Project Miatabusa – Part 1: What The Hell Are We Thinking?

With all due respect to the rest of the project car stable, this is destined to be the single most awesome project car in the short history of MotoIQ. The dirt cheap, bantamweight, nimble and perfectly balanced Miata needs no introduction in this crowd. Neither does the giant-slaying, brass-ball-swinging, 10,000-rpm-spinning king of the squidbikes, the Suzuki Hayabusa GSX1300R. What they both desperately need, though is an introduction to each other.

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If we were to simply bolt a flywheel to the Hayabusa's crank (nothing simple about that, really) we'd end up with a situation like the dyno chart above. The Hayabusa's 1.3-liter engine is actually weaker than the Miata's 1.8 until you're past the Miata's redline. If we take power off the Hayabusa's input shaft, though, the gear reduction between the crank and flywheel bumps up the torque and pulls down the rpm. Of course, this is flywheel rpm on the chart below, and since there's a gear between the crank and flywheel, the engine itself will still be spinning to 10,500 rpm. Oh the sweet music… (The Hayabusa's dyno data, by the way, comes from Sport Rider Magazine's fantastic dyno chart database.)

In a way, these two machines couldn't be more different. One is terminally underpowered, lithe, nimble, cute as a button, and driven by someone so secure they don't care the rest of the world thinks they're a hairdresser. The other is only ridden by someone who does.

Somehow, though, their key elements of greatness dovetail perfectly. The Miata's racecar chassis is paired to a stout but anemic econobox engine, and the car's lightweight, “gram strategy” development mantra is seriously dented by the engine's bulky, cast-iron block. The Hayabusa engine, like most sportbikes, is like a little bundle of race engine technology in a convenient, bargain-priced, crash-recovery package. Unlike most sportbike engines, though, the Hayabusa's unusually large displacement makes it a relative torque monster. Its 99 lb-ft toruqe output is nearly a dead match for the original Miata's 100 lb-ft, but its 171 hp (or 197 if you drop the big bucks on a 2008 or newer engine) will give the Miata the longitudinal urgency it so desperately needs.

Even better, the 'Busa's little bundle of power is a bantamweight 135 lbs. That's less than half the weight of a Miata engine. With the proper application of weight weeniedom, we're confident we can drag the Miatabusa's curb weight down to 2,000 pounds. That's Lotus Elise power, Lotus Elise weight, and even a slightly Lotus Eliseish rear weight bias, all at much less than Lotus Elise prices.

The idea for this project was the inevitable result of sharing the pits with the nutjob creators of the MetroGnome and the Angry Hamster, the two bike-powered creations that dominate our Best Engineered Cars of Lemons series. The sight of the lowly, pedestrian Geo Metro, coupled with the spine quivering F-1 soundtrack of its CBR-1000R engine, is a prescription for obsession. The thought of that sound coming from a Miata is hard to shake.

Alone, I would have the self control to keep the Hayabusa and Miata apart, but stick me with twisted geniuses like Alex Vendler (the Gnome's creator) and Tim Taylor (father of the Hamster) and I'm as suggestible as a sorority girl at Spring break. Nerds gone wild.

Here's how it's gonna work:

The Hayabusa's monsterous 171-hp I-4 sits transversely under the rider's massive stones, sending power through a 6-speed dog box and on to a chain drive, like virtually every sport bike on the planet. This layout is far from ideal for a car. Most bikes carry tall first gears, since a motorcycle's ability to handle short first gears is limited by their tendency to flip over backward. The rest of the Hayabusa's gears are pretty tall too, since the bike is geared to go nearly 200 mph. Then there's the lack of a reverse gear, and of course, the fact that there is simply no practical way to connect a chain to the rear wheels of a Miata.

Most FR Hayabusa installations are like this Starlet we spotted at a rally in New Zealand. The 'Busa's fragile transmission is retained (in the dirt this is probably strong enough), and the sprocket is replaced with a driveshaft. In our car, the engine will actually face the other way, with the timing chain against the firewall and the header on the right side.

The typical solution for front engine/rear-drive Hayabusa conversions is pretty simple. Replace the output sprocket on the transmission with a driveshaft flange, turn the engine north/south and you only need an extra-long driveshaft to make everything work. Unless you want to back up… Adding reverse means sticking a reversing gearbox (roughly $1500 from Quaife) in the middle of the driveshaft.

Endurance racing a bike-powered car gives Alex Vendler a uniquely experienced perspective on this peculiar form of hybrid, though, and it only took three or four destroyed gearboxes for him figure out that while their engines are surprisingly robust, sportbike gears simply aren't beefy enough for car-sized loads. Then, in the midst of one of his many mid-race transmission rebuilds, the answer hit him.

4-cylinder sportbikes all transfer power from the crank to the gearbox through a big reduction gear. That gear both reduces the rpm at the clutch by a factor of 1.596 and increases the torque by the same amount, making both much more appropriate for a car. While the Hayabusa redlines at 10,5000 rpm, its clutch never sees more than 6500. That makes the input shaft, not the crank, the ideal place pull power for a Miata transmission.

