Gen3 Raptor Issues: Intake Collapse

Whenever tuning a vehicle, it is always important to understand the limitations that certain factory parts may pose.  We can frequently achieve reliable power and torque gains over the stock calibration without installing any physical modifications, but when developing our performance calibrations for the Gen3 Raptor, we noticed some interesting issues with the intake system after only adding a small amount of power.

As we began to increase airflow and boost pressure targets, we saw an intermittent loss of power on the dyno – sometimes in the ballpark of 10-20WHP, in other cases as much as 70-100WHP!  Boost pressure was down, wastegate command was up, and along with additional wastegate position came significant increases in exhaust manifold pressure.  We initially suspected a poor fitting intercooler coupling or charge pipe, but smoke testing the intake tract of our 2021 Raptor showed no leaks.  It was only after doing back-to-back pulls on the dyno with the hood open and hood closes that we narrowed in on the culprit.

Airflow Through the Intake System

Before even passing through the filter, airflow entering the engine must navigate a few obstacles.  Air will flow through the upper grille, past the flexible plastic radiator shroud trim piece, through the snorkel and into the lower airbox.  Once through the filter and out of the upper airbox, airflow is split to feed each turbo on either side of the engine. 

[Edited image showing airflow through the cross section of grille/shroud/snorkel]

At factory power and boost levels, this system works fine.  But when we crank up the boost, the snorkel and radiator shroud trim pieces can become big problems.

The Snorkel

The leading edge of the snorkel that rises from the lower airbox to overtop of the radiator core support is constructed of very soft, very thin rubber.  Making things worse, a significant portion of the cross sectional area of this piece seals against the hood of the truck, making the actual airflow path pretty thin.  Here we sprayed the inside of the hood with a contact indicator – you can see around the outer edge where the snorkel rubber seals against the hood.

When airflow and boost targets are raised, the wastegate closes to force more exhaust gas flow through the turbine of the turbocharger to increase shaft speed and the speed of the compressor wheel.  This extra speed on the compressor will drive up boost pressure, but will also decrease compressor inlet pressure as more air is sucked into the turbo through a relatively long and restrictive intake system.  This decrease in pressure will work all the way up into the airbox, past the filter, and into the snorkel.  The soft rubber material on the snorkel is not stiff enough to avoid deforming under negative pressure and will begin to actually pinch off the area that air can flow into the airbox!  As engine bay temperatures increase during hard driving and/or hot summer weather, the part becomes more malleable and the collapse becomes even more extreme.  Check out the footage we captured of the snorkel collapsing in the video below.

The clips for the snorkel are also a potential weak point. If the intake system has never been disassembled and these “Christmas-tree clips” have not been pulled and reinstalled, the snorkel is held in place reasonably well.  However, in extreme conditions they can lift and allow the snorkel to collapse further.  When pushing our development vehicle particularly hard, the snorkel collapsed enough to fracture the hard plastic supports within the snorkel!

This is where the COBB Intake scoop bracket comes in.

This bracket uses two pieces to sandwich the soft plastic snorkel to the core support, replacing the fragile plastic clips with hardware and preventing all deformation under load.  Follow the link below to the install instructions – all you need is a flathead screwdriver to pop the old clips out and the provided Allen key to secure the hardware.

installed snorkel Bracket

The Radiator Shroud Trim Piece

Even with the snorkel secured using our bracket kit, we still found there was a measurable restriction in the airflow path.  As air passes through the grille, it has to make a tight bend around the radiator shroud trim piece.  This material is similarly thing and flexible to the mouth of the snorkel, and at high load, was found to deform and partially block off airflow.  We unfortunately could not find a way to capture this movement on camera, but testing on the dyno led us to a few different solutions.

We have seen two different radiator shroud designs from the factory on Gen3 Raptors and F150s.  One shroud poses a performance disadvantage, and one is fine to use without removing or zip-tieing.

Solution 1: Zipties add Horsepower!

The soft shroud can be drileld in order to allow a zip tie to hold the shroud down to the grille structure hidden below.

Solution 2: Turn it into a Lotus, “Simplify then add lightness”

The easiest solution is to simply remove the shroud trim piece.  It is secured with a handful of trim clips that can be popped out using a flathead screwdriver. 

