How to finish a V6 swap into a Miata: wiring, fuel, exhaust and more | Project Endurance Race Miata

Update by Staff Writer to the Mazda Miata project car
Feb 18, 2022

Photography by Tom Suddard

Story by Tom Suddard and Wayne Presley

Thanks to a freshly painted interior and two sets of new Konigs and tires on the shelf, our LFX-swapped, NA-chassis Miata endurance racer was getting closer and closer to the track.

One problem: We hadn’t yet heard it run–well, not since swapping in the V6.

We’d finish this final phase of the swap at Very Cool Parts, leaning heavily on shop owner Wayne Presley’s expertise (and penchant for grueling hours of hard labor) to get our Miata ready for the track.


How to (Easily) Rewire a Car

Step one? Rewire the entire Miata.

Our V8 Roadsters swap kit included an engine harness that links our LFX to the factory chassis wiring. Then we had to just splice a few wires back to the original Miata wiring: power, ground, key, etc.

There was a problem with this plan: As we discussed in a previous update, our car’s harness was trashed. After years of racing and multiple engines, our wiring harness resembled a hacked-up mess. Then add in every extra light, radio, cool suit, dash display, video camera, and data system added over the years.

That’s why we threw it all away and decided to rewire the car from scratch with a Racepak SmartWire Power Control Module.

This solid-state puck of aluminum would replace every fuse and relay in the car, theoretically creating the simplest, most durable, most serviceable wiring harness possible. All we’d have to do is run a new wire from the SmartWire to literally every single electrical device on the car–as well as that V8 Roadsters swap harness.

Was this a tedious week-long process filled with crimp tools and heat shrink? No, not really. The control module comes with an eight-switch keypad for manual control and hard-wired inputs for other switches or ECU control.

The installation was very straightforward. First, find a suitable mounting location and then make individual runs of wire from the connectors to items to be powered.

We mounted the control module on the transmission tunnel near the firewall. Take the time to write down the output assigned to each pin location.

We did a loose pull of the wires to each location before making a neat bundle inside a nylon sheath. Once all the wires were run and terminated, you make the final battery wire connection and connect to the computer using the Racepak software.

Each output can be configured to be momentary, latched or flashing. Setting up the switching via the software offers even more options, allowing devices to be controlled via external switches, the ECU and even from the Rackpak dashes data.

Be prepared to spend a minimum of an hour configuring all of the outputs and inputs. Once you set up the first channel, the rest goes fairly quickly.

The Miata didn't need all of the inputs and outputs, so we put in a barrier strip to easily add functions in the future. The switch panel was mounted on the driver’s side of our Racepack dash for easy access.


What Battery for a Miata Race Car?

We also took this opportunity to shave some weight and add some durability, swapping out our standard Miata battery for a powersports model from Odyssey. Besides coming with a sweet hold-down, this AGM battery is lighter and more durable than a standard parts store piece.

No, it’s not as light as a lithium battery–figure about 15 pounds for this PC680 model–but for an endurance car that will need to restart after every fuel stop, we prefer a traditional, bulletproof lead-acid battery. Unlike a lithium battery, this can be carelessly jump started, run at low or high voltage, left out in the elements, and generally abused without fear of serious damage.


Does a V6 Miata Need a Special Cooling System?

We also needed a cooling system. V8 Roadsters provided a radiator and coolant expansion tank in the kit, but the rest was up to us.

Fortunately, V8RPerformance, the brand’s turn-key car building division, is located only a few miles away from our shop. Owner Al Ludington graciously modified our LFX’s rear coolant pipe, then gave us some pointers for installing the radiator.

The first pointer? Find new lower radiator mounts.

V8 Roadster’s kit is designed for NB Miatas, so we needed to match its radiator mounts in order to install the radiator. Treasure Coast Miata had used NB lower radiator mounts in stock, which fit with minor modification. We fabricated top mounts from ¼-inch aluminum plate.

Once the radiator was mounted, there was one last thing to weld: the upper radiator hose neck on our LFX’s block. Wayne simply cut and welded it at about a 45-degree angle in order to clear the radiator fans. We used OEM NB-chassis Miata fans as they bolt right up to the V8 Roadsters radiator.


Welding done, we hooked up the hoses, screwed the coolant expansion tank to the firewall, and added water: Success!


How to fuel a V6-Powered Miata?

We’d fed our engine water, and now it was time for air and fuel. Both were remarkably easy: Wayne built a simple air intake out of some scrap aluminum and silicon couplers, only pausing to weld a MAF mount into one of the tubes.

To feed our LFX fuel, we swapped out the OEM fuel pump for a high-flow model from the V8 Roadsters kit, which connected to the OEM fuel lines. Those lines needed some minor re-routing in the engine bay, but two quick disconnect adapters later they were hooked to the LFX’s high pressure fuel pump.


How to Build an Exhaust for a V6 Miata?

Wiring, air, fuel, water were handled. There was one last piece of the puzzle: an exhaust system.

