Building a Championship Winning Rallycross Engine – Julian’s thoughts
One of our latest engines is in the Citroen DS3 driven by Liam Doran. I started the project by inspecting all the available modern Citroen engines to identify the strongest example – a key requisite for a competitive Rallycross entry, especially as the rules stipulate that a stock block must be used.
At this point, I needed to check the strength and density of the materials and of the deck strength to ensure a reliable gasket seal – at 200bar+ cylinder pressures deck distortion can seriously wreck all my hard work!
Luckily though, Citroen produce a factory diesel engine that is just about perfect and I decided that this was the ideal platform from which to build. Using the diesel crank as a dummy, I experimented with compression heights, pin diameters and rod lengths until I found a combination that I was happy with.
With the bottom end taking shape nicely, it was time to have a look at the cylinder head (I have free rein with this as the rules allow any head to be used). I could have constructed my own billet alloy head but I decided that would eat in to the budget and time available too much, so I looked to the WRC head as a starting point. As you may be aware, WRC engines use a 34mm restrictor whereas Rallycross engines use 45mm so it needed a complete re-work to make it suitable. With this in mind, I decided to start with standard road car 16v heads and have them heat treated to improve the material qualities and then machine off any excess material using our XYZ mill. Once the drawing is created, we can machine 60% of the ports and chambers saving huge amounts of time in the process. Indeed, I would estimate that it’s around 10 times faster than doing it by hand, and every head is identical. As I had a combination of valve springs, seats and caps already sorted out, and valves made to suit, I just had a final cam choice to make and the workshop team could start building a prototype. I have experimented with cam lobe design over the years by performing back-to-back testing on our Superflow Dynometer, so choosing the right cams was pretty straightforward.
By now, Arrows had started manufacturing cranks and rods which are the best around in my opinion. I can be sure that, once fitted in a strong block with a billet cradle type sump, it won’t be Arrows parts that let me down. In fact in our YB engines we have never had a crank failure since I designed the billet dry sump and heavy duty crankshaft. Omega make L1 Bridge Pistons for my engines which again I completely trust. I always use the best components in our engines right down to the washers and bolts – not necessarily the cheapest route to take but winning Championships calls for power and reliability rather than receiving penalty points from the FiA for changing an engine!
In all, the only original Citroen parts left are the block casting, head casting front cover and timing belt sprocket. Every other part is made, machined or replaced.
Now, with a fully built engine assembly, we can design the manifolds around the engine’s position in the car. Eventually, a completed engine can make its first dyno run and, with the engine controlled by a Pectel Sq6, I spend 2-3 days testing and tuning it. The result? 2nd in the ERC…..not bad for my first attempt!