COBB Shroud

Here is a sneak peak of a part we have in development to provide a solution to the performance limitation of the factory shroud!

The Data

These three dyno pulls come from the same truck, in the same condition, running the same tune.  In red, we can see a healthy pull with a nice power curve.  In green, we see a pull that starts strong but tapers towards redline.  In blue, we see a pull that hits a wall and loses a ton of power.  This difference is entirely due to the intake and shroud collapsing and restricting airflow.

Airbox Pressure

When developing calibrations and parts for the vehicles we support, we frequently find the need to add additional sensors (pressure, temperature, etc.) in order to better understand the limitations and needs of the mechanical system.  For this investigation, one sensor that proved to be very valuable was a pressure sensor in the lower airbox, pre-filter which allowed us to see a shift in the pressure of the airbox as things closed up.

Hood Open

This represents the best-case scenario for the intake system – the snorkel can pull air out of the full cross-sectional area without obstruction from the hood, and without the need to pull air through the grille and around the shroud.  From a baseline of ambient barometric pressure of approximately 14.65psi, we saw a maximum drop of 0.24psi to 14.41psi during a WOT pull.  While this is a very small drop in airbox pressure, it still does show that the fundamental design of the snorkel and lower airbox represents a small performance restriction.

Hood Closed

The difference here is massive – airbox pressure reaches a maximum depression of over 1.5psi below barometric and 1.35psi below the hood-open scenario, bottoming out at 13.06psi!  As a reminder, this is airbox pressure Before the filter!  Power loss on this run was about 70WHP.  In subsequent tests during the product development cycle, we saw even larger pressure drops in the airbox to ~12.8psi, which accounted for power losses of around 100WHP.

Zip-Tied

With the installation of the COBB Intake Scoop Bracket and the shroud zip-tied, we see an improvement in airbox pressure of 1.0psi relative to the unsecured example above, clocking in at 14.06psi, or 0.59psi below barometric pressure and just 0.35psi below the ideal open-hood scenario.  A huge improvement that picked up nearly all of the lost power, coming up about 5WHP short of the open-hood run.  There is no escaping the fact that the airflow path with the hood closed is not terrific, but the performance improvement offered by these simple, cheap, and easy to install parts is substantial.

Effect of Airbox Pressure on Compressor Pressure Ratio

Through product development on our supported Raptor and F150 EcoBoost platforms we have partnered with Garrett to reverse engineer compressor efficiency data for the various factory turbochargers.  Combined with measurement of compressor inlet and outlet pressures using external dataloggers, we’re able to understand more about when to push the turbos harder, and when to back off.  We’re also able to compare measured data to the factory ECU’s modeled compressor efficiency to create optimized calibrations and perfect parts.

A compressor map uses axes of compressor pressure ratio (outlet pressure/inlet pressure) and airflow mass.  Testing of the turbocharger in a sensored lab-environment allows for measurement of turbo shaft speed and calculation efficiency islands on the plot of the compressor map.  While we are keeping this compressor data internal, the general principles still apply.  For a fixed compressor outlet pressure, lowering compressor inlet pressure increases the pressure ratio, which increases shaft speed.

With a properly functioning intake system, we have found that compressor inlet pressure on our OTS maps will reach between 1.0-1.8psi below barometric pressure, depending on ambient conditions.  At higher elevations (lower barometric pressure) and high ambient air temperatures (low air density), inlet pressure will be lower as the turbos work harder to hit torque and airmass targets.  At sea level and cool temperatures, inlet pressure will be higher as the turbos are able to hit torque and airmass targets more easily.  In all conditions, we carefully adjust associated limiters in the calibration to ensure safe power gains that do not overspeed and damage the shaft, bearings, or wheels of the turbocharger.

Understanding this, the importance of the Intake Scoop Bracket and shroud relate not only to performance but reliability as well.  Obstructing airflow into the airbox will have a compounding effect on compressor inlet pressure and can, in extreme examples, double or more the pressure depression seen at the compressor inlet.  This worsening of pressure ratio will drive up turbo speed to potentially dangerous levels as the ECU tries to hit torque and airflow targets.

