What kind of power can a stock 6.7L Cummins turbo support? What about a performance-oriented replacement? The following data represents an in-depth look at a stock HE300VG turbo compared to the Calibrated Power Stealth STR turbo bolted to our 2015 6.7 Cummins running on our Dyno-mite water brake engine dyno. This is experimental data gathered under real-world circumstances, with reasonable attempts made to control as many variables as possible for consistency. Each "cell" contains a data point that represents an average value obtained by holding the engine at a steady engine speed (RPM) and horsepower (hp) for several seconds. A stock turbo and a Stealth STR were both run steadily at each of the data points shown in the charts below. Installation, instrumentation, and operation of this engine through a gauntlet of tests took several weeks of time and burned over 40 gallons of fuel. Data was logged using an EFI live scan tool and several external inputs for additional temperature and analog parameters.
1. Turbo Descriptions
- OEM Stock HE300VG- Bone stock turbo. A custom calibration allowed us to push this turbo to its practical limits, a significant power increase over stock but still left a little on the table. The stock charger then became the limiting factor, with a little more room in the stock fuel system if there was enough air to support complete combustion.
- Calibrated Power STR- This turbo is designed as a performance-oriented direct drop-in replacement for the stock unit, utilizing a billet extended-tip compressor wheel, custom turbine wheel, and high-flow vane technology. With no change to the previous tune, this charger pushed out an impressive 38 more HP than the stock while imparting much less stress on the engine than the OEM turbo. With additional tuning and fuel system mods, it was able to provide enough airflow push the engine over the 600 hp mark.
2. Steady-State Test Data
2a. Actual Horsepower:
The engine dyno allows us to create an RPM "wall". The operator can command a maximum RPM, and the water brake will ramp up the load to hold the engine at that RPM. With steady control of the throttle percentage, we were able to get close, but not perfect to the target horsepower for each cell. This chart shows the actual values that were achieved. Also worth noting are the values at the highest level of each turbo's performance may not have hit the target HP level in all cases, this shows the max sustained HP at the given RPM. Overall, we were able to push the stock turbo to a peak sustained HP of 544 (EGT Limited), and the STR to 614 HP (Shaft Speed Limited) at 2750 RPM. The STR was also able to fill out more cells in some of the lower RPM ranges.
2b. Actual Torque:
The same commentary mentioned above applies to the torque values. Do the math- it should add up! Keep in mind actual engine RPM throughout the test varies +/- 20 rpm from the target.
2c. Intro to Lambda:
Lambda directly contributes to most of the data sets shown below. The lambda charts show how clean the truck is able to run at any given horsepower and RPM. Lambda refers to the air: fuel ratio during combustion which is measured by a wideband O2 sensor in the downpipe. Lambda of 1.0 equals combustion of 14.6 AFR, 1.1 =16.06 AFR, 1.2 =17.52 AFR etc. If fuel is mixed perfectly with air, 14.6:1 combustion results in clean diesel combustion. Unfortunately perfect mixing is not a reality on this engine (by a long shot) so in order to run clean we need a Lambda value closer 1.15 ( 16.79AFR). The ability the engine has to maintain Lambda 1.15 or higher is a great indicator of the engine's overall happiness in operation. Small differences in lambda (.05) have significant impact on smoke output, EGT, and of course, reliability. The turbo has the largest impact on AFR at any HP/RPM because it directly dictates the amount of fresh airflow coming into the engine. The highest lambda value that our hardware was able to output is 1.45, so assume any value of 1.45 is at least that lean- well within the comfortable operating range of the engine.
2c. Trends in Lambda:
The yellow and red cells (lower numbers) represent lambda levels low enough to cause smokiness at that RPM/Load. Factory engineers typically maintain rich lambda limits of 1.10-1.15 at peak power production to keep particulate (black smoke) production low and keep emissions control systems happy. Throughout most of the chart, you see very little change in lambda, because of the limits of our sensor and because both the stock and STR turbos are able to supply plenty of air to keep the engine happy at those RPM and horsepower levels. The trends in this range (2250-3000 rpm, 100-350hp) are fairly consistent between both turbos across all of the data. As the horsepower demand increases, you can see that the STR is able to supply the additional air needed to obtain a clean burn at those horsepower levels. Not only does the STR allow for a higher (leaner) lambda in the 550 hp range, but it also allows enough air to push the engine to a higher peak horsepower.
2d. Intro to EGTs:
Exhaust gas Temperature is a quick indicator of overall safe operating range. Generally, 1350-1500°F is considered safe in the long term as long as coolant/oil temps are also maintaining a reasonable temperature. In a performance application it’s normal to see EGT's move to 1600°F and beyond for short intervals. The main turbo concern in these performance applications is melting a turbine or oil seals. This process happens when EGT's exceed 1500°F continuously and the oil cooling the turbine shaft is unable to dissipate heat quickly enough.
