Some thing that u should keep in mind when changing exhaust manifolds.
MANIFOLDS
Turbocharger exhaust manifolds play a important roll in the performance of a TurboCharger
Sitting between the cylinder head and the turbocharger of ur engine, the exhaust manifold is more then just a peice of metal that connects ur engine to ur turbo to create boost. The exhaust manifold is a carefully
designed item that has its own unique role to play in transferring heat energy in the most efficient method possible directly to ur turbine wheel.
Changing exhaust manifold in the quest for more power and improved response is a modification path thats only been trodden for the past couple of years. The low cost of fabrication in certain Asian countries gas driven the price of manifolds down so an exhaust manifold swap is something that many more enthusiasts now consider.
There are plenty of tricks and pitfalls when it comes to selecting an upgraded turbocharger manifold however before you go parting with ur hard-earned money you need to know the basics and judge your selection
MATERIAL
Forget about design for a moment as its the material used in turbocharger exhaust manifolds that has the biggest bearing on whether the manifold will be successful or not. living in a unique high temperature environment and capable of rapidly heating up from ambient through to red hot its the exhaust manifold material that takes the biggest buffeting from a durbility viewpoint.
The very best manifold for a turbocharger exhaust manifold is cast iron. Its be en proven time and time over the years and evidenced by the fact that every single turbocharged vehicle manufacturer cast their ehxuat manifolds from iron/steel blends. Another reason for this is that the high production lines that manufacturers undertake also makes casting the manifolds the cheapest and simplest form of production. Cast manifolds are a win on both hands.
Coming in close second is wat is referred to as "steam pipe". These small cast bends and straight sections were originally made for high pressure pipeline industry and come with a neat little ground edge on each end for neat and eazy fabrication.
Being extremely thick walled cast pipe, steam pipe isnt easily affected by heat from welding bends together and once cleaned up on a sanding belt can be heat coated or high temperature painted to look fairly decent. designed to withstand temperatures of 600 degrees celsius on a continouous basis and anything up to 800 degrees it is rare to ever see a steam pipe manifold fall apart or crack due to heat unless its due to poor fabrication and poor welding process. TIG welded properly you will most likely never see a problem with a steam pipe manifold
Stainless steel manifolds although looking good to the naked eye no where near have the strength and durability of cast iron or steam pipe manifolds
then why bother making exhaust manifolds out of stainless steel. the answear is simple: looks sell, a manifold which looks the best must be the best? a properly designed and welded manifold made of Schedule 10, 3mm thick 316 stainless steel will go the distance lasting many years while still looking ok, but this steel has none of the temperature ratings as cast iron and steam pipe.
Rated at a continuous heat of 400 derees and intermittent handeling up to 600 degrees an improperly tuned engine can push the limits of stainless steel to breaking points causing it to crack
DESIGN
For a Daily driven street car where response is most important then oversall power, the shorter the exhuast manifold runers the greater the response and torque the engine will generate. But by making all the runners of equal length will allow all the cylinders to breathe equally therefore less restriction.Reason is retaining heat in the exhaust gas. if heat is losted B4 the gas reaches the turbo the gas contracts and is less effective in pushing the turbine wheel. The thicker and less heat-conductive the material used in the exhaust manifold the greater the potential energy to drive the turbo charger henve the more power you will end up making. Another way of keeping the heat in the system is to ensure that all runners going to the turbine housing as short as possible which gives less potential loss of heat. Any time you see an exhaust manifold glow from heat think of it as a lost opportunity to make power.
So why do some of the most highest performance turbocharger kits come with long sweeping tuned length runners? this comes down to potential management of back pressure. The turbine housing is the biggest restriction in the exhaust system. Regardless of the manifold design the exhaust gas will have to enter the turbine housing threw the turbo flange which is a small hole, hit the turbine wheel/blades and then flow all around the turbine housing. The size of the housinghas very big influence on back pressure.
Cast iron which doesnt look to flash but is king while some of the highly polished stainless steel manifolds have inferior strength and performance. ALL OF THESE POINTS TO SWAPING A TURBOCHARGER EXHAUST MANIFOLD FROM THE CAST IRON ITEM TO SOMETHING FROM AN AFTERMARKET AS A DEFINITE BACKWARDS STEP IF YOU ARE KEEPING THE STANDARD TURBO.