Jack McCornack's 100 MPG DIY sports car

I first read about this guy a few years ago and have been following his blog as he develops a 100 mpg sports car using a small diesel Kubota tractor engine. I have no personal connection to Jack nor his company; just sharing a story that might be of interest to gearheads.

Jack writes well and has many entertaining stories about his mishaps and small victories while building this car. And if you want to build one yourself, all the info is there on his site. I’d give it a try myself if I had the garage, time, and money.


PS…he’s written over 100 blog “updates” which can be found on the Mother Earth News website under the Green Transportation section. Jack says he’s been importing those uodates to his website, but so far he’s imported only about 30. So to read the rest of the story, look for the “MAX” car updates at Mother Earth New, Green Transportation section.

A diesel engine in a Se7en? Blasphemy!!

A pollution pig, I bet. And 0-60 mph in over 18 seconds? A joke, not a ‘sports car’.

Now wait. If some one claims to make ( be working on ) a drivable sports car that gets 100 mpg but then needs “helpful suggestions” on how to get more horsepower to make it perform properly, then the whole idea sounds like this guy has way too much free time on his hands and the gullibility factor of those taking this project seriously is rather high.

Would have did a little better on the engine,a European company makes a small diesel engine suitable for a large touring bike,cant recall the name offhand,but probably 3-4 times more powerful then the Kubota.Just don’t think a tractor engine is that suitable for a car(if you want more power out of the diesel engine,go see the Mennonite mechanics,they know how to turn a diesel up-Kevin

You are right. The only reason you might choose a tractor motor to power a car is in series hybrid and that’s a stretch. ( though i did read somewhere that someone is considering it) Essentially, they are generator motors. The 3 cyl 1.8L in an L3400hst Kubota tractor only produces 35hp at 2300 rpm. . That certainly is inadequate for a car considering the weight of it’s all iron construction, but fine if all it does is run a pump which it does in this case. 1.8L gas motors in cars can get 40 plus mpg highway and produce as much as 100 hp more. Why bother ? Tractor motors are made for durability and very limited rpm range, even less so then a truck diesel. And, it was already mentioned, they are comparably dirty. Could you even licence one for the road ? I think you could do better with a Briggs and Stratton push rod then a tractor diesel.

@dagosa, yes the Kubota is short on HP, but it does have more torque than a gas engine of the same HP, and it’s stingy on fuel. I think one of Jack’s posts shows another guy who built a similar car using a 660 cc gas engine from a Japanese car (?).

And yes, you can license it for the road as a homebuilt replica, as long as it’s a replica of a model year that’s exempt from emissions testing. I think he licensed his as a replica of a 1950’s Lola, or something like that.

Well said Guys,a lot of good choices out there,series hybrid does change the game a certain amount,this engine I referred to ,would probaly be tailormade for this app.But honestly if I was doing this,I would go with a small V-6 or nice 4cyl.with an overdrive,the mileage wouldnt be as good,but the fun factor would sure be there.I dont think 30+mpg would be unreasonable,if you could keep the curb weight low-Kevin

Given that a diesel has no throttling losses at partial power settings, you would expect to see less fuel economy loss as the engine is “upsized” than you would with a gasser.

Meaning, that instead of a 100MPG car that can’t get out of its own way, one would be better served to build a 75MPG runabout with tolerable acceleration from a substantially larger engine.

Before you start working on a home-built car, first, better check with your State Motor Vehicle Dept concerning safety and emissions requirements…

If you start with a Seven, you don’t need much engine. It weighs about 1200 pounds. If you use a small tractor engine, you get a sports car that makes a Sprite look like a muscle car. A Seven with a Kubota engine seems like it should be called a Tata7.

