24 replies to this topic

[blackoctagon]

I was able to get a new (secondhand) exhaust maifold from a member on here.
The following details what I did to modify and get rid of the hateful EGR take off on No.4 cylinder.
I am aware that equal length manifolds and various configurations (4-2-1, 4-1) are both available and an improvement on this boat anchor, but I like to make the best of things. And I have a ton of time on my hands.

The first operation is to remove the manifold to downpipe studs. Penetraing oil overnight and the 'double nut' method was used to get them out.


Next was the obligatory weighing: 4.649kg


Casting flash is dead weight, as is the comically thick boss for attaching the heat shield lower bolt to.


More flash removal


Immediatley we have lost dead weight: 4.498kg now - 0.15kg of nothing removed.


Some pre modification shots showing surface roughness:











With the outside shaved it is time to turn attention to the important bits.
Some calipers can help you transfer the dimensions of the port to a drawing or a table to make sure you match all the ports to the same dimensions. Clearly this will be limited with a cast manifold due to the 3D shape the ports take, as opposed to nice tubular manifolds, but you can do the first inch or so with care.





There are two schools of thought (and both have merit) on exhaust ports - 1. you match the manifold port to the head's port and lose no energy in the transition OR 2. you leave a step, whereby the exhaust port is slightly smaller than the manfold port and lose energy in the spreading jet flow that ocurrs, but have some defence agaist unwanted high pressure waves reflecting back up the manifold (anti-reversion lip is a name used for this).
I recommend using bore guages for this rather than calipers, but you can use calipers.




You can see the exhaust port is smaller than the manifold port, and for no reason other than trial-and-error i'm choosing theory 2 here; Anti reversion. The reasoning here is simple - you can grind a bit out, but you can't grind a bit back in.



With the manifold ready it's time to fit a plate into the EGR channel and tack it up.


Adjust to length (leave a lip, dont dig intothe manifold surface)


Lay weld on the back


Add metal to the port:

[blackoctagon]

you'll need to re-shape no.4 manifold port:


Apply some engineers blue:


Scribe off an exhaust gasket (I have no in-progess photos):


Then it's time to get stuck in to the manifold ports and pipes with a cutter and sanding mandrels to get rid of the surface roughness:


Two shiny shots:




The all important weight: 4.415kg


Gasket in position (marked, but not dowelled - I elected not to do that for now)


Comparion of the manifold from the car:


Three pictures of the manifold on what I believe is a Tullet downpipe and cat (let me know if i'm wrong, please):







WHATS NOT SHOWN: The 'bumps' in the manifold ports that allow casting thickness and clearance for the manifold studs were also shaved down during the port smoothing.
Prior to removal they can be seen in the first post during the measurement phase - the paint was used to highlight them.


How does it feel? It feels better in the mid-range, but thats using a bum-dyno and I also removed the EGR from the software and that could affect it.

Edited by blackoctagon, 05 July 2020 - 08:25 PM.

[smiley]

Interesting.

Your downpipe looks oem. Tullet is stainless.

 

[blackoctagon]

Okay, i'll take that onboard.
So the OEM system is a flexi pipe (thats what's under the wrap) that goes down into a small welded-in cat?
The part does not look OEM, and i was told by the previous owner it was Tullet, but that was maybe only the backbox (it is certainly not OEM). It has no sign of the pre-cat, or having anything removed inside.
However, if it not a well-regarded part (so it is OEM) then i'll feel better about scrapping it when I make the 4-2-1.

Edited by blackoctagon, 05 July 2020 - 08:49 PM.

[smiley]

When the precat has been removed cleanly, it will not show any traces of previously being there.

 

[Arno]

Nice work!

 

Hopefully the manifold won't develop the classic cracks on/in the 'Y' sections like many do..

 

Bye, Arno.

[blackoctagon]

The new manifold is sound - I had a look in each valley, top and bottom, for cracking.

The manifold that came off the car also appears to be in good condition, although i've not cleaned it up thoroughly yet.
I'll probably just sell it on after i've got the dimensions from it, as I don't feel there is a 'Mk.2 modified part' to be had from the cast manifold.
It's a grim design and I doubt the development team were proud of it, other than it meeting emissions specifications.

[smiley]

It was fit for purpose on the Vectra/Astra. and just ended up on the VX, as part of the Z22SE package.

