Inside KC Turbos 6.4L Power Stroke Turbo Development

A behind-the-scenes look at how we test, validate, and build real power

Why KC Turbos Entered the 6.4 Market

KC Turbos is officially entering the 6.4L Power Stroke turbo market. While it may look like we arrived late, that delay was intentional. Instead of releasing a product just to have something on the shelf, we spent years testing, learning, and validating data to make sure anything we release actually improves real-world performance.

The 6.4 platform quickly proved why so many enthusiasts love it. Even with basic tuning, these trucks are incredibly responsive and fun to drive. That made them the perfect platform to push stock fuel to its limit and then methodically evaluate airflow, efficiency, spool, and drivability.

"Well... she is dirty but finally up and running. Thanks Ian Gebbia for the hookup on the nice truck. 2008 f250 CCSB 4x4 Black Harley Davidson Edition. (Without the flames!) We bought her with a blown motor and completely went through it."

Our goal from the beginning was simple: build the best drop-in compound turbo options possible, backed by real data.

Baseline Setup and Testing Philosophy

Before changing anything, we established a consistent baseline. This allowed us to compare every change accurately and understand exactly where gains or losses were coming from.

Test Truck Configuration

• KDD Stage 2 engine (cam, rods, o-ringed heads, ARP 625s, delipped and coated pistons)
• Full billet Warren 5R transmission (built in-house)
• New Alliant HPFP with all new fuel lines
• New Pure Power injectors
• AD4G
• Titan fuel tank with sump
• Stock rebuilt turbos (initial testing)
• No Limit intake
• BD exhaust manifolds and up-pipes
• Odawg ported intake manifold
• New body mounts

Baseline Dyno Results

Using an H&S tuner on the 300 tune:

• 590HP & 1296TQ

Boost & Back Pressure Numbers:

• 2500rpms 41psi boost and 47psi ebp
• 3500rpms 33psi boost and 65psi ebp

This established our reference point before any turbo or fueling changes.

"Let me say WOW... I can now see why guys love these things so much. It is hard to believe how fun they are with just a tune!"

Tuning Platform: H&S vs EZ Lynk

Early testing was done with H&S tuning. While it produced slightly higher peak torque due to aggressive timing, it lacked consistency and drivability. The truck would not downshift properly on the dyno and behaved poorly on the street.

We transitioned to EZ Lynk custom tuning from Chris Buhidar at Truck Source Diesel, which immediately improved:

• Shift quality
• Throttle control
• Street drivability
• Data logging and repeatability

Common Customer Question:

Why did KC switch tuning platforms?
EZ Lynk allowed consistent before-and-after testing and better real-world drivability, which is critical when evaluating turbo performance.

Stock Turbos vs Drop-In LP Compressor Wheels

73mm Drop-In LP Wheel (Stock Turbine, Stock HP Turbo)

We started with a 73mm drop-in low pressure compressor wheel while keeping the stock turbine and stock HP turbo.

Fueling:

• Stock injectors & HPFP
• 2200 microseconds (rail pressure beginning to fall to ~22.5k psi)

Results:

• 628 HP / 1268 TQ
• Peak gain of 38 HP
• At redline, gains reached 100 HP over stock

Boost & Back Pressure Numbers:

• 2500rpms 40psi boost and 48psi ebp
• 3500rpms 44psi boost and 73psi ebp

Key Observations:

• No noticeable difference in street spool
• Significant improvement in top-end airflow
• The KC 73mm LP added a lot of power from about 2500rpm and up.
• At 2500rpms the stock turbos start running out of air and drop boost all the way to redline.

We used both loaded and non-loaded dyno tests to simulate real-world driving, from track launches to light throttle roll-ins.

"So what is the green graph? Well this is how we like to test spool up. The Blue graph would be more of a representation of a boosted launch at the track with a lot of load. The Green graph is just rolling into the throttle at 2000rpms with very little load."

VGT Compounds vs T4 Single Turbo (S364.5)

To directly compare setups, we tested our 73mm LP/stock HP truck against a similarly built 6.4 running a non-VGT S364.5/73/91 single turbo.

Loaded Dyno Results

• S364.5: 633 HP / 1224 TQ
• Stock HP/73mm LP: 621 HP / 1253 TQ

Real-World Breakdown

• The S364.5 made slightly more top-end horsepower (between 8-11hp)
• Compounds brought peak torque 300-400 RPM sooner
• Below 2000 RPM, the S364.5 was effectively unusable due to lag
• The S364.5 would just smoke and downshift so we had to start it much later in the rpms
• That is why we love doing "non-loaded 2000rpms pulls"

Boost and EBP Comparison Highlights

At 2000 RPM:

• S364.5: 7 PSI boost / 9 PSI EBP
• Compounds: 31 PSI boost / 45 PSI EBP

Compounds consistently spooled 500-600 RPM faster and produced dramatically more usable torque.

