Let me give you a switchable option... Back in the very late 90's or early 2000s I turbocharged an 1995 Escort GT (Mazda Protege). It had a similar 1.8 BP engine to the '94-'97 Miata and the 323 GTX manifold and IHI turbo fit perfectly. I wanted two power/boost levels, a low wastegate of 6-7 psi for bad weather and a high boost of 12-14 psi. I built an MBC and used a Mazda or maybe it was a Mitsubishi solenoid that Mazda uses under hood.

The easiest way I can explain it without diagrams is that the pressure output hose from the compressor went into the solenoid NO (normally open) side and out the other to the wastegate. My boost signal line might have been attached after the intercooler just before the throttle butterfly plate to compensate for pressure loss through the IC. I tapped into +12v under the dash (cigarette lighter?) with a fuse near the source. That fed into a tiny toggle switch mounted on the A-pillar boost gauge pod. The UP position sent power to the solenoid under the hood which energized it forcing the pressure source through the NC solenoid output over to the MBC. I built the MBC using a small spring, a ball bearing, and a few locally sourced brass fittings just like Dodge turbo guys recommended back in the day. By loosening the lock-nut I could make the brass body smaller compressing the spring and increasing boost. Once I set the MBC for high boost, if the switch was down it ran on wastegate for bad weather or if I had to hand the keys over to another driver. Flip the switch up and you got whatever pressure you set the MBC to with faster spool up as well.

All that was required was a tiny bleed hole drilled in one spot on the brass valve to allow the MBC to wastegate line to bleed down IIRC. I will search for diagrams. I think I have a solenoid similar to this in my spare parts if you want me to check. I'm fairly certain I saw one a few weeks ago but am not sure if I have a matching plug. Then you need a Y splitter vacuum fitting after the solenoid to connect the wastegate pressure, base boost hose and MBC hose output that connects to the wastegate. You might even want two MBCs so you can set boost for both elevations, only what happens if you forget to flip the switch down before heading to lower elevation? Has FM figured out how to add a boost cut similar to the rev limiter or could it run lean and harm the engine? Ideally a MAP based switch over point would be nice but I have no idea how to design a setup like that. Read up or ask FM if there's any boost cut safety net to prevent more than 12-14 psi.

 

Here's link showing a DIY MBC with the vent hole drilled like I mentioned. Only mine wasn't a T fitting. Mine was straight through and the spring was compressed by screwing the input piece further into a straight brass fitting. I'll leave that up to you but you could probably buy a cheap MBC and add the drilled hole if needed. I think the purpose was to bleed off pressure allowing the wastegate to close because once the pressure source through the MBC stops, the spring presses the ball back against the input hole and there needed to be a place to vent that pressure or the wastegate could stick open too long until pressure bled off, if it ever could bleed down without flipping the switch back to low boost mode if that makes any sense. If you draw the described setup and think it through it should make more sense.

 

I would go with FM's recommendation. We can nitpick about the details and physics of how, but at altitude you need more boost to get the same airflow or absolute pressure that you get at sea level. Without closed-loop control, your plan seems fine to me.

 

Yesterday, we took the NC to our local AutoX. We're not at all into being competitive, and this is certainly not the NC's mission, but it's fun and we like to support our local club. It's also useful for testing considering it's close enough to home for me to drive the NA there if I want to.

I think the turbo actually made the NC slower at AutoX...

Our AutoX pad is concrete and is very low grip. Combined with our 320TW tires, and an open diff, hilarity ensued. The course was entirely 2nd gear, and passing 4500 rpm at full throttle would light the tires, even in a straight line. The way the course was laid out, this happened at exactly the wrong time, just about everywhere. But, I was laughing out loud driving the thing.

My wife, pulling up to the start box



And out on track



It was definitely a learning experience, and pointed out some things I need to change, and that's exactly why we do the AutoX aside from just having fun. A limited slip diff has definitely moved way up the priority list, and a more aggressive alignment is needed. Neither of those will sacrifice any streetability. I'm debating about sway bars. They would definitely help, but I'm not convinced they won't negatively impact the ride quality. A second set of wheels with 200TW rubber is also on the list.

The other thing this provided was a good torture test. It was in the mid 90s by the time we wrapped up, and with us co-driving the car never got shut off between runs. I could definitely feel some heat soak reducing power, but it wasn't enough to keep it from blowing off the tires. Temps never got frisky, and the car just worked.

 

Here's a super cool Lotus Elan that showed up yesterday and took FTD... nice to see the Miata's spiritual ancestor still kicking ***.

 

Good stuff! It's hilarious to hear you couldn't go past 4500 without wheel spin, but good to hear that the kit is working well and passed the torture test. I like how sleeper the car looks.

That Elan looks insanely cool. Thanks for sharing!

 

Yeah, that Elan is quite the eye-grabber. Gorgeous (subjectively) and functional!

320TW with that power level and an open diff sounds like great entertainment haha. That could be a fun class. Minimum p2w ratio coupled with >320TW tires

 

That Elan is SICK

 

Ted's car is definitely badass. Looks like he's finishing up more details for Nationals next month (I don't remember seeing those canards on the front at spring Phoenix events). If you think the Lotus is cool, you should see the way he clamshelled an Airstream to turn it into the hauler for that car!

 

I saw the Airstream parked outside the venue, but didn't get to see it loading or unloading. I was pretty sure there was some trickery afoot.

 

Hello Roda! You had the car after some time now. Did you have any issues with AC on and high temps after long drives on hot days? Any noticeable brake pedal feel during spirited drives? I'm on a similar boat (har har) as you and looking to make a fun turbo street car.

 

Since this is not a daily driver, I haven't put a ton of miles on, and we mostly take the car when temps are nice for top down. That said, no temp issues ever, even at a 90*F autoX. No brake issues either. Underhood temps are definitely higher than pre-turbo, and I needed gloves to grab the hood prop rod at the end of that 90* autoX day!

Driving it on the street is awesome... the car is just great fun with this much power. I do wish the intake noise was a little quieter, though. It's not boy racer whistling BOV obnoxious, but it's obvious things are anything but stock under the hood.

 

Autocrossed the NC yesterday... well my wife did, anyway. I was driving our Capri. Even though we just do this for fun, I think I'm going to add sway bars to the NC. And my wife wants track tires for it for Christmas...