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Basic Hands-On - How to Fit a Boost Gauge

Step-by-step fitting a mechanical boost gauge - properly...

By Michael Knowling

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The humble boost gauge has become a must-have accessory for turbocharged and supercharged cars. It doesn't matter if you drive a Pulsar ET or a Jaguar XJ-R, a boost gauge is essential for observing peak boost levels, tuning and general faultfinding. All too often, though, we see boost gauges hastily fitted; poor gauge mounting, inappropriate pressure pick-up locations, or nasty wiring and plumbing.

In this story we'll give you a step-by-step tour of how to properly install a boost gauge. While it's aimed primarily at automotive 'newbies', the more experienced tweakers might learn a thing or two as well...

What Sort of Boost Gauge Do You Need?

A little bit of consideration is essential before handing over your cash for a particular boost gauge.

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First, you need to decide the maximum boost pressure you plan to run. If, for example, you expect to run no more than 1.2 Bar (17.4 psi), a gauge that reads up to 1.5 Bar (21.8 psi) is perfect - it's good to have a little bit of headroom so you can recognise overboost conditions.

You may be aware that some boost gauges read positive pressure and engine vacuum; a vacuum scale is very handy for setting up an adjustable blow-off valve, general tuning and faultfinding. Speaking from experience, it's also the easiest way to identify when a small hose has popped off the intake manifold. Certainly, where possible, go for a boost gauge that shows vacuum.

Note that the sweep of the needle is important for viewing accuracy - the greater the sweep, the more likely you'll make a correct reading. Another important factor is the diameter of the gauge - the most common diameter is 52mm, but smaller 40mm units can also be bought. Unless mounting space is an issue (which you'll need to determine before making a purchase) we'd opt for the larger size gauge, as they're considerably easier to read.

And what's better - a metric or imperial boost gauge? This comes down mainly to personal preference. Psi (pounds per square inch), kilopascals (kPa) and Bar are the most common pressure units - whichever of these suits you is fine.

Your budget will determine whether you'll be using a mechanical or, possibly, an electronic boost gauge. Mechanical boost gauges, which use an internal Bourdin tube, can be bought brand new from as little as $40. Electronic boost gauges, however, can run to several hundred dollars. As the name implies, an electronic boost gauge is fed an electrical signal from a remote pressure sensor. Electronic boost gauges have a few advantages - they generally have slightly better accuracy, most have a peak recall function and, because the gauge is fed an electric signal from a remote pressure sensor, there's no requirement to run an air hose into the cabin.

Installing a Mechanical Boost Gauge

Where to Find the Optimal Pressure Source

The best spot to connect a boost gauge is the intake manifold; most typically by 'T-ing' into one of the existing manifold hoses. So why is the intake manifold best, you ask?

First, manifold vacuum won't register on boost/vacuum gauges unless you make a connection downstream of the throttle body (ie inside the intake manifold). Only boost pressure can be measured along the path between the turbocharger and throttle body.

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The second advantage of connecting the boost gauge to the intake manifold is you'll receive a more accurate representation of actual boost - the amount of positive pressure received by the engine. Connecting a boost gauge anywhere between the turbo compressor and throttle will give a too-high boost reading. Thanks to the inevitable restriction through the intercooler core, intercooler plumbing and throttle body, an engine suffers a boost pressure loss on-route to the manifold.

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Using a 1994 Subaru Impreza WRX as a demo, we T'd our boost gauge (a second-hand Japanese import job) into one of the hose connections on the topside of the manifold. The hose we selected was one that connects to the purge control solenoid on the underside of the manifold.

Note that - instead of cutting the factory solenoid hose - we fitted a T-piece to the existing end and added another short length of hose to hook onto the manifold connection. This means, if we later decide to remove the boost gauge, the original hose will be long enough to reattach to the manifold connection. The third port of the T-piece is dedicated to the boost gauge.

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And what sort of T-pieces should be used? Brass fittings are certainly the most durable but for a completely stealth installation you can't go past the plastic T-pieces used on 993cc Daihatsu Charade carby turbo engines. We're led to believe that the same T-pieces are fitted to atmo versions as well. These beaut little T-pieces can be picked up from the wreckers for free!

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To ensure none of your boost gauge hoses pop apart, the vacuum hose clamps used on the Daihatsu Charade Turbo are, again, ideal and available for free. We didn't bother with these since we don't expect to run mega boost.

Four millimetre rubber vacuum hose - available from auto parts stores for around $3 per metre - is fine for running from the manifold to the in-cabin gauge. Vacuum hose offers OE-level durability and it integrates into the underbonnet scenery perfectly.

Routing the Hose

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There are a couple of things to remember when routing the hose from the manifold to the in-cabin gauge. The hose should not be kinked, it should be kept well away from intense heat sources and it should not foul the operation of any moving parts - particularly the throttle!

We routed our boost gauge hose around the factory blow-off valve tube, beneath the top-mount intercooler, parallel with some factory tubes that run along the firewall and, finally, though the firewall. A couple of cable ties were used to secure the hose into position.

