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The Best Turbocharger for the 4BT and 6BT Engine

Cummins 4BT 140 HP Engine Unit Arrival 1When looking at more horsepower and torque for a Cummins 4BT or 6BT Performance mod you are definitely going to want to focus on a larger turbo or setting up a twin turbo situation. For the laymen out there, a turbocharger compresses the air flowing into the diesel engine, squeezing more air into the cylinder, meaning more fuel can be added. The more air and fuel the engine can burn is what gives the engine its power.

Comprising of a turbine and a compressor, connected by a common shaft, it’s supported on a bearing system. Its purpose is to convert waste energy from the exhaust into compressed air, allowing the engine to burn more fuel, therefore producing more power and efficiency.

Debate rages across countless forums around the planet about which turbocharger performs greatest in the 4BT (3.9 L) and 6BT (5.9 L). Though it’s impossible to arrive at a definitive conclusion we thought we’d put this guide out there to help you out - or possibly confuse you even further!

We've concentrated on the three main players. Cummins Holset, Garrett and Borg Warner.

Cummins Holset

Holset is a world leader in turbocharger technology and their turbochargers are a major element in the Cummins brand engine success throughout the world. The UK Company has been around since 1952 and now has over 3,000 employees.

Taken from Jay K. Miller’s consummate book, Turbo, here’s a bit of history on Holset and some reasons why their turbochargers are amongst the best in the world.

“Today Holset is known as Cummins Turbo Technologies. While the name is relatively new, Holset has been a part of the Cummins Engine Company family for over 30 years now. Holset is a world leader in turbocharger technology and Holset turbochargers are an important element in the Cummins brand engine success throughout the world. But Holset brand turbochargers are applied to many commercial applications beyond Cummins brand engines, and include major names such as Volvo, Iveco, Detroit Diesel, and Scania among others.”
Holset engineers are very familiar with racing and performance. There assistance in the turbocharger used on the Banks land speed record setting truck "Sidewinder" is but one example.

Holset’s current state-of-the-art Turbos are the 'X-Range" family. From the HX20 for engines of up to 4 liters in displacement, to the HX83 suitable for engines as large as 25 liters, these turbochargers carry advanced design features that include better aerodynamics, improved durability, 'map width enhancement', and sound suppression. Holset also boasts a patented variable geometry turbine design they believe will surpass traditional designs in long-term durability. This new variable geometry design was first utilized on the 2007 Dodge-Cummins turbo diesel Pick up truck.

Overall, Holset have a lot of strengths that their rivals perhaps don’t have. For a start, Holsets spool very fast, far quicker than its counterparts. We've heard the Holset turbine wheel described as a work of art and it flows incredibly well, despite its small housing. The turbine wheel in the HX40, for example, with the right configuration, spools up to 500 rpms faster. With an upgrade of housing you can get a lot more flow psi which will bring you much greater horsepower.

Though Holset don’t employ extended tip technology, their compressors display greater efficiency than their counterparts. They use a Superback technology which makes their compressors incredibly durable. As one Holset engineer once told us, “They’re designed to be overworked and underpaid”.

So let us have a look at the Holset Turbochargers more closely.

Holset HX30

These are suitable for engines of up to 5 liters capacity that require up to 0.35kg/s of air at 3.1. It’s market is medium duty applications on vehicles such as many 4 cylinder automotives, busses and agricultural and industrial applications.

Holset HX30 TurboIt comes with a wide range of standard features such as a choice of turbine housing materials to counteract different temperatures. The compressor and turbine wheels are chosen specifically to obtain optimum airflow, efficiency and pressure ratio. The compressor, along with its housing, have fully adjustable orientation. Other standard features include fully floating twin hydrodynamic journal bearings, high pressure ratio capability and symmetrical twin flow turbine housing. Optional features are Map Width Enhancement, sound suppression baffles and water cooled bearing housings.

