Let's talk Exhaust Manifolds.
You'll find in the Diesel Performance Realm, there are several materials used to manufacture exhaust manifolds.
The majority of exhaust manifolds used for Cummins, Powerstroke and Duramax applications are made from a ductile Cast iron that is mixed with silicon molybdenum (SiMo) to increase lifespan while exposed to high heat. The SiMo mixture helps prevent the cast iron from cracking from the expansion and contraction from the constant heating and cooling. From the factory most Cummins, Powerstroke and Duramax engines regardless of year use a ductile cast iron for the exhaust manifold material primarily due to its low cost to manufacture and durability.
One of the short comings of a cast iron exhaust manifold is that they tend to crack over time due to expansion and contraction during heat cycles.
To combat this, many aftermarket exhaust manifold manufacturers (like us, mdcdiesel) make multi piece exhaust manifolds with expansion joints to allow for the manifold to move as it needs to to avoid cracking. Our 2nd gen style exhaust manifolds as well as our 3rd gen style exhaust manifolds for our cummins turbo kits are all multi piece manifolds. We have found these to be far more durable and less susceptible to cracking when compared to cast iron 1 piece manifolds.
Our standard cast iron exhaust manifolds contain a higher SiMo content than all of our competitors making them the least likely to crack amongst all factory and aftermarket cast iron exhaust manifolds.
In the Performance Diesel world however, there are a couple more options other than just cast iron that offer a bit more than ductile iron alone can.
One material seen on single 1 piece exhaust manifolds is Stainless Steel
We get questions with concerns that we chose to cast our High Flow Stainless Exhaust manifolds out of AISI 316 Stainless Steel instead of Machining and welding our manifolds out of a 1018 Steel like our competitors.
Simply put, casting with AISI 316 Stainless Steel creates a superior exhaust manifold in many aspects when compared to a machined and welded 1018 Steel manifold.
Let's start with casting vs machining a billet manifold.
There are several benefits to castings over machined/welded manifolds.
To start with the obvious reasons, it is much more cost efficient and leaves less room for errors in the manufacturing process to cast a manifold meaning you will have a lower probability of a manifold being flawed. We can produce castings of manifolds significantly faster than machining. To machine a manifold requires several operations on several pieces of steel that can take over an hour per operation and is incredibly wasteful in material. Factor in human error in the welding process and you have a much higher chance for something to go wrong.
Right off the bat, casting has a clear advantage when looking to produce a consistent quality exhaust manifold that can be manufactured quick enough to keep up with demand. Something our competitors struggle to do.
We chose to manufacture our exhaust manifolds using AISI 316 Stainless Steel to create a highly durable and corrosion resistant exhaust manifold.
Let's compare our manifolds material to our competitor:
AISI 316 Stainless Steel (ours) Vs 1018 Steel (theirs.
Aside from the obvious corrosion resistance benefits of stainless steel,
Lets focus on the strengths and weaknesses of each material.
Generally, an exhaust manifold is not thought to need much structural strength because all it is doing is funneling exhaust right? WRONG! Not only is an exhaust manifold on a diesel engine funneling super heated, pressurized exhaust, it is also supporting the weight of the turbocharger(s) as well as the weight of the exhaust system all while being bounced around by the movement of the vehicle. Not to mention the strength that it has to maintain under extreme heat!
So lets compare strength properties of AISI 316 Stainless to our competitors 1018 Steel
316 Stainless Ultimate Tensile Strength (UTS): 76,000to 170,000 PSI
1018 Steel Ultimate Tensile Strength(UTS): 63,000 to 70,000 PSI
The 316 Stainless even on its lowest end of the UTS scale is still far superior to the 1018 our competitors manufacture with.
Now lets take the argument a step further:
Metals begin to weaken and change structure under prolonged heat exposure.
This can be measured by the Latent Heat Of Fusion which is measured in Joules Per Gram (j/g)
Compare 316 stainless to 1018 Steel under Heat Stress
316 Stainless Steel: 290 j/g
1018 Steel: 270 j/g
Another Win for Our Cummins 316 Stainless Exhaust Manifolds
Maximum operating temperature: The Temperature where molecular structure begins to change and weaken under prolonged heat exposure.
316 Stainless Steel: 1090F
1018 Steel: 750F
Meaning 316 Stainless takes more heat for a longer period of time before the material's structure begins to change and weaken.
Simply put, 316 Stainless is a better material all around for an exhaust manifold. This does not mean that a welded 1018 manifold does not work, because it certainly does. However the Science does not lie that the 316 stainless exhaust manifolds we produce are of higher quality.