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Most cars have several firewall openings that are capped with a rubber or plastic seal; we'll use one of these to pass through our boost hose. Have a good look under the bonnet to find a seal that isn't too crowded with wires and/or cables and prise it away from the firewall using a flat-blade screwdriver - be careful not to strain any wires or cables from inside the car. Next, enlarge the hole in the seal so it's big enough to accept the 4mm rubber hose - this can be done with a pair of pointed scissors or a razor. With that done, refit the seal to the firewall and poke the boost hose through to the cabin.

Note - in no circumstance should a hose, cable or electrical wire rub against the metal edge of the firewall hole. Install a firewall seal from another vehicle or use a sheath (even just a short length of rubber hose) to avoid contact with that sharp metal edge.

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Working from the driver's foot well, route the hose beneath the dashboard so it won't foul the operation of the pedals, steering, handbrake or any other controls. Again, use cable ties to secure the hose at various locations to prevent it falling somewhere it shouldn't - like on your feet while driving! Cut the hose in the vicinity where you intend to mount the gauge.

Wiring

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Most boost gauges incorporate a bulb for nighttime illumination. Two electrical terminals can usually be found on the back of the gauge - one for a positive electrical feed and one for Earth. Note that - unless LEDs are used for illumination rather than a conventional bulb - you can hook either terminal to the positive or Earth wire. A conventional bulb is not polarity conscious.

So where should you wire the illumination bulb to? Well, the best place is to connect into the existing instrument illumination circuit - this enables the boost gauge to illuminate along with the rest of the vehicle's instruments.

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In the case of this particular install, we powered the boost gauge bulb from the illumination circuit for the cigarette lighter (which is immediately alongside). We simply T'd into the two wires that are used to illuminate the cigarette lighter surround and ran the appropriate length of hook-up wire to the ashtray area. To enable easy installation and future removal, we soldered two short lengths of wire onto the gauge terminals and adjoined the ends with a pair of spade connectors.

Note that most (all?) cars' electrical systems will be able to cope with an extra bulb without being overloaded.

Mounting the Gauge

A boost gauge can be mounted in a number of positions.

The most common positions to mount a boost gauge are the A-pillar, on the steering column, around the instrument binnacle or in various nooks and crannies throughout the dashboard. Each of these approaches has advantages and disadvantages in terms of viewing ease, prominence for theft, aesthetics, safety, legality and - of course - installation difficulty.

Aiming for the stealthiest possible installation, we opted to fit our boost gauge in the cavity normally dedicated to the WRX's ashtray. Unfortunately, the ashtray area is a fair way out of the driver's normal viewing line, which means it's a little difficult to watch while on the move. We were prepared to make this compromise, though; the gauge was primarily for tuning purposes and, as mentioned, we wanted the most integrated look possible.

Note - a standard 52mm boost gauge will not fit inside the WRX's ashtray cavity. A 40mm gauge was used in this case.

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Having settled upon a gauge location the next consideration was fitting and integration. Our approach was to whip up a replacement faceplate for the ashtray opening and fit the gauge directly to the faceplate. The faceplate had to be the same colour and texture as the surrounding dash trim, and it turns out the black ABS Jiffy boxes available from Jaycar Electronics fit the bill perfectly. A 158 x 95 x 53mm Jiffy box (part number HB-6011 UB1) was big enough for us to cut out a new faceplate from either its lid or base - it cost just $4.40.

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Using the Jiffy box lid as the platform, we marked out the approximate dimensions of the ashtray opening and cut it out using a small saw. The Jiffy-box-lid-come-faceplate was then filed down so it fit snugly into the ashtray cavity.

With the outside edges of the faceplate shaped to suit, we can now bore a hole to pass through the boost gauge. We drilled a circle of holes through the plastic, broke the centre out and filed the hole until it was large enough to accept the boost gauge. Note that we decided to mount the gauge in the centreline of the centre console, which meant the gauge sits to one side of the faceplate. This leaves room for a high-low boost switch alongside the gauge - you never know....

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To attach the boost gauge to the faceplate we used the retaining bracket that was included in our purchase (brackets are available separately from most gauge outlets). The U-shaped bracket is passed over two threaded fittings on the back of the gauge and two nuts are fastened on top of the bracket. As the nuts are tightened, the bracket pushes against the rear of the faceplate and - in doing so - pulls the gauge face securely against the front of the faceplate.

With the faceplate completed, we plug our new wiring connector into the back of the gauge and check that it illuminates along with the other dash lights.

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The final task is to secure the faceplate to the dashboard. This can be done by fabricating custom metal brackets, but we opted for some extra high strength epoxy - it's much easier and, depending how much you use, it's damn near as strong. Don't apply so much glue it's impossible to remove the faceplate however - you may later need to access the back of the gauge if its bulb blows.

Job done!

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Footnote - a week after fitting our boost gauge, three WRX enthusiasts commented on the "neat, factory look" look of the install. We're certainly happy with its integration (both inside the cabin and under the bonnet), but the low mounting location and the small gauge diameter make viewing relatively difficult; there's no such thing as a casual glance at this boost gauge. If you're running a highly stressed engine and you need to keep a permanent eye on boost pressure, we'd advise mounting a larger (52mm) gauge closer to your driving line of sight.

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