The Weirdest Honda Insight On The Internet Gets 70 MPG With A 20 HP Mini Excavator Diesel Engine

One of the most fuel-efficient cars ever sold in America is the first-generation Honda Insight. When it launched in December 1999, the Environmental Protection Agency slapped it with an incredible 70 mpg highway rating. Sadly, only some people in real life hit that lofty number, and then the EPA later revised its methods, knocking the Insight down to a still impressive 61 mpg rating. As it turns out, there’s a super weird way to get 70 mpg out of an Insight, and it’s by tearing out the hybrid system and installing a cute 20 HP diesel engine meant for a mini excavator.

This wild project comes from the madman behind the Robot Cantina YouTube channel. The charismatic host, who calls himself Jimbo, loves creating silly things just for the fun of it. Jimbo is the mastermind behind such projects as making a hybrid golf cart, giving a Saturn ‘Cylinder Deactivation’ through deleting its engine’s valvetrain, and powering a first-generation Honda Insight with a Harbor Freight Predator 212cc engine. If you’re not in the know, that’s an engine commonly used in go-karts and minibike builds.

Jimbo is pretty upfront when he says that these projects aren’t done for real practicality. He doesn’t seriously expect you to gut your car to replace its engine with a Predator mill. Jimbo also doesn’t expect you to do what he did in the video below, and fill your former hybrid car with a 20HP 719cc Kubota D722 diesel engine, which normally goes into mini excavators and other things that aren’t cars.

We’ve been fans of Robot Cantina for a while, and I’ve been periodically writing stories about Jimbo’s videos since 2021. His projects are just that alluring!

Before Jimbo begins explaining how he squeezed 70 mpg out of this thing, he talks about why he put Predator engines and then a Kubota engine in an Insight in the first place. As I noted above, even with the EPA’s corrected numbers, the Insight gets epic fuel economy. It took Toyota until 2015 to finally unseat the Insight as the mpg champion, and even then, the Prius only beat the Insight’s post-2008 revised fuel economy numbers.

To some, the first-generation Insight remains the holy grail of hybrids. It was designed with a lightweight aluminum structure, was originally equipped with a manual transmission, and its drag coefficient of 0.25 made it the most slippery car on the market at the time. The original Insight is even DIY-friendly, and something I like is that due to Honda’s design of the hybrid system, the car can still start and drive even after a battery failure. It’s no surprise that there’s no shortage of Insights with well over 300,000 miles.

Jimbo acknowledges all of that as true. However, he chose the Honda Insight because the same traits that make it a great hybrid also make it a great platform for stupid engine swaps. The low weight and slippery aero are great if you’re trying to fit a lawnmower engine into a car and have any sort of expectation of hitting the speed limit. Amazingly, Jimbo found out that a stock Predator 212 is good for 39 mph in an Insight.

In previous episodes, Jimbo worked through a bunch of experiments involving the three-cylinder Kubota D722 engine. This is an interesting little powerplant. It comes from Kubota’s Super Mini engine family, which launched in 1983. Kubota says it was the world’s smallest multi-cylinder diesel engine series with a 62.2mm stroke and around 200cc per cylinder, depending on configuration.

At first, the engine was bolted into a Saturn, but that car was so worn out that it had to be scrapped before the project was finished. Then, Jimbo tried to fit a Volkswagen 1.6-liter diesel into the Insight shell, but found that the engine was so big that it was a tight fit.

Jimbo says that fitting the Kubota into the Insight was a difficult task.

The big hurdle was with the transmission, as the Insight’s manual transmission ratios were incompatible with the low-power diesel. But that’s where the earlier attempted Saturn swap comes in, because the Saturn MP3 five-speed close-ratio manual transmission is a good fit for the diesel. Mounting the engine and transmission wasn’t hard, either, as Jimbo just made custom mounts that bolted into the Insight’s existing structure without modification. Click here to watch those videos.

Last summer, Jimbo added an AMR500 supercharger to the Kubota, and he targeted an output of 30 HP to 35 HP. That was good for a 55 mph sprint in about 20 seconds. Viewers have been begging Jimbo to go with a turbo, so this year, he tossed the AMR500 out and replaced it with an RHB31 turbocharger. This turbo slowed the car down and made it take around 10 seconds longer to reach 55 mph.

Jimbo is correcting some of those faults in this video. First, there’s a loose bolt that needs to be tightened. He also needs to fix a pinhole in his welding work, both of which appeared to cause a massive boost leak. Some welding and some sanding fixed the holes created from the original welding job.

Next, Jimbo notes that the engine was getting way too much fuel during the initial turbo testing. This is bad, and not just because of the black, dirty smoke. Exhaust gas temperatures (EGT) were dangerously high. In extreme instances, super high EGT values can lead to melted pistons or even a catastrophic engine failure, followed by an instant unscheduled roadside disassembly. Fixing this on the Kubota was as easy as adjusting the fuel rack limiter screw.

With that out of the way, Jimbo takes the car out onto the road. I was a bit impressed that this little engine was powerful enough to allow the Insight to maintain 64 mph on level ground. Jimbo noted that EGTs were about 750 degrees during this, which was safe for the engine.

Next, Jimbo settles into a cruise of 55 mph for the mpg run. The car’s doing great in this setting with EGTs down to about 560 degrees. Sadly, the turbo isn’t doing so hot here as it’s putting out only 2.5 PSI. Worse, Jimbo notes, is that fifth gear is so tall in the Saturn MP3 transmission that the engine loses all boost and has to spend a bunch of time rebuilding it.

Despite this, Jimbo completes his mileage test. To measure fuel economy, he measured the weight of the diesel at the beginning of the run and at the end of the run. The Insight carried about 12.8 kilograms (about 28.2 pounds) of diesel at the start of the run. Once Jimbo got home, he measured again, finding that the car burned through roughly 2.8 kilos (6.1 pounds) of diesel.

Before revealing the final score, Jimbo then hopped into a stock Honda Insight and took that on a 60-mile loop of rural Kansas. During the drive, he notes that his car’s hybrid battery is bad, so it’s not performing as well as it could. Still, after 60 miles of driving the speed limit, the car returned to home base with a score of 62.7 mpg.

The diesel returned a great 70.1 mpg. For reference, my 2006 Smart Fortwo diesel will get 70 mpg all day. Amusingly, the 800cc three-cylinder turbodiesel in my Smart is only slightly larger than the Kubota engine and makes only 40 HP. So this turbodiesel Insight is not far off!

Yet, as Jimbo notes, the current turbo is so inefficient at its job that the supercharger did better, scoring 78.6 mpg with far better performance. Jimbo says he’s not done yet, as he’s currently looking into using a snail from a Smart Fortwo diesel or one from a Volkswagen Lupo TDI. So, maybe there’s still a chance for a better turbo to beat the supercharger.

Again, none of this is really all that practical. It’s all in the name of fun, and I dig it. Everyone’s into K20 swaps and lithium conversions, yet here’s Jimbo over here slinging the dumbest, yet seemingly most fun swaps on the Internet. Click here to watch more of Jimbo’s madness.

H/T to the Drive!