Aurelia exhaust manifolds

[chriswgawne]

I hesitate bringing this up as it seems a little pedantic and dry but I have a genuine issue which I hope there is a solution to.
My problem - self created of course - is that whilst i have rebuilt the original engine and transaxle for my B21S project with new components etc, I am concerned that once I am on the road, I will inevitably compare the 'drive' to that of my B12 which I find to be perfect.  The 2.3 litre B12 is great  whether on short or long journeys on local or trunk roads but will the 2 litre B21 be good enough? .
I have a spare B12 engine which just needs re-assembling, a spare B12 transaxle which is in good order and a set of 3rd srs B20 front brake backplates, drums etc so potentially I can almost turn the B21S into a B12.
The only part of this conversion I have any doubt or  problem with really is establishing what is the best combination of exhaust manifold + front downpipe which fits the cylinder head and also connects to the centre section correctly? Ideally I would like to be able to change engine without changing the downpipe.
It seems to me that there are basically just 2 types of exhaust manifold  - lets call them  'small' & 'large '. The 'small' version appeared first on B10 and I believe that all of them had a 2 bolt 'manifold to downpipe' flange.
The 'large' version first appeared I believe on B22 and 3rd srs B20 with a 3 bolt 'manifold to downpipe' flange and then subsequently a 4 bolt 'manifold to downpipe flange' appeared on 4th, 5th & 6th srs B20, B24 etc.
The 'small' manifold has smaller i/d  ports (28mm at the head and 34mm at the downpipe flange) whereas the 'large' manifold has 34mm ports at the head and 38mm at the downpipe flange. The manifold inlet centres are 122mm for all variants but the inlet stud centres vary from 56mm for the 'small' to 62mm  for the 'large'. On the 'large' manifold the downpipe flange is about 55mm nearer the ground than that of the 'small' manifold so the downpipes are very different..
IMASAF dont list a system for B21 but as the OE B21 exhaust manifolds are of the 'small' type I believe the original system used 38mm o/d pipe for the centre and rear section.
For my IRS B21S, from choice for the main underfloor party of the system I would much prefer the larger 45mm dia o/d centre and rear sections as  fitted to the IRS B22 (according to IMASAF.  The B22 is by far the most powerful 2 litre Aurelia variant) particularly if I fit a 2.3l B12 engine. But the B22 appears to use a 'large' 3 bolt manifold as does my B12.
Writing this it seems that I will have to order a B22 system complete and then for the B21 engine get a downpipe made especially with a 2 bolt flange but  to connect to a 45mm centre section.
Can anyone help me with advice please?
Chris

[williamcorke]

Hi Chris,

I think your logic is correct and you might have to have a special downpipe made.

My B10 with B12 engine uses an IMASAF B22 exhaust. 3-bolt flanges. I've not measured the downpipe diameter, but can do so if that's helpful.

Best, William

[Sebastien]

Chris,

My Aurelia berlina B12 has the standard 3 bolt (B12) manifold.
My Aurelia cabriolet (B50 with fitted B12 engine) has the original 2 bolt (B10) manifold.
Both have kind of non-standard exhaust lines - long stories!
Both perform remarkably well, with their B12 engines (unmodified). Behaviour is slightly different, but difficult to quantify.

Cannot say much more, as I am away from the cars at the moment, and cannot measure the size of the downpipes....

[chriswgawne]

Hi Chris,

I think your logic is correct and you might have to have a special downpipe made.

My B10 with B12 engine uses an IMASAF B22 exhaust. 3-bolt flanges. I've not measured the downpipe diameter, but can do so if that's helpful.

Best, William

Ah, I had forgotten you had a B12 engine in your B10 William. You have confirmed a minor point in that all along I have assumed the mounting points on the body for the underfloor sections are the same for B10 & B22 ( & presumably B21 as well).
It would be helpful if you could measure the downpipe o/d and also the straight section under the floor please when you have a minute.
Many thanks
Chris

[chriswgawne]

Chris,

My Aurelia berlina B12 has the standard 3 bolt (B12) manifold.
My Aurelia cabriolet (B50 with fitted B12 engine) has the original 2 bolt (B10) manifold.
Both have kind of non-standard exhaust lines - long stories!
Both perform remarkably well, with their B12 engines (unmodified). Behaviour is slightly different, but difficult to quantify.

Cannot say much more, as I am away from the cars at the moment, and cannot measure the size of the downpipes....

And another part of the equation drops into place thanks Sebastien in that the B10 2 bolt manifold fits the B12 engine. Thank you.
Chris

[chriswgawne]

Hi Chris,

I think your logic is correct and you might have to have a special downpipe made.

My B10 with B12 engine uses an IMASAF B22 exhaust. 3-bolt flanges. I've not measured the downpipe diameter, but can do so if that's helpful.

Best, William

Do you happen to know what stamping is on your exhaust manifolds William? Are they B12 or maybe B22 ( if they are not the same).
Chris

[chriswgawne]

Hi Chris,

I think your logic is correct and you might have to have a special downpipe made.

My B10 with B12 engine uses an IMASAF B22 exhaust. 3-bolt flanges. I've not measured the downpipe diameter, but can do so if that's helpful.

Best, William

Hello William,
Could you possibly measure the downpipe diameter please? And also if visible, what markings are there on your exhaust manifolds please?
I 'think' B!2, B22 and early 3rd srs B20 all used the same manifold with a 3 bolt flange but I may well be wrong.
Thanks in anticipation of your help.
Chris

[williamcorke]

Hi Chris,

Sorry I didn't see your earlier request on 7th July - perhaps email me another time as well as I don't necessarily see every forum post.

I tried to measure the downpipe just below the manifold and got readings between 41 and 43mm. Access isn't the easiest. I can't see any markings on the manifolds. On my shelf there are a pair of 3-bolt manifolds with B20 markings. Perhaps from my S3 B20? Not sure where they came from...

Let me know if you want more measurements on the B12 engine / exhaust, or the 3-bolt B20 manifolds.

Best, W