TECH: SIGNAL BOOST

Building a hybrid turbocharger.

Read through the pages of many motoring magazines and you’ll note a significant number of the cars featured are being propelled by engines making use of hybrid turbochargers. Essentially, this means the host car’s snailshaped bhp booster differs from manufacturer specification in some way or another, usually through the introduction of aftermarket components or, as is increasingly the case, custom modifications deemed necessary in the drive for more power. Changes can also be applied in the interests of taking a tired turbocharger and affording it modern levels of performance and reliability, ordinarily by way of fitting current-specification parts in place of worn original items. Doing so is usually cheaper than forking out for a new turbocharger.

There are lots of ways to modify a turbo, although the choice of changes available can differ depending on the design of the unit being worked on. That said, the end result is almost always the same, with quicker boost response in the lower rev range and extra airflow capacity for more power at high rpm. By mixing, matching and machining uprated parts, independent forced induction specialists are able to provide owners of performance cars with precisely profiled turbochargers helping to deliver vastly increased power and torque.

When approaching a company offering hybrid turbocharger conversion services, ensure you enquire about the extent of work being carried out. With ‘hybrid’ simply referring to a shift from OEM specification, a tiny change may be given the same label as a thorough rebuild. The latter is demonstrated across the following pages.

Spotting a bargain on ebay, we bought a Garrett turbocharger originally prepared for racing by Northampton-based engineering firm, Turbo Technics. “The unit was modified to deliver the power of a T34, but with much faster spooling,” confirmed one of the company’s representatives. The work took place many years ago, and though the seller claimed the part had been collecting dust since being treated to a full service and new oil seals, there was no way of confirming the performance part’s recent history. We had to assume this mystery Garrett was in need of overhaul.

With this in mind, we commissioned the forced induction wizards at Dorset-based turbocharger supply, repair and modifying specialist, Turbo Dynamics, to demonstrate exactly what's involved in the strip, evaluation, upgrade and rebuild of a tired old turbocharger when a customer asks for the part they're in possession of to be brought up to date with modern internals.

HYBRID THEORY

A hybrid turbo should be capable of delivering 15-30% over its previously rated maximum bhp, but other equally important performance improvements can be observed following installation. Reduced lag and significantly better response across the rev range are just two benefits, while designing the turbo to custom specification (as opposed to relying on an ‘off the shelf’ solution) will ensure the finished product is perfectly suited to your Porsche's engine, fuelling and airflow equipment. Of course, other apparatus, including cams, injectors, intakes, intercoolers, ECUS and exhausts, will have a huge impact on overall performance, but even owners of turbo Porsches with standard engine equipment will see improvements after fitting a hybrid.

As mentioned at the beginning of this article, when referring to turbochargers, the term ‘hybrid’ is used to describe any unit modified from original specification. In other words, what one person presents as a hybrid might be totally different to what someone else presents using the same label. The term originates from the days when altering a turbo by using parts from other units was common practice. For example, if turbo A was too big for the intended application, but turbo B was too small, it was accepted that taking parts from A and shoving them inside B would result in a fit for purpose ‘hybrid’. Advances in manufacturing processes and the advent of specialist components (ball bearing cores etc.) means the days of taking the front off a turbo and bolting it onto the back of another are long gone, even if ‘hybrid’ remains the word most commonly used to describe an altered turbocharger.

It’s claimed this turbocharger had been serviced prior to being put into storage and subsequently offered on ebay. Even if that’s true, we could be introducing the unit to an entirely new engine operating environment. In short, without dismantling, there’s no sure-fire way of us being able to check whether this turbocharger is in sound working order.

An unusual serial number revealed little about the history of our new turbocharger, but after much in the way of head-scratching, Turbo Technics confirmed the unit was originally built for a heavily modified competition car owned by Racing Technologies Norfolk (RTN), the company now known as trackday organiser, Palmersport.

To begin with, the turbocharger was fully stripped, before each of its component parts were checked and measured for wear. The turbine shaft surface, thrust bearing face, thrust collar and journal bearings were all checked for signs of heat damage, material transfer or scoring. Reclaimable parts were put to one side.

Keen to avoid any nasty surprises (and hopeful of extracting more power out of the newly acquired part), we contacted the hybrid turbo heroes at Turbo Dynamics and requested a strip and rebuild of our bhp booster to custom specification. The firm has what’s thought to be the largest catalogue of hybrid turbochargers anywhere in the world. Our Garrett was in safe hands.

Turbo Technics engineers upgraded the unit with a 360° thrust bearing, a T35 compressor wheel and a .48 A/R turbine housing for faster response. RTN wanted a punchy turbocharger that could deliver between 340bhp and 380bhp at 1.6bar boost. The T3 fitment is commonly used across a wide range of OEM and aftermarket applications.

The blades of the compressor and turbine wheels were examined in order to identify any foreign object damage or contact with their respective housings. The housings themselves were also looked at in detail. In this photograph, we can see scoring and material transfer on the turbine shaft.

A high-performance compressor wheel was selected for use in our upgraded turbocharger. A device known as a clock gauge was called upon to ensure the new part was held correctly before any machine work began. The compass-like instrument consists of an extendable arm, a fine point and a ball tip. The ball was rested on the hub of the new compressor wheel. The wheel then span, and the clock gauge let the operator know whether the new part needed to be re-seated before reprofiling work.

Because the guys at Turbo Dynamics were building our Garrett to custom specification, the centre bore of the new compressor wheel was modified to match the diameter of the turbine shaft then inserted into it. In the instance of the shaft being smaller than the core, the core could have been sleeved to suit the diameter of the shaft.

