The current exhaust set up has quite the bark, it consists of 100cell cats , x-pipe mid pipe and xkr rear box with the outer valves in the closed position

Any ideas on how to tame the sound down without changing the cats perhaps resonators ?

 

I'm thinking of trying 400 cell cats instead of the 100 cells, the noise is really getting to me...

 

Sorry but I couldn't resist........

 

I don't have any hard facts, but I would think resonators would cause less back pressure than denser cats; and with a supercharger the goal is to reduce impedance to the free flow of the exhaust and move as much air as possible into and out of the engine.

Most resonators are flow through with perforated walls containing packing around the circumference. But with cats, if you increase the numerical count of the cat, it stands to reason you also increase back pressure.

Here's a couple pics of the internal difference between the two converter cores. 200 core on left, 400 center, and resonator on right

Source: Cat cell count = ? - EvoXForums.com - Mitsubishi Lancer Evolution X Forums

 

Anyone cut open a Jag resonator or know if both banks flow through through one box for space convenience but remain independent internally or if they are actually open inside, essentially acting as a crossover? I have 200 cell cats with the X-pipe & valves always open on XKR OEM back box and as GordoCatCar noted, it would be interesting to see how it would sound with a resonator in place. Just don't know if I would have just a single resonator (I still have the original dual resonators) or resonator + x-pipe. Steve, do you still have your XK resonator section, have you even put it in place with the 100 cell cats and without the X-pipe?

 

Put some glass packs on, that'd take away the worst of it. And since they are straight through, no added pressures.

 

I have no experience with them and have not heard the term in years (except on classic American muscle cars) but a quick look up on how they affect exhaust indicates they reduce back pressure but are not effective at reducing noise unless they have internal louvers, then the results are the opposite due to internal turbulence generated.

 

The oem resonators actually merge each bank and do not run independently

Unfortunately I can't try the original xk rear box as It was part of swap to acquire the xkr rear box

I might try cambo's idea of 400cell cats

 

To reduce noise, it HAS to be baffled somehow. The less baffling, the more noise. You will need to accept at least SOME flow reduction to rid yourself of raucous irritants. Since evidently the rest of the system is upgraded, the slight amount of turbulence created in negligible.

 

No I meant do you still have the XK single resonator to swap out your X pipe?

And the so the OEM resonators are also effectively an x-pipe?

 

You can also see some comparisons of different cell counts here >Sureflo Exhaust - Catalytic Converters

Now the thing about the cats on the Jags, is they are a pair of cats per bank, there's a 600 cell brick, with a 400 cell brick behind. Here's a cutaway;



That in itself has a huge muffling effect. Probably why the XKR's, F-Types, etc have more or less a straight through exhaust when the valves open, but "not that loud".

By ditching the cats or putting in 100 cell, 200 cell, that muffling effect is greatly reduced.

I think the pendulum can swing too far the other way as well and too little backpressure could also be a problem. Going to make some more tests in the next week or two and see what we can find out.

 

Some mufflers and silencer work by baffling the sound waves in a manner where one wave cancels another. Different baffle layouts result in different sounds and sound levels. And Cee Jay is correct in that the more baffles the more back pressure.

However, another method is sound absorption wherein the sound wave travels through a perforated or louvered straight pipe and is absorbed into a medium like fiberglass or steel wool. These systems are in theory less restrictive than baffling. I have read that on an engine like a small block Chevy with a mild street cam one 90 degree bend in an exhaust pipe is equal to 6 feet of straight pipe. So its easy to see how baffled silencers can quickly lose flow ratings compared to a straight through system like a cherry bomb.

In addition, I have read that on naturally aspirated engines a minimum level of back pressure is helpful as it aids in the "scavenging" of spent gases from the combustion chamber: and that sometimes when a freer flowing straight pipe replaces a normally configured exhaust system, it is quite possible to discover that torque values may actually decrease in the lower rpm ranges. However, that may not the case with forced induction engines where the more efficient the exhaust manifold and wider the tail pipe and the lower the back pressure becomes, and the easier it is to pump more air through the engine resulting in higher power ratings.

Finally, I have experience with using exhaust pipe venturi to counter act a "drone" occurring within a specific rpm range like highway cruising. Apparently the venturi, which temporarily reduces the internal diameter of the exhaust pipe changes the flow characteristics of the gases enough that it can sometimes kill a drone by altering the speed of the exhaust gases as they enter the muffler. Obviously, these work best utilized just prior to the muffler or silencer. So it is often helpful to determine if the objectionable sound is due to an excessive noise level, or an irritating drone in specific rpm range.

