I got an idea... (High revving, rear/mid engine miata content)

So I was thinking on the drive home the other day, Hayabusa miata. I know it has been done before but I have a different idea on how to go about it.

First off, I doubt there is much likelihood of this being street legal. It might be possible, but for maximum performance my idea is picturing a track/autocross only car.

First step, weight reduction. remove anything and everything possible. I have seen various weights for stripped down miatas, with this I am hoping I could get it down to 1700-1800lbs once it's done including the Hayabusa engine, but with a roll bar and everything that might be optimistic.

Second, engine mounting and drive: I picture taking out the fuel tank, rear deck, soft top frame and putting the Hayabusa engine back there, with the chain running straight down to turn the rear diff. Just bolt a sprocket onto the input of the diff instead of the driveshaft.

So all said and done, if it worked out how I picture it right now, it would be a car with a 10:1 or less pound to horsepower ratio, revving to a furious 10-11,000 rpms.

With no reverse...

Some snowmobile engines run backwards to have a reverse gear. I wonder what needs to be done to do that with a busa?

In reply to AWSX1686:

I have had this idea before. It will be tight with a drivers seat in place but you can likely make it happen. Nice thing about this way is the ease of sprocket swaps depending on the event or use for that day.

If I'm not mistaken, mounting it where the fuel tank was with the sprocket straight above the pinion flange will require running it sprocket forward...Which means you would have 6 reverse gears and no forward. Or does the diff sit further back in the car than I realize?

RossD wrote: Some snowmobile engines run backwards to have a reverse gear. I wonder what needs to be done to do that with a busa?

Why not use a small electric motor and put the transmission in neutral to accomplish that?

The rotation of the busa motor and output sprocket means the full motor would need to be in front of the diff input. Save space by locking the diff and putting the drive sprocket on one of the output shafts. Then you can run the busa transverse.

I too, have spent considerable time thinking about this.

Honestly though, in a fr car like a Miata, I'd put the busa motor in stock location. There are easy solutions to adapt the output of the bike to a driveshaft.

Put a starter on the diff with a ring gear and severely lightened flywheel for reverse if you must.

There is a reverse coupler thing they make for those giant adult go kart things. Not sure it would handle the abuse though.

I seem to recall that the LeMons Miata that ran the 3-cylinder Polaris snowmobile engine that was mounted up front and reused the stock driveshaft and rear diff was claiming something like a 600-800lb weight loss, so you're goal of getting it down to 1600-1700lbs is certainly feasible, although you'll have to run a cooling system for the 'Busa engine.

z31maniac wrote:

RossD wrote: Some snowmobile engines run backwards to have a reverse gear. I wonder what needs to be done to do that with a busa?

Why not use a small electric motor and put the transmission in neutral to accomplish that?

Maybe. Trying to keep it lightweight though.

z31maniac wrote:

RossD wrote: Some snowmobile engines run backwards to have a reverse gear. I wonder what needs to be done to do that with a busa?

Why not use a small electric motor and put the transmission in neutral to accomplish that?

You could do like Ben_Modified did with the Daihatsu HiJet and run the alternator off the differential pinion and then use it as a reverse motor.

Robbie wrote: Save space by locking the diff and putting the drive sprocket on one of the output shafts. Then you can run the busa transverse.

I didn't think of that. Even better for drifting right?

Robbie wrote: Honestly though, in a fr car like a Miata, I'd put the busa motor in stock location. There are easy solutions to adapt the output of the bike to a driveshaft.

I really like the rear/mid idea...

Robbie wrote: Put a starter on the diff with a ring gear and severely lightened flywheel for reverse if you must.

That could work too.

What rules regarding weight removal? DP Miatas are around 1700lbs with a BP motor.

I'd put the motor in or near the stock location. With the motor set back behind the front axle line, it would be a front-mid engined car. Helluva lot easier than putting the engine where the fuel tank would go.

AWSX1686 wrote: I really like the rear/mid idea...

We do too... Wanna work on a challenge car?

You're a bit far but we are putting a GSXR-1000 motor in this:

There's a gentleman involved in rally, Randy Z. I believe, who has a fastback conversion done to a miata with a 13B mounted mid ship that's for sale. You'll need to contact Kennedy Engineering regarding transmission mounting options, but picking this up, though it wouldn't be a Busa motor, would be the easy button.

ProDarwin wrote: What rules regarding weight removal? DP Miatas are around 1700lbs with a BP motor.

Personal rules or track rules? Personally since it would be track use only I would strip out EVERYTHING uneccesary. Down to all the wiring since there any wiring for the busa motor would be added, and the battery would likely be moved anyway. No idea what track rules are though.

captdownshift wrote: There's a gentleman involved in rally, Randy Z. I believe, who has a fastback conversion done to a miata with a 13B mounted mid ship that's for sale. You'll need to contact Kennedy Engineering regarding transmission mounting options, but picking this up, though it wouldn't be a Busa motor, would be the easy button.

Sounds neat! Personally, I'm not looking to spend a lot (who is?) and I like the fun of building it myself.

Robbie wrote:

AWSX1686 wrote: I really like the rear/mid idea...