The modified compressor wheel was test-fitted to the new ‘turbine shaft and wheel’ combo. The new shaft was designed to accommodate staggered gap oil seal rings, parts which prevent leaks even when exhaust back pressure is reduced through the removal of catalytic converters or an exhaust system making use of larger-than-standard pipework.

Despite its age, our turbocharger can be equipped with thoroughly modern parts. This picture shows the very latest compressor wheel design, chosen for its lightweight billet construction and a performance rating of 450bhp. Adding to performance advantages, this particular compressor wheel is twenty percent lighter and much stronger than an equivalent cast part.

Parts that were heavily affected by oil residue and carbon build-up — such as the compressor cover and turbine housing — were exposed to a high temperature propane burner. This treatment eradicates any stubborn contaminants. The treated parts were then subjected to the full force of a heavy duty media blaster.

Although many of our turbocharger's parts were no longer fit for purpose, some could be reused. Reclaimable items (including half-moon clamp plates) were chemically cleaned, hot-washed and media blasted in order to fully remove dirt, grease, carbon deposits and any corrosion.

A new compressor back plate (the part that sits between the compressor wheel and bearing housing) was selected and held in a lathe in advance of modification. The component needed to be machined to a custom profile to allow for the larger diameter of the new compressor wheel.

With measurements taken, lathe is called into action! In this photograph, you can see the back plate being modified to match the overall dimensions of the compressor wheel’s larger-than-standard exducer, as well as its extended tip technology.

Turbochargers get hot. Red hot. Any metal surface can be affected by heat, but it’s of vital importance a turbocharger flange face is perfectly flat in order to prevent the leak of exhaust gases. This is especially important if you’re planning to attach a second-hand turbocharger to a new or replacement exhaust manifold.

As highlighted in the previous picture, any warping to the flange face that may have been caused by prolonged exposure to heat is completely eradicated at this stage of the build. Not addressing this part of the process is a huge oversight, and one that's likely to deliver unwanted results. You won’t need to worry if you’re working with swish new turbocharger hardware, but it stands to reason that an older turbocharger subjected to an unknown number of operational cycles will benefit from the restoration of its mating surfaces. You have been warned!

Here's the modified turbine wheel. Look at any dyno graph and you’ll see linear power tends to drop suddenly when a turbocharger reaches its limit. Cut-back blades allow for greater performance in the upper rev range, enabling the turbo to continue producing power long after it would ordinarily be able to do so.

With the shaft’s perfect fit confirmed, the turbine wheel could be prepared for action. Its blades were cut to a specific angle so that back pressure at high rpm is reduced. A large grinding wheel was used to make the change — a modification greatly improving top-end performance by allowing the turbine to flow air beyond its usual limit.

The end result is a precisely modified, application-specific compressor cover designed to leave only the slightest of gaps between its core and the edges of the uprated compressor wheel. Perhaps unsurprisingly, the work involved is often described as being something of an art form!

To recap, all of the parts you can see in this image are brand new, and most were modified to meet the custom specification of our turbocharger. The compressor wheel, compressor back plate, heat shield, journal bearings, back plate o-ring, thrust bearing, screws, bolts, thrust collar, thrust flinger, actuator c-clip, oil seal rings, snap ring, turbine shaft, bearing housing and nose nut (fitted to the end of the turbine shaft) were all selected and/or altered to suit our requirements. It was now time to bring the pieces together.

The compressor cover profile was modified to suit the new, larger, billet compressor wheel. This stage of the process was carried out by engineers who used special measuring equipment to manage fine tolerance whilst removing material until the correct, constant clearance between the housing and wheel was achieved.

When exhaust gases spin the turbine wheel, pressure pushes it against the thrust bearing. The greater surface area the bearing has, the more capable it is of preventing exhaust pressure from pushing the turbine inwards (hence our 360° bearing). Here’s a photograph of the finished bearing housing.

The bearing housing was drilled, countersink holes were added and locating pins were installed. While the design of our modified Garrett turbocharger dictates that the new thrust bearing must be screwed firmly into place, there are some performance applications where the thrust bearing doesn’t require a fastener.

A completely new bearing housing designed to support a motorsportspecification 360° thrust bearing was chosen before being modified to suit our needs. The bearing itself has a larger surface area, which enables it to handle higher levels of boost pressure when compared to the performance of a traditional 270° bearing.

Wow! The amount of time and effort Turbo Dynamics has invested in modifying and rebuilding our formerly tired turbocharger is clear to see. The completed assembly can now boast a better than factory finish, hugely improved reliability and massively increased performance across the entire rev range. All that's left for us to do is find an engine to mate the unit to!

Specific values were programmed into the VSR to allow it to take into account each and every modification applied to the parts it was being asked to monitor. This fine-tuning ensured the machine returned accurate information the technician could then use to make final changes to our modified turbocharger.

The turbocharger core (bearing housing, bearing assembly, compressor wheel, back plate, turbine shaft and wheel) was assembled before being mounted to the Turbo Dynamics in-house Vibration Sorting Rig (VSR), a tool using compressed air to spin the turbocharger up to operating speed. A magnet was then dragged across the compressor wheel, activity which sent information to a series of sensors, allowing the company’s technicians to instantly identify where any unwanted imbalances may have been present.

Any detected imbalances were cancelled-out through a process involving the removal of small sections of material from the turbine shaft nose nut. Once this stage of the build was complete, the remaining parts of the turbocharger could be fitted. The unit was then ready to be boxed and shipped.