The first task was getting exhaust from the LFX’s integrated manifolds to the bottom of the car, and V8 Roadsters’ kit included two custom-made downpipes. Sadly, one didn’t quite fit–we needed to TIG a notch into the driver’s side pipe to clear the steering shaft–but after that they bolted right on.

The rest of the exhaust system would need to be built, so we hopped on Summit Racing’s website and filled our cart with nearly $500 worth of stainless steel tubing, bends, V-bands and a muffler. Then Wayne went to work, TIGing together a beautiful exhaust system with 2.25-inch primaries and a 2.5-inch tailpipe and muffler.

Curious how to build your own exhaust like this? We covered the process in depth after LS swapping our 350Z, but the basics are the same for any car: Plan your system, order the bends, tubes and couplers you’ll need, then cut and weld until finished.

[Project Car: LS-swapped 2003 Nissan 350Z]

We’re big fans of Summit Racing’s house brand for these exhaust builds, as their tubing is reasonably priced, welds extremely well, and arrives on our doorstep a day or two after ordering.


So Why Didn’t Our V6 Miata Run?

Wiring, cooling, fuel, air, exhaust–OMG, we’re done! After programming the Racepak electronics, we hopped in, pressed the start button, and finally heard our LFX roar to life.

Well, half of it, anyway. A quick check of our coil wiring scared us. And then a compression and leak-down test terrified us: The sound we heard was the sound of an engine without compression on its right bank of cylinders. Every junkyard engine is a gamble, and we’d lost this round.

Rather than tear apart a junkyard engine of unknown provenance, we decided to simply chuck it in the corner and install a replacement. After all, that’s one reason we originally chose the LFX swap: a cheap, plentiful supply of engines in every junkyard.

Yeah, that was before Covid, the new car shortage, the used car shortage, etc., etc. That’s also before we were up against a deadline to get this car on the dyno and finished.

$800 Camaro engines suddenly became $2000 Camaro engines, and there was no guarantee we wouldn’t be burned again with a junkyard motor. We could have bought a less expensive LFX–numerous variants appeared in GM products with slightly less horsepower–but we wanted to start with the full 323 horsepower a 2012-’16 Camaro would offer.

So we took a different path to arrive at our new engine: A Copart auction 45 minutes from Very Cool Parts, where we became the proud new owners of a wrecked 2012 Camaro LT for just $2250, totaling $3007 once fees were added.

We’d paid a slight premium for our engine, but now had an entire Camaro worth of sensors, wiring and other goodies, as well as plenty of interior parts and scrap metal to recoup our purchase.

Shoutout to Wayne, again the hero of this story, for playing into overtime and completing the engine swap himself. A week after the Camaro arrived, our Miata was finally a running, driving car. We could finally hit the dyno.


How Much Power Does a V6 Miata Make?

The V8 Roadsters swap kit uses the OEM Camaro PCM, so Wayne plugged in our HP Tuners MPVI2 and fired up the laptop. This tuning software let him do two things: Disable the security features that would have prevented our Miata from starting, as well as load a more aggressive tune from his own LFX-swapped Exocet.

[Exocets done three different ways]

Why not go to a standalone ECU? There aren’t many on the market that can control a direct-injected engine with variable cam timing, especially not for the cost of free like our Camaro unit.

Tune loaded, dyno on, foot to the floor, we got a glimpse of our endurance racing future: 281 horsepower and 247 ft-lbs. of torque. Let’s update some safety gear, make our Miata pretty, and hit the track–that’s in our next update. 

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View comments on the GRM forums
Jerry From LA
Jerry From LA SuperDork
2/19/22 7:18 p.m.

Funny but adding the drivetrain loss to the chassis dyno numbers comes out to 323 horsepower.

What will you do with the dead motor?  I would rebuild it as the primary engine and keep the Copart motor as the spare.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/19/22 9:00 p.m.

In reply to Jerry From LA :

How did you determine drivetrain loss? It's neither a percentage nor a fixed value, but a combination. It's 281 measured RWHP, that's the one number that's reliable - and applicable. 

This update reads like real life meeting forum lore :) The engines are cheap, until they're not. The wiring is easy, until it's not. The parts bolt in, until they don't. This is what it's really like :)

2/19/22 10:08 p.m.
Keith Tanner said:

In reply to Jerry From LA :


This update reads like real life meeting forum lore :) The engines are cheap, until they're not. The wiring is easy, until it's not. The parts bolt in, until they don't. This is what it's really like :)

I think I am going to print and post that on my shop wall because it describes every single project that has ever found its way into my shop. 

67LS1 Reader
2/20/22 4:08 p.m.

Being the owner of an LFX powered 1966 Chevelle, I went through almost all of these issues. I used the factory harness and fab'd just about everything else. 


carguy123 UltimaDork
2/20/22 6:00 p.m.

You do know they make a twin turbo version of that engine don't you?

I've driven several and it do make me smile!

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