We do too... Wanna work on a challenge car? You're a bit far but we are putting a GSXR-1000 motor in this:

Looks sweet! You are a bit far. I'd consider a GSXR1000 motor if I could find one cheap. How did you find yours?

In reply to AWSX1686:

GRM, GRM, GRM, and a little bit of ebay, and other forums.

I bought an 03-04 one on GRM like 3 years ago that was a "re-buildable core" but came with all ancillaries and wiring harness and ecu and a few other motorcycle bits like grips, switches, seat, levers, etc for $300. The internals of that motor are J.U.N.K.

I bought an 01-02 one from the dwarf cars forum that is running but has a busted 2nd gear. Guy included wiring, ecu, tbs, and an unistalled but good gearset. Less than $500 shipped from California.

Best deal so far is an 01-02 motor that the seller claimed is good, but was unwilling to ship. $20 won the ebay auction (I am very sure the ebay auction got no other bids because of the seller's anti-shipping stance - a similar motor sold the same week on ebay with "free shipping" for $1000). A fellow GRMer graciously picked it up and shipped it to me. $135 or so total.

Robbie wrote: In reply to AWSX1686: GRM, GRM, GRM, and a little bit of ebay, and other forums. I bought an 03-04 one on GRM like 3 years ago that was a "re-buildable core" but came with all ancillaries and wiring harness and ecu and a few other motorcycle bits like grips, switches, seat, levers, etc for $300. The internals of that motor are J.U.N.K. I bought an 01-02 one from the dwarf cars forum that is running but has a busted 2nd gear. Guy included wiring, ecu, tbs, and an unistalled but good gearset. Less than $500 shipped from California. Best deal so far is an 01-02 motor that the seller claimed is good, but was unwilling to ship. $20 won the ebay auction (I am very sure the ebay auction got no other bids because of the seller's anti-shipping stance - a similar motor sold the same week on ebay with "free shipping" for $1000). A fellow GRMer graciously picked it up and shipped it to me. $135 or so total.

Nice. Time to start searching!

AWSX1686 wrote: So I was thinking on the drive home the other day, Hayabusa miata. I know it has been done before but I have a different idea on how to go about it. First off, I doubt there is much likelihood of this being street legal. It might be possible, but for maximum performance my idea is picturing a track/autocross only car. First step, weight reduction. remove anything and everything possible. I have seen various weights for stripped down miatas, with this I am hoping I could get it down to 1700-1800lbs once it's done including the Hayabusa engine, but with a roll bar and everything that might be optimistic. Second, engine mounting and drive: I picture taking out the fuel tank, rear deck, soft top frame and putting the Hayabusa engine back there, with the chain running straight down to turn the rear diff. Just bolt a sprocket onto the input of the diff instead of the driveshaft. So all said and done, if it worked out how I picture it right now, it would be a car with a 10:1 or less pound to horsepower ratio, revving to a furious 10-11,000 rpms. With no reverse...

Since I seem to own the world's largest collection of Miata bits in compromising positions, I will offer this up as an illustration of the real estate you are planing to re-zone.

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You could pretty much build the project as a sub-assembly directly on to the suspension and offer it up into the car.

If you are willing to run a spool you can put a sprocket on the axle flange and do away with the need for the pinion entirely. That way you can also resolve the "too many gears in one direction" problem by flipping the engine from one side to the other.

Nobody says the differential has to be mounted facing forward either

What about mounting the diff upside down? Or flipping it over to put the ring on the other side?

Putting a bike engine in a car has always been a dream of mine, and I've researched and designed several possibilities.

If you are set on using the miata I would suggest mounting the engine in the front. You will get better weight distribution and attaching a driveshaft is relatively simple. This would be the easy button.

If mid-engine is your desire, and if there is enough room to mount the motor in front of the rear axle I would take out the differential. The dif is going to cause a fairly large power loss from the right angle gear drive, and its heavy (weight is your enemy). A better solution is to use a spool with a sprocket on it and drive from the output shaft of the motor straight to the spool. You will need to suspend the spool from the chassis on bearings of some sort and then attach it to the axles. I have seen guys who took a differential out of its housing, found a way to seal it to keep the oil inside, and then put a sprocket in place of the ring gear. This way you could have an open rear end, or one with a limited slip.

For reverse the cheapest and easiest way is to use a starter motor and a flex plate on the dif/spool). I've looked at several other ways but none seemed any better than this.

There are other challenges such as oil starvation of the Hayabusa. Google is your friend. People have gone this way before. Use their experience to help your project be a success.

I have been thinking about a similar project nearly every day since picking up the miata shell/scrap from Quasimofo in the $2017 classifieds a few weeks ago.

Since the diff/engine do not spin favorably to get 6 forward and no reverse gears......is there a junkyard differential which could be fitted to the miata subframe which rotates the opposite direction?

Edit: new posts in the time I typed mine, off to the wonderful world of google!

singleslammer wrote: What about mounting the diff upside down? Or flipping it over to put the ring on the other side?

Careful with this as diffs are carefully engineered with oil level and thrust angles and stuff in the stock orientation.

I would probably work long enough for a challenge weekend though!