I Don’t Want a 1.8 Swap

Just say no to 1.8 swaps

The other day I was chatting with a guy about his bitchin Miata with ITBs. He put them on his BP engine and got a lot of power and cool sounds. Forced induction would probably have been cheaper, but I totally understand wanting NA power and better throttle response. I also understand doing things just to be different, and thought I found someone who was a kindred spirit. So I asked him about putting ITBs on a 1.6, and what kind of power did he think I’d get from that? He said the 1.6 head doesn’t flow enough, and I should be swapping in a 1.8 anyway.

I’m so fucking sick of that. Everyone says don’t tune a 1.6, swap in a 1.8. But I don’t want a 1.8! I already have a 1.8 in my race car, and I don’t want one in my dual-duty street car. It won’t make sense to most people, but I’d rather have a 1.6 making 125 hp than a 1.8 making 140 or whatever. Here’s why.

NASA TT6

949 Racing built a Miata for NASA ST/TT race series. The NASA classing is based on power-to-weight ratio, and Miatas typically fall into the TT6/ST6 class, which is 18-20 lbs/hp depending on how you configure it. 949 chose the NB body because the rules give a slight bump in power for using base-trim model bodywork (BTM), and the NB has better aero, than a NA. This calculates out to a car that weighs around 2460 lbs and puts out about 138 hp. These are easy numbers to get from a BP 1.8 Miata engine with tuning. However, they swapped a 1.6 engine into the NB chassis. Huh?

Emilio says “We don’t tune 1.6s, we swap in 1.8s,” but there he went and made himself a liar. In this case, it made a lot of sense. The 1.6 engine is lighter and shorter, and this gives the car a better front/rear weight balance. In theory it should have worked better, but they later abandoned the 1.6 engine project because it was going to be too expensive to hit the horsepower number. But theoretically, a 1.6 should be better than a 1.8 if making the same power.

And so I’ve been thinking about competing in NASA TT6 as well, but I’m not going to build an engine that has 138 hp. First, that’s an expensive build, but more importantly, I want to use aero, and that means the car isn’t allowed the same power to weight ratio. If I use all the aero allowed (airdam, roof, wing), then the engine can make a maximum of 125 hp. I’m guessing my 1.6 has 110-112 as it sits, and so I can probably hit 125 without breaking the bank.

Also, I have a low milage engine. My dad bought the car when he was 69 and never drove it hard, and so the engine has had a low-stress life. Tuning this engine makes more sense to me than swapping in something of unknown origin.

Taking scalps

One of the joys of driving a momentum car is embarrassing more powerful, expensive cars. Hunting down BMWs, Corvettes, Mustangs, and Porches, and passing them (or harassing them into giving you a point-by), is great fun. For lack of a better phrase, I’ll call this “taking scalps”.

You don’t earn a scalp for passing slower cars, only faster ones. But when you’re in a Miata, pretty much every other car on track is a scalp for the taking. If you’re driving a 1.6 Miata, then 1.8 and later Miatas are scalps, as well!

Accentuating the 1.6-ness

I want to build the ultimate expression of a NA 1.6 Miata. The early 1.6s differ slightly from other Miatas in several ways, and my goal is to exaggerate all of these differences.

  • Higher redline – The 1.6 revved to 7200 rpm, which was later reduced to 7000 in the 1.8. I have a Megasquirt PNP2 and programmed my redline to 7500 rpm. I won’t go higher than that without modifications.
  • Higher compression ratio – The 1.6 had a 9.3:1 ratio, compared to the 1.8 NAs which were 9.0:1. I got a spare head and decked it, which should bring the compression up to around 10.3:1, which is close to what the later 1.8 BP engines have.
  • Lighter clutch and flywheel – The 1.6 has a smaller clutch and flywheel, which means less rotating mass and a revvier engine (at least without load). I bought the lightest aftermarket clutch and flywheel I could find.
  • Shorter final drive – The NA6 4.3:1 diff ratio is shorter than the 4.1 ratio used in the later 5-speed 1.8s. My original plan was to exaggerate this difference further by using a ring and pinion from a 4.78:1, but I’m fairly certain this will annoy the shit out of me on the street. For my county club track, a 4.1 makes a bit more sense (fewer shifts), so I’m on the fence for which way to go here.
  • Hotter cams – The 1.6 has a slightly more aggressive cam profile than the 1.8s. I went one louder and bought Kelford 264 B-grinds with 9mm lift. It will be interesting to see how much power these add.
  • Smaller brakes – The early cars came with smaller disc brakes. On a street-oriented car that does the occasional time trial, I don’t see the need for larger brakes (and more rotating mass), so I’m leaving them stock. I’ll duct the rotors, but that’s it.
  • Smaller wheels – The NA6 always came with 14” hoops, and it wasn’t until 1995 that 15” was an option. If I’m being serious about performance, then 15s are a better choice. They are available in wider widths and have better rubber selection. But if this is an exercise in nostalgia and 1.6-ness, then 14×7 RPF1s are obviously that way to go.
  • Less power – Do you ever floor the accelerator, and then pat the center console or the steering wheel, urging your Miata to giddy-up? The 1.6s have less power and torque, and it leaves you always wanting more. Somehow that’s part of the charm. A naturally aspirated 1.6 is never going to overwhelm anyone with power, so this is a goal that’s impossible not to achieve.
  • Less weight – Miatas got heavier through the years, and to keep with the theme I’ll reduce weight where I can. There’s no secret recipe, just concentrate on the ounces, and the pounds will take care of themselves.
  • Simpler – The earliest Miatas were the simplest, with fewer electronics and conveniences. Mine has a manual steering rack, manual mirrors and windows, no ABS, and came with only a single airbag (now gone because I replaced the steering wheel). Anything I do to further modify this car will stay in the same theme of keeping things simple, pure, and analog.