2d. Trends in EGTs:
Exhaust gas temperatures were measured with a thermocouple probe into the stream of exhaust gasses in the exhaust manifold. This data set opens the door to a lot of variables, but most importantly shows a trend in the overall behavior of the system. These tests were all done with the engine at full operating temperature, generally running top to bottom, left to right. As the engine was working harder in the higher-horsepower cells, we had to interrupt the tests and let the engine idle to cool down. Keep in mind each cell represents several seconds at that horsepower level- with a longer sustained pull at that level the temperature would climb. The stock turbo was unable to produce enough air to keep Lambda in a satisfactory range around the 500-550hp mark. That rich fuel mixture caused EGT’s to rise to an unsafe level very quickly. You can make 550hp with the stock turbo. Should you? Absolutely not. The STR was able to keep EGTs significantly lower from 450 horsepower and above, allowing the engine to comfortably make more HP while maintaining lower EGT’s than the stock unit.
2e. Intro to MAF:
Mass air flow is a direct indicator of how much clean horsepower an engine can make. Fuel is important too, but as we saw with lambda and EGT, if the air isn’t there to support the fuel then we create undesirable operating conditions. A boost measurement is affected by temperature, altitude, and other atmospheric conditions. A mass air flow number remains true regardless of atmospheric conditions. MAF data was logged from the engine computer via the OBDII port.
2e. Trends in MAF:
Take a look at the last chart- the difference between MAF with each turbo. Aside from the cells where the STR exceeded the capabilities of the stock unit, the values are very consistent. This is exactly what we would expect- MAF is a direct indicator of how much clean horsepower an engine can make. As HP demand climbs however, the STR can provide more air flow where the stock unit is beginning to run out.
2f. Intro to Fuel Rate (MM3):
Fuel rate is another direct indicator of how much horsepower an engine can make. Fuel rate is measured in MM3, or microliters. More fuel = more potential horsepower. MM3 data was logged from the engine computer via the OBDII port.
2f. Trends in Fuel Rate:
Just like with our MAF data, for the bulk of the chart the fuel rate remains the same between the two turbos, because they are well within a comfortable operating range- plenty of air so the computer only gives the engine enough fuel to make that HP level. However, as the engine pushes to the 400 hp+ region, we start to run out of air. With less air, the only way to make more power is to increase the fuel rate, but at the cost of a lower (richer) lambda and its undesirable side effects. Where the stock turbo runs out of air, the STR is still within a normal operating range, and therefore can provide enough air for clean horsepower. This is where you find the green cells on the comparison chart, there is enough air for a clean burn so the engine computer doesn’t have to throw as much fuel into the mix to achieve that horsepower. Note the 211 values at the bottom of each chart, the max fuel rate that each tune would allow. The bottom line is, with the same fuel rate, the STR was able to make significantly more power.
An interesting note- compare the MAF from left to right with the fuel rate from left to right in the middle of the chart. As RPM increases, so does air flow (engine is an air pump). Conversely, fuel rate drops as RPM increases. We’re making the same horsepower, even though both fuel rate and MAF are indicators of clean horsepower potential. If you recall, the lambda reading was maxed out in this region at 1.45. From the trends we see with MAF and fuel rate, if the lambda readings weren’t maxed out, the values at the top right of the charts would be significantly more lean, likely somewhere near 5.0 lambda or higher
2g. Intro to Efficiency:
Efficiency can be expressed as energy output divided by energy input. Fortunately, this ECM gives us a reliable fuel quantity value so we can accurately estimate how much potential energy is going into the engine. The dyno measures effective energy output. With these two values, we can estimate efficiency. The energy input that doesn’t turn directly into horsepower is lost, mostly as heat, into the engine’s cooling system and out the exhaust.
2g. Trends in Efficiency:
For this data set, we can see a practical trend where the engine is operating at its most fuel efficient range with both turbos. This makes sense, with an obvious increase near the middle of the RPM and HP range. If you imagine driving your truck in a fuel efficient manner, I’m sure having the pedal buried, screaming down the road at 3000 rpm doesn’t come to mind. There are some strong outliers near each corner of the chart, in operating ranges where most people don’t -or can’t- run their engine. Remember that this data was collected on an engine dyno- if these tests were run on a chassis dyno, it is unlikely you would be able to get the engine screaming at 3000 rpm while only sending 100 hp through the drivetrain. When running this test, the load valve barely opened up at all to achieve 100 hp at 3000 RPM. This means that the engine makes almost 100 hp on just enough fuel to keep the engine spinning that fast. Following the same trends as the other data, where the stock turbo was running out of air, the STR allows the engine to burn the fuel it is given more efficiently. Pay particular attention to the cells where the fuel rate was 211 or close to it. The engine does a lot better of a job making power with that fuel when the STR is the one feeding it air.
