon as there is vacuum. That stops flooding from becoming terminal. Just as well with only one carb. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Hi Richard, I didn't realise there was no condenser at the distributor end, must look at this tomorrow night. The condenser on the MG distributor is an external one, as the original one was replaced with a modern one, and the diameter of the modern one was to big to fit inside the distributor. (short and fat as opposed to thin and long). Thanks Ashley for the info on the modern coils. Will borrow a Bosch parts book tomorrow and browse. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Hi Richard, Just been down to the shed in my jim jams, brrrrrrrr, cold! There is a condensor mounted on the bottom of the distributor to protect the points. I assume this is factory. The condenser on the coil is on the switch side, not on the coil side. I think this lines up with Bill's explaination of helping the radio? Bill, definitley a coil problem causing the car not to start, I switched coils and the car started again instantly, within half a turn of the crank. Marty | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Sorry Richard, meant not on the points side. Message is too short? Hmmm, will add a few more words!!! Marty Cutler | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 271 Registered: 4-2003 |
Martin, All cars have a heavy-duty condenser/capacitor across the points, or in the equivalent position with electronic or digital ignition. It is not there to protect the points, but is a fundamental element of the critically-damped three-element LRC (inductance, resistor and capacitor) low tension circuit. The resistor, ie the balast resistor, is sometimes shorted for starting like on a Silver Shadow, to compensate for the low battery voltage while cranking. Deleting the resistor permanently makes the circuit highly underdamped, and the spark goes all over the place. That is quite unsafe once the motor has started. The circuit would ring (oscillate), and you would end up with multiple weak sparks instead of a strong single spark. If the cap is faulty or missing, the points would be left alone to break the current by arcing. Not only would the points fry, but the very fast interruption of the LT current would cause a very high negative voltage spike to appear across the coil itself each time the points open. That would stress the coil's insulation on both LT and HT sides and it would eventually explode. I have seen that many times. Furthermore, the spark would be intense but far too short. Even before the bits destroy themselves, the motor would be likely to run very poorly if either the resistor or the capacitor were deleted. The cap on the power side of the coil is a light duty, low internal resistance one mainly for radio suppression, but also to reduce cable voltage drops and loss of spark when the points close. It is not entirel essential but worthwhile.} | ||
| John Dare Unregistered guest Posted From: 144.138.194.110 |
Forget ignition points with gapping/synch. hassles and fit a "Pertronix" solid state (no wires/black boxes etc)an "Optima" (or equiv.) "spiral cell" type battery along with a "Bosch" blue coil housed INSIDE (you cant "tell") a stock standard "Lucas" coil casing,finishing off with a "Burlen" solid state conversion to your fuel pump. From the Matterhorn to Maitland,from Zanzibar to Zeehan; first start every time. Remember you read it here first! | ||
| Ashley James Unregistered guest Posted From: 62.252.44.82 |
Have a look at www.classicheads.com You get a new baseplate for the distributor, nothing electronic external and just use the standard coil. As an alternative to Burlen, www.flexolite.co.uk who do the spin off filter conversions also do a fuel pump conversion to Holley or another. They have lots of racing customers with a pathological loathing of SU fuel pumps. | ||
| John Dare Unregistered guest Posted From: 144.138.194.51 |
Thank you Ashley. It is indeed reassuring to hear of new modifications/alternatives as tried and proven in the real world. As Sir Henry did declare; "Take the best that there is and make it better". How true it is. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Got a hold of some Bosch GT40 coils, non resistor, will look at fitting them on the weekend. A thought occured that I could fit them inside the shell of the original coil that had failed, extra heat protection? The condensors in the Bosch catalogue all look too small, and they don't list specifications, only part numbers and a picture, with a reference to the part numbers matching up with vehicles at the front of the book. I would be afraid to fit a too small capacity condensor. Any thoughts? I installed an electronic ignition in my old Mercedes, trying to overcome stalling issues, but unfortunately that was actually carb issues in the end, the carb was the same as used in the Starfire 4 Holden Torana, horrible! | ||
| John Dare Unregistered guest Posted From: 144.138.194.240 |
