the Turbo Exhaust Manifold

originally posted on astand.com.au 8 February 2023 by Dave, moved here 31 March 2024

In addition to correctly aligning and supporting the turbocharger the basic premise of a turbocharger exhaust manifold is to carry the exhaust gas (and its untapped energy) from the engine to the turbine inlet. Simple right?

A turbocharger exhaust manifold requires a good balance between the desired power delivery band, durability, and cost. Aesthetics may also be an important consideration.

The exhaust manifold runners are passageways through which exhaust gases travel at a significant proportion of the speed of sound, and design factors such as their internal diameter and length can greatly affect the turbocharger (and therefore engine) performance characteristics.

Fundamentally we are recovering energy from the exhaust gas to power the turbocharger. The punch in the exhaust gas needs to be preserved in order to spool the turbo efficiently and consists of three closely interrelated components: temperature, pressure and velocity. These three forms of exhaust gas energy may be interchanged from one form to another as is the case in the turbine housing where the volute carefully trades off temperature and pressure for an increase in velocity (mechanical energy) to drive the turbine wheel.

Runner Tube Diameter

When it comes to optimising your turbocharged internal combustion engine for maximum performance, choosing the right size turbo, turbine housing and exhaust manifold is crucial. The primary size of a turbo manifold refers to the internal diameter of the individual cylinder runners that carry exhaust gases from the engine to the turbocharger.

A larger primary diameter can allow for more exhaust gas flow and potentially increase manifold boost pressure, resulting in improved performance. However, it's important to note that a larger exhaust primary size can also cause increased turbo lag, as the turbocharger will require a higher engine RPM to build up sufficient turbocharger shaft RPM to reach desired boost levels.

On the other hand, a smaller primary size can reduce turbo lag and potentially improve low-end torque, but it may also limit the maximum boost pressure and overall performance of the engine.

So, how do you choose the right primary size for your turbo manifold? It's important to consider a number of factors, including the size of your engine, the boost level you're aiming for, and the type of driving you'll be doing. For example, if you have a high output engine and are aiming for high boost levels, a larger primary size may be more suitable. On the other hand, if you have a smaller engine and are mostly driving in the city, a smaller primary size may be a better fit.

Choosing the right primary size for your turbo manifold requires a careful balance of several factors. By considering the size of your engine, power goals, and driving needs, you can select a primary size that will help optimise the performance of your turbocharged engine.

As a general rule exhaust runner diameter should be of the same internal diameter as the exhaust port where it exits the head. Exhaust gas velocity is very important for turbos which implies relatively small and well flowing exhaust primary diameters. A larger diameter primary will hurt exhaust gas velocity.

A larger diameter will also reduce the cylinder scavenging effect when the exhaust valve closes. Both of these result in an increase in exhaust gas residual which is bad for horsepower due to less room available in the combustion chamber for the next charge, heating of the inlet charge and creating an increased tendency for knock or abnormal combustion.

In certain cases a larger runner diameter may increase high RPM output and because of this it can be tempting to opt for a larger diameter exhaust manifold runner. However, my advice is: if you are unsure, select the smaller diameter if you have a choice - especially for a daily.

Sharp turns and sudden variations in runner diameter need to be avoided in order to maintain the energy in the exhaust gas.

Pipe Size and Wall Thickness

Pipe sizes are based on a historical “nominal bore” specification originally set to give the same “nominal” inside diameter (ID) based on wall thickness standards back in the day.

Nominal bore pipe, also known as "nominal pipe size" (NPS), refers to the size of pipes used in a piping system. The NPS is the "nominal" or "name" size of the pipe, and it does not refer to the actual outside or inside diameter of the pipe. Instead, it is a standardised size that was used to identify the pipe and its compatibility with other components in the system, such as fittings and valves.

As the wall thicknesses evolved over time with better metallurgy and manufacturing process the wall thickness changed and therefore NPS became only indirectly related to ID and OD.

A set of interrelated standards and “schedules” emerged over the years helping to clear up the mess. A table needs to be referred to find the actual ID and OD for any specific size of pipe.

Therefore, in order to determine the actual ID and OD of a pipe with a specific “nominal bore”, it is necessary to refer to a table or consult the pipe manufacturer's technical specifications. Such tables can be found in pipe standards such as ASME/ANSI B36.10 and B36.19. A simplified table is listed below with common turbo manifold sizes.

Pipe (or steam pipe as it is sometimes affectionately referred to) needs to be specified in both “nominal bore” and “schedule” i.e. wall thickness. Common Schedules for a turbo manifold are Schedules 5, 10, 30, and 40.

How thick should turbo exhaust manifold plumbing be? Mild steel needs to be thicker than either stainless steel or exotic metals such as titanium or Inconel.

Runner Length

The longer the manifold runner length, the greater the volume available to be filled by the exhaust gas pulse. With the exhaust gases being shared between the cylinder volume and the exhaust runner volume, a longer runner will result in a lower cylinder pressure at the end of the exhaust stroke. Obviously this is very beneficial to the engine and is one of the main techniques to achieve a positive pressure ratio across the inlet and exhaust of a turbocharged engine. Longer runner lengths are very healthy for an internal combustion engine.

A long runner manifold will raise the rpm powerband for a given setup similar to a larger A/R turbine housing and have the effect of narrowing the powerband. Caveat here that the shorter manifold being compared is not causing massive reversion and scavenging issues. A long runner manifold, just like a larger A/R turbine housing, will result in less flow through the wastegate for the same overall power output. Any wastegate flow by definition is inefficient use of the exhaust gas, however the trade-off is a wider rpm powerband.

