EXHAUST MANIFOLDS
The most important part of your exhaust when it comes to fine tuning your torque curves!
Manifolds are massivly miss understood, fitting just any you find isnt always a good idea, what manifold you choose widley depends on your current engine spec, fitting the wrong manilfold to the wrong engine can have negative effects (ryan (c2vts) can vouch for this) here is why,
Firstly there are two types of manifold, 4-2-1 and 4-1 branch types for 4 cylinder engines. As a rule fitting a 4-2-1 is the most sensable for road use because they are designed to increase torque low down and in the mid range!
the reason why is because on a 4 cylinder engine the first and last cylinder are connected (untampered with by 2 and 4) and this means as the gasses are exspelled down the first header it will pass number 4 cylinder making a big vacume in that header, then when number 4 releases its gasses the gasses will be pulled out causing the scavenge effect again which will give a huge torque increase (gasses dont have to be traveling fast down the header to cause this effect due to there being only 1 other header to cause the effect on). Hence why they give good low-end torque.
The length of each header effects the timing and the strenght of the scavenge affect which inturn effects where in the rev range this effect will be at its best!
A 4-1 manifold however works differently, the scavenge effect is still there but it is massivley reduced in the lower rev range because-
say number 4 has released its gasses down the header and into the collecter where all 4 branches meet, there will still be a vacume caused but it will be split between 3 branches instead of one which means the gasses in number 4 header would have to travel alot lot faster to take effect, this will only ocour in the higher rev range giving increased bhp at high revs.
some race cars etc will use this manifold beacuse of this.
another fact is that the manifold branches should be the same size and shape as the exhaust port they are mated too other wise unwanted turbulance and speed changes will be caused, all aiding the loss of power and torque!
