While we prepare our first (prototype-engined) car for its first appearance on track and work continues on our first of a (very) limited run of customer cars, I thought you might be interested to learn of a second associated project.
A Bit of Background …
Before their V12, Jaguar’s racing and practically all road cars were powered by the powerful and renowned XK straight-six double overhead-cam unit. This engine had its origins in pencilled sketches drawn during the London blitz by Sir William Lyons and his engine designers; William Heynes (Chief Engineer), Walter Hassan and Claude Baily. These sketches and original designs were followed by working prototypes as early as 1943. The first 3,442cc production unit saw the light of day in the beautiful XK120 of 1947. The same basic engine continued production into the 1990s.
As can be seen from the drawing reproduced below, both inlet and exhaust valves were inclined towards the centre-line of the hemispherical combustion chamber at 35°. This was changed to 30° inlet and 45° exhaust for the ultimate “wide angle” head used in racing engines. The valve angle was modified simply to allow the use of larger inlet valves.
In the 1950s/60s this hemispherical type of combustion chamber was considered ideal for high-performance engines because of reduced valve “shrouding” compared to a “flat-head” design and a low surface-area to volume ratio. As can be seen from the following photo taken of a head I sectioned, the ports and valves were arranged more or less in-line across the engine. However, Weslake worked closely with Jaguar when the engine was being designed and he introduced a curvature to the inlet port in an attempt to allow charge movement inside the cylinder (“swirl”). This was done to aid combustion efficiency and is evident in the photo.
Weslake’s modification, whilst introducing swirl, was compromised by the need to place the spark-plug off to the side so as not to interfere with the valves. A central spark-plug would have been ideal in this situation. Many designers of similar engines tried to improve the situation by introducing a second spark-plug on the other side of the chamber but this was never really successful.
As owners will testify, these engines seem to prefer richer mixtures and rather a lot of ignition advance (10° and more). This generally indicates combustion is not as good as it could be. In the end, the “wide-angle” racing head probably reached the end of its potential because it could breathe better than it could burn.
Food for thought …
A bit more food for thought
See the very limited water passages in the above photo as well as the large amounts of metal in the casting? Square exhaust ports? Nowadays, and especially with the advent of 3D visualisation using tools such as CAD, it is possible to design optimal ports and heads with far greater and more efficient cooling surfaces – as well as optimal air flow characteristics. Wheras the thinnest port walls in the head are more than 10mm thick, today’s cylinder heads tend to be closer to 4.5/5.0mm with considerably increased cooling surfaces. Whilst I don’t pretend to be any sort of expert in this field, it seems to me that the port shapes, by today’s standards, could also be improved?
What do you think? Comments are enabled for this blog entry and, as long as you don’t try to sell me viagra (or worse) I will add them below.
One thing which did work in the 6-cylinder engine’s favour may have been the side-entry and curvature of the inlet port which assisted combustion. In common with other engine designers of the period, Jaguar’s Claude Baily anticipated that further improvements could be made to his basic design by making use of down-draft porting. Baily adopted this when he designed his successor to the XK engine – the quad-cam V12. In theory, there just had to be a benefit of down-draft porting but Baily (and other designers) found these benefits weren’t achieved in practice. Flow may have been excellent but this arrangement simply didn’t allow useful swirl/charge-movement within the cylinder and combustion suffered as a result. Others who wandered down this cul-de-sac included Ferrari, BRM, Matra and Ford – Jaguar wasn’t alone in this.
A breakthrough came from work carried out by GM’s Sam Heron in the 1950s (Heron became famous for his work with aircraft piston engines and the sodium-cooled exhaust valve). Rover were probably the first to adopt his scheme which consisted of a flat cylinder head with the combustion chamber in the piston crown – a feature later adopted by Jaguar in their first SOHC V12 engines. These heads became known as “Heron Heads”. In the mid-1960s Ford (of England) adopted the Heron layout for their entire range of engines. A close relative of the Heron layout was the very successful Repco V8 engine that powered Jack Brabham’s team to two F1 World Championships in 1966/67.
Jaguar later improved the efficiency of their V12 further by adopting Michael May’s “Fireball” combustion chamber. In this design, the exhaust valve is deeply recessed into the head, forming a compact oval chamber with the spark plug at one end. As the piston advances up the bore it forces some of the charge into a shallow channel around the flush inlet valve from where it is squeezed tangentially. This creates a high-speed vortex in the combustion chamber – LOTS of movement and “squish” here …Why was this basic layout found to be better? The increased combustion efficiency as a result of increased swirl and charge-movement may have pointed the way. It is all very well having superb flow, but this is to no avail unless the charge can be adequately and rapidly burnt.
