I've been googling the issue rather aggressively. According to the Walker diagram:



(and some additional research) Our systems are 2" from the end of the downpipe (apparently 1.75" in from each manifold) and remain 2" until the pipes exist the resonator upon which time the pipe is reduced to 1.75"

One of our users just got a custom exhaust system not too long ago, (which you can read about in this thread here) and if I'm understanding everything alright, then that system is 2" until the cats, then 2.25" afterward.

I've tried to figure out what effect, if any, that would have on performance - whether it would be worth it to go 2.25" if any section of my exhaust would be 2" - even if it's just that very first section.

The consensus appears to be that I wouldn't gain anything, so my options would be to get that quarter inch and be happy at 2", or replace/modify the stock downpipes so that they exit at 2.25". I've decided to bump exhaust up on my priority list because I have an exhaust manifold gasket leak (rear right,) and according to one mechanic, "if the gasket has failed, the manifold is probably cracked too" which only makes things more interesting, and raises further questions.

Barring the necessary facilities to get in there and take a look for myself, how accurate is that assessment? Outside of eBay and junkyards are there any recommended sources for manifolds? How likely is it that the manifold can be welded if it is, in fact, cracked?

Chances are I'll have to find a new primary mechanic, too. They completely misdiagnosed the clunk: that rear transmission bushing is shot, so onto the shopping list that goes as well.

 

I did 2" all the way some 20 yeas ago on the HE.

We dont have cats etc, so a very simple process.

I have only ever seen damaged/cracked/broken V12 exhaust manifold where some CLOWN has done so trying to get the pipes out, never cracked from casting issues.

I reckon your guy is covering himself, and prepping you for a HUGE $$ bill to do those gaskets.

The exhaust manifolds do come loose over time at the head mounting area, hence the gasket issues, then add to that the restrictions that build up in the system (blocked cats/mufflers/etc) as the exhaust components age, put undue pressure on the weakest link, the gaskets.

 

On my '88 V12 I recall 2.25" from the front converters to mid-converters and then to the front mufflers. Exiting the forward mufflers it was 1.875" all the way to the rear.

The engine is only 326 CID. Not like a big 455 V8 where 2.5" pipes are needed

I can't imagine increasing the pipe size would give any seat-of-the-pants improvement. But, if you're gonna have a custom system fabricated anyway...well...it couldn't hurt.

Cheers
DD

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There is just about 0 chance the exhaust manifold is cracked. But what a royal pita to change the gaskets. I bet I put 20 hours into changing all of mine.

 

Excellent responses, thanks.

Grant, if you had good experience with a 2" system, that makes me a little less worried about the downpipes being 2". I've got to keep the cats, but I hardly think they're causing much damage to my performance. Removing them for straight tube is illegal, and emptying them is (according to some research I've been doing) might actually do more harm than good.

Doug, if our '88s are at all alike (and I sure hope they are) then my situation just got a bit brighter. I'm not looking for "seat of the pants" satisfaction so much as I am "breathing room." My goals are not so much to turn my XJS into some sort of amazing performer, but reaching the low hanging fruit that is 300 HP is definitely on the agenda and having a little extra potential is just icing on the cake. I'll have to give the pipes another gentle squeeze with a measuring tool more accurate than my own fat hands tomorrow.

I'll be getting a third opinion next week, this time from a specialist's shop.

 

I agree, if the gasket is bad, you will have a leak there. The weakest point already broke, so why would the manifold be cracked?

I would say you are correct in thinking there is no point in expanding the rest of the system larger than a bottleneck further up the system. If you are getting a custom exhaust there shouldn't be any harm in increasing the size a bit, but it won't do much for you, like others have said.

As for the 1.75 manifolds, you have to remember there are two of these split between 3 cylinders on each side, so that 1.75 isn't a bottle neck. The smallest point starts where those converge as long as it is smaller than 1.75 combined.

The aerodynamics and flow are obviously more complicated than just saying 1.75+1.75 but this is just a street car, so it is good enough for estimating.

I would be more concerned with higher flowing cats and other parts of the muffling/silencing system to increase power than an extra 1/4 of room in the exhaust pipes. If you can delete those or upgrade them to higher flowing, it would probably have a more dramatic effect than slightly larger pipes, as would getting down to two mufflers instead of four

If you are talking 1/8-2" vs 2.5" it is a little more considerable, but still those cats can block up a good bit, especially old ones

 

I guess I'm not too concerned about the downpipes' inlets so much as I am the exits, but at any rate it looks like 2" is what I'm working with.

