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How Engineers Sought To Destroy Turbo Lag With Science

Here at eEuro, we love turbochargers.  Most of our favorite engines are turbocharged.  Even our favorite non-turbo engines (such as the BMW M50) are also getting boosted these days thanks to a younger crowd of enthusiasts, suitable compression rations, and falling prices on the used car market.  However, if you’ve ever turned up the tap on a turbo car you’ll notice something very quickly, nothing.  For a moment nothing will happen.  We call this enemy ‘turbo lag’, and it must be stopped at all cost if we are ever going to keep up with our Naturally Aspirated challengers on both the street and the track.

A history of turbo lag

Originally, turbochargers were looked at as a way to add power to large engines that maintained a constant RPM, and occasionally for racing applications for a select group of ballsy drivers who could cope with the lag. In the 60’s a few manufacturers were messing around with the idea of adding turbos to street cars, but the world just wasn’t ready.  GM put what was basically an industrial turbocharger on their Cutlass, but because of a high compression ratio it ran like an epileptic water buffalo without water/methanol injection* (and swiftly died like one, too).  A similar setup had a brief stint in the Corvair as well.

After those failures, the Germans thought they would have a go.  In the 70’s they put some of their racing experience into street turbo systems, notably on the BMW 2002 and Porsche 911.  However, these systems were descended from some of the most ridiculous racing cars ever made and were not what you would call ‘street ready’.  The large laggy turbos were horrendously hesitant and made cruising around town difficult. The 911 Turbo was also the fastest production car ever made at the time.  Fun, but not what you’d take the kids to school with.

Effective for the track, scary for the street. The powerband on early turbo cars resembled drinking from a firehouse.  Great for crab fishing boats, not so much for street cars.

Around the same time, the ailing car maker Saab needed a hook.  They couldn’t afford to design a whole new engine, so they put a small T3 turbocharger on the Saab 99 triumph derived 4cyl “B” engine.  After all, they had some experience in the airplane industry where turbos were used to keep piston engines breathing at high altitudes.  Shortly after equipping the 99 with a turbocharger, Saab introduced the Automatic Pressure Control system.  The APC greatly increased driveability by having a variety of valves and a knock sensor now controlling the boost pressure.  The result was a peppy car that made useable power from a small engine, while still being able to compensate for low octane fuel, poor maintenance, and otherwise non-perfect conditions witnessed in the real world.

Building on Saab’s success

With turbocharging for the street now coming into play, engineers started the crusade for better optimization and the complete annihilation of turbo lag.  Getting a turbocharged engine to spool up fast without running out of huff is a trick.  It’s all about moving air as efficiently as possible, and a well designed exhaust manifold is crucial to making an efficient and powerful turbo engine (see Twin Scroll below).  However an exhaust manifold is something that has to be very specifically designed for every individual engine and application, so in order to make a more plug-and-play option, manufacturers started trying some different techniques.

Multiple Turbochargers

Parallel Turbocharging – Perhaps the most straightforward to cut down on turbo lag is to use a smaller turbo that can spool faster.  For those with power goals, sometimes a single small turbo isn’t enough, so adding a second small turbocharger can make up for the added volume without increasing lag.  The downside of a parallel twin turbo setup is an increase in complexity.  Sometimes engine bay packaging won’t allow for this setup, and too much turbo piping will quickly nullify any gains in lag by introducing too much volume to compress before the engine can take advantage of the extra manifold pressure.

This great illustration shows a single stage sequential setup

Sequential Turbocharging – Taking a slightly different approach, a system using sequential turbochargers reduces lag with one small turbo and one larger one.  Many of these systems take care of the task a little differently, but generally the small turbo spools early at low RPM for nice driveability.  After a good head of steam is built up, the smaller turbo passes the baton to the larger one, which continues the induction party through the upper RPM ranges.  The infamous Toyota 2JZ GTE is one of the better known engines to feature sequential turbocharging.  The power delivery characteristics of this engine makes it one of the most recognizable engine codes in the world.

In a single stage sequential system, both turbos are connected to the engine independently, which causes some tricky piping to pull off.  In a dual stage system, the smaller turbo and the larger turbo are plumbed in sequence, which also takes some clever manifold design and ducting to pull off.  Although not terribly common, dual stage sequential turbocharger systems are most popular in diesel applications.  An example of which can be found in the BMW E60 535d.

The Twin Scroll Turbocharger

Taking the basic principle of a sequential turbocharger system and compressing it down into a single unit is the basis of the Twin Scroll turbo design.  With two different sized air channels (aka scrolls) in the turbine exhaust housing, the twin scroll turbo can utilize both a high velocity/low volume channel, and a low velocity/high volume channel without the need for multiple center rotating housings.  However, there is an even better advantage to the twin scroll design, and that has to do with the exhaust manifold being divided based on cylinder firing order.  This keeps the pressure pulses from adjacent cylinders from interfering with each other.