Regret I dont have part nos.for the "Bosch" coil and sundry pieces, since my conversion was performed by a R-R professional with over 30 years "in the field" experience. I never feel comfortable asking such people to openly "share" their knowledge, much of which they have, in many cases, taken years to research and acquire. Doubtless there are electrical geniuses out there who can ably tell us all, how this presumably simple "conversion" would have been performed without any difficulty. | ||
| Ashley James Unregistered guest Posted From: 62.255.0.5 |
I would not worry about the capacitor on the coil, I do not think mine is connected and the radio is barely affected.It is only to reduce interference to the radio. AM is very susceptible. I did see some brand new condensers fo that type on the shelves at Montagues www.bentleyspecialists.com if you are interested. Old capacitors were made by rolling up aluminium foil and waxed paper, modern ones are rolled metalised polypropylene and therefore smaller. They used to be called condensors and are now called capacitors. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 273 Registered: 4-2003 |
Martin, I just came back from Geneva where I fiddled with a Silver Dawn long-boot for three days (brake servo overhaul, oil & filter service, transmission service and band-throttle linkage adjustment, new brake linings, chassis lubrication rectification, head retighten, tappets, B6ES plugs and more including, guess what... new ignition (not supressor) condensor/capacitor. The poor guy, a friend of mine, had bought a new distributor cap and used it one day last week before I had time to go down there. He bought it in desperation at the recommendation of the local agent because of lousy running, and at a cost of A$450 (price in the UK £40-100 by the way). It was frazzled, the newish points had burned to bits and the coil was dodgy ! I found that the capacitor had been pierced by I don't know whom but can guess. A new one cost $4, a small Bosch one (I'll find the part number tomorrow) from a generic garage 100m from his home, and fitted the original 50-year-old bracket perfectly. On later cars like his (eg 1954 at least), the ignition capacitor is screwed to the distributor casing at the front under the lip below the distributor cap. Just for fun, I disconnected the cap and ran the motor briefly. It ran, but like a tractor. Put the cap back and it ran ever so smoothly. Have a good look under the rim of your distributor for an ignition capacitor. | ||
| John Dare Unregistered guest Posted From: 144.138.194.55 |
Lucky am I that my "R" type has got TWO capacitors/condensors bolted to the distributor with a handbook notation suggesting that these be alternated on a periodic basis. Although I am not an electrical genius, I have nevertheless been able to follow this extraordinarily simple direction which is a giggle to perform. It is clear to see how (particularly when common dist./points symptoms arise) this procedure allows identification/isolation of a "suspect" capacitor/condensor and likely avoidance of the problems/expense as described. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Hi Richard, Would like to know the part number you used for your new capacitor. I fitted 2 new coils this afternoon, and started it briefly in the shed, seemed to fire up alright. It does have the large capacitor under the distributor, but as my points are OK, I think the coil may just have been on the way out. Have fitted 2 bright red coils, which fitted OK on the original bracket, will paint them black in due course. The MG failed to proceed this morning............ | ||
| Bill Vatter Unregistered guest Posted From: 68.211.3.193 |
Martin, Your comment: "A thought occured that I could fit them inside the shell of the original coil that had failed, extra heat protection?" That's probably not a good idea. Coils generate heat. It is simply resistive heating from the current passing through, about 4 amps. Insulating the coil will surely make things worse. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 274 Registered: 4-2003 |
Martin, The Bosch ignition capacitor box is marked: Exp.-No 02 111 1237 33 0805-085 I suspect that the 0 805 is the important number as it is in larger type as shown above. You will either need to replace the connector to suit the stud on the distributor, or to fit a spade adaptor to the stud to suit the new capacitor. Either is a trivial exercise and a connector or adaptor costs a few cents if you don't have one already. I fitted a new connector to the capacitor so that it looks original.} | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 275 Registered: 4-2003 |
Just to avoid confusion, this is the ignition capacitor and its location. All the others are just suppressors, mainly for the radio reception, and are not essential for engine running. Ignition caps are best replaced every 10 years or so.  | ||
| Ashley James Unregistered guest Posted From: 62.255.0.5 |
Richard that is the early type, later cars have bracket on the side of the distributor with two long capacitors on them. I think I may have a working spare if anyone would like to make an offer. modern replacements are smaller and look wrong. If there is oil around the top of the old type it should be replaced. They are oil cooled so that if they are leaking electrically, they get too hot and oil escapes. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 276 Registered: 4-2003 |