Common Misconception About Single Turbo Swaps on a 6.4 Powerstroke:
Single turbos spool just like stock compounds.

Real data shows this is not accurate, especially below 2500 RPM! The graph below shows the difference in power you can expect when single swapping your 6.4 Powerstroke.

"If you like lugging the motor and cruising around 1500-2500rpms then it is no contest and will enjoy upwards of 600 more ft/lbs of tq. 73/stock Compounds are for you. But with a higher stall converter and some tuning to keep the rpms up and easier to downshift... then the s364.5 is gonna make more top end power efficiently... 8-10hp."

When looking at the graph above, this is a good way to test spool up difference between 2 turbos and represent max hp under less load like blowing through 1st and 2nd gear without the tq converter locked in higher gears. Or rolling into the throttle at lower rpms in OD. You can see the compounds come right to life with ease... where the s364.5 really struggles. The compounds come to life about 500-600rpms quicker.

At 2500rpms the compounds make almost 600 ft/lbs of tq more... if you could get the s364.5 to not downshift at 1500rpms that gap would be even greater. The compounds make more peak HP, but even under light load the s364.5 makes a little more top end power starting at 3200rpms.

Now the boost/ebp numbers that I know everyone is wondering about.

Boost & Back Pressure Numbers @ 2,000RPM:

• S364.5 Made 7psi boost and 9psi ebp
• Compounds Made 31psi boost and 45psi ebp

Boost & Back Pressure Numbers @ 2,500RPM:

• S364.5 Made 34psi boost and 38ebp
• Compounds Made 40psi boost and 48ebp
  

Boost & Back Pressure Numbers @ 3,000RPM:

• S364.5 Made 39psi boost and 52ebp
  
• Compounds Made 42psi boost and 66ebp

Boost & Back Pressure Numbers @ 3,500RPM:

• S364.5 Made 34psi boost and 60 ebp
• Compounds Made 44psi boost and 73 ebp

So the s364.5 obviously spooled much slower, WAY SLOWER. Those who say they spool just like stock are CRAZY... but the s364.5 was able to make more top power with less boost and less ebp. So it is "more efficient" and "safer" for top end power.

"NOTE: The weird dip on the loaded s364.5 graph seems to be a common issue we see with some peoples EZ Lynk tuning. It happened on every run but did not affect any of the peak numbers or data we posted. It was after peak tq and before peak hp. We were not working on the tunes for that truck... just wanted dyno numbers."

Refining the LP Turbo: KC Modified VGT Testing

We then refined the low pressure turbo using:

• 71mm point mill compressor wheel
• Ported 4-inch inlet cover
• Anti-surge ring
• Stock turbine
• Stock HP turbo

These changes resulted in:

• +17 HP and +53 TQ peak power
• Faster spool
• +75 HP and +150 TQ at 2000 RPM

Despite being a smaller wheel, efficiency improved across the entire powerband.

Why This Matters:
Bigger is not always better. Compressor efficiency and turbine balance matter more than size alone.

"Despite being a smaller wheel, efficiency improved across the powerband."

"During the non-loaded 2000rpms test there was virtually no difference at all. Is this where the misconception that Non-VGT single swapping a 6.4 Powerstroke 'spools just like stock'?"

Fuel System Limitations and Larger LP Compressors

As compressor sizes increased, fueling became the limiting factor.

75mm LP Compressor Results

• 662 HP / 1329 TQ
• 50 PSI of Boost & EBP Sensor maxed out at 73 PSI

Power gains began to show diminishing returns without additional fueling.

The 1st graph shown above is the loaded dyno run. No noticeable lag at all from the 75 vs 71mm. Carried basically the same power band, but achieved an increase of 22 HP and 33 TQ.

The 2nd graph shown above is the “non-loaded” 2000rpm roll. You can see a little bit of lag this time, not enough to really matter, but it is there. On the streets and driving there is no noticeable difference in lag.

This 3rd dyno graph above is just for reference, another 6.4 we had on the dyno recently with an s364.5. The 75mm drop in low pressure spools WAY faster, making 700ft/lbs more than the s364.5 at 2100rpms and makes 29hp more all throughout the rpm band to redline. Now the compounds are making 10-15psi more of boost/ebp all across rpm band. That is going to add a lot of heat/pressure in the motor. I would like to get an s369 to see how it compares in power to the drop in 75mm atmo.

