500 (Classic) Ducati powered Fiat from California

I have taken on the crazy task of trying to stuff a Ducati 1100 air cooled L-Twin engine in the back of my 1974 Fiat 500R, and solving all the challenges that will arise because of it!

Introduction

Hello everyone! As the title suggests, I have decided to build a Ducati powered Fiat 500!

I have restored a couple classic American cars, some motorcycles, and have designed and built a handful of Formula SAE cars in college, but I've always wanted to build my own motorcycle powered road car.

After a decade of working at one of the largest Spacecraft companies in California, I decided to take a break and try to make this dream a reality. The car needed to be simple, lightweight, relatively cheap, and older than 1975 (California smog requirements). I have a buddy that did this with an old Honda S600, and another with a Fiat 850 spider, but I wanted to do something a bit less roadster-y. Then, as the wife and I were honeymooning in Italy, I saw the answer: the old Cinquecento.

There seems to be lots of motorcycle powered 500s for hill climbing, and Z-Cars is one of the most popular swaps out there with their Suzuki Hayabusa swapped "Fiabusa" (they also make a Subaru swapped "Fubaru"), but I wanted to keep the motor air cooled (no big radiator up front) and Italian. After a bit of research, I decided to go with my favorite motorcycle engine, a Ducati 1100. The last of the big, air cooled, dry clutch L-Twins. How hilariously adorable would this car be with that noise coming out of it?

I spent a few months looking for the perfect car - something that was driveable, in decent shape, but not too nice, so I could save money up front and put it toward the swap. I ended up with a clean-ish 1974 Fiat 500 R with about 26,000 km on the clock that had been imported into the states a few years prior. The body was in pretty good shape until a strap broke towing it home resulting in a head on collision with the trailer. I minimally repaired the damaged area so I could turn the wheels full lock again, then drove it around a few months until I got the car registered.

Now that the car is legal, it's time to begin the project!

I'm looking forward to sharing the progress and hearing everyone's thoughts on the build. I have reached out to a few people individually and everyone has been extremely nice and very helpful!
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Hey Everyone - I wanted to give a quick Powertrain Update to go through my process of engine and differential selection!


Although I loved the idea of using the Ducati 1100 engine, I did look at a few other options including an electric motor and a much more powerful, water cooled Ducati 1098 engine. I even looked at a Moto Guzzi engine using a VW Beatle transaxle since Fiat had considered using Moto Guzzi on the 500s once upon a time, but that engine isn't very fast revving and I decided I really wanted sequential shifting! My first thought was to look at how these different options would make the car accelerate - so I made a "Tractive Force" plot, showing the vehicle's acceleration (in G) for a given speed in each gear. This is also a helpful tool for choosing my final drive.

1669227683283.png


I plotted the stock Fiat 500, just to remind myself of what I was comparing everything to. I was actually seriously considering an electric option similar to what Electric Classic Cars offer, but I decided it was going to be too slow for my liking. Maybe I can do a more custom electric option for a future Fiat 500 when I have more time to figure out how electrons work. The Ducati 1098 option would definitely be the fastest option of the bunch, but this power curve isn't nearly as smooth, and I really don't need the car to be that fast, even if I were to play with different gearing. Plus, I really didn't want to package a water-cooling system. The air-cooled Ducati 1100 seemed like the right balance between power and drivability. I'll need to add a couple of fans and open the rear of the car to get some good flow over the heads and oil cooler, but rejecting heat through oil coolers in the engine compartment should be much easier than rejecting heat through a radiator. This will be future me's problem.

Now that I've fixated on a particular engine, I need to figure out how I'm going to get this power to the ground, and ideally, go backwards. In the past, I've made a handful of chain-drive differentials but I no longer have access to free CNCs. You can buy complete chain diffs but they still require you to make custom diff mounts. These would also require a separate reverse solution, which I could solve with an electric motor or a reversing gearbox. I could also look at using a small differential from a car or ATV, but these options would just take up too much space, and since I'm trying to keep the back seat (a requirement from our dog, Peach), I really need to be space conscious. Below shows the Z-Car's solution - I'm trying to do the same in half the space!

1669227713921.png


So, I started looking for an off the shelf differential (not a spool rear end for dune buggies) that included a reverse. There seemed to be 2 players in this game: Quaife and Elite.