Could it be possible to design a down-draft, hemispherical head with the necessary degree of charge-movement to allow combustion to match this design’s superior flow characteristics?
More food for thought …
So where is all this leading to? Can’t you guess?
Having installed my unique quad-cam prototype V12 in my re-creation I thought it would be rude not to attempt to produce at least one engine of my own. I am offering to build customer cars as a means of contributing towards the cost of my project and thought it might be interesting to go the whole hog and perhaps include our own design of quad-cam V12 for someone wanting the ultimate replica.
I am at a very early stage in the project but thought there may be interest out there as the engine project unfolds and I plan to report on progress via this blog.
I do have an ulterior motive ….
Whilst I have access to a number of gifted individuals who know far more about engines than I ever will, I do know there are many fellow-enthusiasts out there who share my passion for all things Automotive and who could usefully contribute to a project such as this. I welcome your comments, suggestions, criticisms and guidance. Leave a comment below or message me here. Those of you who use Facebook can find me here – https://www.facebook.com/Building-The-Legend-860208630703278
The plan is to end up with a quad-cam V12 which is inspired by (but with no attempt to exactly replicate) Jaguar’s prototype “XJ13” V12. We aren’t trying to re-invent any wheels here or produce anything approaching “state of the art” but, instead, produce a reliable fast-road/race engine which celebrates and is inspired by the basic architecture of Baily’s 1960’s prototype quad-cam and legendary 6-cylinder engine.
To this end, we are setting ourselves some basic ground rules:
- Whilst Being inspired by the basic architecture of Baily’s quad-cam (as well as other engine designs of the period), opportunities to improve gas flow, combustion and overall efficiency in the light of current knowledge will be taken. Whilst cosmetically similar, these will not be exact copies of Jaguar’s quad-cam prototype engine – instead, they are inspired by this engine.
- As was the case with the original XJ13 prototype engine, cam drive will be via duplex chain.
- Two-valve, hemispherical head design.
- Fully programmable fuel injection & ignition (prototype quad-cam uses a pair of 6-cyl distributors and Lucas mechanical fuel metering unit). Alternatively, downdraught carbs may be considered.
- Normally aspirated.
Because we are starting with an almost clean sheet of paper, we have the opportunity to go back to first principles and consider things such as optimal valve sizes, port configuration, charge movement and spark-plug positioning etc.
The first step was to draw up a pair of heads combining a typical SOHC V12 mounting face with a basic 6-cyl DOHC design just to see if everything could be made to fit. After all, we don’t want to end up with head studs coinciding with inlet/exhaust ports! Also, we needed to make sure it was practicable and possible to mate up with oil and water passageways. There are also practical considerations to consider such as being able to access head nuts – bearing in mind each SOHC V12 head is fastened down by four rows of head studs but only two in the prototype quad-cam and XK 6-cyl.
This is what we took our inspiration from::
The V12 head is not only longer than the 6-cyl head, but the bore positions are different. Positions of water and oil passages are very different between the XK 6-cyl and SOHC V12. The biggest challenge was combining the two heads so that the V12 stud pattern was maintained. It became evident very early on that the new quad-cam engine will have unique cam covers as well as custom cams. Fortunately, some items can be sourced “off the shelf”.
The following pictures of the prototype quad-cam V12 show what we took our inspiration from:
In contrast to the 6-cyl XK head and the SOHC V12, oil is fed to the quad-cam heads via a drilling passing from the gallery to each head.
The following pictures show the general layout of these initial designs. They are just preliminary designs with no attempt to optimise things like port configuration, spark-plug location etc. They showed it would be possible to design our own heads which would bolt straight on to a typical SOHC block. Discussions with a local foundry highlighted considerations we need to build into the design. Further discussions with pattern-makers confirmed it will be possible to produce the necessary patterns etc.
Where to go next?
Having gained some confidence that everything can be made to fit, our attention can now turn to optimal port design – still remaining within the “ground rules” established above. We now need to consider the following questions:
- To what purpose will this engine be put? Full race? Gentle plodding? Something in between? In other words. where in the rev range should we aim for maximum torque and power to occur?
- Which aspect(s) will have the greatest impact on power? Inlet? Exhaust? CR?
We will explore these aspects and report in a future blog article – watch this space!
Any thoughts of your own? Please feel free to comment – we would appreciate any contributions.