I have been shown a similar exhaust manifold project in an XJ40, which is a half-way house between the factory design and doc's racing solution. Read the following link and you'll get the idea.
http://www.xj40.com/viewtopic.php?f=23&t=4526
This involves removing the heat shield to make more room for the slightly extended manifolds, and making the manifold as tight as possible to minimize alterations that may be needed to accommodate the new, but larger headers.

 

Here is a picture of the factory exhaust manifolds and how they link up with the downpipe equipped with pre-cat.



One way of making the manifolds longer and stronger is to do away with the Pre-Cat, as experimented by our counterparts in Denmark, which has no problem passing MOT air test. See story here:
Jaguar World Monthly Forum - Clogged downpipe cats ??

Not sure if disengaging the air injection and EGR valve would mess up with the fuel and engine management system. To retain these would need to drill more holes to reconnect to several inlet pipes which too have to be slightly relocated if we were to give more room to the larger headers.

Would really appreciate some heads up from members with more in-depth knowledge about these.

 

The picture above is of the exhaust system from an AJ6 engine, as it appears to have a single Zirconia oxygen sensor mounted at the junction immediately before the outlet. The exhaust system for an AJ16 engine has two Titania oxygen sensors downstream of the downpipe catalysts.


If an aftermarket exhaust is to be designed that replaces both the manifolds and the downpipes, then this provides much greater flexibility in pipe length and routing, if the only common features are the head flange and the outlet flange at the side of the gearbox. There would be no need to preserve a joint in the current location of the cast manifolds and downpipe. However, a joint will be required somewhere. I would not recommend trying to make a single part which replaces all the parts shown in the photo.


An AJ16 engine will run fine without the air injection system fitted (un-machined bosses near outlet flange of cast manifolds), though in markets where OBD11 software is enabled, then this will trigger a check engine light.


I can't remember after 20 years how the failure mode effects management strategy works in relation to EGR. If a std AJ16 engine exhaust system with EGR (machined cast boss on back of rear manifold) was replaced with a custom made one without EGR, then the EGR temp sensor will detect the absence of hot EGR flowing. However, I doubt the engine ECU will do anything as a result. It might retard the ignition timing to values more appropriate to operation without EGR. If not the ignition timing will be a bit more advance than appropriate. However, EGR is only used at part load, so this would not affect wide open throttle operation. In markets where OBD11 is enabled then the engine ECU would also turn on the check engine light. An ECU could be reprogrammed as a European specification to disable any secondary air and EGR diagnostics, if acceptable to the owner.


I have had built an exhaust system for my own car that replaces both the cast manifolds and downpipes. This has no downpipe catalysts and is still capable of passing a UK MoT using the underfloor catalysts only.


Please note that AJ16 exhaust systems were available with both olive type and slip fit joints between downpipe and under floor systems, depending on production supplier.

 

Andy, thanks for the very helpful insight. As far as I understand the EGR valve is external to the exhaust manifold and belongs to the "Exhaust Recirculation System". Can we still maintain the link between the EGR as well as the temperature sensor to the custom-made rear exhaust manifold, or fool the system in some way if we abandon the EGR altogether? Would this affect the Supercharged system, as mine is a 4.0 Supercharged edition. Sorry if these seem silly questions.

 

If you get H & S to replicate the EGR outlet flange on the rear of the custom made exhaust manifolds then there is no reason why you can't continue to have EGR functioning on your car. However, please note that this flange will not be fully machined on any second-hand exhaust manifolds in the UK because EGR was never part of the spec. I would therefore suggest that they buy an EGR gasket EBC10827 which they can measure to determine the spacing for the studs and the size of the outlet hole.

 

Thanks Andy for the advice. Assuming that we can do away with the heat shield, and relocate some of the piping nearby, it would be possible to custom-make a pair of larger front and rear exhaust manifolds that would be stronger and able to improve air flow. The workshop will have to think of whether to combine the front and rear manifolds, and where and how to merge with the downpipes. Hopefully all these parts are not made into one but in various pieces to enable friendlier packing and shipment. I also hope I can retain the EGR function, which means the workshop has to think of allowing EGR valve access. It would greatly help if H&S can find an X300 in the UK to (a) fully size up space and length available for the larger headers (b) assess any alterations needed to enable installation of the larger headers; and (c) break up all parts to facilitate reassembling them in the destination hometown.

 

I would strongly recommend against trying to make both exhaust manifolds in a single assembly. The thermal expansion along the entire length of the manifold would be enormous. It will create huge stresses in a single part. I almost guarantee it would fail.

 

Thanks, Andy. Hayward & Scott in the UK is attempting to make a prototype exhaust manifolds with improvements to reduce risk of cracking and improve heat dissipation for my X300 4.0 SC RHD as described above. I believe that all engine layout and exhaust arrangements are the same in the X300 series. Would any X300 buddy in the UK living near Hayward & Scott volunteer to let them verify the measurements and space available for a pair of larger headers? Their workshop is in Burst Millis Industrial Estate, Basildon, Essex. Ask for Ian if you can make them a visit. Any help greatly appreciated.