Quaife is a good brand, I've used their diffs before, and they have a pretty smart design that doesn't really need to be serviced; however, with this configuration, the engine needs to be mounted in-line with the diff, as it would be in the motorcycle. There are lots of these units out in the wild, including the Z-Car and my buddy's Fiat, but they all have much more space available. To package this, I'd have to run the diff in front of the engine, and squeeze everything together to give me as equal length axles as possible, reducing CV angls, while still fitting the engine in the car. This configuration gives me lots of room in the engine bay but it wouldn't be the most aesthetically appealing to me as it hides the engine’s exposed belts and dry clutch. It also would be more challenging to package a symmetric rear suspension.

1669227743352.png


Elite is a brand I've never heard of, and can't find a ton of information on, but they use a tried-and-true clutch pack style differential. I would eventually need to service this unit, but I don't expect that I'll put enough miles on it for this to be an issue. What's unique about this differential is it requires the engine to be mounted in a transverse orientation. This would allow me to directly mount the engine to the diff, eliminating the need to run a chain or belt, as required with the Quaife. However, after thinking about the critical alignment between the two units, and how low I'd have to put the engine in the car to line up with the diff, I thought an indirect drive solution would be best. It also allows me to reduce the input torque as seen by the differential, something that was a concern from the manufacturer.

1669227751744.png


I started to weigh the pros and cons of each solution, moving things around in CAD to see what packaged the best, making some wood templates to check fit and even buying a blown motor to put in the car and see what would actually work. After a pseudo–Design Review with my buddies, we decided that the Elite differential gave me the most options, packaged the best (especially from a chassis design side) and would deliver on the aesthetics potential of the engine bay.

1669227762780.png


So, there you go, I ordered the Elite option, which should hopefully arrive in January. In the meantime, I can focus on the front suspension, tearing down the donor bike, and designing the rear frame!

For the next update, I'll talk about wheels, tires, and front suspension stuff. I'm looking forward to hearing everyone's thoughts and continuing to get lots of help on this project!

-Bobby

Bounding Bambino

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Apr 13, 2021
Messages
164
Location
Ontario, Canada
This is a great update. I am also looking forward to your use with the Elite diff, if it works well in your application, and holds up to the abuse, it might be something i would consider for a future project. Getting the power out of the output shaft of a motorcycle engine too the wheels is always a tricky design challenge, especially when wanting reverse. Which is a given need in most swap applications.

Touching on what you were discussing earlier; a motorcycle engine in a car is i would say can be a win/loose scenario. Usually more pluses then negatives always lol. But the narrow power band and awkward gearing ratio can be tricky to get around and perfect.
I always found when a motorcycle engine is used in a car, the typical factory ratio between gears was a bit close to my liking and i always preferred to have a decent rev range between gear to get a good "harmonic range" lol. Obvioulsy the ratios are designed for peak performance range on a motorcycle. But im sure many of us 4 wheeled folks enjoy using the full rev range when rowing thru the gears. Its just the nature of the beast. There are some awesome motorcycle engines out there nowadays, and their attractive secondary market price makes it very tempting for a cost/lb/power ratio.

If space does become an issue, dont forget plan b. Alot of the racing fiats, have the upward hood hinge system for optimized airflow on the performance engines. Something like that might help with the odd tight squeeze if need be, while still looking "fiat heritage correct" lol.
 

the hobbler

Well-known member
Joined
Jul 25, 2012
Messages
3,644
This is a great update. I am also looking forward to your use with the Elite diff, if it works well in your application, and holds up to the abuse, it might be something i would consider for a future project. Getting the power out of the output shaft of a motorcycle engine too the wheels is always a tricky design challenge, especially when wanting reverse. Which is a given need in most swap applications.

Touching on what you were discussing earlier; a motorcycle engine in a car is i would say can be a win/loose scenario. Usually more pluses then negatives always lol. But the narrow power band and awkward gearing ratio can be tricky to get around and perfect.
I always found when a motorcycle engine is used in a car, the typical factory ratio between gears was a bit close to my liking and i always preferred to have a decent rev range between gear to get a good "harmonic range" lol. Obvioulsy the ratios are designed for peak performance range on a motorcycle. But im sure many of us 4 wheeled folks enjoy using the full rev range when rowing thru the gears. Its just the nature of the beast. There are some awesome motorcycle engines out there nowadays, and their attractive secondary market price makes it very tempting for a cost/lb/power ratio.