Here are the boost & ebp numbers for the graphs above:

75mm (Graph #1)

• 2000 RPMs: 29/42
• 2500 RPMs: 38/48
• 3000 RPMs: 43/68
• 3500 RPMs: 50/73 (ebp maxed out)

71mm (Graph #2)

• 2000 RPMs: 30/46
• 2500 RPMs: 40/49
• 3000 RPMs: 44/68
• 3500 RPMs: 44/73  (ebp maxed out)

S364.5 (Graph #3)

• 2000 RPMs: 7/9
• 2500 RPMs: 34/38
• 3000 RPMs: 39/52
• 3500 RPMs: 34/60

The boost numbers are similar down low between the 71mm and 75mm, but the 75mm is more in the center of the compressor map which will make it run cooler. Up top the 71 runs out of steam.

Dual Fuel Pumps: What They Do and Do Not Do

We added a Midwest dual fueler system to stabilize rail pressure with our Stock HP Turbo and KC 75mm LP turbo with 4" inlet, ported cover, anti-surge ring with modified MAP groove. Larger upgraded s400 turbine wheel and bearings.

Key Results:

• At 2200 microseconds, zero horsepower gain vs a single pump
• Rail pressure stability improved significantly
• Power only increased when fuel demand exceeded what a single pump could supply

Final Dual Pump Results:

• 2200 us: 667 HP / 1327 TQ
• 2400 us: 708 HP / 1426 TQ

Previously, we were starting to drop Rail Pressure on a 2200us tune even with a brand new fuel system on a single pump. Made good power but could be inconsistent in between dyno runs so it was making it hard to do back to back testing. Well that problem is solved now.

 "Blue run was dual fuelers on the same tunes as before, 2200us. ZERO power gain over a single pump. This is still a good representation of what stock would expect if you can hold rail. As long as you hold rail on a single pump then there are no gains. We get asked that question a lot..Green run was turning the truck up to 2400us... which is a pretty hot tune for these trucks. A 2200us tune made 667hp & 1327tq. A 2400us tune made 708hp & 1426tq."

Boost & Back Pressure Numbers:

• 2900rpms: 50psi boost and 69psi ebp (Maxed out MAP Sensor)
• 3300rpms: 50psi boost and 73psi ebp (Maxed out EBP & MAP Sensors)

We need to get a higher reading map/ebp gauge on the truck. 2900rpms maxes at 50psi at 2900rpms with 69ebp. It just barely maxes out the ebp gauge at 73psi at 3300rpms. If I was to guess it is 53-55psi boost and 75psi ebp comparing to previous runs and how they rise.

"Here is a pic of WOT on the dyno. Yes that is the smokiest part of the dyno run. NO FUEL LEFT. As clean as stock. Truck runs SUPER CLEAN at WOT. This setup is just begging for more fuel which is crazy because it still has the stock HP turbo."

Pushing Drop-In LP Turbos With 100 Percent Injectors

With twin pumps holding rail pressure solid, we stepped up to 100 percent over injectors.

To fairly compare fueling we ended up testing:

• Stock nozzles at 2400 us
• 100 percent nozzles at 1300 us

Both setups produced nearly identical curves until higher RPM, where larger nozzles maintained efficiency.

Incremental Fueling Results

• 1300 us: 742 HP / 1411 TQ - extremely clean
• 1800 us: 857 HP / 1675 TQ - very clean
• 2200 us: 946 HP / 1787 TQ - black haze
• 2300 us: 1002 HP / 1866 TQ - heavy smoke

Street preference was the 1800 us tune due to cleanliness and drivability.

"We pushed stock fuel as far as it could go… twin pumps and 2400us tune and still ran as clean as stock. At WOT there was ZERO SMOKE. So we decided to bump up to 100% nozzles to see how much power we can squeeze out of a drop in LP turbo upgrade. First thing we wanted to test it to see if we could pull enough fuel out of the 100% nozzles to mimic “stock fueling” hp. We settled on 1300us with 100% nozzles to being fairly close to stock nozzles on a 2400us tune… both with twin pumps holding rock solid rail pressure. See CHART 1 for the results, you can see they make nearly the exact same HP and follow the same curve until about 2800rpms… which makes perfect sense because at the higher rpms the 100% nozzles can dump the fuel quicker keeping the spray in the “sweet spot” where the stock nozzles have to spray so far down the back side of the stroke they lose efficiency. The stock nozzles did make a tiny bit more tq… but it is close enough to be attributed to normal variance in dyno runs."

Chart 1
Red – STOCK NOZZLES: 708hp and 1426tq
Blue – 100% nozzles: 742hp 1411tq


Now onto the fun part. We slowly started bumping up the Pulse Width to see how much power we could squeeze out of this unit. We actually went up 200us at a time but the graph looks weird with all the dyno runs on there and makes it harder to read. So we are only posting 4 graphs, but the data was consistent… as we kept adding fuel the power kept going up until 2300us where we started hitting diminishing returns and it got pretty smokey.

High Pressure Turbo Testing: Data Over Assumptions

This phase took years of testing and verification.