Ashley, Correct. Early ones are as in the diagram: I believe that Martin's Mk VI is like that from his description, so I posted the diagram to end any confusion. Later ones have a pair of long caps on a bracket screwed to the side of the distributor casing. Even later distributors, like that on my R-Type, have a single long cap screwed to the casing in the same place but without the bracket. | ||
| John Dare Unregistered guest Posted From: 144.138.194.47 |
My TWO capacitors are as per the foregoing illus. except that they are mounted vertically (approx.) and as per FACTORY recommendation, these should be alternated periodically. If you only have one capacitor you can fit a spare nearby (rather like the "twin" coil arrangement) so that in the event of burning points you can swap over the "spare" capacitor to see if this assists in any way, thereby avoiding "frazzled"(?) dist. caps et al. Pity the poor fellow in Europe who allegedly went to "the dealer" (for his largely avoidable problem) given that todays average mechanic(incl. dealers/s!) was born at least 25 years AFTER the introduction of MkV1/"R"type cars. You really have to wonder/giggle about such incredible stories, BUT remember - When all else fails, "Read the Instructions". In the official factory handbook that is; see periodic alternation of capacitors. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
Thanks Richard, I will order a new one tomorrow. My plug leads seem to be original. Modern car leads (silicon?) seem to break down regularly, especially the thin ones on Toyota's, and the horrible long extension caps on Hyundai Excels, what are your thoughts on the solid copper leads and caps lasting indefinitely? Marty | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 292 Registered: 4-2003 |
Hi Martin, I personally prefer resistive HT leads and straight NGK B6ES plugs. My resistive leads have lasted decades and there is no radio interference at all. However, there is no right or wrong. The trick with resistive leads is to poke 1 1/2 cm of solid copper wire down the centre at each end to give a good contact to the terminal. If you don't, the resistive core will slowly burn back inside, due to internal arcing, and the leads will fail sooner or later. You can resuscitate a bad resistive lead by shortening each end by 2cm and inserting the copper wire. However, I did some work on a Silver Dawn last week which has standard copper leads: no problem. Another good solution is to use resistive plugs (BR6ES or Champion RN8 etc) with copper HT leads. Otherwise you can apply an in-line resistor (if you can still buy them) between the coil and distributor. Then I would use copper HT leads and B6ES plugs. In any case, the HT leads should last a very long time. I always use the original black right-angle plug caps, but that's just for appearance. There are many ways to skin a cat. | ||
| John Dare Unregistered guest Posted From: 144.138.194.12 |
Ive got "Magnecore" leads (as recommended by John Vawser) these being connected to genuine (n.o.s)PINK "Lodge" brand spark plugs via genuine (n.o.s)"Lucas" right angle type plug connectors. | ||
| Ashley James Unregistered guest Posted From: 62.255.0.5 |
I thought that, originally, the cars had straight Champion spark plug connectors - That is what is one both my MKVIs. I do not understand why anyone should recommend resistive leads as they do eventually deteriorate whereas copper cored ones do not. It does not matter either way but I would be interested in the thinking behind it. Originally interference with AM radio was a major problem but with DAB or FM, it makes no difference. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 295 Registered: 4-2003 |
Ashley, you are correct I believe. I would definitely start with plain copper HT leads. I could not actually recommend resistive leads, but have indeed used carbon cored resistive leads successfully for almost 34 years on my R-Type. Remember, I was 14 at the time and probably fitted them for all the wrong reasons. Correctly terminated as described before, they do seem to last forever. Mine are no doubt overdue for replacement, but still work fine, and the car has travelled 300,000 of its over 400,000 miles without a lead change. I believe there were two lines of logic, both probably flawed. Expeiences retrofitting MW radios to early Holdens had some bearing also. Besides, someone gave me 20 metres of carbon leads free of charge. First, York Motors in Sydney recommended and fitted Champion RN8 resistor-type plugs back then. This was a dealer serviced car at the time. They became very hard to buy, so I used N8 as per the Grey book around 1970. The first logic was to use resistive leads to compensate for straight plugs. Probably error number one. Secondly, the original radio, which still works perfectly, had some spark noise on distant stations with N8s, which was cured by resistive HT leads (note the FM and CD systems are completely