 

No to having the single xk mid pipe resonator that was deleted to make way for the x-pipe

In regards to the oem resonators acting as a merger I'm truly unsure how effective it would actually be in a flow sense compared to an x-pipe.

 

I was planning to update the thread I started regarding what to do after having my cats removed once I have an 'after' dyno result but from what I have experienced over the last month you have to be very careful when playing around with the exhaust setup because you can wreck performance.

After the cats were cut out on my '07 XKR I was left with straight through down pipes, the standard dual resonators and the standard back box. I was left with a very noisy and unpleasant exhaust note. As replacing the cats would have been quite expensive, the guy who services my car took me down to the muffler shop who had removed the cats and we had a play stuffing various resonators in to the tail pipe. At the time I had wired the valves shut to stop some of the noise so we were able to just put one resonator in each of the remaining exhaust pipes. Even though I could see straight through the resonators they did make quite a difference to the exhaust volume when the engine was revved.

There isn't much space to play with under the car and as I didn't want the rear box mutilated in case I had to give up and return to factory standard they removed the dual resonator section and fitted an H pipe followed by two pairs of resonators fitted either side of the factory mount that spans the void where the standard exhaust goes (see pics). It isn't as flash as some of the off the shelf solutions but does the job for now.

This has made a big difference to the volume and has also made it slightly deeper. The bark at full throttle was still quite pronounced but it was higher up the rev range and only at its worst after the exhaust valve opened so I could at least drive it around town again without drawing too much attention to myself.

The car did feel that it was significantly down on power though. When I had the smaller supercharger pulley put on last year it had made the engine very free revving but I felt that with the modified exhaust it was now back to or worse than factory standard. I had a dyno done last week and the peak rear wheel power was 255KW/342bhp which seems quite low.

With the help of Cambo and his tuning guru I put a tune on the car last weekend and it has transformed the car. Based on my butt dyno I have got the lost power back and more. He was also able to modify the profile of the exhaust valve opening points in the tune so that it is only open up until 1500rpm and closed above that so I no longer have to listen to the bark.

For the times when I want to act more like my shoe size and not my age I have fitted a switch across fuse 19 and located it in my center console cubby so that I can flick it and hear the louder deeper exhaust sound which is great for cruising around town on a hot summers day with the window down.

I had a play down a quiet road after the resonators were first fitted and was struggling to beat 5.5 secs 0-60. The first run I did after the tune was a 4.9 and traction seems to be the limiting factor now. I was also interested to discover that after the tune but with my switch set so that the exhaust valve was permanently open I was struggling to beat 5.5 secs again so it looks like you can open up the exhaust too much. I am hoping to get some runs done on the dyno next week with the valve both opened and closed to see of that shows a difference as well.

 

This is the standard program for the active exhaust in the XKR;



For u102768's car it's now a little different, basically like this;



So the valves only open at high rpms when you back off to get some overrun noise.

We're a little surprised that the car is making more power with the valves closed, since the tune now in his car has the exhaust backpressure values taken from another XKR with no cats that was actually measured with gauges. So obviously the different exhaust layout on this car has a different back pressure to the test car. In an ideal situation the actual pressure of the exhaust would be measured with gauges or pressure sensors and then those numbers input into the tune file, but this is not the easiest thing to do...

Also the way the cats are removed is probably having an effect. The original test car has gutted cats (so the large chamber is still there) and this is making a different backpressure profile to a straight downpipe.

The exhaust on the test car is a copy of the XKR-S layout (here's a pic of the "special" factory exhaust from the 2008 XKR-S and the PalmerSport XKR)



The XKR that the tune was developed on has a copy of this exhaust layout on it. And then the gutted cats. Yes it is LOUD.

I'm lead to believe that the location of the crossover, and the type of crossover, be it an X or an H also has quite an effect on the backpressure and overall "tuning" of the exhaust. You can see the x-pipe on the 2008 XKR-S exhaust is some ways rearward, and if you look at a 5.0L XKR-S exhaust it's even further back.

So in all, there is a bit of a black art to the exhausts, especially when the specific backpressure of the exhaust is part of the engine tune in the PCM... if the backpressure numbers aren't right in the tune, it's not going to make full power.

 

To be fair, I only had two or three attempts doing a 0-60 with the exhaust valve open because it is so loud at full throttle that I didn't want to hang around too long but the dyno will hopefully show whether it is just my driving or whether there really is a difference.

All in all I am very pleased that I have got my pocket rocket back