DIY 2-Stage Boost Controller

Story by Martin Donnon and Hot 4’s Magazine

Original Scans courtesy of Damien D.

It’s a well known fact. Anyone who doesn’t agree that it’s incredibly cheap and easy to extract additional power from turbo cars has rocks in their head. The problem is, exactly how do you do it? There are numerous ways of winding up the wick in a turbo car, but exactly which one should you use and how far can you go before running into reliability/tuning problems?

The key to improving a turbo car’s output is simply to raise the turbo boost pressure. This has a number of effects. Firstly, the inlet pressure in the inlet manifold ( and exhaust manifold for that manner) is higher, giving the effect of stuffing more air and fuel into the combustion chamber

Theoretically, the overall result should be that the engine produces more power as you’ve pumped a higher volume of gas into the cylinder. If it were that easy, then everyone would run massive amounts of boost in their turbo engines, but as you might have guessed, increasing the boost introduces some potential problems.

Cast your mind back to earlier articles about water injection. I have written that cover the basics of density ratio. The higher the boost pressure, the hotter the inlet air charge becomes and the less power the engine will make.


You have to make sure that your charge cooling system (intercooler or water injection) can cope with the additional temperature load of increased boost. If you believe that your particular car may be marginal (that is small or no intercooler), then it is best to keep boost increases down to a minimum.

There is also the problem with thermal and mechanical load on an engine. You must be aware that increasing the boost pressure puts more thermal stress on the pistons in particular, and also results in more heat being transferred into the engine oil. Not to mention the turbocharger itself, which has its internal shaft rpm increased by as much as 20,000rpm during a simple boost-up. Detonation is often a big problem with boosted engines – listen for it, have it checked on a dyno, and at first sigh of a problem, consider lowering the boost pressure (or not raising it as much).

There is no easy answer here of course, because the number of ways of controlling boost alone is simply enormous. Almost every serious player in the local workshop scene is able to offer some form of electronic boost controller – allowing the engine to have its boost increased by means of a microprocessor-controlled solenoid mounted in-line with the wastegate actuator hose. These systems are generally very good, although it pays to have an experienced fitter to avoid such problems as surging engine or nasty overboost. The only real drama with the electronic controllers is their price. Most systems range from $700 – $1500 for a complete package.

As I said, they are great but it isn’t the end of the world if you can’t afford one. There are other options, and the DIY 2-way boost controller presented here would have to be one of my favourites. To show you exactly how to build your own, we followed the installation of the 2-way boost system on a ‘typical’ cheap turbo car – the Nissan Pulsar ET Turbo.

The concept

The 2 – way boost controller is a pretty simple device. It consists of a dash-mounted switch that controls a solenoid fitted in-line with the wastegate actuator hose. The whole idea is that when the switch is in the ‘off’ position, the solenoid is closed and all the air goes to the actuator, giving standard boost pressure.

However, when the switch is set to the ‘on’ position, the solenoid is opened and allows a controlled amount of air to bleed through a screwdriver-adjustable ‘bleed valve’ out to the atmosphere. The end result will be a boost change at the flick of a switch, allowing a nice, quiet standard mode for relaxed driving.

Parts required

With this system, we used a conventional in/out solenoid from Goyen Controls, who can be found in most states of Australia. Rated to 120psi, there was no chance that this would have problems holding boost on an ET turbo! From there, a quick trip to the brass-fitting shop secured a quarter-inch brass barb with a 0.125-inch gas fitting thread and, of course, the all-important screw-type bleed-off fitting. If you can’t find the bleed fitting at a brass fitting shop, no doubt your local performance workshop should have one at a reasonable rate. An easier solution still would be to buy the complete 2-stage boost valve kit from Turbosmart, which is pretty good value too, but I know some of you guys like to do things the hard way.

Now all the under-bonnet stuff is out of the way, we need to concentrate on the interior side of things and select the right switch for the job. We settled on a simple and small 2-way steel switch that would neither be too obvious nor too hard to find when mounted in the dash. A trip to the local auto supplies shop also netted us a conventional 12-volt automotive relay and some spade connectors. With all of the parts sourced for a grand total of $85, we were ready to start work.


The first thing you have to do is identify the pressure line going from the turbocharger to the wastegate actuator canister. it is a rubber line (in almost every case) and is pretty easy to locate. This is where the tee piece has to go. It can be slotted in the actuator line (when you have cut it with side cutters) once you have soldered one end up and drilled a 1.5mm hole in it.

Soldering one end of the tee piece is a pretty simple affair, requiring only a simple soldering iron and a small length of solder. So don’t go to town applying too much solder as it makes it harder to drill. Once done, it’s a simple matter of drilling the freshly soldered end to 1.5mm by using (guess what?) a 1.5mm drill bit. When finished and ready to install, it is important to make sure the drilled end of the tee piece is inserted facing the turbo compressor rather than the actuator.

I have assumed that you could learn how to screw the bleed valve into one side of the solenoid from the photos, and the standard bass barb into the other. From there, it’s simply a matter of working out exactly where you want to mount the solenoid and bleed valve in the engine bay ( I would strongly suggest a place you have easy access to). With the Pulsar, you will notice there was an overboost blow-off valve in the inlet manifold. This has to be either plugged or welded up like the example in the photographs. Thankfully not many cars have crap like that on them.

Once you have joined the tee piece and the barb on the solenoid together with a piece of high-pressure fuel hose ( using small screw-type clamps on all hose joints ) you’re ready to start wiring. There is not much you can say about the wiring other than look at the diagram I have supplied (picturing the numbered underside of the relay). It really is that easy.


Always make sure that you fully close (wind in) the bleed valve and at least have a boost gauge fitted before you attempt to set the boost up on the high side of the switch. Setting the car up on a dyno is ideal (exactly what we did in this instance), as it allows accurate measurement of the air/fuel ratio and easy detection of any detonation. The best part is that you can get a before and after dyno printout.

   With the switch set on the high position, it’s simply a matter of winding the bleed screw out half a turn at a time until the boost gauges indicates the desired boost level, or until fuel lean-out of detonation starts to occur. On our Pulsar test bed, we doubled the standard boost to 13 psi (from 6.5psi) using the 2-way switch, and as you can see from the dyno graph, we achieved a very respectable power increase.

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