"This is probably not a post that many are going to be happy about, but I am not here just to throw random parts in a turbo and sell them like you see so many other companies do. If I was that kind of guy I would have made some 6.4 turbos 3 years ago that looked/sounded cool and made sure they were bigger than every other option on the market. Instead we spent the last 3 years testing, learning, developing new parts, and testing again."

Core Conclusion

With a stock-frame LP turbo, anything larger than a 6x6 or 7x7 59mm HP compressor wheel reduces performance. (Results are different if you put a larger Precision or Borg Warner in place of your LP Turbo).

We kept the LP turbo the same for this testing. This is the best performing LP turbo we have tested so far out of any drop in turbo out there available on the market. New KC Stage 2 75mm with larger turbine wheel.

We tested:

• Multiple compressor sizes and blade counts
  • 6x6, 7x7, 11, and 10 blade wheels 
  • varying inducer, exducer, and wheel height sizes
• Multiple turbine wheels sizes & designs
• Ported covers
• Gated and non-gated setups

Here are some of the graphs that show the data the best all with stock and/or modified covers:

• Green - 976hp - 100% stock HP
• Brown - 1026hp - Typical 7x7 59mm compressor with stock turbine wheel.
• Red - 1006hp – Larger 59mm compressor with stock turbine wheel
• Yellow - 995hp – Larger 59mm compressor with larger turbine wheel
• Blue - 979hp - Larger 61mm compressor with larger turbine wheel
  

*Not featured on the graph above but good to mention:
992hp 66mm Larger ported cover, 4” inlet, larger turbine wheel
985hp 68mm Larger ported cover, 4” inlet, larger turbine wheel

So going bigger than a 59mm 7x7 compressor wheel with stock turbine did not increase power... it actually hurt HP.

Why Bigger HP Turbos Hurt Performance

Here is what we know and have learned and why this is happening. The LP turbo is actually the restriction... not the HP turbo. The problem is the turbine side of the LP turbo is restricting into the HP turbo. Turbos don't care about PSI, they only care about pressure ratios. This is pressure in vs pressure out. As the pressure ratio drops you will flow less air through the turbine housing and not drive the turbo as hard.

Measured pressures:

• Pre-HP EBP: ~90 PSI
• Between turbos: ~60 PSI
• Post-LP: ~15 PSI

This results in:

• HP turbo driven at ~1.5 pressure ratio
• LP turbo driven at ~4.0 pressure ratio

The LP turbine becomes the restriction, over-driving the LP and under-driving the HP.

That is too high of a pressure ratio for the LP turbo and not high enough for the HP turbo. The LP turbo is being driven TOO HARD and can't keep up on the turbine side. This is causing a backup in between the two turbos. This results in the HP turbo NOT BEING DRIVEN HARD ENOUGH. So increasing the size of the HP turbo actually hurts because you are not driving it very hard so making it bigger loses efficiency in the lower end of the compressor maps and turbine maps. The 59mm 7x7 with stock turbine is the most we can squeeze out of it.

The changes we have seen so far that great improved performance over everything else on the market was with the addition of the larger turbine wheel on the LP turbo, a more efficient compressor wheel on the LP turbo, and larger 4" ported inlet to increase efficiency and flow through the LP turbo.

What Actually Improved Performance:

• Larger LP turbine wheel
• More efficient LP compressor
• 4-inch ported LP inlet

Gates reduced EBP but did not increase horsepower. Adding gates helped to bring down EBP, but did not increase power. Gates are actually blowing off excess pressure which could be converted into horsepower if done properly.

Additional Low Pressure Turbo Testing: Just An FYI

Also in case you wanted to see them. Here are our results from the our initial LP turbo testing.

• Light green - 628hp - 73mm with stock cover STOCK TURBINE
• Red - 640hp - 71mm flank mill with upgraded cover STOCK TURBINE
• Light blue - 655hp - 71mm point mill wheel with upgraded cover STOCK TURBINE
• Orange - 655hp - 75mm point mill wheel STOCK TURBINE
• Dark Blue - 667hp - our 75mm point mill wheel, upgraded cover, LARGER TURBINE WHEEL (it also had lower EBP and more boost over the stock turbine wheel)

After that we just added bigger injectors

• Dark Green - 708hp - DUAL hpfp to flow more fuel
• Brown - 976hp - Dual hpfp and 100% injectors
  

What Comes Next

The next step is increasing flow through the LP turbine housing:

• Larger A/R turbine housing (20 percent or more)
• Integrated wastegate provision (didn't make much change in our recent testing)

This development is ongoing and will take time, but it is the key to unlocking further gains from larger HP turbos.

Final Takeaway

This program reinforced a critical lesson:

Power in the 6.4 platform comes from balance, efficiency, and pressure control, not just bigger parts.

Every KC Turbos product released from this testing exists because it worked better, not because it looked impressive on paper.