Before I get into how to fit one, ask yourself a simple question. Why do you need one?
If you are just upping the boost to say 19psi using the actuator, then quite frankly apart from looking nice you probably don’t need one permanently. You only need one for setting the boost up the right level to start with, which will make things considerably easier. The boost should be checked if you’ve modified the fuelling as adding fuelling can often increase boost due to the small internal wastegate on the GT15 turbos.
The hardest bit of fitting a boost gauge is finding somewhere for the gauge to be mounted.
The next hardest bit is routing the pipework and electrics to the gauge.
And finally the easiest part is fitting the boost gauge into the engine pipework.
Ok so the first task is to find somewhere for the gauge. When doing this bear in mind that the gauge often has a significant depth (2 inches or more), and so any area has to be able to accommodate the depth of the gauge. The gauge should usually be easily visible without moving your head when sat in the normal driving position, so fitting it in the glovebox isn’t usually a good plan! Remember that as well as the gauge being a significant depth, it will also need pipework from the engine bay and an electrical connection to illuminate it at night (or in the day for some gauges), so don’t put it a totally inaccessible place! Fit the gauge but don’t secure it in place just yet, you’ll need to remove and refit it a few times to connect the pipework and the electrics.
The next task is to route the pipework from the gauge to the engine bay. The MAP sensor is typically the easiest boost source to get at and this lives on the bulkhead. So you need to route the pipework that came with the gauge to the MAP sensor. The route taken is up to you, however if you have small bore pipework for the gauge the heater control cable that controls the hot water flow to the heater matrix is often in a relatively easy location to get at. You can force the pipe through the heater cable grommet to maintain a water resistant seal so water can’t get into the cabin.
Once the pipework is routed into the engine bay through a suitable grommet then the battle is half won.
Next job is to connect up the gauge illumination. Most gauges will have a backlight on them so that they are visable at night. This backlight should come on when the side/headlights are switched on just like the rest of the dash illumination. The easiest way to do this is to take a feed from one of the dash illumination lights. This might be the cigar lighter illumination, or the heater dial illumination, it really is whatever is easiest for you to get access too. Typically the wire colour is red with a black tracer/stripe, for the dash illumination. You’ll also need to complete the circuit by connecting the earth connection to a suitable earth (usually black on both the gauge and car wiring), or a clean metal connection on something bolted to the car metalwork, (whichever is easier to get at).
Some gauges have a smoked appearance which needs to be backlit during the day and at night so the dial can be seen, often the gauge also has a night illumination level which is dimmer than the daylight level so as to not blind the driver at night. These gauges need an ignition switched positive supply to provide the daytime illumination (and a night illumination too in some cases). Handily the cigar lighter provides such an ignition switched power feed, as well as earth and night time illumination feeds (COLOUR??), so as you can imagine this is a common spot for people to wire their gauges into. The radio connection also carries all of these power feeds too so use whichever is easier to get at. You can use other places to hook the wiring into too but the ones I’ve listed are the most common and easiest to get at usually.
Now you are on the home stretch! All you need to do now is to fit the gauge T-piece into the MAP sensor hose and off you go. The MAP sensor has a short length of 8mm fuel hose joining the hard plastic pipe to the MAP sensor. Either remove this pipe, cut it in half and fit the T-piece in between or remove the pipe from the MAP sensor, connect it to the T piece, and add a short length of fuel hose to join the T piece to the MAP sensor. Many T pieces have an outlet with a small diameter restriction in one outlet. This is to minimise gauge noise/rattling and so this outlet should be connected to the gauge.
Now you are done. Take the car out for a drive and once you reach 3rd or 4th gear accelerate hard and note the boost readings or get an assistant to help. Max boost on a standard turbo is typically reached at ~3k rpm. My max recommended boost on a standard turbo is 19psi. Any higher than this and you’ll be asking for trouble in my opinion.
You can’t measure the boost accurately in 1st or 2nd gear as you typically aren’t in these gears for long enough to get a good reliable reading. You also have to be driving at speed and apply full throttle to measure boost as otherwise you’ll get an inaccurate reading. You can’t measure boost by simply revving the engine. It just doesn’t work, you might get a boost reading but it isn’t going to be anywhere near the same as the one that you’ll achieve on the road, the engine has to be under full load to generate its maximum boost.
OK so what about a temporary gauge installation?
Well this is what I do when I’m setting up the boost on cars that don’t have a gauge fitted.
I install the T piece to the MAP sensor as previously mentioned, but then I have a longer length of pipe. I run this up through the bonnet/windscreen gap (there is a nice rubber seal here so it doesn’t crush the pipe when I close the bonnet), and in through the drivers/passenger window. I then either wedge or tape the gauge in position so that I can see it and then I set up the boost level as I’ve described elsewhere. Simple yet effective.