EQUAL LENGTH tubular vs. LOG manifolds (headers) - SCAVENGING and EXHAUST BACK PRESSURE EXPLAINED

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In today's video we will be talking about exhaust manifolds or headers. First we will explain what exhaust manifolds do and then we will compare OEM short cast manifolds and equal length tubular welded aftermarket ones, like my awesome turbo manifold from Weldspeed right here, to see how different designs and materials influence the performance of your engine.

So as the name very obviously suggests the exhaust manifold has the very simple task of providing a pathway for exhaust gasses. They connect your cylinder head to the rest of your vehicle's exhaust system. An exhaust manifold is always bolted directly to the cylinder head of your engine and when the exhaust gasses exist from the exhaust port the exhaust manifold is the first thing they see. So obviously the exhaust manifold doesn't actually DO anything, it's a passive part, or a collection of pathways through which exhaust gasses flow, but despite it's passive nature, the design of the exhaust manifold can play a very important part in the p
Now exhaust manifolds will be very different on naturally aspirated and turbocharged engines. This is obviously due to the fact that a turbocharged engine will have a turbo bolted to the exhaust manifold and then the rest of the exhaust system will be bolted to the turbine housing of the turbocharger. On a naturally aspirated engine the exhaust manifold will be connected directly to the rest of the exhaust system, without a turbocharger in between.

Now the design of the exhaust manifold mainly influences 2 things: Scavenging and exhaust back pressure and these two things then influence the power, torque, responsiveness and efficiency of the engine.

So what is exhaust Scavenging? In the simplest possible terms exhaust Scavenging is using the exiting of the exhaust gasses to ease the entry of the intake air into the engine to improve performance.

But scavenging can not occur the entire time the engine is running. It's heavily dependent on the camshaft specs of your engine but most of all it is determined by your exhaust manifold.

Now the scavenging effect is obviously very important for naturally aspirated engines because they depend on the pressure of the atmosphere to get air into the chambers. On a well tuned engine the negative pressure wave can decrease the chamber pressure by as much as 7 psi at a relatively narrow rpm band. On the other hand turbocharged engines don't really depend on the scavenging effect because they can add one or two atmospheres of additional pressure to the intake air over a broad rpm range which greatly improves cylinder filling as soon as the intake valve starts to open resulting in dramatic power increase. Another factor is valve overlap. Although valve overlap isn't necessarily all bad for turbocharged engines having too much of it is counter-productive because you're essentially wasting the work of the turbocharger. By having the intake and exhaust valve open at the same time for too long you're actually sending valuable pressurized intake air into the exhaust which means that you're wasting boost.

Now let's look at our cast and our tubular turbo manifold to see exactly how this play out in practice. Probably the first thing you will notice is the difference in length of the runners. The cast manifold has much shorter runners and in addition to this the runners are of unequal length which increases back pressure.

To understand how runner length influences back pressure we must understand that pressure inside the manifold spikes every time an exhaust valve opens. By making the runner longer we are enabling a more free flowing form of the runner and by making all the runners of equal length we ensure that each exhaust pulse takes an equal amount of time to reach the turbocharger.

The final difference between the two manifolds is the material itself. OEM cast manifolds are usually cast using nodular iron and most will have trouble resisting temperatures beyond 850 degrees Celsius for prolonged periods of time. Tubular aftermarket manifolds usually employ stainless steel. 304 stainless steel is a good and common choice while 321 stainless steel is an even better choice.

Stainless steel welded manifolds also have much smoother internal surface compared to the rough surface of cast iron. This of course helps increase gas speed and reduce back pressure.

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