If you haven’t read the introduction post already, I’m building a hybrid sports car in my garage. Yes, an entire sports car, top to bottom.
It’s intended to be "open-source" when complete, with every part fabricated off-site. That means every component needs a specification, the whole build planned out before any custom hardware is ordered.
The engine is a focal point of the project, especially how it relates to the car’s other components. Let’s talk about it.
For my project, I’m using a BMW K1600 motorcycle engine. It’s a 1.6-liter, makes 160 horsepower, and revs out to 8,500 rpm. It weighs 226 pounds, including the six-speed sequential transmission, which makes it pretty light overall. It’s a great foundation for any lightweight sports car.
However, there are some drawbacks to the K1600 mill, but not ones necessarily unique to this engine. I grappled with these issues for a long time without actually buying an engine, but eventually I caved. A low-mileage example is now in my garage, finally hoisted up on an engine crane so I can take some better scans and figure out how this is all going to work.
Key Details
I’ve mentioned before how this engine spins the wrong way compared to a car engine. In terms of planning, that hasn’t complicated things a ton. Other smaller quirks are really the bulk of what I’m figuring out now.
The first big decision was general engine placement. I knew I was going mid-engine, so everything sort of revolved around that. I got a rough and ready scan of the motor on its shipping dolly to help figure out how it might all work.
This scan isn’t perfect, but it gives me the size of the engine as well as some locations of key components. Easily the most important part of this whole engine placement issue is the transmission's output shaft. Since this unit is normally installed on a motorcycle, the output is not centered. This may not sound like a big deal, but it is.
I want my wheelbase as short as possible. When I started, I set a goal of less than 90 inches, which quickly became impossible. It all has to do with the rear subframe and the passenger area.
The passenger area must be a certain length to accommodate the driver. I have a human analog in my CAD model that’s about my size. I’ve also taken rough scans of a few cars to lay over my own interior, ensuring my work isn’t way off the mark. Front to rear, I give the passengers 54 inches to stretch their legs. A little gets taken out due to the seat and pedal placement, but that’s a chat for another project journal.
My front axle centerline sits 15 inches ahead of this 54-inch passenger area. That’s as close as I’m comfortable putting the front wheels to the frame. Now that the front axle centerline and passenger volume are fixed, the wheelbase is completely at the mercy of the engine and rear differential placement.
It’s hard to enumerate exactly how complex this issue is. It’s not complex conceptually, but there are so many facets to it.
So I wanted to keep the wheelbase short. First things first, I moved the engine right up against the firewall. Okay, that got me a few inches. Now it’s time to select a differential to get power to the rear wheels. I really wanted to use the diff out of the Audi A4 B8 like I did in the front—commonality of axles and wheel bearings would be nice (the Audi stuff is also very cheap)—but it just didn’t pan out.
The Audi diff’s tail section proved to be too long. As a result, the wheelbase would be lengthened artificially, for no good reason, since the differential mounts more or less right up to the transmission output shaft.
I looked through a ton of options and eventually landed on a BMW differential, the so-called “small case” diff installed in a ton of cars including the BMW Z3, the earlier E30, and the E36 3 Series. It has a very short distance between the output flanges and the input shaft—the shortest I’ve seen—and it’s available in the high gearing ratios this motorcycle engine needs.
Okay, so I found the diff. It looks like it could mount okay, and the ratio is perfect. The wheelbase is still too long, though. At this point, I wondered how much I could angle the CV axles forward to shorten the wheelbase. Anything past 15-20 degrees and your CV axles won’t last long.
Even modest angles presented another problem. The BMW drivetrain sat centered in the car up to this point, and since the output shaft of the transmission is offset, the differential is not centered in the car. This isn’t an issue if your axle angles are relatively straight, but once they're tilted forward, the angle is more extreme on one side.
A decision had to be made. Lengthen the wheelbase to improve the CV angle, live with severely angled and short CV axles, or move the engine over to center the differential. While there’s no perfect decision, I went with the third one.
The bad parts: the single heaviest part of the car is now off the vehicle centerline, the engine mounting is more annoying, and plumbing for things like the intake and exhaust is trickier. I’m sure there are others I have yet to discover as well.
The good outweighs the bad. Yes, the engine is now off the centerline of the car which is a mass distribution issue, but the transmission output location means the engine moves to the passenger’s side. The weight of the driver offsets the engine and there’s room on that end for a fuel tank, too. The actual corner weights of the car will be set with a half tank of fuel and the driver in place. I think that will be plenty to get the mass equation sorted out.
Now I have equal axle lengths which means I can order two of the same custom axle, which will save time and money. Likewise, I can run my angle right up to 15 degrees to minimize the wheelbase. Sure, the intake and exhaust are a little wonky but those parts have to be custom anyway. In the end, the wheelbase adds up to 98 inches, which is longer than a Miata but shorter than a BRZ/GR86.
If you want a very short wheelbase in a mid-engined car, you need to make many creative compromises regarding the passenger area, the rear transaxle, or both. This is where I’ll briefly touch on a fourth option, which is to mount the engine and differential on the car's centerline.
The issue is that there’s no good way to do it when you really examine the problem. You end up on an inescapable spectrum between sketchy and expensive, where you’re always adding weight to solve the problem. The wheelbase advantage of this more central layout is also negligible in the end. There are a few pros, yes, but it’s just not worth the trouble.
Direct drive with an offset engine is the lightest, cheapest, simplest, and most reliable option.
This was supposed to be a post about the engine, right? It’s easy to get off track, though. Few decisions on this project exist in a vacuum. Once the scans start to land in CAD, tough decisions have to be made.
It really makes you feel for engineers developing sports cars that use a lot of parts from other cars. Those guys don’t have the luxury of developing new transaxles or engines to optimize the entire car as a package, they're handed parts they must use and have to make the best of it. They’re also dealing with tons of other constraints like crash safety, fuel economy, internal engineering requirements, and—god forgive me for saying this—marketing. It’s a tremendous challenge.
And if the car produced by all these compromises and challenges is not particularly well received? What a punch in the gut.
I always keep that in mind when I do this. This project is difficult, expensive, and time-consuming, but in the end it’s a drop in the bucket compared to an actual production car.
There’s a lot more work to be done before I get the next update out the door, but it will be more substantive in terms of actual progress. I’m currently in the thick of finding suppliers and sponsors, which isn’t very exciting from the outside, but I have to make more design progress to move that process forward as well.
See you next time.
