I'm working on. 97 xk8. No supercharger here
After looking under the car I'll be doing the x pipe from magnaflow, deleting the 70 lbs Center box! Lol
Probably deleting the smaller ones before the axle to allow my local shop some room to bend pipe up over the axle. They would have a lot of difficulty bending the over axle piece without having to squash it just like the factory piece that's there.
If I still am not happy then I'll replace the last 2 mufflers with magnaflow and do some larger tips.
Going under the axle isn't an option for me. As I'm sure where I drive something would be bound to hit and I'd like to lower the car in the future.
I'm hoping to avoid that popping sound a few others have posted. It's just not my thing. Low tones and some growl is all I'm after!

 

So you will end up with straight pipes with an X and just rear mufflers. That's exactly what K Westra did in this thread only with an H instead of a X. He has magna flows and they are way to loud for me.

https://www.jaguarforums.com/forum/x...ter-40s-37811/

I just wonder which of the stock mufflers actually would sound best to leave on, the rear most or the middle.

 

About 10 years ago I started working on developing a new exhaust system for my 2003.5 XKR. I started with Borla, who is north of me on the other side of L.A. My car was the mule for the project. But, they decided to not continue making systems for our cars (they already had one for the earlier models) and dropped out. I then chose to go it alone.

The first 6 to 8 prototypes were made by a local muffler shop to my designs. They were all dyno tested and some of them were tweaked a little and re-tested. The first one was an "H" pipe replacing the center dual muffler. It yielded little to no (measurable) power change, but did have an unpleasant increase in noise and tone. I would recommend against that mod.

Incidentally, the center muffler is simply two perforated pipes running side by side through a single case. Nothing special. All of the mufflers/resonators in the OEM system are straight-through perforated pipe designs. No baffles at all. There are no power gains to be realized by replacing them (with the same pipe diameter mufflers). Small differences in sound may be seen with substituting different aftermarket mufflers. Noticeable change in sound requires eliminating OEM mufflers.

Subsequent prototypes all used off-the-shelf "X" fixtures from three different manufacturers (different designs). They all improved the sound with negligible power increase. I started researching patent applications for "X" type exhausts to glean more knowledge. Quite interesting.

This led to the first MacLeod Performance Mk I X-Pipe exhaust systems. These were muffler shop built using off-the-shelf "X" fixtures and mufflers (Magnaflow, as I recall), 2.25 inch aluminized pressure-bent steel pipes in a straight back under-axle design. These consistently dyno tested with about 12 rwhp increase on forum (Road Fly) member cars. The tone and sound level received lots of positive comments.

None of the manufactured "X"s are true crossed-pipes, but instead are either stamped out "clam shell" approximations, or simply bent and siamesed pipes. (Both much cheaper to manufacture.)

This led to some flow bench testing with the different styles. The manufactured fixtures could not begin to equal the extraction effect of a true crossed-pipe "X". Probably because they had, in effect, a small plenum chamber at the point of intersection. I was also able to determine that the angle of intersection, as well as the distance from the cats, affected the power increase, sound level and tone (acoustic noise-cancelling effect of crossed-pipes).

A lot more experimentation and testing, now dyno, flow bench and sound meter, led to a much improved MacLeod Performance Mk II X-Pipe version: laminar flow 2.5 inch mandrel bent 0.065 T304 SS tubing, true cross-pipe X, straight back under axle, custom mufflers, etc. Consistent 23-24 rwhp dyno tests increase on 4.2 XKRs. Several owners reported 10-15% improvement in mpg. Many accolades on the forums about the sound level and tone. It also stiffens the chassis quite a bit.

All this just goes to say that there is a lot more to improving our exhaust systems than just changing out a few of the parts. The entire system, from the exhaust manifold to the tips at the exit, functions as a single unit. Each part affects the sound and flow in a synergistic manner. (1+1+1+1=8 not 4)

Passing on the following lessons learned may help you in experimenting with your own system.

1. The OEM system is a little restrictive (power reducing) on the XK8s. It is grossly restrictive (same system handling 33% more flow) on the XKRs.

2. The most power robbing part of the OEM (and several after-market) exhaust systems is the over-the-axle pipes. On the OEM system they are crimped down to between 0.75 and 1 inch in two places on each pipe. There'll be little to no performance improvement until these are replaced. They also affect the sound (negatively, in my opinion).

3. Pipe diameter: 2.25 minimum, mandrel bent; 2.50 inch noticeably better, mandrel bent. (Muffler/Exhaust shops use pressure benders which crimp the pipe diameter down in the bends. Mandrel bending maintains a constant diameter through the bends. Pressure bends not only restrict, but also disrupt the laminar flow, as do the welds.)

