The 16v and 8v have different flanges and are not interchangeable.
The 8v has 2 types of manifolds, one from a 1.5, the bore stays the same size. The 1.7 bore tapers out, leaving room to remove the nuts.
The 16v have 2 types but the same size, one for the cat, which are crossover, unequal lengths and end with a 2 bolt end. The other is 4 into 2 into 1 with a sleeve fit.
I have a 16v ser cat cammed engine. I went from 4-2-1 system to cat system, even with a restrictive cat I found this system to be better, more torque and higher end flow, due to the crossover, even unequal.
There is an equal length 16v manifold available from CSC, only for cat "2 bolt end" but you can also buy a decat centre section, allowing no mods to the manifold to be made.
I have a 2 ¼” SS system from the cat back, I want to buy a CSC manifold eventually.
The 16v will have even less low down torque due to it being 16v, however, again due to how a 16v engine works it will flow more at higher rpm's and out perform the 8v.
The 8v will have more low down torque and will be restrictive at higher rpm, either different cams and or porting can change this but its swings and round abouts as you will probably loos some of the low down torque.
Torque can be increased by a longer stroke, to list only one option, but that’s a whole different discussion, the boxers are knows for their short stroke = quick revving engines.
What about fitting the 16v with carbs, or different ECU, cams become very expensive for this engine.
I think from what you have said you maybe happier with an 8v, cams, porting & K&N air filter. Quick cheap option is to cut the 8v 1.7 manifold where it goes into 1 and weld on the rest of the 16v system, or get a 2 1/4" after that, which is exactly what I had on my 8v, kent fast road cams btw, that thing loved to rev.
K&N air filters will loose you low down torque but give a freer revving engine, you can reduce this by using trumpets. You will gain in induction noise but can loose with engine soak.
I found it best to cut the bottom of the airbox off completely, fit a K&N panel filter and feed cold air to the bottom of the airbox via tubing from either sides of the inner wings.
If you go too large a bore you will loose necessary back pressure.
There used to be a crossover for the 8v made by ansa, “hens teeth” comes to mind, but these were for the 1.5, as in same size bore, not flared out and suffered with low ground clearance.
Hope that helps
Last edited by BigAl33; 08-06-10 at 00:28.