retrofitting non-cat manifolds to later models

[Dave80GTSi]

I did a search on this topic, but did not find anything.

The later model 986 exhaust manifolds with integral cats are a flow horrorshow. See enclosed pinch point, shown in second picture.

Putting the differing oxygen sensor location issues aside for the moment, has anyone ever retrofitted a later model (like, just for a random theoretical example, a 2000 S) with the exhaust system from an earlier model 1998-1999 or so non-S model with no-cat manifolds?

My question centers around whether or not the early non-cat manifolds (first picture), combined with their complimentary secondary pipes which run from the manifold outlet flange to the rear muffler inlet, are dimensionally interchangeable between the early no-cat and the later cat models.

Is it all just a simple bolt-on?

Thanks - DM

[The Radium King]

i think so - it was done with the supercharger kits where they needed the clearance. as you state, you will need different mid-pipes as the header lengths are different. also check the IDs of your primary and secondary pipes to make sure they are not smaller for the 2.5 cars - i'd looked into this previously and i think they are.

also, if you are looking to replace your current exhaust w something oem, look at the 987 stuff - available cheap (folks pull it off to put on aftermarket - ask at planet-9 which is the go-to 987 forum) flows MUCH better than 986 stuff, sounds better, and bolts right up.

of course, if you put on catless headers, you are going to have to lengthen the wires on your post-cat o2 sensors to extend them to mount after the secondary cats. which means adding bungs as well. or find some other work-around for the emissions cel (lots of threads on that here).

[BYprodriver]

Might want to measure the exhaust port dimensions for comparision to see if you will have sealing problems. Bigger tube size reduces low rpm torque, can't afford much of that on a 2.5L

[Dave80GTSi]

Thanks, guys, for your kind review and comments. Very useful stuff.

I am sufficiently intrigued by this whole concept that I intend to do some independent investigation and "What-If" work. It might take a month or two due to other life priorities which are going on, but if do I end up finding anything significant which can add to the data base of knowledge, I will report back to this thread accordingly.

Thanks - DM

[Troutman]

It's simple if all your manifold studs come out without breaking.
I had 8 out of the 12 snap off.
I don't know what else you can do prior to undoing them other than giving them a good tap. You can't get any fluid at them and heating them up did nothing for me.
Good luck though, sometimes they all come straight out.

[edc]

Why would you want to put that standard non cat manifold on the car? Those are standard across the range in UK and Europe and even the cheap and simple aftermarket £100 eBay manifolds work much better.

[Dave80GTSi]

It's simple if all your manifold studs come out without breaking.
I had 8 out of the 12 snap off.
I don't know what else you can do prior to undoing them other than giving them a good tap. You can't get any fluid at them and heating them up did nothing for me.
Good luck though, sometimes they all come straight out.

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Yup, good point, I have already had both of my manifolds off of the car so to hollow them out and then to ceramic coat them. That’s one of mine shown in my earlier posting (I had them coated gold since I thought that it was a nice break from the standard black or silver).

Even though all 12 of my bolts thankfully came right out of the heads without a care, nevertheless I will be replacing these bolts with short studs and flange nuts the next time that I am poking around under there, just so to protect the aluminum threads in the head from the potential for wear / damage due to frequent removal.

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Why would you want to put that standard non cat manifold on the car? Those are standard across the range in UK and Europe and even the cheap and simple aftermarket £100 eBay manifolds work much better.

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That’s certainly a fair and obvious question.

Several reasons:

a) I like the general philosophical approach of returning a USA-spec car to its original unencumbered Euro-spec status, using as many OEM parts as can reasonably be done. And the OEM Euro-spec manifolds are plentiful and cheap over here in the States as they were used on our earlier 2.5 / 2.7 engines.

b) I’ve heard various anecdotal stories of some of the cheap and cheerful Chinese eBay headers not fitting so well. Seems like their use could become a bit of a roll of the dice, depending upon which flavour I just happen to receive this week.

c) Aside from the potential for fitment issues, I don’t like the way that these headers splay out and away from the block. See marked-up picture. Again, I’ve heard second-hand stories about how various components which are located beyond the heads can get cooked by these protruding header tubes, especially after the car is shut down and stationary after a long run.

d) It looks as if there could perhaps be some interference between the headers and the various undercar cover trays, as shown in the picture? Maybe, maybe not. But even so, they do seem to end up much closer than the OEM set-up, and I don’t want to modify or to cut these panels.

So, all in all, recognizing that the headers would appear to flow better than the Euro-spec manifolds, I would be willing to compromise so to accept the more compact design of the Euro manifolds.

Thanks - DM

[edc]

Here's a first hand sorry for you. I've used the eBay manifolds on 2 of my Boxsters which I've had since 2012. I've had no issues with them.