Has anybody tried fitting a 156 manifold to a 147 1.6L. I was thinking of doing this convertion with a slight increase in diameter on the overall pipes. Maybe a increase of 0.15inch and a long pipe connecting to the 2 to 1 pipes. Please advise
The diameter of the header pipes is crucial. The bigger the pipes the more gas volume they can shift but if they're too big the gas speed is reduced (think about the difference if you blow through a straw and then through a drainpipe).
Reduced gas speed means that at low rpm the gas won't have enougn energy to leave the combustion area completely .. which means poor cylinder filling on the next cycle. You car will suffer a loss of pickup ("grunt") until it all starts working at higher rpm.
The length of the pipes and how they are joined is also crucial as the valves opening and closing sets up pressure waves in the exhaust that are reflected back from the first point where the exhaust "opens" into a space - usually the primary silencer.
The wavelength of the wave depends on the rpm. At high rpm the wave is shorter, so a short header pipe suits higher rpm. At a "peak efficiency rpm" point the wave is exactly the same length as the header pipe so when the exhaust valve opens there's a nice negative pressure point sitting right behind it that will suck that exhaust gas out. Nice one!
At different (i.e low rpm) the wave is longer and instead of coinciding at the valve, the low pressure point is reflected back down the exhaust.. so it's not where it's doing the most good.
So at low rpm a longer header pipe is what you want.
Obviously the manufacturer works out what rpms they want the engine to be most efficient at (e.g 2/3 max rpm) and they modify the header pipe sizes and length to be "spot on" at that rpm.
If you want more top-end efficiency (at the expense of tractibility) you should fit slightly bigger and shorter header pipes. If you want to crooooze.. then narrow, longer pipes are where it's at.
To complicate things, the way the pipes are joined to each other also affects scavenging. Ideally you want 4 pipes and 4 silencers.. but manufacturers don't like the cost/weight so they blend the pipes together.
If they're a bit cheapskate they'll join all four pipes into a single collector. That does the job and it saves weight and cost.
If you wanted to be more classy then you may want to join the pipes into pairs before joining the two bigger pipes into one collector. Depending on the firing order, you would siamesed TDC1 with BDC2 (1 with 3, and 2 with 4 ) so that the wave set up by one cylinder assists it's "twin" cylinder in its next operation (or rather doesn't interfere with it so much).
Exhuasts are quite complex.. If you want to get less back pressure, I'd start at the back box (or get rid of the middle silencer).. a lot less hassle and less chance to upset the way the exhaust is doing its thing.
I've been thinking of modifying the exhaust from the backbox forward with the goal of having a well-tuned overall system. It's hard trying to find a good mix of low noise, top-end power and torque; no wonder manufacturers spend so much time setting up their exhaust systems.
By changing to a less restrictive backbox or removing the middle silencer, wouldn't it shift the powerband further up? Or it backpressure really necessary for low-end torque?
After the gas has passed through the first silencer the back-pressure effect that helps the inlet process has largely done it's thing.
Really then you want to get rid of the gas as quickly as possible so that it doesn't get in the way of the next bit of gas trying to get out.
Most silencers contain a lot of interlinked (by metal tubes) chambers.. the chambers are there to damp out the noise but obviously the mouth of each tube becomes a pressure obstacle to the gas leaving each chamber.
A free-flowing ("staight-through") silencer is where it's at. It provides less (practically none) resistance to all the gas being extracted.. but obviously it's a tad noisy.
If you take out the middle silencer that will have more effect on the breathing than fitting a lairy back-box. Keep the same diameter pipe into the rear silencer (to keep gas speed up) but then after the rear silencer, go as big as you like. You can't do much to influence things that far back.