My plan is (after sleeping on it,) to go with a 2.25" system anyway, even if there will be no benefit over a 2" system. Who knows - one day I may lose my mind and pour unfathomable amounts of money into headers.

Deleting the cats isn't going to be an option for me, but I will step down from four silencers to just two, and I'm leaning on a Super Turbo-style. Since my goal is a mild sleeper, it's important that my exhaust sound humble until I hammer it on. My research certainly makes Dynomax seem like the obvious choice there.

The cat pre-heaters have got to go, though. There's nothing that can save them. Their fate is sealed.

 

I have twin 2.5" from the down pipes and if I was do it again I would go 2.25", it's a bit loud and 2.5" goes under the diff causing scraping on large bumps.

 

Got a third opinion today. Going to the exhaust shop as planned didn't go... as... planned.

Ended up back at my old mechanic's shop, and they were willing to take on the job, saying they had someone who "might just be crazy enough to do it," though they did warn me it would be expensive. This I knew. Problem is, they couldn't find the gaskets.

My local shop didn't want to take on the job, fearing that - should it not go according to plan - it would just become too much for them to handle. Somehow, they were able to order the gasket.

I did some research, and expected one gasket per manifold, and instead found that the gaskets were a bit less elaborate.



It seems number 3 up there, the current part number being EBC10199, is what I'm after. Endless cross-referencing has found a couple aftermarket solutions, such as , (they're apparently the same company,) and finally , which has apparently been discontinued.

I don't want to do this on the cheap (believing that one gets what one pays for, even though all of these options save for the genuine OEM part are less than the $75 per side quoted by my regular mechanic,) but I also don't want to wait for a package to come in from overseas.

What's my best option here? Are there alternatives I should be considering?

 

Hopefully they ordered three gaskets, might as well change them all on that manifold if it has to come off.

You can do this yourself in your garage with not any specialized tools, but it is NO FUN and will eat up an entire weekend (then again, you'll save a lot of money). I've had all of my manifolds off (and had them ceramic coated). The rears are more difficult than the fronts, so it's going to be a job.

When one of mine blew a gasket I didn't want to wait a week for parts, so I just made some with thin sheet copper. Two years later they seem to be holding up well.

I understand that you feel like you are running out of options, but I'd be concerned about the shop's attitude that it may be "too much for them to handle". Where does that leave you...with one manifold off and an unusable car and a big bill for nothing???

Just wondering, are you into astronomy?

Thanks,

John
1987 XJ-S V12

 

The cats are very likely the greatest restriction in the system and you are not likely to accomplish anything with larger pipes unless you go with high flow cats. I have not used them but I see high flow cats readily available through Summit Racing or Jegs.

 

JCR, my local shop (the ones who said it would be "too much") aren't doing the work. I'm not even having them order the part as they said that one gasket was $75 bucks, and to do one manifold with the EBC10199-style gaskets would cost less than $20 - I have no idea WTF they're actually ordering. I honestly think I might just go across the river to my old shop for all the work I can't do myself - which is unfortunately quite a bit being that it's winter and I'm sadly without garage for the time being.

I guess the root of the question was whether there were alternatives to the part I've found that are known to fit but will not otherwise come up in the average search. With my other vehicles, I'm used to having to "lie" to get parts - the Lincoln often necessitated asking for parts for an '89 Mustang LX, and the exhaust gasket in my Royal Enfield Bullet is usually found on the oil cooling system of radial piston aircraft engines (though I can't remember which ones off the top of my head.)

Though I am fascinated by space, the name is kind of a theme, originally started with "Stargazer LSC" on the Mark VII boards, named for the Lincoln four-point star. When I got my Bullet, I kept the theme with "Royal Stargazer" on the RE boards so "Stargazer XJS" is just a logical extension of that. Maybe I'll get lucky and be able to use the handle "Stargazer +8" one day

Dleit, rear cats are on the list of things to be replaced, as well as the pre-heaters, but I'm keeping the stock downpipe and cats combo for now.

 

jcr i ported and had ceramic coating inside and out, exhaust manifolds.

been 20ys and still look perfect, also did away with those pesky heat shields(coating reduces heat radiation better than tinny shields).

i am into astrology, as an abstract for nothing else to do. retired.