Removing midbox P300

[IvoDC]

I want to give my P300 MY21 a bit 'beafier' exhaust note. But I also want to keep the original look. I certainly don't want to make it a boy racer, just make the sound match the looks a little more

So i was thinking to replace the midbox (the small muffler between the cats and the backbox) by a straight pipe and keep the rest of the exhaust system all original. And it will not influence the MOT so no worries there.
But this type of mufflers, aside of reducing the sound level, mainly cuts down the higher frequencies. So I fear the exhaust not will not only raise a bit, but also get sharper and tinnier.

i read every post in the Technical section of the forum, but didn't find any info about it. Is there someone who did this on their P300, and what was the effect on the exhaust sound?

[MajorTom]

I guess there are not enough P300s on here but I suggest to take a look in the various FB groups where I've seen a number of P300 exhaust upgrades with pretty good results.

Considering what a stellar job Jaguar did on the exhaust sound design of the V8 and above all the V6 it's a bit odd that they didn't make more of the P300 exhaust. I've seen some guys online making them sound a bit more like Fiat 500 Abarths which I think sound pretty cool for four bangers and better than the stock P300.

[IvoDC]

I didn't expect many reactions, because I noticed almost everyone on this forum drives around with the V 3.0 or the V8 5.0 given the low road taxes in the U.K.. In Belgium, many opt for the smaller 4-pot, given the insanely high road taxes for the V8. F.e.; for the F-type V8 575bhp you already have to pay € 17,340 in taxes (insurance not included) just to be able to put the car on the road. The following years will cost you € 3,646/year in road tax. And you can be sure that road taxes will increase slightly every year. Add to that your insurance, maintenance and fuel and you understand that we in Belgium think twice before purchasing the F-Type V8 .

When I had my Speedsters, I've thoroughly studied the technology behind mufflers/backboxes. And modified several of them. I tried by dividing soundwaves to make a 4-pot sound like a V8. But unfortunately it didn't work because the turbo distorted the sound waves. A problem you don't have using a compressor/supercharger. That's why a naturally aspirated or supercharged engine sounds much better than a turbo engine. A physical law that even the Jaguar engineers could not change

[MajorTom]

Yeah I know Belgium is pretty crazy in that respect. Denmark and Norway are really bad too when it comes to car taxes. Fortunately there are many countries where you can have a bigger engine without getting ruined.

Sounds like you know your stuff then. You can definitely make the P300 sound better anyhow. Also check out the US forum if you haven't yet. The P300 is pretty rare over there too but there's much more talk about mods.

[IvoDC]

Yeah, being an engineer, I like to delve into the technical details of things. You could call it professional deformity I found a US topic of a guy who removed the midbox. Something I will definitely do too as this can easily be reversed to its original state.. Too bad he didn't provide a recording before and after. Also someone opened up a P300 backbox and posted pictures of it. Interesting to see. It gave me some ideas which will definitely make it sound better. Slight problem; my TIG welder broke down some time ago, and I'm out of gas for it. First have to solve that

[cj10jeeper]

I’ll be interested to see what you do and how it turns out, from a learning perspective.
4 cylinders naturally just doesn’t sound as good as 6 or 8 and the factory puts a lot of effort into overcoming the issues around frequency of firing, firing order, and in the F Type a turbo in there too, etc.
We are all too familiar with the result of tinny obnoxious sound of any small 4 cylinder engine razing around with near open exhausts Focus, Corsa, etc.
If you remove the front silencer and open the backbox, what will you do to remove the high frequencies and tune it?
Genuinely interested, not critical.

[IvoDC]

Today I ordered the parts for replacing the midbox with a straight 65mm pipe. In addition to improving the sound, it will also reduce back pressure as the line diameter in the midbox reduces from 65 to 60mm.
To be continued ......

[relaxitscool]

Hi, how did you go on with this? I have a MY19 P300 which I think is pre GPF. To my ear it sounds pretty good, pops and crackles etc.
Could it be the GPF ?

[IvoDC]

So, after driving without midbox for ca 3000km, it's time to write down my experience.
That midbox is described by JLR as a resonator, not a silencer. When removing it, I noticed the following. The internal diameter of the resonator is about 20 smaller then the exhaust pipe itself. Thus creating extra backpressure by restricting the gasflow. If you look into it, you see a straight pipe, which over a distance of about 10cm is perforated with very small holes. The area around that pipe is packed with acoustic fibers. In theory it should remove some of the highest frequencies and thus also lowering the sound level a bit.

The results I noticed from removing this resonator.
1) The exhaust volume is raised a little bit, but not much. It sounds a bit deeper/louder at low revs (e.g. at idle), but once you're driving you hardly notice a difference. On the highway switching on/off the active exhaust, I could tell the tone changed a bit, but my wife didn't hear any difference. There is no droning at any revs, so that's positive. Did not had the opportunity to check the sound during a tunnel run.
2) On my car in it's standard form, I didn't get any pops of bangs. After removing the resonator, you get some pops when coming of the accelerator. I noticed this when descending a steep slope without using the accelerator. But again you don't notice it during normal quiet driving.
3) Without the tube restriction and the active exhaust valve open, I can feel a better torque curve in high revs. Once you pass 4000 rpm, the car feels quicker; more alive. But below that it doesn't feel different to me.

I would like to open the backbox (large silencer) and weld in a patch tube so the gasses of the biggest tube go straight out, but then again that would not only raise the volume, but also make the tone sharper and raspier. So I have to modify the existing resonator chambers in that backbox following my plan (see drawings) and eventually lower the tone further by introducing somewhere after the cats a long resonator without restriction of the flow. In the drawing I added you can see the inside construction (before) like it is now, and what I plan to do with it (After)

To do all this, I'm looking to buy somewhere a second hand P300 backbox for a reasonable price. So when the result isn't what i hoped for, I can go back to the existing situation without losing too much money.

PS: in the original photo of the opened silencer I've found, the owner who posted them had drawn 2 curved pipes in white ink on the still closed side chambers. But that's not correct. As far as I can see the pipes just end in those side chambers and input and output pipe are not connected. The side chambers funtion as a resonator chamber.