4. The sound and performance (a little) can be improved with a muffler shop built system. It will be cheaper than a complete cat-back after-market T304 SS exhaust system, but you'll be getting a lot less, also.

I would suggest, if you are going that way, to not waste your time and money cutting and patching pieces of the OEM system. At least replace everything between the cats and rear mufflers with an "X" fixture and under axle pipes--eliminating the center resonators as well. If possible, replace the rear mufflers also and increase the entire system diameter to 2.25 inches. The system will still be a mix of steel and possibly low grade 409 SS parts, but it will sound better.

Bending the pipes correctly to fit up next to the rear cross member without touching it (which will produce a drone inside the car) and not decreasing the ground clearance is a bit tricky.

Be sure to get a price for doing all this before you start. It can add up quickly.

If you do this and still want to change the sound level afterwards, let me know. There are some other tricks to change the sound level and tone.

Hope this helps.

 

Thank you. Wonderfully comprehensive. I am curious about the loss of low-end torque for an XKR coupe.

Disconnect everything beginning with the central muffler and just let the exhaust flow out of the bottom of the car had the same low-end power loss as removing the central muffler and clamping in temp pipes to the before rear axle muffler-ettes whose crimped OEM pipes go over the axle and straight out with no mufflers.

Now just remove the rear mufflers and there is no loss of low-end torque.

So is replacing the center muffler with an X-pipe going to bring back the back-pressure needed for low end torque?

I'd argue any exhaust system exceeding $1,000 should not be bothered with before the lower pulley upgrade on gen1 XKR coupes.

 

SanDeigoMac, thanks for the detailed info and you certainly appear to know what your talking about. I for one am only interested in a little more "refined" sound. Without losing any of the power my XK8 has I would like some rumble from the pipes, not anywhere near a full hot rod which the car is not, just a pleasant low rumble when idling and a bit of bark when accelaterating medium to hard. Roads are not bad where I live and this is just a hobby car not a DD. With this in mine what would your suggestion be for a moderate priced modification to the exhaust?

From reading all I can I'm leaning toward the following:

H crossover near the front with straight pipes all the way back to a quiet performance mufflers at the rear, maybe a magna flow, not sure. Unsure about over or under the axle as while roads are not a huge issue I don't want clearance issues so over the axle may be a better solution as HP gains is not what I'm after.

 

Love the write up, thanks! But an exhaust system that stiffens the chassis? I'm not buying that. Exhaust systems are typically mounted with hangers that flex for the system to move a bit to avoid pipe breakage, deliberately taking away any likelihood of stiffening the chassis. Nothing short of a welded in roll cage or perhaps some custom stiffening plates attached at key points will do anything to truly add rigidity.

 

I was thinking magnaflow x pipe and magnaflow rears. I believe K Westra used flowmasters 40 which are way more aggressive then what I would also want.

 

We never saw any low end power loss on the dyno as back pressure was progressively reduced. The shape of the power curves stayed the same, virtually identical, only higher, until around 4800-5000 rpm where they would then start to increase their slope, versus falling off, as the exhaust systems' efficiency improved.

I'd heard, before I started on this venture, that reducing back pressure would reduce low end torque, so I was particularly looking for it. Never happened. I could never find any substantiation for this (urban myth?) in any literature or research.

The center mufflers, as are all the OEM mufflers/resonators, are straight through perforated pipe designs. None of them should have any more than a minuscule effect on back pressure. Back pressure (aft of the cats) is easily measured by installing a small fitting just aft of the pipe-to-cat flange and running a line to a pressure gauge.

"Butt Dynos" are notoriously inaccurate. That's why I spent a bundle on dyno testing. Also, I've seen estimates of at least 25+ hp being necessary in our two ton cars before even the most sensitive of butts could tell the difference. Our XKRs are around 10 lbs per hp, so even 50 more hp only reduces that to .89 lbs per hp. Not a lot of difference when moving 4000 lbs.

Consider this; we could be in a plane together with you blindfolded and I could do a 360 degree barrel roll, fully inverted at the top, and you would swear we were flying straight and level the entire time. :-)

Regarding the lower pulley change on 4.0 cars; the superchargers on our cars are already at the edge of their performance envelope. Spinning them faster, while providing slightly more boost also rapidly raises the induction charge temperature and lowers their adiabatic efficiency. I think the factory already has them pretty much at their best speed for overall use. If the car is only being used for the dragstrip, well, maybe then it's worth trying it.

This is why I started the twin screw conversion modification back in 1987.

