During my journey to replace the lower wishbone bushings, I've had the opportunity to drop the downpipes and various other things.

There were three things that stuck out during the dismantling process regarding the exhaust:

1. The intermediates are rusted out in spots
2. Both downpipes were barely connected to the headers with large gaps
3. Both downpipes have the honeycomb mesh

When I fix these and get the car back running, what might the expected impact of these changes be? Would it be anything noticeable or largely just a happy feeling that she's breathing better?

I have pictures of removing the honeycomb from one of the pipes I did yesterday if anyone's interested and getting ready to do the other one today if anyone wants pictures of a specific part, but it's really easy to do, just, you know, gotta wear a damn mask.

 

It seems like you're on a path and I don't mean to throw a "what I did" out in a way meant to drive ya nutts,,, but this is what I did. From the downpipes, my situation was pretty messy right back to the middle mufflers. One muffler was an aftermarket, which wasn't terrible, but I replaced that too.

For reasons of heat, proximity to a lot in the very front of the tunnel and because I could git inspection stickers pretty readily here in the city, then because the PO did a hack job on both sides at that point with different aftermarket cats and clamps, I eliminated the cats altogether. I went on a search for down pipes that didn't include cats from the European market on Ebay (super cheap). With help (thanks forum members) I was able to find 2 OEM used downpipes, no cats, and I had a bung and O2 sensors fitted. This meant an altered and lengthened intermediate pipe which a local muffler guy (I'm not pleased with his work) fabbed up for me.

Honestly, I don't know how she rightly drove before this work and the cats (if gutted and left and messes fixed) but so far I am pleased with the results of straight pipes, no cats and or style everything else from the front back...

Really though, minimizing heat and proximity to so many other parts in that area in the front of the tunnel was a big reason for me. Heat barrels, cats, just seemed like toooo much. Jus say'n.

 

Would love to see pictures to assess the degree of difficulty...

 

Well, if you closed the gaps and fixed the rusted-out sections, it should be quieter.

If the converters were plugged-up, removing the honeycomb should result in more power, noticeably. If they were not actually plugged-up, you might feel a slight increase in power.

Any exhaust leak upstream of the oxygen sensors (such as the gap in the downpipes you mentioned) might throw-off the sensor readings. So, fixing that might make a difference

Cheers
DD

 

I'll get ya more pics tonight for each step, but it's pretty simple - takes about 30 minutes. I use a 3/8" bit and cordless Dewalt drill with needless pliers. Essentially, I start by drilling 3 holes, triangle pattern, then it's just a matter of using the needless nose to grab and twist the little bits of metal out.


1989 XJS V12 downpipes before honeycomb removal

1989 XJS V12 downpipe honeycomb sleeves after removal

1989 XJS V12 downpipes after honeycomb removal, before cleanup

 

Thanks. How hard was it to remove the downpipes in the first place? Is it feasible to do this as a stand alone project, or do you have to remove so much other stuff that it's only worth doing if you're tackling other items as well?

 

This project started to replace a leaky P/S hose that lead to a "while I'm in there" fix of the suspension. The first thing I did was to loosen up the intermediate pipes and all the other exhaust clamps - the thought here was to be able to have a little wiggle when getting the pipes out. My passenger side one fell out as it was pretty rusted at the rear exhaust clamp. Next was the removal of the air cleaner housings, then I removed the remote coolant tank (rusty as heck) - this may not be necessary for just the down pipes. Those two things make it pretty easy to get at two of the nuts on the driver side pipes (LHD) with a 9/16" speed ratchet. It probably helps to squirt a little WD40 or such on the long nuts, but mine were about half-way off to begin with so came off really easy. Passenger side was pretty much the same, but just had to remove the air cleaner housing to get the space. The other two bolts you gotta come up from underneath the beast. I used about a 2 foot extension with a deep socket to easily get at the rear bolts, but switched to a regular 9/16" socket to get at the forward two bolts. These were a little tricky to find, but there's just enough of a hole to get up in there - a strong floor lamp really helps find the space. If I remember correctly, there are a couple of heat shields and a mounting bracket on each side that get in the way a bit - below is a picture of the passenger side mounting bracket that gets in the way. I can't remember if the rack gets in the way, I had already dropped mine before going after the down pipes. Then it's just a matter of wiggling the down pipes - the exhaust flanges kinda flop around and generally just get in the way, but a little wiggle goes a long way.


1989 XJS V12 passenger side (LHD) bracket that gets in the way when removing down pipe

I'm wondering if my intermediates were originally stainless and just got replaced with the cheapo aluminized crap because all my other pipes and cats are stainless...

 

Finally got the chance to remove the other down pipe honeycomb. Took a different route this time as I had some time to think about things. Screw the 3 hole triangle thing, takes too long. Drilling as many holes as you can - the key is being able to get the drill bit to grip into the honeycomb and blast it out. Then it's just a screw driver to ply the metal tube from the side and a crescent wrench to yank it out.


1989 XJS V12 down pipe - drilling out the honey comb

1989 XJS V12 down pipe - piece of honeycomb cake now

1989 XJS V12 down pipe - crescent wrench to grab it

1989 XJS V12 down pipe - rip it out