Luckely that makes them p*ss cheap to replace for other oem.

 

[blackoctagon]

I know - i've been there. Still heartbreaking when you have see a sub-optimal part go out the door though.

I've taken the part as far as I can, or at least as far as I can be bothered.
Finding out that downpipe is not a good one has at least meant that I can make the 4-2-1 now without worrying about expensive waste.

Edited by blackoctagon, 06 July 2020 - 12:42 PM.

[smiley]

You planning to make a 4-2-1 to mate with the oem downpipe, or will you also custimize that one?

If you do manage to pull off a 4-2-1 within the available space, you may thing about creating more and put them up for sale.

 

[blackoctagon]

Keeping a cat on the car means that the primaries and secondaries will be sub-optimal without cutting into the car's clam, and i'm drawing a line there.
The primaries will be around twice as long as the OEM, the secondaries around a OEM-cat's length, then you need some length in the tertiary before the cat, otherwise you get a nasty reflection wave right back into the secondaries. You still will, of course, but you can place it to leave some distance, and hence wave travel time.
The design I have in my head will be significantly better than OEM, but not as good as having a free hand on the packaging. But it's a road car for me.

I would consider moving the cat to the backbox space to lengthen the secondaries and put a bend between the tertiary and the cat matrix. The bend between the tertiary and the cat would slightly attenuate the reflection wave strength (not by much though).


I would not be producing them, but i'm happy to let someone else do it if they like the design.


I did have one ineresting/cost-reducing idea though - I had a Citroen AX GT a long time ago. It had a 4-2-1 manifold from the factory by using a single-looking downpipe, which was actually packed with two semi-circular pipes. From the outside it looked like a 4-1, but the 2 was in the downpipe and the tertiary came before the flexi joint.
It still leaves the manifold as a problem, as it's 1-2, 3-4 pairing is not great and I can't see a path to changing it without heavy modification, and that effort is probably better spent on a new manifold.

[oakmere]

I use the tsudo header (weapon r) from the US to form a 4-2-1 system. It is far from optimal but works much better than the OEM and is cheap and reliable. Do a search there is quite a few examples.

[blackoctagon]

Thank you for pointing me to that - i've had a cursory look and it looks like it would fit, albeit with some downpipe surgery.

I'll see where they are for sale.

[vocky]

thats not a standard downpipe/car section, the cat is similar to the Tullet, so probably is a Tullet

[blackoctagon]

Thank you for weighing in on this.
I have one count for and one count against.
It certainly does not have the hallmarks of an OEM/mass produced pipe, but I have no point of reference, and it is a Lotus with all that that entails.....
I do believe it is an aftermarket piece, and the previous owner said it was Tullet, but ill hold tight in case anyone else wishes to speak up.


It dosn't matter so much now as I ordered a 4-2-1 from California today, so when the time comes i'll either be selling a manifold and Tullet, or a manifold and OEM.

[The Batman]

it doesnt look oem but looks way to big to be tullet, here is a pic of tullet 200cell downpipe

 

[smiley]

Did i miss a lesson in science class?
How does a stainless pipe get rusted?

 

[aquilaproejct]

Not all stainless is created equal. 😉

[blackoctagon]

Smiley,
Most stainless steels will take a light film of corrosion if they are heat cycled in an oxygen containing atmosphere. There are plenty of grades that stay quite 'metal looking' and a lot that take an oxide layer.

The stainless that OEMs use is often a 400 series and it can look okay for along time, but aftermarket will often use 300 series stainless and they behave differently (they even behave quite differently to each other).

The downpipe is one of those grades. I have access to ways of testing, but i'm not bothered now that i've sorted out where I am going on the exhaust idea.
I might take it to work when it is next off, just to answer the question though.

[martinf]

Thank you for weighing in on this.
I have one count for and one count against.
It certainly does not have the hallmarks of an OEM/mass produced pipe, but I have no point of reference, and it is a Lotus with all that that entails.....
I do believe it is an aftermarket piece, and the previous owner said it was Tullet, but ill hold tight in case anyone else wishes to speak up.


It dosn't matter so much now as I ordered a 4-2-1 from California today, so when the time comes i'll either be selling a manifold and Tullet, or a manifold and OEM.

 

Hi, I'd be interested to know what 4-2-1 you have ordered from the US, would you be able to post a link? Thanks.