A much simpler way to get maximum acceleration is to downshift to about 4500 rpm, then WOT (wide open throttle) and shift back to Drive. Let the tranny do the shifting close to red line.

These cars will never be very good off the line, simply too heavy and rev-limited from a stop.

 

Magna flow, flow master, heck I can't keep anything straight. Been searching for magna flow 40 , duh, no wounder I can't find anything. Wife just handed me my drugs maybe that will help....

 

Thank you SanDiegoMac. I can assure you on my car the loss of low end torque is easily noticeable by me and business partner, so no urban myth. Seen it twice: first time when we disconnected the whole thing (sound was hilarious though), second time now when everything connected but central muffler replaced with clamped on pipes that goto before axle muffler-ettes, over axle OEM and straight to back with no rear mufflers. That center goes back on and I'll bet anyone $100 my low-end torque returns at low-end.

As I said, low end loss made up for at high end so if I ran canyons more than city streets, I'd keep it that way. :-)

 

We actually discovered this by accident. After the first Mk II X-Pipe systems had been installed for awhile, I got a few calls about the rear hanger rod, just in front of the rear muffler (same location as stock), breaking. I use a 3/8 inch stainless steel rod (everything on the system is SS, even the fasteners) which is really strong, and couldn't understand how they could have broken. I thought maybe we'd gotten some bad material. I tested some of the rods and could find no problems. The rods do go through a rubber donut that is bolted to the chassis, but that doesn't allow much movement.

Continued investigation into the cause revealed a common thread--all of the owners had a steeply sloped driveway that they would approach at an angle to avoid scraping the front of the car! One of the guys was in L.A. and I went up there to watch him enter his driveway. Wow! The car visibly flexed as it transversed between the street and driveway apron at an angle. Further observations showed the convertibles were a lot worse than the coupes--as expected.

The Mk II X-Pipe system is made with .065 thick T304 SS tubing--heavier gauge than used in roll cages, with two inch slip joints fastened with TorcTite clamps. It does not flex! Frankly, I had not thought about this being a problem. Other exhaust systems use .042 thick tubing.

I then doubled the attachment rods and welded them together. I also sent out extra rods to be added to any cars that reported a problem.

Next, we took a few cars, unbolted the attach rods in front of the mufflers, and jacked up one corner until a second wheel came off the ground. This really showed just how flexible is the chassis. Try it, you'll be surprised. We did my Ranger pickup and my daughter's Corolla for comparison--virtually no flex at all--the wheels came off the ground about together.

We bolted the attach rods back to the chassis and jacked it up again. There was about 3 inches less flex with the Mk II X-Pipe bolted to the car. The center hangers bolt to the front of the rear cross member, are more flexible than the rear rods, and don't contribute much. But, between where the pipes bolt to the cats and the front of the mufflers, the system is extremely rigid, it does not flex, and transfers this rigidity to the chassis. It is like adding a second sub-frame to the chassis.

A few convertible owners have also told me they could see the difference with less cowl flexing around the windshield--I think they're talking about the A pillars.

This should improve the handling, but I haven't checked that out

 

I find this really, really interesting. Especially on a supercharged car where there is extra positive induction pressure clearing the cylinders. Theoretically, it makes no sense (at least to me). I can't fathom what could be causing it.

Are you able to tell the rpm range where it is different?

Do you have any way to inspect the inside of your center muffler--short of cutting it open :-) -- such as an inspection camera? Search results for: 'inspection camera'
I bought a good one at Costco for a great price some time ago.

Are there any dynos in your area? Seeing the difference in the power curves, would be really enlightening. We used to have "Dyno Days" out here where we could get a group of cars together for a Saturday morning and the shop would run them through like an assembly line for $40 cash, 3 good "pulls" each car.

You have me stumped.

 

For those concerned about the under-axle routing and reduced ground clearance:

I have 2 1/2" straight back pipes that route under-axle. Yes, ground clearance is obviously reduced, and I was always conscious of it at first. To tell the truth though, the only times they have dragged were when I was entering inclined driveways with slope changes abrupt enough that any of us would have taken them slowly anyway.

Bottom line: The reduced ground clearance is not enough of a reason to put up with the OEM over-the-axle crimped pipes. Now, if you want to lower the suspension too - well, all bets are off then. Good luck!

 

I've also never had an issue. Never scraped anything, ever.

The way I've run the pipes, they are at their lowest only between the wheels. This results in them always maintaining the same ground clearance as long as the wheels are also going over any bumps, such as speed bumps, culverts, etc.

The only time it could be an issue is if something were sticking up between the wheels and thus the car would not be going up the same distance.