One problem with many turbochargers is that they’re not designed for optimum performance unless the engine has been set at very specific conditions. This can lead to excessive fuel usage and higher emissions. Holset have overcome that and the HX30 comes equipped with an integrated wastegate, which helps the engine to function with that smaller turbine housing, mentioned earlier, giving it a quicker spool time and less drag in boost.

The HX30’s major USP is it’s durability. Yes, you can go and buy a brand new one from the manufacturer, but if you spend a bit of time looking for a well-maintained used HX30 that’s for sale, you will not only save hundreds of dollars but a turbocharger that is just as reliable as the new one. The Holset HX30 is the standard turbo we offer on our 4BT Crate Engine models.

Holset HX35

Suitable for both 4- and 6-cylinder engines, the HX35 is designed for a power range of between 300 - 500 HP. It’s ideal for 1.8 - 2.5 L engines. With it’s simple oil-cooled construction it’s ideal for use in motorbikes too.

Thanks to it's low price and oil-only core it is a very popular choice as both an upgrade turbocharger and a unit for turbocharging an originally naturally aspirated engine. Its high flow aerodynamics can provide up to 0.46 kg/s at 3.0:1 pressure ration.

It comes with all of the standard and optional features that you get with the HX30 plus a CNG Engine version. Basically this is an upgraded HX30, just as good but a bit bigger for more boost. The HX35 is a popular choice for twin turbo setups.

Holset HX40

Designed for the bigger engine (7 to 10 liter), it is aimed at the medium/light heavy automotive sector, it has many of the features found in the heavy duty range and is widely used in busses and large vehicles across Europe and North America.

It comes with all of the standard and optional features of the HX35 but also optional are a Command Valve Boost Control, High Durability Impellers and the Holset Exhaust Waste Management Turbine Exducer. Like everything Holset manufactures, this turbo is built to last and thrives in high intensity situations. Made with the highest quality materials it’s designed to cope with the heat and stress of frequent use.

Another reason why it’s so popular is the ease in which it can be installed, much more so than its competitors. Remarkably, the HX40 has a similar spool times as the smaller HX35, meaning there is very little difference in throttle lags despite it being bigger. And of course, its price.

Yes, there are more powerful turbo chargers out there but you’d be looking to double, or even triple your budget to obtain one.

It’s good to remember though, even being very competitively priced, a new HX40 will cost twice of a used/reconditioned one.

Honorable mentions similar to the HX35 include the HX50, HX55, HY35W, HY40V, HY40W, HE341 and HE351VE. Really you can't go wrong with Holset when supping up a 4BT or 6BT Engine. The turbos are reliable and will supply that much needed boost when you need it. With older mechanical setups without aftertreatment controls (EGR, DPF, DEF) you won't run the risk of regeneration issues clogging the turbocharger. Cummins ISX Engines faced regeneration issues where exhaust particles not fully burned off clogged the turbocharger and ruined the unit.


Let’s turn our attention to our second manufacturer, Garrett.

Cliff Garrett formed this company in 1936 with a $5,000 loan and some help from friends. Initially run out of a tiny one room office in LA it grew to become one of the biggest names in the engine boosting industry. Its turbos can be found in Audi, BMW, Daimler, Chrysler, Fiat, Ford, Renault, Saab and Volkswagen.

Garret has long had the reputation for revolutionising turbocharger technologies and is responsible for many industry firsts, including its leading-edge patented dual-ball bearing turbos for high performance vehicles. For many teams racing in the World Rally, CART, Le Mans and Pikes Peak, a Garrett Turbo is their preferred choice.

They claim that their latest model, the G25 - 660, is the most powerful small frame turbocharger on the market.

Garrett G25 - 660

The G series are designed for top end optimal performance. Garrett have a history in delivering that and their turbos have been in the engines of cars who have won major racing competitions, such as the Indy 500, the Formula 1 World Championship, Le Mans, the World Rally Championship and countless others.

