Project MR2 Turbo: Adding a Boost Gauge So We Can Turn up the Power

Update by J.G. Pasterjak to the Toyota MR2 Turbo project car
Oct 2, 2020

Making more power with turbocharged cars is easy, right? Just add boost and the rest works itself out. Right? That’s how it works, isn’t it?

Well, sort of. That’s the spirit of the rule, anyway. The more complex answer is that while simply increasing the boost compresses more air into the engine, that additional air requires additional fuel to ignite, and a whole host of other potential adjustments to properly ignite the mixture and prevent an explosive disaster.

However, if your goal is to increase the boost on your turbocharged engine, first you need to know precisely how much you have. For more recent cars—anything from the OBDII era or newer—that data is typically available right from the on-board computer. Plug in to the data port with the proper protocols and anything the computer knows you can know, too.

But for folks with pre-OBD vehicles, like the ones that exist with a larger complement of analog controls not linked by a central brain, things can be a bit more complex. Actually, that’s weird to say. Things are “mechanically” more complex, since accessing that data requires more than simply looking at what the car is already monitoring.

So, if the goal with our 1991 Toyota MR2 Turbo is going to include additional power, more boost will certainly be a part of that equation. While turbocharged MR2s from that year originally came with dash-mounted boost gauges from the factory, the fact that our car’s original Gen2 3SGTE engine had been swapped for a more technologically complex Gen4 3SGTE engine by Prime Performance means that the retention of the factory boost gauge is not possible. In any case, the factory boost gauge isn’t even graduated numerically, and we needed something more precise than “low” and “high.”

Our answer came from Stack in the form of a 2-1/16-inch diameter digital boost gauge and electronic boost controller. The $360 kit lets you not only precisely monitor your boost levels, but also adjust the boost through a solenoid-actuated purge valve you install in the wastegate actuation vacuum line. When installed in Auto Meter’s integral A-pillar gauge pod, it’s a clean and easy way to monitor and eventually adjust boost in out project car.

1. Installing the interior component of the system requires passing this wiring connector from the engine compartment to the dash of the car. It’s about ¾-inch wide, so it’s not exactly tiny, but it can usually be passed through an existing grommet or seam.

2. In our case, we had a very convenient pass-through right behind the driver’s seat. Remove some trim and pull the carpet off of the vertical firewall, and the grommet protecting the main wiring harness is clearly visible. The rubber seal also has a generous area around the main harness to fish additional wires through.

3. We poked a piece of stiff wire through from the interior of the car into the engine bay.

4. After securing the pulling wire around end of the harness, we pulled it through the grommet and into the interior of the car. It emerged into the cabin like a tiny, wiry baby, ready to face the world with turbocharged fury.

5. The harness was easy to run along the sill of the car after pulling off the rim and carpet clip. If you’re installing this setup in a front-engined car, you’ll have zero issues. For our MR2, the harness was just barely long enough to extend through the cabin and to the dash while still keeping the wires hidden behind trim. When pulling the wires through, take care that you leave enough wire in the engine bay to properly hook up the devices that need to be located there. Also, don’t replace the trim or carpet until the final step, as you’ll be making lots of small tweaks in both directions to get everything to fit properly.

6. One major component of the system is this solenoid-based boost controller. You’ll need to place it where it can be reached by the wiring harness as well as a vacuum line from the turbo to the wastegate.

7. The other key component in the engine bay is this pressure sensor, which actually reads the pressure signal from the intake tract and relays it to the gauge inside the car. It needs to be mounted in a position-stable location with the nipple facing downward to prevent condensation from collecting. A small bracket is included, but some light modifications might be necessary to mount it in an ideal location.

8. This vacuum hose delivers pressure from the turbocharger to the wastegate to actuate the latter and relieve the boost pressure. If the wastegate was not present, the turbo would just keep increasing the pressure inside the engine until, well, the pressure got out somewhere else, probably either the head gasket or the side of the block. The wastegate is a diaphragm tuned to actuate the pressure bleed when a certain amount of pressure hits the diaphragm. So, to increase the boost, all we need to do is “fool” the diaphragm into thinking it’s not seeing as much pressure.