The pipes, at their lowest point, are about a 1/2 inch higher than the bottom of the front valance. So, anything would hit the valance before the pipes.

 

Normal torque on my car felt readily around 2300, by 2800 it is full bore and fulfilled my muscle-car desires that were unfulfilled in prior 2000 Porsche Boxster (small engine 2.7 with auto) which had no torque or exciting engine anything below 4000 rpm. I have celebrated that torque in 30% of my first gear starts over 2 years of daily driving over 9000 miles. Really, I am into it. As important to me as the car's beauty and the key compensation for its GT only level handling.

I will report back this week when an X pipe gets clamped on.

Are all X-Pipes created equal?

 

In my normal "cart-before-the-horse" mode, I've ordered one of the new Dynamax x-flow mufflers to replace the front muffler. It's basically a perforated x-pipe inside a big glasspack. I'm planning 2.25" straight pipes from there back under the axle to a set of 4" Borla resonated exhaust tips. I'm hoping it'll sound as good as the "H" pipe and turbo tubes on my XK8. Any thoughts?

 

Assuming it is as you describe, like adding a second sub frame, I would wonder what kind of pressure is put on the headers, and by extension the motor mounts, transmission mounts, etc. in situations where the car flexes. All that energy has to go somewhere. Chassis stiffening should probably be something that only involves the chassis, not the drivetrain.

 

That's all true, and a great observation on your part. I thought about that too, and debated about just adding a flexible pipe section between the X and the rear chassis mounts (always an easy option). I spoke with the guys that had the broken hangers about that as an alternate fix, but they liked the improvement to the car with the stiffer chassis so much that they all opted for the stronger hangers instead. Some of those cars have now had the Mk II X-Pipe on them since 2007 and have driven around 100K plus miles since installation. We've been watching the very areas you mentioned, plus a few, and no problems reported so far. The cats are also bolted at the bottom, right behind the pipe flange, and their whole attachment is quite strong.

The weakest point in the whole system is still the hangers and if anything were to break it should be them. Also, the extra load is spread out over a lot of areas, as you mention, and not concentrated at any single point, that certainly helps. Another factor is that the exhaust system is only adding stiffness to the chassis, reinforcing it, not taking all the load by itself. Kind of like adding a leaf to a spring to stiffen it.

Anyway, all I can really say is that after seven years and a bunch of miles on several cars we've monitored carefully, there have been no problems.

I like the thoroughness and depth of your thinking. That's a real asset when making any kind of modification.

 

First, NO, all X-Pipes are not created equal--not by a long shot. The differences we saw on both the flow bench and dyno were pretty enlightening.

Secondly, your post answers a lot of questions. And I think I understand more about what you're talking about--that nice "torquey" feel of big bore V8s at low rpm. Corvettes have always seemed to excel in that area.

We always dyno test in 4th gear (closest to direct on our cars) starting at 4500 rpm. Any lower rpm and the trans will downshift at WOT, which throws all kinds of artifacts, spikes, and totally invalidates the test. (I think my car can show over 700 hp doing that!! :-) :-) :-) ) Pretty unrealistic, though I know some people count those as genuine, like to sell air filters. :-)

So, I have no (reliable) data below that rpm. It's part of the power curve, but it's not at WOT and only shows what's produced at partial throttle. We could devise a test to measure any difference, say test a specific low rpm range at 50% throttle opening as shown on the car's ECU, but since it would be at reduced throttle, it's easily compensated for by just opening the throttle more. It would be more to satisfy curiosity than pursuing more power.

I don't know what would cause that, lots of variables here, but I take your word for it. Still, to get more power, all else being equal, there needs to be more air and fuel crammed into the cylinder (other than reducing parasitic losses), so, who knows what's happening? Could it be that maybe something (O2 sensors?) is causing the timing to retard? That could make it feel "torquier" at low end. You could watch the ignition advance with a Scan Tool or other ECM monitor. Maybe some really smart engineer has the answer.

 

I have a MacLeod system and can attest it does make the car feel stiffer. I have to enter my driveway at a steep angle and haven't broken anything yet. However I got his quietest version (with two mid mufflers be side) and am having a clearance problem near the axle because the car is lowered. I went quiet because of the Nameless cats. Were I to do this again I'd likely take the extra sound over the grind of the pipes hitting the ground on my driveway. I should point out this is really the only time they hit, driveway goes up then down into a steep decline and car hits at the peak unless I hit it at a sharp angle which I often don't hit right. The system (before the scratches) was stunning to look at.


I also have a smaller upper pulley but while the power did come up nicely so did the heat suggesting I'll eventually need something like a chiller killer or bite the bullet and get a different kind of supercharger.