There's an amazing amount of stuff going on in a Hayabusa's crankcase. The silver can on the end (yellow arrow) of the beautiful, fully couterweighted forged crankshaft is simultanously the outer shell of the generator and the trigger wheel for the crank angle sensor. The big gear in the middle of the crankshaft (dark blue arrow) drives the engine's single balance shaft. The small gear all the way on the far end of the crank (light blue) is for the starter. The big gear we really care about, (orange arrow) is the primary drive gear, which drives the even bigger primary driven gear (the other orange arrow) that's wrapped around the clutch at a ratio of 1.596:1. The green arrow is the transmission's input shaft. We'll be replacing this with a longer shaft (without any gears) that will extend out past where the Hayabusa's current clutch sits, and will hold a MIata flywheel and clutch. The purple arrow is the transmission's output shaft, which will just be removed.

So, if you just replace the bike's clutch and input shaft with a slightly longer shaft, you can bolt a Miata flywheel to the end of it. Then you can make an adaptor to bolt the Miata's bellhousing to the Hayabusa's clutch cover flange. Suddenly you have big, car-sized gears backing up your bike engine.

Comparing the SolidWorks model (above) with measurements we made by taping IKEA measuring tapes to the bottom of our $1500 Miata (right), revealed the Hayabusa sump and Miata steering rack really wanted to occupy the same space.

If “just” fabbing up this new shaft and bellhousing adaptor seem daunting, remember we're also sharing our pit with Tim Taylor. Go read about his Magna-powered Honda Z600 and all your doubts about this project's feasibility will vanish.

Work started with ones and zeros. Tim made a rough solid model of the adaptor in SolidWorks before we even had a Miata. This gave us enough information to start to approximate where big things, like the oil sump, were going to fit.

As soon as I saw the model, I found a clean, reasonably straight, 130,000-mile 1996 Miata with a factory Torsen LSD for sale for $1500 and pounced on it. Within a day the car was up in the air and we were comparing the model to the reality of where it would have to fit. This first approximation revealed a few very crucial issues. The Hayabusa's oil sump, for example, wanted to occupy the same space as the steering rack. The exhaust ports also dumped right into the frame rail, leaving no room for the factory header, and little space for a custom one. The offset engine ran right into the Miata's front subframe, and, finally, the lowest point on the sump, a V-shaped bulge on the bottom intended to keep the oil supply steady during wheelies, hung dangerously low.

The oil pan is off in this photo, but you can still see how a portion of it sticks forward (to the right). That portion used to hit the steering rack, until we rotated the engne, removing the slant that it normally sits at when in the bike. The simple shaft on the right was used to align the Miata's input shaft with the Hayabusa's.

With no SoilidWorks model of the engine bay, it was hard to know how to resolve these problems. We would have to re-enter the real world. After removing the engine and front subframe, we stuffed a spare Hayabusa case into the engine bay. Tim whipped up an alignment shaft that indexed on the Miata's input shaft, and pressed into one of the Hayabusa's input shaft bearings. With that alignment set, we just jammed a half-inch spacer between the bellhousing and engine case to approximate the thickness of the as-yet-unbuilt adapter flange.

With these key dimensions set, the solution to 3/4 of our interference problems was as simple as rotating the engine until it was standing straight up. This lifted the offending portion of the pan off the steering rack, pulled the exhuast ports off the frame rail, and pulled the deep vee of the sump up flush with the bottom of the bellhousing.

Since this rotation approximates what the Hayabusa does during a mild wheelie, the pickup is positioned perfectly for this layout. And, since the oil level in the sump is limited by the the need to keep the crank out of the oil, raising the crank with this rotation also increases the engine's oil capacity, which will help keep temperatures down.

This rotation did nothing to fix the interference between the subframe and the engine case, though. There is simply no way to avoid making a new subframe. While we're in there, though, our new subframe will be lighter, stiffer, will have more negative camber adjustment range, and will use the higher caster geometry from the NB Miata. If you're gonna do it, do it right…

That's the look of satisfaction and relief you get when a simple rotation of your Hayabusa engine makes it fit in your Miata…

One last detail you should know before the obsession sets in: we're not greedy. This concept is too good to keep for ourselves, so we're making tooling to reproduce the adaptor and subframe. Miatabusas for everyone! And I really shouldn't have to point this out, but just in case all this Miata talk is causing cranial misfire, the Hayabusa engine/Miata trans combo will bring featherweight horsepower to any front-engine, rear-drive car you can dream up. Toyota Corona, Lada Riva Estate, Borgward Isabella, the possibilities are endless. When there are eventually parts available, you'll find them at Miatabusa.com. Meanwhile, while there is only one, you'll find it here.

All the Miatabusa you can handle: