Hi guys. This has been mentioned briefly on here when talking about induction kits I think.
I got thinking about this after J77 spoke of our friend who had his 156 tuned in a big way recently and the "problems" he encountered.
Reading stuff I found on the net, I realised this subject is really interesting (to me anyway).
Basically, big bore exhausts are a no-no as the exhaust gases cool down too quickly. We all know cooler air/gas is more dense, so the engine has to work harder to push the gases out. A narrower exhaust doesn't allow the gas to cool as much and due to the bore size, is forced out more quickly.
This brings me to another interesting point. Exhaust gas is forced out in "pulses", like a blob of gas. The front of the pulse is high pressure, while the end is very low pressure, almost like a vaccuum. The vaccuum basically helps pull the next pulse through and therefore pulls more air etc through the cylinders. If the exhaust bore is too big, the pulse slows and cools which retards this effect.
Not to say we should be driving around with drinking straws on our cars as too much restriction would make the top end suffer, obviously.
Another thing I learned was about exhaust manifold design. Going by the text, the 155 has a very good design with (on the 4 cylinder engines) cylinder 1 and 4 connected and 2 and 3 connected. This helps the engine timing make the pulses all meet at the correct times when the exhaust becomes one pipe. If you look at the manifold (or headers as this design seems to be called), you will notice that the pipes are bent in such a way that they are of equal length (or very close). This again is good design.
If you've ever owned a Mini or Metro (I owned 2 MG's eek!
), you probably will have noticed that the manifold was a lump of iron bolted onto the side of the engine. If you think about this, you will realise that pipes 2 and 3 are much shorter than 1 and 4....bad design.
Another thing I learned was that longer, fatter exhaust headers lend to more low down power and the shorter, thinner ones (as found on the 16v compared to the 8v) give higher revs and more power at these higher revs. Maybe one of the reasons the 16v redlines at 7000rpm instead of the 6300rpm of the 8v.
Not sure what my point is really, but I found this subject quite fascinating when I was reading about it. I think it made me realise that the Alfa intake and exhaust design is in fact very good.
Thanks for reading,