The exhaust manifold for the S63 V8 crosses between cylinder banks to ensure alternating firing order and keep pressure waves from colliding. Each channel is separate as it enters the turbos different scrolls.

For example, on a 4cyl engine, the first scroll will be a high velocity/low volume channel, and will be hooked up to cylinders 3 and 2 which fire in opposite time.  The second low velocity/high volume scroll will be powered by cylinders 1 and 4 which fire in opposite time.  For the best exhaust manifold tuning, you don’t want these pressure waves colliding in the collector, that results in poor flow, heat buildup, and pressure resonance.  If each pressure wave is allowed space to zoom straight out, you’ll make more power and less heat around the exhaust valves.

I recently wrote an article about the BMW N63 V8, which has a top mounted single scroll twin turbo system.  However, The S63 found in M cars utilizes twin scroll turbochargers, and the manifold crosses the engine to different cylinders so that pressure waves are each allowed time to evacuate.  The result is a major boost in power and reduction in turbo lag with this simple design implementation.

Variable Turbochargers

This is when things start to get complicated, and there are a few names for these.  Variable Geometry, Variable Nozzle Turbine, and Variable Vane are all used to describe this technology.  In a variable turbocharger, there is a series of vanes inside the turbine housing that twist and move to control the airflow to the turbine wheel.  Each vane has a small pivot shaft that goes outside of the turbocharger and connects to a ring that has a few degrees of rotation built into its function.  The degree of rotation on the control ring is influenced by an actuator mounted near the center of the turbo between the turbine and compressor housing.

There are a lot of moving parts in a variable turbo that must maintain precision in extreme temperatures and chemical exposure.

At low RPM, the actuator will push the ring to force the pivoting vanes inside to close the passageways to the turbine wheel.  Because the exhaust is being forced into smaller channels, it must pick up speed (this is known as the venturi effect).  This allows for a reduction in turbo lag, but also.  As RPM climbs, the engine computer will signal to the actuator to rotate the ring in order to allow the vanes to open up, forcing a more direct (and hire volume) of air into the turbine wheel.  Variable vane turbochargers are popular on large diesel applications, but are complicated.  Thus, their use is not as prolific as the twin scroll design.  Industry pioneer Borg-Warner has actually recently come up with a design that combines a Twin Scroll with a Variable Geometry turbocharger.

The first use in street cars goes back to the Shelby CSX which was a wonder of technology at the time.  With a near complete elimination of turbo lag, max torque was available as low as 2100rpm all the way to the redline.  However, it was not without its problems.  Because of the location that the variable vanes need to live in, they would get fouled by carbon and heat, and many users swapped to conventional style turbos.  This car also had composite wheels called ‘Fiber Rides’.  You should read more about the CSX if you get a chance, moving on.

Hybrid Super/Turbocharging

Only recently has the future of turbocharging been integrated with electric motors, but the technology has major advantages that haven’t been fully realized yet in modern production.  Volvo is currently picking up the task of reducing turbo lag by incorporating added superchargers (pully drive) to their Drive-E turbo engines, with electrically spooled turbochargers on the horizon in their not-yet-realized ‘Drive-E Triple Boost‘ engine.  That system borrows electric supercharging technology from the ERS systems used on modern turbo Formula 1 cars.  There are a lot of ways to approach electric turbocharging which I won’t go into too much depth here (perhaps another article), but Volvo specifically uses a third electrically powered supercharger to force extra air into the standard single scroll twin turbos on the compressor side.  The engine has not made it into complete production yet, but more than likely we will see these concepts slowly leak into the consumer market, and make exhaust driven turbochargers obsolete.

Turbocharger Anti-lag

I would be remiss to not mention the technology of anti-lag, a complex system designed to never let the turbo spool down.  There are a few approaches to this (again perhaps for another article) but the main idea is to always keep exhaust flowing, through the turbine housing.  This means that the car doesn’t spool down on throttle lift, and sounds like a machine gun unloading into a box of fireworks.  This is a very popular system in top tier rally cars.


*The water/methanol injectable solution for the “Jetfire Turbo Rocket” engine was called ‘Turbo Rocket Fluid”.  I love that.

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One thought on “How Engineers Sought To Destroy Turbo Lag With Science

  1. As a broad overview of turbo technology, on a vendor blog, this was a nice write up. A lot of people are still in medieval times where this technology is concerned. I just bought my first turbo car (and I’ve owned 40+ over 35 yrs) this past year. I have always avoided them like the plague, but I knew Volvo and a few other makes had a good track record. And older Volvos and Saabs can be had for very reasonable prices and are a great deal (if you can work on them yourself). So, I took the plunge. I love my S40 and the 1.9T is a sweet little engine that makes peak torque from 1800-4800 RPM and it averages 25 MPG. I know there’s more power to be had, but it’s my daily driver, so I don’t want to compromise reliability or economy, so it will remain stock. I do plan on doing a 1.8T AWP swap into my Mk3 Jetta though. So you can count me as a convert.

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