Boost Leaks Test & When Do You See A Power Loss

Boost Leaks Test & When Do You See A Power Loss


Let’s talk about boost leaks. If you are new to forced induction, a boost leak is a term referring to the loss of air mass - by leakage - between your compressor (turbocharger or supercharger) and your engine.  Since our superchargers and turbochargers both require engine power to generate the extra airflow they give us, if we allow that air to escape the system, it is simply given away, reducing the overall efficiency of the system, leading to more heat, shorter component life cycles, and lower overall performance.  Those of us who have built race cars before have battled them.  Sometimes they are easy to find, other times not so much.

We had a ton of fun doing this test, but what needs to be pulled from the data is the conceptual understanding of what is happening due to a boost leak.  For our more advanced readers, you can consider this information in your use of intake wastegate boost control strategies.

While you are going to see small changes between most of the tests, the message there isn’t to suggest that you’re “safe with just a few” boost leaks - it is to show you “what boost leaks do” when they stack up.  Many people feel if there is not something obvious, that their system is good enough.  By helping you understand the impact of leaks, we hope that you’ll routinely inspect your charge pipe system as a part of your maintenance and/or vehicle preparation checklists.

In the tests ahead, you’ll see that we put together a basic experiment to test how various sized leaks can affect a turbocharged engine. How much harder does a turbocharger have to work to keep up with the set targeted boost? How much horsepower and torque get affected with a little or big leak? How does the engine behave? What does the exhaust back pressure look like during a pull? 

One thing to remember here:  while these tests and the video feature circular boost leaks, the test is about surface area and the overall concept of leakage, not shape of a given leak.  Many small leaks can quickly add up to a large one.  Leaks you don’t expect such as a welded seam splitting on the back side of your intercooler, pinhole leak on a weld, a ripped wastegate diaphragm, can quickly cause these conditions.  This test is to help you understand what your engine, and if turbocharged, turbo, are experiencing.  Try to visualize a split in an intercooler core along the weld of the end tank to the core.  Seen those before?  So have we.

While in this test, we reference inches, let’s use Metric so that we can calculate using whole numbers to try and relate the sizes to something easier to visualize:

A 4mm hole has an area of 12.57 mm^2.  Ignoring the fluid dynamics of how the leak will flow, looking at that area, it is not hard to imagine a 1mm welded seam split that is a half inch (12.7mm) long.  Well guess what, that’s roughly the same area (or same size leak) as an ⅛” hole:  1mm x 12.7mm long = 12.7mm^2.

A 1mm seam split as small as 6” would have 20% more overall leak area than even a full ½” hole!

  • 12.7mm (0.500”) Hole = 126.7mm^2 (0.196 sq/in) Leak Area
  • 1.0mm (0.039”) x 6.00” (152.4mm) Seam Split = 152.4mm^2 (0.236 sq/in) Leak Area (↑ 20.28%)

It’s not hard to imagine a split along the weld of an intercooler or charge pipe, an old exhaust manifold collector, wastegate diaphragm, intake manifold, throttle body, coupler tears, cracked vacuum lines, etc.  Add all of those up and these things happen quickly.


To start this test off we needed a few things. A good engine and an otherwise-efficient turbo combination. We wanted to create a scenario that many people could encounter: a 6670 Precision at a very casual ~650hp.  We pressure tested and dynoed our system as a baseline and then gradually stepped up the severity of our boost leak by opening a larger and larger hole near the outlet of the compressor.  We started around a ⅛” (~3-4mm) hole and went up to a 1” (~25+mm) hole in the charge pipe.  The results we found are interesting!  Let’s take a look.

Intentional Starting Variables: 

  • Test 1:  Closed System w/o Leaks
  • Test 2:  ~⅛” Hole in Hot Side Charge Pipe
  • Test 3:  ~¼” Hole in Hot Side Charge Pipe
  • Test 4:  ~½” Hole in Hot Side Charge Pipe
  • Test 5:  ~¾” Hole in Hot Side Charge Pipe
  • Test 6:  ~1” Hole in Hot Side Charge Pipe
  • Test 7:  ~1” Hole w/ More Aggressive Boost Control

Constants & Conditions:

Test 1:  No Leaks

Test Results:

  • Peak Horsepower:  694.6hp @ 8,100rpm
  • Peak Torque:  465.6tq @ 7,200rpm
  • Peak Exhaust Pressure:  17.9psi
  • Peak Turbocharger Speed:  86,227rpm
  • Peak Boost:  15.1psi

Here is our baseline run.  The resulting power was 694.6hp @ 8,100rpm and 465.6tq @ 7,200rpm.  Exhaust pressure peaked at 17.9psi, crossing 1:1 at approximately 620hp & 6,950rpm, and the shaft speed of the turbocharger was 86,227rpm. Solid start so far. 

The first engine dyno pull is complete and data has been collected, now let’s get started with the first boost leak attempt. For this next dyno pull we will be drilling a ⅛” hole into the charge pipe. 

Test 2:  ⅛” Leak

Test Results:

  • Peak Horsepower:  684.5hp (↓ 1.45%)
  • Peak Torque:  468.2tq (↑ 0.56%)
  • Peak Exhaust Pressure:  18.1psi (↑ 1.12%)
  • Peak Turbocharger Speed:  86,557rpm (↑ 0.38%)
  • Peak Boost:  15.3psi (↑ 1.32%)

No Leaks:  Blue Line

⅛” Leak:  Red Line


Test 3:  ¼” Leak

Test Results:

  • Peak Horsepower:  681.0hp (↓ 1.96%)
  • Peak Torque:  462.9tq (↓ 0.58%)
  • Peak Exhaust Pressure:  18.6psi (↑ 3.91%)
  • Peak Turbocharger Speed:  86,981rpm (↑ 0.87%)
  • Peak Boost:  15.3psi (↑ 1.32%)

Let’s take a look at the dyno graph for the 3rd pull which is the 2nd boost leak run. Please keep in mind that the most recent run is always the red color line in each of the following graphs.

