A hot rod is, by a definition I just made up, a small vehicle with large vehicle parts. Years ago, someone figured out that you could take the engine out of a truck and put it in a roadster, and bam! Fast car. This modification has spanned the history of the automobile, from a hundred years ago in the California deserts to any one of a thousand LS engine swaps happening as you read this.
All you really need for a hot rod is the big vehicle engine, but you would be well advised to consider also carrying over the rest of the drivetrain (so that it will not explode when you hit the gas pedal) and the brakes (so that you will not explode when you hit a wall). A lot of people forget the brakes. Dodge forgot the brakes on first generation Viper, which has led to many sphincter clenching moments, including one for me at the end of the straight at Willow Springs.
But I digress.
The bigger the donor vehicle and the smaller the recipient vehicle, the hotter the rod. Putting a 1.8 liter in your Honda CRX? Mild hot rod. Putting a Space Shuttle SRB in your Peel P50? The hottest of rods.
But, I like to ignore the rules, I like to start sentences with conjunctions, and I like to string independent clauses together with commas into one long run-on sentence.
I also like motorcycles. I like 13,000 RPMs, I like sequential shifting, and I like a 180 horsepower engine complete with a transmission that is small enough to be carry-on luggage. We live in a golden age of two-wheeled death machines; power-to-weight ratios that are laughably ill-advised. Three and a half pounds for every horsepower on a two million dollar supercar? How about two and a half for a sportbike you can buy used for $3500 on any craigslist page in the country. A little discount suitcase-sized power plant just waiting to be dropped into a car.
The trick with the motorcycle engine powered car is that the car needs to be light. Part of this will be taken care of by the fact that you’re trading out whatever iron engine and transmission fatty are in there with a 130lb all-in-one screaming power unit. So start as small as possible; I chose a Honda S600, but you could also use an old Datsun, an early Triumph, a VW Beetle, basically anything built outside the U.S. before 1975. This is more of a personal preference, but: The less it looks like a hot rod, the better; you hear motorcycle engines all the time but when you drive by at 12,000 RPMs in a rusted shitbox, people do that cartoon thing where they rub their eyes and it makes that squeegee sound.
“This sounds really complicated and expensive, Matt; how do I attach the driveshaft, and what about reverse?” Fret not, fledgling fabricator. Getting the power from the bike sprocket to the wheels is easier than it sounds. There are companies that make adapters from the output shaft to common U-joints. I made my own with a spare sprocket, a tube, and a waterjet plate. You could also go chain drive to a rear axle, or even run the engine longitudinally and put a sprocket straight on the diff. As far as reverse, you can drop a grand and a half on a reverse gearbox, but the cheaper way is to use a starter motor from a larger car that engages a gear on the diff. You just put the car in neutral and press a button that turns on the starter motor. You could also just go without reverse and push the car backward, which is what I did, and which I do not recommend if you plan on having a broken leg sometime in the future.
There are a few other things to figure out, but you’d be surprised how much information there is out there. I have some pictures and info that explain how I did my motorcycle engine car, and I’m willing to help if you have any specific questions. There is also a whole group of people doing this in the cheapest possible way, and then posting how to do it on message boards. I’m talking of course about the Locost crowd. Locost, for those of you who don’t know, is a type of Lotus 7 replica built by people who want a racecar but all they have is a 30-year-old Ford Escort and a pile steel tubes they reclaimed from a warehouse fire. A lot of these people use bike engines because they are cheap and light, and also because of the glorious, glorious revs. These forums (both the UK one and the US one) are a goldmine of information on how to custom make anything you’d need. They are in stark contrast to the sportbike forums, where you will search through thousands of arrogant posts from ignorant teenage bro-tools to find absolutely no useful information of any kind.
Thanks to these and other idiotic and reckless motorcyclists, there are salvaged motorcycle engines with 150+ horsepower literally just lying around in the street. Literally! Okay, not literally, but they’re plentiful, and they’re cheap. It’s time to redefine hot rod. Go buy a liter bike engine, a welder, a 12 pack of High Life, and a rusty Fiat. Or build a Baja Bug with a bike engine, or add 80 horsepower to a salvage title MGB.
Go do an eBay search for “no engine” in pre-1975 cars and see what blank canvas you can find. Do it now; your boss is busy and you don’t really have anything that needs to be done today. The information is out there, the engines are out there, and the cars are out there. Do your part to make the world a better place.