Just to be clear. Though goobs of torque is where it’s at in truck application , horsepower IS the measure of choice in modern compact tractors, not massive amounts of torque. Though it does have more torque then another 35 hp gas engine, that isn’t the point. The motor is used to run a pump for it’s hydrostatic transmission and the power generated through the transmission is a direct result of the rpm and the number of horsepower available ompared to other diesels. Tractor operators are not swayed by huge horsepower ratings and is impressed only by that available at the end of a pto shaft compared to other diesels. Also, the diesel is not chosen for it’s torque alone but for it’s eonomy of operation and longevity…and in boat applications, it’s safety. All diesels are not torque monsters nor are they meant to be but neither am I saying they aren’t better then a comparable horsepower gas motor. Just don’t automatically think a diesel is chosen for it’s torque rating alone. It isn’t and many gas motors could be just as effective as they have in the past, but not as economically, dependability, with ease of maintenance and safety…

Since we’re on the subject, this is from Jack’s website amd describes his thought process for selecting the Kubota engine. (He originally intended to enter the car in the Automotive X Prize competition).

"Well, hopefully the X PRIZE will inspire development of higher efficiency automotive powerplants, but if you want 100mpg at 55 miles per hour or better with current off-the-shelf internal combustion technology, you need to get your car’s cruise power demands down to 8 horsepower or so, which…well, it isn’t rocket science, you can do it and we can do it, it starts getting tough when you get much faster. But we could all be cruising along at 55 at 100mpg with 10 horse engines if we didn’t mind taking six minutes to get up to speed, and dropping down to a trot when we hit a hill.

Light weight helps a bunch; it lowers rolling resistance, and acceleration and hillclimbing ability are directly proportional to weight (half the weight gives double the acceleration and double the grade), but still, some excess power is required. The problem is to find (or develop) a powerplant that has high efficiency in the 8 to 10 horsepower range, yet can crank out four or five times that power on demand. This 1100cc Kubota is rated at 32 horsepower and runs generators day after day at 50% power and high efficiency (.42 pounds of fuel per horsepower per hour), and though it’s going to take a bit of tweaking to get the power range we want, it’s a good basic engine to start with. The turbocharger is a big help, it adds a bunch of high end oomph without much (maybe not any) cost in economy at low power, and there are other off-the-shelf widgets that will further broaden the power range we’ll keep you up to date as we work with them and who knows, maybe we’ll invent something.

And maybe we’ll have to, because one of the interesting features of the Automotive X PRIZE is a new standard for measuring fuel economy; a unit they call the MPGe. That’s Miles Per Gallon Equivalent, using pump gas as a baseline. Diesel fuel has slightly more available energy per gallon than gasoline, so we (and the other diesels) need to get slightly better than 100mpg to reach 100MPGe. We don’t yet know exactly how much, but the X PRIZE Foundation only dropped the green flag on August 1 and some details are still being worked out."

Good luck,but its tough to reinvent the wheel,given the current state of 45-50 mpg cars(full featured I may add) the law of diminishing returns must certainly play a factor( a Mack truck can gross53K lbs and return 6-7 mpg on the highway)-Kevin

@Kevin, the car has already demonstrated over 100 mpg. I think he said he got around 120 mpg on one run. Mainly by minimizing aero drag.

Who here would want this car as a daily driver? Slow, polluting, 2 seats, open…not a lot to recommend it.
And while ‘100 mpg’ SOUNDS neat, like Kevin said there’s diminishing returns at work here. Going from 19 mpg to 25 mpg save more gas per mile driven than going from 50 mpg to 100 mpg.

@jesmed,he may have been hypermiling too.What I was implying was a scheme to make this thing more practical,I dont know where he got this engine ,But I have a suspicion if you bought this engine new,you would pay a pretty penny for it,not to mention the emissions.I think people used to be able to obtain 60 mpg from a diesel Rabbit pickup(what would happen if you used this running gear in this little machine(with auxiliary tanks these little trucks would go coast to coast on a fillup) What would happen if you used a Cruze 1.4 engine,or a good 1.8mill,still be fairly economical and lot more fun to drive.Diesels are great engines,but as Smokey Yunick said there are disadvantages to real small diesel engines,I guess thats one reason you dont see a lot of Diesel motorcycles.I do understand the advantages of the low frontal drag and low weight of this machine and He is to be lauded,no telling how far He can go with this concept(keep up the good work)-Kevin