No Leaks:  Green Line

⅛” Leak:  Blue Line

¼” Leak:  Red Line

Test 4:  ½” Leak

Now we are starting to see larger compounding effects as these leaks stack up.  Both exhaust pressure and turbocharger speed are increasing while power and torque continue to suffer. Let's drill a larger hole and see what happens.


Test Results:

  • Peak Horsepower:  658.7hp (↓ 5.17%)
  • Peak Torque:  446.9tq (↓ 4.02%)
  • Peak Exhaust Pressure:  19.9psi (↑ 11.73%)
  • Peak Turbocharger Speed:  88,524rpm (↑ 2.66%)
  • Peak Boost:  14.4psi (↓ 4.64%)

No Leaks:  Magenta Line

⅛” Leak:  Green Line

¼” Leak:  Blue Line

½” Leak:  Red Line

Test 5:  ¾” Leak

Test Results:

  • Peak Horsepower:  635.6hp  (↓ 8.49%)
  • Peak Torque:  429.6tq (↓ 7.73%)
  • Peak Exhaust Pressure:  21.4psi (↑ 19.55%)
  • Peak Turbocharger Speed:  90,006rpm (↑ 4.38%)
  • Peak Boost:  13.7psi (↓ 9.27%)

No Leaks:  Orange Line

⅛” Leak:  Magenta Line

¼” Leak:  Green Line

½” Leak:  Blue Line

¾” Leak:  Red Line

Test 6:  1” Leak

Test Results:

  • Peak Horsepower:  569.4hp (↓18.02%)
  • Peak Torque:  387.1tq (↓16.86%)
  • Peak Exhaust Pressure:  24.0psi (↑34.08%)
  • Peak Turbocharger Speed:  93,491rpm (↑8.42%)
  • Peak Boost: 12.3psi (↓18.54%)

No Leaks:  Cyan Line

⅛” Leak:  Orange Line

¼” Leak:  Magenta Line

½” Leak:  Green Line

¾” Leak:  Blue Line

1” Leak:  Red Line

Test 7:  From Bad to Worse

As you can tell from the increasing turbocharger speed, our wastegate and boost controller have been attempting to accelerate the turbocharger attempt to chase down the target boost.  

We wanted to give an example of what happens when you try to cover up a leak with overworking your components.  While we are intentionally choosing the most drastic test here, again the idea is that you will understand the concept and trends that occur with leaks and how to relate that to your own vehicle and racing program.

Since the leak became so large that it didn’t even achieve 13psi peak boost during Test #6, we were more aggressive on the boost controller to try and chase down that original 15psi target to demonstrate how different the operating conditions of the engine are.

Test Results:

  • Peak Horsepower:  626.1hp (↓9.86% from Test 1 / No Leaks)
  • Peak Torque:  430.4tq (↓7.56%)
  • Peak Exhaust Pressure:  30.1psi (↑68.16%)
  • Peak Turbocharger Speed:  102,186rpm (↑18.51%)
  • Peak Boost: 15.6psi (↑3.31%)

No Leaks:  Blue Line

1” Leak w/ More Aggressive Boost Control:  Red Line

Here we can see the overall difference between the first run of this test with no leak that made 694.6hp at 15.1psi peak boost versus 626.1hp at a similar-but-very-different-looking 15.6psi peak boost with the 1” hole in the hot side charge pipe.

Even with more boost pressure, we are making almost 10% less power, we’re almost at a 2:1 exhaust pressure ratio, and we’ve asked the turbocharger for almost 20% more shaft speed to make this 10% less power. During valve overlap, your exhaust system is stalling spent/burnt mixture in your cylinders, effectively shrinking the size of your engine, but also substantially increasing the amount of heat that the components are living with.

In Conclusion:

What did we learn here?  We learned that boost leaks come in all shapes and sizes.  We started to think of their combined leakage, rather than assuming it is either obvious or irrelevant.  We learned that we need to think of our engines as complete systems or power units.  One small leak may not mean much of anything empirically, but two, five, ten of them may add up to something.  We learned that just because we don’t have a leak at one point doesn’t mean that a leak cannot develop over time.  The mental approach that we want to take to our projects is that leaks leave power on the table and add unnecessary heat to our power system as a whole, shortening the lifespan of critical components.

We watched torque and power fall.  We watched work (pressure, rpm, and therefore heat) increase, and how singular changes affected multiple different aspects of the engine.  The best case scenario of any leak is “to have more than you need to finish.”  Oil, Fuel, Boost:  the concept is still fundamentally the same.  However, due to the nature of these leaks, we treat them differently.  

Fluids have loaded capacities, are visually easier to identify, as well as have numerous sensor-assisted approaches.  This doesn’t mean that we should ignore the other leaks where turbochargers and superchargers often cover up the mistakes and inefficiencies in the system.  Given the knowledge we now have, we can make this a step of any new build, and consider our usage and come up with a schedule that works for you to have it looked at, even if that is just when you have the car prepared for the next race or do your next oil change.  

Not only that, but this practice can also help prevent an unlucky engine failure due to a compromised water-to-air intercooler, where a failure can put water in your intake tract.  In motorsports and racing, even fixed-location metal items do often have a service life, much less the ones that we ask to handle heat cycles and pressures, so we need to understand that just because we didn’t have a leak before, does not mean we don’t have a leak after usage.

What surprised you the most?  

Is there anything you’d like us to cover or anything we missed here?

Thank you for reading along and we hope that this information was helpful!


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