Add 1.5K RPM to your Jaguar V12 XJ-S!
Sounds too good to be true, right? In less than 10 hours I redesigned, manufactured and installed new trumpet intakes for our 1990 Jaguar XJS which increased airflow and made a quantifiable difference to the usable rev range. Before, the mighty V12 eagerly revved to 5000 RPM, before lumbering up to 6000. With the modified trumpets it roars straight up to redline at 6500. All with only 50 pence of PETG.
Our initial expectation was that this would increase intake noise and provide a bit more excitement for those in the cabin, in addition to the engine being able to suck in more air due to the small increase in cross sectional area of the intakes.
In actuality, there was little to no increase in intake noise and outstanding performance results – making this a perfect modification for those who wish to keep the sophisticated credentials of the XJS.
This idea came about while idly thinking about the pitiful size of the existing air box trumpets. Each 2.65L bank inhaling air at a frightening rate through a 20cm tube with an aperture not much larger than a two pence piece seems like a strange design decision. Coincidentally, the end of the trumpets are jammed right up against the front of the engine bay. Compared with the size of the throttle bodies, it seemed like the intakes were being needlessly limited. AJ6 Engineering have a kit to combat exactly that.
I’m sure the Jaguar Engineers had a good reason for this, but it was lost on us. Our best guess is either for fuel efficiency (more on that in a bit) or reduction of intake noise.
For some time we had been devising a plan to cut the tapered intake trumpets in half, thereby increasing the size of the apertures and giving them more room to breathe. My original plan was to cut the trumpets then hammer them from an oval to a circle. This would afford the ability to turn a cirular trumpet on the lathes at University which would fit on the end. They could even be aluminium just like AJ6!
However, between conception and the present I had gained the ability to 3D print. This meant I could custom design a trumpet to precisely fit the tube – regardless of the shape. An easier and more elegant solution.
The tubes were cut and a pencil rubbing was made of the resulting cross section, which was scanned and imported it into Solidworks. I got to work designing a trumpet before exporting, slicing and printing. Within 3 hours V1.0 was ready for a test fit.
My tolerances were a bit tight and the print ended up cracking as we forced it onto the end of the intake tube. We checked that there was no interference in the engine bay before I reprinted the design 3% larger. V1.1 fit perfectly and a second was printed for the other side. Contact adhesive was perfect for providing a malleable yet strong bond to ensure they didn’t fall off. They look a bit goofy, but V2.0 will rectify that. Time for a test!
After excellent feedback from the driving team to the engineering department, we got wondering why the improvement was so huge. Our thought process was as follows: less restrictive intake = more air = more boom = more power. The car doesn’t have a MAF so how does the ECU know the engine is getting more air? We’re forgetting that half the car runs on those God forsaken vacuum lines.
Less restrictive intake = more air = less vacuum in the manifold at wide open throttle, where the ECU has a vacuum sensor. It therefore increases the fuel supply to compensate agaist a lean mix, giving more power.
This would provide a reason for the Jaguar engineers to make such a small intake. Artificially create a high vacuum at high revs to reduce fuel usage? Who knows. Maybe to stop people revving the engine to bits!
Like I said, it’s just a theory. Maybe the engine will melt and prove us wrong! The XJS is being rallied by Jacoby Jaguars at a HERO event in the coming days, so no doubt we will find out soon.
A V2.0 is on the horizon with better aesthetics and perhaps a more considered trumpet shape, since V1.0 was only approximated.