2h. Intro to Boost:
Although boost pressure is relatively unimportant since this engine is equipped with a mass airflow sensor, it still provides interesting data and is a value most truck owners are more familiar with. Boost in this test was sampled using an aftermarket manifold air pressure (MAP) sensor in the factory location on the intake horn and logged via the engine computer. The reason for the aftermarket sensor was due to exceeding the stock sensor's capabilities.
2h. Trends in Boost:
Given these tests were run with very similar ambient air conditions, the boost numbers make it clear to see that the STR is able to supply much more air to the engine. The STR outperforms the stock turbo by up to 5psi increased boost at the 550+ hp mark.
2i. Intro to Drive to Boost Ratios:
Drive pressure, back pressure, or exhaust manifold pressure- whatever you call it, it is an important metric to observe in relation to boost pressure. A significant imbalance in drive and boost pressures can create an uneven thrust load on the turbo bearings which can cause premature failure. Too much drive pressure limits engine performance, restricting air flow through the entire system. A simple way to look at this data is by creating a ratio of drive pressure divided by boost pressure. Values close to 1.0 are the most ideal. This engine, equipped with variable vane technology, allows superior regulation of drive to boost ratios compared to a fixed-geometry turbo. Drive pressure was measured with a pressure sensor connected to a copper coil plumbed into the exhaust manifold right before the turbine inlet of the turbo. The copper allows the exhaust gases to cool down to protect the sensor. Below are charts for drive pressure and calculated drive to boost ratios.
2i. Trends in Drive to Boost Ratios:
A quick look at the drive pressure charts shows the STR was able to maintain lower drive pressures at higher horsepower levels. The drive to boost table shows a consistent “bubble” of values less than or close to the 1.0 target. In areas where the turbo is operating at the edge of its ideal range (low rpm/ low hp and high rpm/ high hp), this ratio increases considerably. With the improved geometry of the STR, this 1.0 targeted “bubble” grows beyond what the stock turbo was able to achieve.
2j. Intro to Turbo Speed and Holset VGT:
This 6.7L Cummins engine comes equipped from the factory with a turbo speed sensor. Output from the speed sensor in conjunction with precise control over the variable geometry turbo (VGT) allows an aftermarket tuner to make performance and efficiency driven decisions for optimal turbo output. Turbo shaft speeds of up to 135,000 RPM are generally considered safe. Anything above that limit is unsustainable long-term. The factory speed sensor reads up to 150,000 rpm before cutting out (believe me, we found out by experimenting with less-than-ideal vane settings). In general, closing off the vanes increases drive pressure and thus turbo shaft speed, but the shaft speed and drive pressures can quickly climb to unsafe limits. As the engine reaches higher power levels, the vanes open up allowing better exhaust flow while maintaining a high shaft speed. Higher shaft speed result in higher mass air flow rates. In real-world applications, such as taking off from a stop, the vanes will close off, allowing the turbo to spool up faster, and then the vanes open as the engine reaches a higher RPM and power level. Below are charts that show both turbo shaft speed and turbo vane position for each turbo. 0% Vane position = fully opened, and 100% vane position = fully closed, but 100% closed is never used in regular operation.
2j. Trends in Turbo Speed and Holset VGT:
In a quick comparison, it might seem like the airflow of the stock turbo was limited because the RPM wasn’t as high as with the STR. However, In both sets of tests, each turbo was commanded to achieve the same vane position. The high-flow vanes of the STR allow for higher rpms at the same or lower drive pressures.
3. Conclusion
After a gauntlet of testing and data logging to compare the performance of these two turbos side-by-side, it is clear to see that the Calibrated Power Stealth STR compromises very little to the stock turbo at low engine speeds and outputs and that the STR generally out performs the stock unit in every way above 2000RPM and 300HP. The STR can achieve higher peak horsepower and torque by providing better air flow for a cleaner burn, lower EGTs, better efficiency, and lower drive pressures.
If you're interested in learning more about the Stealth STR for 6.7L Cummins click here to visit our website or give us a call at (815)-568-7920 Monday-Firday 9am-5pm (CST) and we would be happy to provide you with any information about this turbo that we can. 
Don't Forget To Check Out Our Other 6.7L Cummins Related Articles!
-
Why EZ Lynk Is The Way To Go For Cummins 6.7L Tuning
-
Must Have Upgrades for Your 2013 - 2018 Cummins 6.7L!
-
What You NEED To Know About 68RFE Tuning