Shorter length tube header manifolds, with their smaller total runner volume are excellent at transferring the pressure energy of the exhaust to the turbine. This allows the turbo to spool quicker than a long runner length manifold, and in some cases thousand’s of RPM sooner. This makes for a great responsive driving experience. Unfortunately this higher pressure also exists inside the cylinder pressing down on a piston that is trying to move up the bore and costing crankshaft power. Short length runners also cause a higher exhaust residual or in other words causes exhaust gases to not completely evacuate the cylinder.

Short tube headers will help provide a fast responsive turbo, while a long tube header just like a larger A/R turbine housing will move your horsepower curve right, and allow you to run a higher compression ratio, more ignition timing advance, and have higher exhaust flow.

It's worth noting that engine knock is a complex phenomenon and it can be caused by several factors other than the exhaust manifold runner length. Some of these factors are:

• boost pressure
• ignition timing
• ignition energy
• camshaft timing
• RPM
• fuel octane
• air fuel ratio
• compression ratio
• combustion chamber shape
• spark plug location(s)
• combustion chamber hot spots
• charge mixture homogeneity
• engine temperature
• intake air temperature
• atmospheric pressure
• oil control

So to recap, runner length is a compromise between the responsiveness of the turbo and the compression or ignition timing you will be able to run without causing end gas detonation or knock. In other words a design trade off between responsiveness and maximum power potential.

Equal Runner Lengths

It’s not often mentioned as to why equal length exhaust manifold runners are a good thing.

Besides a higher output potential because all cylinders are singing together in harmony, tuning simplicity is probably the biggest gain with equal runner lengths. If individual cylinders have different characteristics at different RPM points due to things like unequal length exhaust tube runners then individual cylinder tuning may be required to achieve the most from the engine package. Individual cylinder tuning is very expensive, not much fun and quite a low return in horsepower gains for the effort invested. The alternative to individual cylinder tuning is a less than optimum tune on all other cylinders whilst catering for a safe tune on the worst performing or most knock prone cylinder where the knock limit of the poorest performing cylinder dictates the engine's overall fuelling and ignition timing.

With equal length runners the exhaust pulses will be arriving at the turbine wheel at equal intervals according to the engine firing order. If they arrive sooner or later due to unequal length runners, they may interfere with the exhaust pulses from the next firing cylinder.

Dual Entry Turbine Housing

Also known as twin scroll. A single scroll turbine housing is where all the exhaust runners come together in a single common collector as they enter the turbocharger.

As the name suggests a twin scroll manifold has two entries into the turbine housing with an equal number of cylinders each. The manifold for a twin scroll turbo is arranged such that the cylinders fire into each half alternately, so there is separation between consecutive exhaust stroke events. This keeps the exhaust gas velocity event driving the turbine much higher than a single entry turbine and also greatly assists with cylinder scavenging because there is no interference from adjacent cylinder firing order events.

Ideally dual wastegates are employed with a twin scroll setup to maintain separation of each consecutive exhaust puff.

There is a very slight trade-off with overall maximum turbine efficiency as there is more scroll surface area friction.

Bends

Each 90 degree of bend in a tube is said to incur about a 2-3% pressure drop penalty and also affects pressure wave tuning potential.

Material

Cast Iron: OEM’s have different design goals compared to an enthusiast or race tuner. Cast iron is a terrific material choice for exhaust manifolds having excellent durability and is very low cost when mass produced.

Mild Steel: Don’t underestimate the benefits of mild steel. Mild steel has excellent durability and a low likelihood of warping and or cracking. Mild steel is low cost and is also very easy to work with and fabricate.

Stainless Steel: Whilst stainless steel is a popular choice for turbo exhaust manifolds due to its durability, excellent heat properties and corrosion resistance, it can be prone to cracking.

A property of stainless steel that is not often discussed is its extremely poor thermal conductivity. That’s actually a really great property to have if you want to keep heat inside your turbo manifold and deliver all of that waste heat energy from the Otto cycle exhaust stroke to drive the turbine.

Stainless steel is more difficult to weld and requires back purging to avoid problems with the weld on the inside of the pipe.

The L in the grade of stainless steel such as 304L indicates a low carbon content which helps reduce the tendency for cracking. 321 stainless steel is basically derived from 304 stainless steel. 321 is stabilised with a very small addition of Titanium to provide resistance to intergranular corrosion which can promote cracking. 321 stainless steel has advantages in a high temperature environment and is an excellent choice for many turbo applications.

A thinner walled manifold can be constructed from 321 stainless shaving a couple of kgs without sacrifices in strength or longevity.

Typical chemical composition of 304, 304L & 321 stainless steels:

Image from Wikimedia Commons "Intergranular Corrosion":

Titanium: Titanium alloys are lightweight and corrosion resistant exotic metals, however Titanium is not recommended for turbo manifold applications, at high exhaust gas temperatures Titanium isn't really that strong.

Inconel: Inconel is a fantastic material to use for high temperature exhaust manifold work but is very pricey. It has outstanding corrosion resistance and thermal fatigue properties and therefore copes with heat far better than stainless steel and titanium. It is also stronger at temperature than stainless steel or titanium. Inconel exhaust manifolds can be made even lighter than Titanium by using thin wall tubing without risk of cracking because Inconel is so much more heat and corrosion resistant.

Collector Merge Angle

A 12 to 15 degree convergent angle in a collector typically provides the best velocity and performance gains. However, when space limitations are a constraint, a 20 to 25 degree angle is often used instead.

Wastegate Arrangement

The wastegate system needs to have good flow from all of the exhaust runners to allow good boost control boost and avoid boost creep.

All blogs are copyrighted. No reposting, plagiarising or other forms of flattery is permitted.