Wastegate Position and Exhaust Manifold Pressure

Exhaust manifold pressure, or EMAP, is often overlooked when understanding the performance and calibration of a turbocharged engine.  With the wastegate open, exhaust gas can either pass through it or through the turbine.  With the wastegate closed, exhaust gas is forced to go through the turbine as its only way out of the engine.  Not only does this decrease in flow routes for exhaust gas increase EMAP, it also drives up shaft speed, compressor speed, and airflow through the engine to compound the increase in EMAP.  While EMAP is valuable for driving quick boost response, it is the calibrator’s job to establish a safe balance between intake MAP and EMAP, as EMAP does represent not only a pumping efficiency loss but also reliability concerns as well.

Since the introduction of the Gen2 Raptor 3.5HO and F150 3.5 and continuing into the Gen3 3.5HO and 3.5, Ford has used electronic wastegate actuators.  These actuators replace the traditional pneumatic style wastegate control that used a combination of boost control solenoid(s), pressure reference lines, and wastegate springs and diaphragms.  The net effect is very precise control of boost pressure, but also the potential to create extreme EMAP if the calibrator is not careful.  Whereas extreme high EMAP on a pneumatic style wastegate may be enough to push against the spring and diaphragm pressure to “blow open” the wastegate, the electric motors that control wastegate position in an electronically controlled setup often have much higher holding capacity.  As a result, extremely high EMAP may be encountered if too much wastegate position is used.

The basic framework of the factory boost control logic in the ECU will initially target a torque value based on engine speed and throttle position.  This torque value is translated into an airmass target via the torque model, which after passing through the ECU’s airflow model is transformed into a manifold pressure target.  Turbo modeling is used to calculate the required compressor power to meet the manifold pressure target, which is then translated into a commanded wastegate position.  If torque, airflow, and pressure targets are not met or are exceeded, the boost error system will either increase or decrease commanded wastegate position.  In situations where airflow obstructions occur – like with the snorkel or shroud – boost error adjustments to wastegate position can increase too far and drive up EMAP.  Ford’s turbo modeling also provides us with the helpful tool of modeled EMAP, which we are able to datalog and compare to external sensor data.

Let’s revisit the dyno chart from above and look at the ECU datalogs from these runs.  Again, these are all on the same car, same conditions, and same calibration.

Run1 (Blue) – Snorkel unsecured, shroud unsecured

The dyno plot tells the story clearly – as the deformation of the snorkel and shroud increase through the pull, they hit a critical point of massive obstruction and massive power loss.  We can see in the datalog that the boost error system drives up wastegate position to its maximum value of 100% (fully closed).  The turbos are still unable to hit the pressure targets (TIP Actual several psi below TIP Desired), and boost pressure is some 4psi below where it should be.  Interestingly, EMAP is lower than in Run2 despite the higher wastegate command, but this is explained by the significant decrease in airmass able to enter the engine and be turned into exhaust gas and exhaust manifold pressure.

Run2 (Green) – Snorkel unsecured, shroud zip-tied

Here we can begin to see the cause of the power loss.  As the snorkel deforms and decreases both airbox pressure and compressor inlet pressure, the turbo needs a greater amount of wastegate position to high the pressure targets (TIP Desired vs. TIP Actual).  EMAP shows a significant increase to over 70psiA while boost pressure stays nearly the same.  Here, the increase in EMAP causes the power loss shown in the dyno plot.

Run2 (Red) – COBB Intake Scoop Bracket installed, shroud zip-tied

In the datalog below we can see that wastegate position is well controlled around 72-80%, and EMAP peaks around 62psiA.  Boost pressure follows a smooth curve and throttle inlet pressure targets (TIP Desired Abs.) are easily being met (TIP Actual Abs.)

Conclusion

Every Gen3 Raptor Accessport sold will come with a COBB Intake Scoop Bracket included at no additional cost.  This bracket is a requirment in order to run any of our OTS maps.  For those not yet ready to tune, we will be selling the bracket separately – it is also compatible with earlier model year EcoBoost Raptors and F150s.  Removal of the shroud and installation of the snorkel bracket can be completed in ~10 minutes using only a flathead screwdriver

Links

Map Notes for Gen3 Raptor

8G4625 - Raptor / F150 Intake Scoop Bracket