If space does become an issue, dont forget plan b. Alot of the racing fiats, have the upward hood hinge system for optimized airflow on the performance engines. Something like that might help with the odd tight squeeze if need be, while still looking "fiat heritage correct" lol.
Follow-upon Jacques comments regarding the reason that the engine cover is raised on the 600 based Abarths,and subsequanly, the 500 based variants. The story goes that a test session was being carried out on a 1000tc at (I think) Monza in the mid-sixties by their test driver Mario Poltronieri. The story goes that they found that the more they opened up the engine cover, the quicker the car went---in the end, they gained 6mph with the engine cover horizontal---an early 'boot-spoiler'! Let's face it, to gain that sort of mph out of a 1000cc car for the cost of 6 lengthes of tubing and a few small pieces of steel can only called one thing---"a result" The result was about 30% cooling and 70% earodynamics.
 
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PeachsGarage

PeachsGarage

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Sep 1, 2022
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21
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California
This is a great update. I am also looking forward to your use with the Elite diff, if it works well in your application, and holds up to the abuse, it might be something i would consider for a future project. Getting the power out of the output shaft of a motorcycle engine too the wheels is always a tricky design challenge, especially when wanting reverse. Which is a given need in most swap applications.

Touching on what you were discussing earlier; a motorcycle engine in a car is i would say can be a win/loose scenario. Usually more pluses then negatives always lol. But the narrow power band and awkward gearing ratio can be tricky to get around and perfect.
I always found when a motorcycle engine is used in a car, the typical factory ratio between gears was a bit close to my liking and i always preferred to have a decent rev range between gear to get a good "harmonic range" lol. Obvioulsy the ratios are designed for peak performance range on a motorcycle. But im sure many of us 4 wheeled folks enjoy using the full rev range when rowing thru the gears. Its just the nature of the beast. There are some awesome motorcycle engines out there nowadays, and their attractive secondary market price makes it very tempting for a cost/lb/power ratio.

If space does become an issue, dont forget plan b. Alot of the racing fiats, have the upward hood hinge system for optimized airflow on the performance engines. Something like that might help with the odd tight squeeze if need be, while still looking "fiat heritage correct" lol.

Thanks Jacques! Agree, I'm not a huge fan of the narrow power band that most of the high revving motorcycles have, which is why I like this particular motor - lots of torque, very low in RPMs, and very flat torque curve. Most bikes don't start building power until 5-6000 RPM but this is pretty useable after 3000.

The nice thing about this plot is that you can really visualize how the final drive plays with the driveability of the car. If you want to run through the gears on your way to cruising speed, there's a gear for that. If you just want to just use 3 gears, that's an option as well (at the sacrifice of acceleration). For me, I wanted something that would require lots of shifting, but also wanted 60MPH in top gear to not be spinning too fast. At least I can change out gearing later when I get disappointed with my selection!

Follow-upon Jacques comments regarding the reason that the engine cover is raised on the 600 based Abarths,and subsequanly, the 500 based variants. The story goes that a test session was being carried out on a 1000tc at (I think) Monza in the mid-sixties by their test driver Mario Poltronieri. The story goes that they found that the more they opened up the engine cover, the quicker the car went---in the end, they gained 6mph with the engine cover horizontal---an early 'boot-spoiler'! Let's face it, to gain that sort of mph out of a 1000cc car for the cost of 6 lengthes of tubing and a few small pieces of steel can only called one thing---"a result" The result was about 30% cooling and 70% earodynamics.

Yup! As you guys mention, I've always thought I can mimic an old 1000tc, but hopefully I can package everything internal. I also have thought about just partially cutting out the lid to reveal the dry clutch and timing belts, or leaving it off all together. When I get closer to that point, I will probably ask everyone's opinion on what they like!

Totally makes since with the improved aero by turning that cover horizontal. It's a huge low pressure zone back there and it sounds like it did wonders to cut down drag and draw out engine heat!
 

Goldnrust

Member
Joined
Sep 27, 2020
Messages
88
Very cool project! I've had a couple of Ducatis, 748 and a 800 monster, and the sound and feel of the engines is something special.

I will watch the project with interest!

It looks like you're considering the 'professional' gearbox solutions, but if you did decide you want a more budget option then I've seen some interesting things done on other bike engine builds online using a second starter motor as an electric reverse and a CV joint to remove the challenges of perfect alignment.
 

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PeachsGarage

PeachsGarage

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Very cool project! I've had a couple of Ducatis, 748 and a 800 monster, and the sound and feel of the engines is something special.

I will watch the project with interest!

It looks like you're considering the 'professional' gearbox solutions, but if you did decide you want a more budget option then I've seen some interesting things done on other bike engine builds online using a second starter motor as an electric reverse and a CV joint to remove the challenges of perfect alignment.
Thanks!

That's a pretty clever solution, but too bad there's no space in these cars to run a car differential without loosing the back seat!
 
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