People say that the later Miatas are better cars, but the earlier ones are better Miatas. When you add up all the differences between the 1.6 and 1.8, you get a car that has a slightly different character. Those differences are what I’m accentuating. For sure I could make a better car, but I’m building a better Miata.

And that’s why I’m not swapping in a BP 1.8, using forced induction, or whatever else is the cost effective and logical way to get more power. Yeah, I’m tuning a normally-aspirated 1.6, get over it. And all of y’all are scalps to me!

3 thoughts on “I Don’t Want a 1.8 Swap”

  1. ~125 whp (as measured by TFB Performances dyno at Sonoma Raceway) with a torsen is doable in 1.6 spec miata, a bit more with a Cusco diff. This is with stock header with cleaned up welds, exhaust with a single resonator to keep it in the ~100 db range, head shaved a little to increase compression, selecting a good flowing head and cleaning up any core shift to the degree allowed by the SM rules. Racing beat intake with the turn signal removed, adjustable fuel pressure regulator, have your injectors cleaned and balanced every season (this can be a +4hp gain), and someone who really knows how to tune a 1.6 – X-Factor would be a good shop to talk to. I’m ore unsure about what gains there are in the bottom end, not a whole lot is allowed in the SM rules.

    IMHO there are 3 things that suck about racing the 1.6:
    1. The head castings aren’t as consistent as the later Miatas. So you can spend a lot of money building motors just trying to find a good head (this is for SM where you can’t just port it yourself to clean up core shift).
    2. The ECU is dumb as bricks and you end up tuning it a few times a year to stay at its peak potential.
    3. The AFM flapper door causes AFRs to go all over the place with G load. Left handers you go lean, right handers you go rich. So you end up getting your base tune on the dyno and then add fuel based on how many left handers you’ve have at a given track and how ok you are with your valves melting… If I tuned for a 13.3 AFR, I’d see as high as 15 g load. You’ll never get your peak dyno numbers on track without risking the motor unless you are doing drag racing.

    If I were building for a 1.6 for a class that allowed it, I’d port the head, switch to a megasquirt ECU, swap from the AFM to a MAP sensor.

    To your point about smaller brakes, with a race pad the OEM 1.6 brakes are more than enough for ~125 hp power levels. Never a hint of fade with basic Castrol dot 4. Both for sprint races and enduros. Brembo rotors are worth the extra money, last multiple pad swaps without warping and still go for less than $40.

    I feel like the best drivers are in SM so when I wrecked my 1.6, I decided to stick with the class but switch to the NB2 because of the above issues. However, if money were no object, you could build a weapon of a 1.6 SM and I do miss how light and nimble it was.

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    1. Excellent info. I have a Megasquirt and don’t face the challenges of the stock ECU and AFM, but I had no idea about the variability of head castings, or cleaning and balancing (?) the injectors. Thanks.

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      1. https://www.rcfuelinjection.com is where I’ve had my injectors serviced, do great work and the prices are reasonable.

        To clarify, the goal is to make sure the fuel is atomized well and that even fuel is delivered to each cylinder. I don’t know that the injectors can actually be balanced themselves, but if I noticed one injector was flow tested way different from the other 3, I’d try to swap out injectors until I had a matching set that of 4 that flowed the same. Mazda’s factory tune is extremely conservative to account for variances over time that the ECU is not going to react to. As you tune out that safety factor out in favor of performance, you want to make sure one injector isn’t leaning out 1 cylinder while your average AFR looks safe.

        I’ve seen junkyard motors on the stock ECU/AFM/header with a good tune hit ~118 hp in SM. The Megasquirt will eliminate a ton of 1.6 specific problems. Without having to comply with SM rules, 125 hp should be reasonable to hit without going nuts on the bank account.

        Best of luck with your build!

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