 

The headers I supply are one piece I have done 40.000 k.s trouble free no cracks or leaks driven in 40 degree heat in stop start traffic no problems
Lasted longer than any standard rubbish
I have been extremly busy with my own business and have spoken to my system builder about making them in mild steel which he is prepared to do
I am not going to go out of my way as I make these not for profit but simply because I was sick of cracked factory manifolds
I offer the stainless versions but it seems they are to expensive hence my now getting them priced up in mild steel

 

H&S has found a 4.0 SC model for reference. It would be nice if Doc in Australia can find another more affordable solution made in mild steel. I do not doubt the one-piece-OK observation, and I am also confident in the worksmanship doc has learned to trust. Australia and New Zealand appear to be more liberal in pollution control so there is less concern with air-injection and EGR, and a total solution from manifold to floor-cat would be ideal and I heard would gain around 28 HP, whereas according to Andy disabling the pre-cat and only maintaining the floor-cat would still have no problem passing the MOT test in the UK. I believe that a solution in two manifolds or more pieces (downpipes, etc.), and air-injection-ready and/or EGR-ready would meet different market demands and be packing-n-shipment friendly. My X300 4.0 SC is Hong Kong specific model possibly with specifications same as the UK edition, but I really do not know, and am hence playing safe by deferring to the professionals and keeping the air-injection and EGR features.

 

The XJR6 does not have an EGR valve not sure about the N/A models
The only reason the air injection is there is to speed up the heating up of the underfloor cats if the underfloor carts get replaced with a metal substrate cat it is unnecessary for the air injection as metal substrate cats work much faster than the old fashioned ceramic cats remember our cars are 18 years old now and catalyst technology has leaped ahead from the old ceramic type cats

 

I have double-checked the parts number stamped on the broken exhaust manifolds on my XJR6 X300 4.0 Supercharged - they read 2902AD and 2903AC which means equipped with air injection and with EGR valve. EGR stands for exhaust gas recirculation, so I believe it has a role to make fuel burn more efficiently and reduce harmful emissions to the environment. I am not sure whether disabling EGR would compromise exhaust air testing results, so am inclined to keep the EGR functional feature when sourcing a set of custom-made exhaut manifolds.

 

My XJR6 is a 95 Australian delivered and has no EGR XJ6 X300 N/A engines have an EGR not the XJR6 engines as far as I know

Maybe your manifolds have been changed before when I changed mine the first couple times I used second hand ones from the scrappers 150au a piece never checked the numbers they did work for a short time though

My car only came with 2 O2 sensors up the front of the headers

It had! air injection like I said its there for the firing up of the rear cats

I fit theO2 sensor bungs in the new headers so the O2 sensors still go in the same place they are only a switching type sensor any way

As for EGR it is actually used in most cars to create a vacuum in the engine which makes the rings seal better that's the theory anyway and it also burns the fumes and oil vapours instead of them going to the atmosphere

If you where worried about emissions a new Cat would be a better choice and maybe a retune of the ECU by Andy to lean the car out as they run rich from standard Theory behind that is to cool the exhaust valves with good free flowing headers and cat the valves will run cool anyway so a remap of the ECU from Andy will make it go better run smoother and get out of the hole quicker mine goes exceptionally well

I have also changed my diff gears to N/A 3.54 from 3.27

I figure not many places to do 180MPH anyway best mod I did to the car cruises just on the bottom of the torque curve acceleration is instant no down shifting and waiting to go (LOL)

 

Hey doc, quick question. What is the brand of muffler you have?

 

I was using these as tail AT2525XL Performance Muffler mufflers when I first built my system 2 at the rear with 2 of these Performance Exhaust Resonator as resonators
with these Venom Exhaust Works | Performance Exhaust, High Flow Catalytic Converters as my cats 200 cell versions

I have since had custom tail mufflers fitted as the droning was driving me mad and the sound was just to loud even for me
(showing my age LOL)

On my XJ8 X350 I designed 2 1/4 inch dump pipes using twin venom 200 cell cats culminating into a X-pipe under the car then a large dual in and out resonator with standard rear sections and mufflers sounds very nice like a V8 but not obtrusive

The XJR6 was a learning curve I shouldn't have gone to a twin 2 1/2 inch system with twin cats and twin resonators should have used a single cat and single in twin out resonator 2 1/2 inch in twin 2 inch outs for the tail section would have been substantially cheaper and similar results
one thing with my system if the engine ever does go I will build a killer engine and at least I have the exhaust system to go with It LOL

 

US spec XJR6 engines had EGR for some of the later model years. It would seem that this Hong Kong spec car is built to US emissions spec.


EGR (exhaust gas recirculation) is a technique used to lower NOx emissions. It introduces exhaust gas (CO2 and water vapour) into the intake system. This inert mixture plays no part in the combustion process other than to lower the peak combustion temperatures. NOx formation is strongly influenced by peak combustion temperatures.

 

I assume, therefore, that we need not take the air-injection feature too seriously, but concentrate on the EGR and the floor cat?

 

I just had the same exact mufflers put into my xj6 with no resonators since the mufflers already have the resonators built in. I used to get a slight drone depending on the mph speed and rpms but after about 1500 miles of driving, they seemed to quiet down after they were "broken" in. Even with Aero's resonators you were getting a bad drone in the cabin? Also from the rear cats back I have custom 2 1/4 piping.

 

Have a look at my system then you will see why it droned went to large in the pipes diameter as I said my XJR6 was a learning curve
The factory under floor cats and resonators are a huge restriction not to mention weight as well
The XJ8 system is the perfect combination of power adder and sound with what I have done to it
Just waiting to change the diff gear in the XJ8

 

If only California smog law allowed me to removed my cat converters. Once I'm able to get your stainless steel manifolds I'll have to change them out for the factory ones with cat converters in order to pass smog