concealed). As an aside, I use a plain rotor button with no supressor, which probably didnt help the radio. I have a few with supressor stashed away unused. Part of my message was that leads, all types, usually fail at the terminations. They can usually be repaired if you can part with a centimetre or so at each end, and insert a copper conductor as described if the leads are resistive. So, would you agree that a good solution is to use copper leads with resistive BR6ES plugs ? On plug caps, I recall that MkVI cars had short straight Champion caps originally, whilst R-Types had right-angled caps new, but I may be wrong. Mine has always had the angled caps, at least since 1969 anyhow, and I even bought a few spares from Rolls-Royce once. Again, no big deal. | ||
| Bill Coburn Grand Master Username: bill_coburn Post Number: 221 Registered: 4-2003 |
Probably a furphy but I have understood that copper leads were 'illegal' because of their radiation properties. | ||
| John Dare Unregistered guest Posted From: 144.138.194.251 |
Thanks Bill; Now THAT is a worry!. NO such problems with "Magnecore" leads, being another of mankinds advances that WORK, in the non rarified atmosphere of the real world. The one that most of us live in. | ||
| Ashley James Unregistered guest Posted From: 62.252.40.9 |
One cause of failure of the electronic ignition units on Shadows is the resistance of the plug leads going high. The main cause is age and dry joints though. Bill may be correct regarding legality too. | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
I have access to a "Sun" tune up machine tomorrow night, should be interesting. I will be hooking up my MG Magnette, and if it tells me anything interesting, then I will hook up the Bentley. It does an "electronic" compression test, by shorting one lead out and measuring the drop in rpm. The oscilloscope should tell me the strength of my coil, capacitor, points, etc. Will keep you posted. Wow! I generated a fair bit of discussion with this thread! | ||
| Martin Cutler Unregistered guest Posted From: 211.30.116.216 |
No luck with the Sun Modular Engine Analyser, it did have a positive earth switch, but the solid copper leads sent it into a frenzy, was hoping to get a nice oscilloscope pattern showing the voltage at the coil and condenser, oh well. Maybe some magnacore leads? | ||
| Ashley James Unregistered guest Posted From: 62.252.40.88 |
I wouldn't worry about it, if it is timed correctly and everything is by the book, it will be fine - it is only a truck engine! You could do the voltage checks on the regulator to make sure it is working correctly. | ||
| Jeff Martin Experienced User Username: jeff_r_1 Post Number: 131 Registered: 07-2018 |
I dug through the forum and found this thread, so it makes sense to post here rather then plug things up and start a new one. Bill, I am running this 3.1 ohm coil now and it's getting much to hot, it will fail eventually. (I know this from experience) https://www.holden.co.uk/p/retro-ignition-12v-sports-coil-dlb105-1 My old old Lucus top hat coil is 4.4 ohms. Is it better to install a ballast resister in my new 3.1 ohm coil and bring it near the 4.4 ohms of the original, or install a coil that was meant to have a ballast resistor ? I'm thinking because it's a sports coil with a hotter spark (more secondary windings ?), that even though it was not meant to have a ballast resister, adding one will still give a good spark _ also given the fact of the long dwell period of the duel points. The problem I see is that even with a 2 ohm resister and a coil that requires a ballast resistor, it will still only bring the total resistance 3.5 ohms. This assumes that the ballast resister is 2 ohms and that a coil that requires a ballast resister is 1.5 ohms. Or if there is such a thing as a 2.9 or 3 ohm ballast resister that would bring the 1.5 ohm coil to 4.4 0r 4.5 ohms. | ||
| David Gore Moderator Username: david_gore Post Number: 3781 Registered: 04-2003 |
The fact that the coil is getting hot indicates the current flowing through the coil is too high. The internal resistance of the coil determines the current flowing through the coil by Ohm's Law better known as Resistance=Volts/Current. In your case with the existing coil, reducing the current through the coil is best achieved by adding a suitable ballast resistance in the positive power lead to the coil. This resistor must be mounted appropriately in a location with good airflow to carry the heat generated away. In your present situation, the current with a circuit resistance of 3.1ohms is V/R i.e. 12/3.1= 3.9amps. A circuit resistance of 4.4ohms gives a current of 12/4.4=2.73amps. Accordingly, you need to fit an additional resistor of 1.3ohms with a power rating of at least 50 watts to the coil power feed. The heat generated from each current is calculated using the formula Power=Current x Voltage: The heat generated by a current of 3.9amps is 47watts and by a current of 2.73amps is 33watts; to put it another way, adding a ballast resistor of 1.3ohms to the coil power feed will reduce the heat generated in the coil by approximately 30%. . |