The G25 is designed for engines of 1.4 - 3 litre. The key to the G series power is their fully - machined billet aluminum compressor wheels, featuring next generation aerodynamics which give a larger HP range and maximises boost response. Which it certainly does as you can get up to 660 HP with the G25. It’s also robust as the turbine wheel is made with a Super Alloy, Inconel, that shrugs off long term exposure to high exhaust gas temperatures, and the water cooled CHRA makes sure the housing temperatures are kept to a minimum

Garrett can safely make these claims because they test all of their turbochargers to the extreme. One such test is to take the turbo beyond maximum operating temperature and shut it down hard. Repeat the test four more times and make sure maximum temperatures stay within their strict limits to avoid oil coking or build up inside the center housing.

So, on paper the G25 looks slicker and more powerful than the Holset HX35, though if you look around the turbo forums, many will disagree. There is one huge difference though, the price. A new G25 will set you back over $2,000, whereas you could get a new HX35 Super Turbo for around a third of that, a used one, much less.

GTX Gen II Series

Garrett GTX4202R TurboOne of the USPs of this turbo series is that it has the ability to create more air mass flow, meaning there’s a wider range of power output available when tuning the vehicle. This is managed through using compressor wheels which were designed by using computer simulation technology which helps to optimise the wheel design to reach the highest levels of efficiency and mass flow of each turbo, adding an extra 20% to the HP count.

An 8mm dual ceramic ball bearing cartridge adds optimal turbo response and reliability through its reduction in shaft motion.

As an extra you can add the Honeywell Garrett speed sensor, which allows tuners to understand existing operating conditions, which is essential when fine tuning the turbocharger to its most efficient operating conditions. The compressor housing has been altered to accommodate the sensor.

For the 4BT and 6BT our pick is the GTX4202R, one of the most popular sport compact drag turbos. You get most of the bells and whistles of the GT3582R but with a little bit cheaper cost. It has been revised and is supporting an additional 200hp. The GTX4202R is a great option for a large displacement low/mid boost setup or a lower displacement high boost setup and supports up to 1200hp and has four turbine A/R options. It comes with a compressor anti surge option.

Again, price lets it down, costing $2,500 upwards against $750 for the Holset. Performance wise, you have to say that the Garrett needs to offer much more to merit the difference in price.

GT Series

The GT series is synonymous with the pioneered turbo technology that boosted drag and road racing teams to break countless world records. It comes in both journal and ball bearing options. Among the options is an internally wastegated turbine housing. The cast compressor wheels feature original GT series aerodynamics, providing great durability and lifespan.

For the 4BT we've gone with the GT3582R, the most powerful turbo in the series, giving between 400 - 600 HP. It has a ported shroud compressor housing to increase surge resistance and the turbine housing is cast from high nickel ‘Ni-Resist’ material for extreme applications. It’s dual ball bearing and has an oil and water cooled CHRA. It also works well in twin turbo applications. If you can pony up the dough it's hard to beat this model if you decided to go with Garrett.

Borg Warner

Borg-Warner was formed in 1928 when it merged with Warner Gear, founded by Thomas Warner, and Borg & Beck, founded by Charles Berg. It’s led the way in automotive technology ever since, most notably through co-developing a variable - geometry turbocharger with Porsche, who used it in their 2007 911 Turbo. Ford, Renault, Peugeot and Volkswagen are all major customers.

Borg Warner didn’t enter the turbocharger business until 1998 when they founded K-Warner Turbosystems, and have been aggressively buying up turbocharger tech companies ever since, becoming one of the leading turbo manufacturers in the world.

S300 Series

The 300SX-E family is the only modern technology 1000+HP midframe T4 Journal Bearing turbo available and BW claim that it ‘redefines performance and value for modern turbochargers’.Through modifying their 91S compressor wheel blade shape they were able to increase airflow rate to 98lb/min. It’s features include Integrated speed sensor port, 360 degree thrust bearing, Pre-machined boost port, Forged milled compressor wheel, Flexible compressor cover outlet options and Optimised compressor stage aerodynamics. With its’ smaller dimensions and high airflow rates it’s a popular choice for high- boost and high-power applications.

Utilising the latest generation 69.00mm billet EFR compressor wheel aerodynamics with Motorsport specification 360 degree thrust assemblies, the all new ‘SX-Enhanced’ turbos redefine performance and value for modern turbochargers.

