When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Seems like the right old threat as a place to drop this:
Never thought Id' see David Vizard tackle the V12 combustion chamber, but there it is: https://youtu.be/Nz4cmBbzowQ
Thanks for posting this. I haven't heard much of David Vizard since his regular pieces in Triple C in the 1970s!
I do not doubt his mods provide more top end BHP; but we must remember that the aim of the May head, and Jaguar's development of it, was to provide greatly enhanced mid range cruising economy, not top end power. I have recently been reading Roger Bywater's papers on the development of the V12 (for sale and downloadable from his website and worth every penny) and the big task for the May head was achieving a combination of good lean-burn mid range cruising economy (say speeds between 50 to 95 mph) without losing too much high rev efficiency.
As revs rose, ignition had to be retarded in order to avoid detonation and this affected BHP. Achieving the balance between these requirements took some careful work.
My point being that while the Vizard mods undoubtedly will give a bit more top end power, they must necessarily sacrifice some of the mid range economy, as some of the May head's ability to burn very lean mixtures will be lost as some of the essential characteristics that promote the adequate burning of very lean mixtures are altered.
Now David Vizard is talking to racers; most of us are not, so here is a question: How many of us use the full power the unmodified engine is capable of delivering, and how often?
After making sure that we have the proper gap the change is to cut the ground electrode halfway over the central electrode. I've seen this mentioned from another guy in Australia on his website but was skeptical until I saw it from Visard. Dave does all the R&D with true results. He said the only other plug that made a difference were E3's but I don't think I would use them on the v12.
Originally Posted by Greg in France
Thanks for posting this. I haven't heard much of David Vizard since his regular pieces in Triple C in the 1970s!
I do not doubt his mods provide more top end BHP; but we must remember that the aim of the May head, and Jaguar's development of it, was to provide greatly enhanced mid range cruising economy, not top end power. I have recently been reading Roger Bywater's papers on the development of the V12 (for sale and downloadable from his website and worth every penny) and the big task for the May head was achieving a combination of good lean-burn mid range cruising economy (say speeds between 50 to 95 mph) without losing too much high rev efficiency.
As revs rose, ignition had to be retarded in order to avoid detonation and this affected BHP. Achieving the balance between these requirements took some careful work.
Is this a new publication? I have his Modern Engine Technology book and have been through it thoroughly. I have to admit that I would have preferred to hear more about the v12 but the formulas and techniques mentioned certainly apply. With due respect to his experience and knowledge do you think that perhaps some matters have changed or maybe even improved since his time at Jaguar? I say this because he implied that the basic cam specs hadn't changed since their use in WWII aircraft, that they were basically proven and that there was little room for improvement in that regard.
That being said I believe that cam technology has made vast improvements in the past several decades since his time. Look at Mobeck's cams, probably head and shoulders above a stocker. I do believe the engine as it exists is a good balance that I would not want to upset too much but I also believe improvements can be made without too much of a penalty to that efficiency as well. Could be wrong. Here is a couple pictures of the so called "May B" head that was used in offshore power boat racing with twin turbos to fetch I believe something like 700hp: Evidently widened out to unshroud the valves some
After making sure that we have the proper gap the change is to cut the ground electrode halfway over the central electrode. I've seen this mentioned from another guy in Australia on his website but was skeptical until I saw it from Visard. Dave does all the R&D with true results. He said the only other plug that made a difference were E3's but I don't think I would use them on the v12.
Got a link to the vid or pictures of a plug? I'm having a hard time picturing this process. Thanks
I agree with Greg, a very interesting collection of information about the V12 and it's development. It's worth remembering that a major goal of the HE was an improvement in fuel economy at motorway cruise conditions. It was not about increasing full throttle power. That had to be balanced with the limitations of ignition systems of the day, improvements could have been made with a programmable ignition system like is common today.
This is the Aussie I was referring to but he takes too much off of the ground electrode. You only want to cut it back mid way over the center electrode.
Wouldnt opening up the head(as in that offshore boat)decrease compression ratio?
Of course, but this thread is about making a 6.7l so you won't lose anything there. Most people going this far are getting pistons and rods amongst other things.