The boost control solenoid accomplishes this by blocking the flow of pressure from the turbo to the wastegate, allowing the boost to climb beyond the point it would normally be actuated. Once the boost reaches a level that you’ve programmed into the controller, the solenoid opens and lets that pressure circuit complete itself. When the pressure drops below the desired setting, the solenoid closes again. This adjustment happens several times a second, precisely controlling the boost to your desired level.

9. We made a small bracket to place the boost control solenoid in a proper location so that all the wiring harnesses could reach it, as well as keeping the vacuum line from the turbo to the wastegate as short as possible.

10. Next we needed to plumb in the air pressure sensor for the gauge portion of the system. For this, we needed a source of manifold pressure to take downstream of the throttle body. In our case, we had an easily accessible pressure line coming off the back of the intake manifold to tap into.

11. The kit includes a plastic T connector that slips into the pressurized vacuum hose to route that information to the pressure sensor. We used zip ties on all of the slip-on hose connections as an extra measure of safety.

12. Using the compression fitting included in the kit, we cut an appropriate length of the supplied plastic tubing to route to the pressure sensor.

13. Finally, it was time to finish up inside the car. We routed the harness up the side of the dash and behind the pillar trim.

14. Power for the gauge came from the radio fuse, conveniently located right below the gauge’s final location. Stack also recommends placing an inline 3A fuse in the power lead for the gauge. The additional parts of the harness that we won’t be using at this time were run along the main harness and stashed under the carpet along the door sill.

15. The gauge mounts in this stylish and subtle A-pillar pod that is a custom fit for the MR2. The pod can be affixed to the A-pillar with double-sided tape, screws or plastic fasteners. The fit was so snug and precise, though, that additional fastening was simply a safety consideration.

With the gauge in place, we were able to confirm that our Gen4 3SGTE was producing the full 8psi of boost that Toyota claimed it should.

But this device is about more than just monitoring boost. With the boost control solenoid, you can dial in a specific boost target that the unit will hit before it relieves the pressure. Programming is a little clumsy with only three buttons, but it’s not something you’ll be doing often, so it’s really nothing to complain about.

The unit also features additional wires in the harness that can be used to input a tach signal to the unit, or a gear selection signal, or a number of other parameters commonly output by many standalone ECU systems. The boost controller can then be programmed to be based on rpm, selected gear or a few other factors. As a whole, it’s a lot of power and options for a small, affordable unit.

As for our MR2, we won’t be messing with boost levels until we can get a more accurate picture of air/fuel ratio and some dyno time. Most OEM ECUs can compensate for the additional air that comes from more boost by adding more fuel. But stock ECUs usually have limitations to how much adjustment they can make and are easy to overwhelm with additional boost. Long story short, adding boost before you have a way to properly measure the other engine parameters affected by it can lead to disaster.

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View comments on the GRM forums
300zxfreak Reader
10/4/20 9:26 p.m.

You gotta love the bloody fingers in fig. 11, pure GRM work. 

spandak HalfDork
10/4/20 11:50 p.m.

Stack makes some of the best looking gauges. I put a mechanical boost gauge in my MS3 and added a color film to the face to match the vehicle gauges, it was perfect. It's hard to find decent gauges with white needles apparently

dean1484 GRM+ Memberand MegaDork
10/5/20 7:03 a.m.

Making more power with turbocharged cars is easy, right? Just add boost and the rest works itself out. Right? That’s how it works, isn’t it?


Yes. Things like pistons and rods and bottom ends work there way out of the block. laugh



JG Pasterjak
JG Pasterjak Production/Art Director
10/5/20 9:47 a.m.
300zxfreak said:

You gotta love the bloody fingers in fig. 11, pure GRM work. 

When you live with five parrots, an MR2 engine compartment is a manicure and a hand massage by comparison.

Tyler H (Forum Supporter)
Tyler H (Forum Supporter) GRM+ Memberand UberDork
10/5/20 10:33 a.m.
JG Pasterjak said:
300zxfreak said:

You gotta love the bloody fingers in fig. 11, pure GRM work. 

When you live with five parrots, an MR2 engine compartment is a manicure and a hand massage by comparison.

Even if that 3SGTE was sitting on a pallet, it would still find a way to make you bleed.  Looks good!  I'm betting that Gen4 will love a little extra boost.  Looking forward to the next installment in this series.

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