BW engineers started with their 91S compressor wheel blade shape from the EFR9180 (more on the EFR series later), but trimmed to 75% for an increased airflow rate of 98lb/min. This all-new wheel is packaged in a purpose built SX-E compressor housing with ported-shroud inlet and optional speed sensor port. While Airwerks 80mm Inconel turbine and flexible housing options maintain consistency with existing installations. Smaller overall dimensions than the EFR9180 and higher airflow rates make this an excellent match for high-boost and high-power applications. Oil-Cooled Journal Bearing only.

The S366 SXE Turbo is a good fit for the 4BT or 6BT Engine. It’s features include being available as a complete turbo -or- supercore (no turbine housing) to retrofit existing Airwerks installations. It has forged milled compressor wheels with Optimized compressor blade aerodynamics and all-new severe duty 360 degree thrust bearing. The compressor cover has an integrated speed sensor port and pre-machined boost port.

Price wise, it’s closer to the Holset HX series but still more expensive, at around $950. Way cheaper though than a Garrett turbo for those on a budget.

A popular choice for the 4BT and 6BT is the Borg Warner 171270, which is the result of a joint venture between BW and Holset. The turbine housing has a .7 A/R and the compressor wheel has an OD of 68.6mm and an inducer of 40.6mm. The turbine wheel has an OD of 69.6mm and a 60.7mm exducer.

Smaller than the Holset HX30, with the fuel turned up it can spool 29lbs. A common complaint seems to be that the temperatures it reaches are on the high side.
It’s manufactured for maximum durability to get the most out of your investment and has direct bolt-up for ease of installation. Like all BW and Holset turbos it’s engineered for high efficiency and long service life and comes with a one year/100,000 mile warranty.

The latest innovation from Borg Warner is their much anticipated EFR series.

Borg Warner 7670 EFR Turbo

Borg Warner 7670 EFR TurbochargerThe EFR 7670 is one of the smaller B2 frame turbochargers in Borg Warner's ultra-efficient EFR lineup. The 76mm OD compressor has a 57.2mm inducer at 64lb/min max flow - supporting up to 650hp with very large map width for outstanding power potential even at high boost. The 7670 is one of fastest spooling EFR turbos - and fits in a medium frame size. Ideal for small engines (up to 3L) in single turbo configuration and stock internal or modified engines or when paired with an additional 7670 turbo to create a twin turbo setup that can handle up to 1300 HP! Water cooling is highly recommended.

The unique aspect about the Borg Warner 7670 is that it is a hybrid venture combining Holset, Borg Warner and Honeywell. Borg Warner incorporated some of the turbo housing designs in with Honeywell's precise technical engineering to create a beast of a turbo.

The EFR 7670 seems like it might be an excellent fit for the top end enthusiasts and those willing to incorporate a turbo that is relatively new to the market. It seems great on paper and preliminary reviews are stellar but guys haven't had them in the field long enough to get a fair assessment. The price will break the bank though starting at around $2,000 all the way up to $3,000+ for more powerful models.

Final Conclusions

Of course, all three manufacturers make many more versions of turbochargers but generally each turbo in a series has the same features and further option choices. There is no definitive pick for the 4BT or 6BT as there are many different applications the engine is used for. If you’re on a budget Holset is where you should start as they’re built to last and won’t break the bank. But if you’re thinking of entering Le Mans, you’d be better suited with something from the Garrett range.

For top end mods if we had to make a choice, we would go for the Borg Warner 7670 EFR Turbo, purely because you get all of the engineering expertise of Holset and the tech expertise of Borg Warner and Honeywell, at a reasonable price for a top end turbo. That puppy is nice! For general use turbos to pair up with that 4BT or 6BT mod a Holset HX30 or HX40 will be just fine to get that added boost when you need it. No matter if you have a rat rod or simply a skid steer with a little 4BT or 5.9 L adding a turbo to the setup will make the ride just a little sweeter.

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