This is the Aussie I was referring to but he takes too much off of the ground electrode. You only want to cut it back mid way over the center electrode.
Is this a new publication? I have his Modern Engine Technology book and have been through it thoroughly. I have to admit that I would have preferred to hear more about the v12 but the formulas and techniques mentioned certainly apply. With due respect to his experience and knowledge do you think that perhaps some matters have changed or maybe even improved since his time at Jaguar? I say this because he implied that the basic cam specs hadn't changed since their use in WWII aircraft, that they were basically proven and that there was little room for improvement in that regard.
That being said I believe that cam technology has made vast improvements in the past several decades since his time. Look at Mobeck's cams, probably head and shoulders above a stocker. I do believe the engine as it exists is a good balance that I would not want to upset too much but I also believe improvements can be made without too much of a penalty to that efficiency as well. Could be wrong. Here is a couple pictures of the so called "May B" head that was used in offshore power boat racing with twin turbos to fetch I believe something like 700hp:
Your point on powerboat engines, again, I am sure it is true, but power boat engines are not required to have part throttle driveability and economy! My own view is that it is essential to keep firmly in mind when discussing the V12 HE engine (and indeed all the road going versions of the Jaguar V12 in general) is that it was not made to be modified to produce huge amounts of extra horsepower. It was made to be a sublimely smooth powerplant for fast, quiet, comfortable, well-handling and riding road cars for the relatively wealthy buyer.
Also, it has to be judged by the standards of its time. When the XJ12 and the XJS were launched, both were faster than the competition, much cheaper, quieter, better handling, and infinitely more useable. The Jaguar V12 remains, even today, easily the most reliable, low maintenance and trouble free V12 ever put in a road car.
If more power really is required (and why not if that is what the owner wants) then, as Bywater says on his site, easily the most cost-effective way the V12 can be modified for a lot more power is to have 7 or more litres capacity, even then still using the flathead two valve heads: https://www.jagweb.com/aj6eng/stretching.php
Last edited by Greg in France; Jun 8, 2025 at 07:24 AM.
Your point on powerboat engines, again, I am sure it is true, but power boat engines are not required to have part throttle driveability and economy! My own view is that it is essential to keep firmly in mind when discussing the V12 HE engine (and indeed all the road going versions of the Jaguar V12 in general) is that it was not made to be modified to produce huge amounts of extra horsepower. It was made to be a sublimely smooth powerplant for fast, quiet, comfortable, well-handling and riding road cars for the relatively wealthy buyer.
Also, it has to be judged by the standards of its time. When the XJ12 and the XJS were launched, both were faster than the competition, much cheaper, quieter, better handling, and infinitely more useable. The Jaguar V12 remains, even today, easily the most reliable, low maintenance and trouble free V12 ever put in a road car.
Yes, that is understood, I only mention these things due to the fact that the discussion I thought was revolving around a stroker engine and the desire for more performance. Perhaps I'm mistaken here. I do remember however Bywater mentioning with regard to the performance of the standard engine that it was a little hampered by the need to pass then current emissions and that was one of the reasons he sold his "super enhanced ecu". More power does not always mean a decline in fuel efficiency. Back in '05 I walked into a Ford showroom to see their latest offerings one of which was the Ford 500. It had a 3.0l v6 rated at 215 hp if I remember correctly with a highway mileage rating of 29 mpg. At the time I had a '96 Taurus with a 3.0L that I had impounded and the owner surrendered which I repaired. That car gave me 29 mph on the highway. So power and efficiency are not always apposed to each other. My thought at the time was "if they can increase power that much without a penalty in economy than the inverse is also true".
I would agree that as it sits it is fairly good and probably the only changes I would make would be to clean up the intake, perhaps equalize the runners (like the plus torque) and build an exhaust like the "plus torque" exhausts that Bywater sells. Otherwise I would prefer a cam that offers more lift with a little less duration to augment midrange torque.
Phil Lockner, a gentleman in South Africa installed a megasqrirt system with DIS on a pre-HE engine and was able to tune it for better economy showing that with a more modern management system and proper tuning improvement is possible. Perhaps that is again why Bywater offers the 'enhanced ECU" by itself, or used to, never mind as a supplement to the other upgrades that he offers.
