Spark plug tech. has long been sort of a "snake oil" solution to automotive preformance. All of the new Platinum plugs etc. pale in comparison to what went on with "improved" spark plugs early in the automotive scene. Get a look at a great early collection of spark plugs and you will see why I say that. Jaguar was not imune to playing with spark plugs and arrangement but thankfully they were smart--they had to be for they were lean in staff and money. So to answer one of the questions here as requested---"Had it been done in the past by the factory, making it a legal modification for vintage racing"? Well the first thing I did was pulled out my "D" service manual and suppliments and of course there is no mention of a twin system or dual fire system. I then looked at papers submitted by W.M. Heynes to the Institution of Mechanical Engineers one in 1953 on the XK engine development and the second in 1960 on his life as an Automobile Engineer. In both he goes into great detail about modifications to the Jaguar XK engine but also never mentions twin plugs.

So were they tested---yes tested but never used! Very good pictures of a twin plug head can be found in Graham Robeson's Jaguar DTYPE and XKSS work ISBN 0 85045 498 0 pub 1983 on PP56-57. No mention is made in the work about their employment however. If you go to Andrew Whyte's Jaguar Sports Racing & Works Competition Cars from 1954 (rev ed) ISBN 0 85429 319 1 p 81 he describes from factory notes that the twin plug 35/40 head was developed but the twin plug usage was abandoned. Further on page 84 from team notes the second plug was left off the 1955 team cars and plugs fitted to the "new" positions only (or only six plugs if you will). The drawing for that head is XK 1987 for those who want a copy. From the Autocar Magazine reprinted on page 509 of Whyte comes the most compelling evidence. When describing the specs for the 1956 D-Type it states in part... " The same cylinder head as used last year-developed by Harry Weslake-is retained with inlet valves inclined at 35 deg and exhaust at 40 deg from the cylinder line. Two plugs per cylnder WERE part of the origional design but development experience PROVED the dual ignition offered NO advantages and only one plug is now used.

Further searching through Powered by Jaguar by Nye ISBN 0 900549 56 4 pub 1980------Jaguars in Competition by Harvey ISBN 0 85045 323 2 pub 1979 or Jaguar C-Type and D-Type Brooklands Books ISBN 1 85520 3464 failed to produce one example of a car actually running on the road much less racing on twin plugs. Further Jaguar Technical bulletins make no mention of ignition parts required for twin ignition although the new head is covered for production cars.

So I am not sure what a racing body will do with this type of information-It seems though that the system was played with--abandoned--and the casting used retained the second plug hole but blanked off. As far as Mr Hagopian's approach to re-invent this system it would appear the factory took a simpler approach from the pictures found in Robeson's pictures (which are no doubt from Lynx) They seemed to simply drill and insert a plug bung but an actual head would provide the answer. My understanding of race bodies is that factory or production cars must race as delivered or developed. I would think this was just the left overs from an experiment that Jaguar seemed to think failed and would not be allowed much like crank triggering is not allowed. Yes Jaguar eventually used crank triggers but not at that time. As a footnote Jaguar also experimented at that time with 4 valve cylinders and abandoned that also--at that time only to bring it back later. Good luck.

George Camp

Submitted by msmith5242@com… on Wed, 04/11/2007 - 17:57

Ill throw in my two cents..I've not seen the whole thread, but I used (3)standard GM type double ended coils on my '67 GT2 E. They are fired with a crank trigger system. On my engine, each coil fires two cylinders, but one cyl. is on the power stroke, the other on the exhaust, so it is termed a waste spark system. One could easily wire up, say a Motec M48 controller to fire (6) double ended coils. Of course this would probably never fly in vintage, but it would be a working solution. I have also read what Mike Frank noted, that at high compression, ie. >11:1 etc., with a hemi, the dual plugs can help power due to the shrouding effect of the high dome.

As a side note, I have a weird Jaguar Tank head, (used on the 4.2L Jag powered UK light tanks), that has been modified with 4mm surface gap spark plugs that angle closer to the chamber center. This type head might have been a good starting point for G. Hagopian's variable cam setup as the tappets are larger in diameter than the standard XK head. I've never run it so I don't know if it makes more power, but the plugs sure are pricey.

M. Smith
'67 E roadster SCCA GT2

Submitted by SE98-32482CJ on Sun, 04/01/2007 - 18:34

Point is Dr Klat was ex factory. There are cars with a bifuracted lead system. You are correct the leads have to be percise but his quest was for cheap! Since Weslake and Heynes saw no improvement if the second does not fire what will the difference be. As far as Porter goes I have over 500 manuals and books but have no Porter works as they are almost always suspect and so are not good for reference. My and others opinion. George Camp

Submitted by SE98-32482CJ on Sun, 04/01/2007 - 18:34

Point is Dr Klat was ex factory. There are cars with a bifuracted lead system. You are correct the leads have to be percise but his quest was for cheap! Since Weslake and Heynes saw no improvement if the second does not fire what will the difference be. As far as Porter goes I have over 500 manuals and books but have no Porter works as they are almost always suspect and so are not good for reference. My and others opinion. George Camp

Submitted by mfrank@westnet.com on Sun, 04/01/2007 - 17:34

A few points.

The twin plug design was attempted again in 1964, by Dr. Samir Klat. This was done during the development of the Lumsden/Sargent car for Le Mans. In Porter's Definitive E-Type, there is a lengthy article about this car, including photographs of the head and distributor. It was NOT run at Le Mans. Quoting Klat "We realized that if we ran this engine at Le Mans, we wouldn't be able to get homologation. What we did inside the engine, people wouldn't spot, perhaps...asymmetric pistons, for example...but a 12-pt distributor is very visible. There is no way you can hide it, and Jaguars were interested in helping us the other way around. They made sure we would never get that passed." In other words, Jaguar blocked them from introducing something that wasn't available to the factory team.

As to the value of it, they found that it was beneficial when increasing compression beyond 11:1. As you know the hemispherical head is not an ideal starting point for a high compression motor, due to the fact that the piston has to be very tall, creating an "orange peel" shape to the combustion chamber. As a data point, dual plugs are used in all current Mercedes motors to control flame propagation.

You CANNOT simply increase the spark and dual-lead the plugs. The reason is that voltage will always take the path of least resistance. This will be through the plug experiencing the highest local pressure. The other plug will see little or no spark. (This also occurs in lost spark ignitions, in fact very little spark is lost). To trigger two plugs, you need a 12 point distributor and dual coils, or you need an electronic control system. The photos in Porter show an interesting distributor, sort of like two six cylinder distributors stacked above a common breaker plate.

Submitted by SE98-32482CJ on Sat, 03/31/2007 - 12:38

Mr. Hagopian you might want to look as SAE paper 580094. Also see Roger Bywaters papers on the wide angle head on Jag Web etc. You might want to look at WWW.MikeHawthorn.org.uk/d-type.php which gives a great quick development of the wide angle 35/40 head. All tend to agree that twin plugs offer nothing worth while. As far as racing rules go it seems that the 35/40 head can be fitted because of homologation to the E in all racing bodies but the twin plugs are not addressed as thay were never offered---but they might let you run them if you show them the documents that no benefit will be gained. As far as firing twin plugs the easiest way would be to boost the spark and bifurcate the leads which would be easy if not cluttered. George Camp