The often talked about, but rarely accomplished...

13B based 4 Rotor

How many times have you heard someone say that they were going to build their own 4 rotor engine, and that they heard it was as easy as stacking the rotor sections of a coupla engines together?

Well here's a 4 Rotor I built, but it's the result of a couple years of brainstorming, much frustration, and several thousand cans of RC Cola. The engine design uses modified 13B eccentric shafts, and the two engines are joined by a custom made 3/8" thick aluminum adapter plate. It was all backyard engineering at the time, but very effective.

The Outlaw Dirt Cars were one of the few places that the 4 rotor car was allowed to run, so that's where we ran it. In the form shown here, output was around 550hp. Even at this low power level, the engine held it's own at times against the $25,000. aluminum blocked V-8s it ran against. Turbos were not allowed, so plain 'ol low octane pump gas was used. Race gas seemed to hurt power and make the headers glow. Thought was given to increasing the compression ratio by filling in the depressions on the rotor flanks, either by metal spraying them or bolting on some pieces of metal, but it was never done. We had planned to slip in some nitro, which would have helped with the relatively low compression ratio, but never got around to it.

Both the engine and the car were specially designed & fabricated by myself, and burned up a couple years worth of free, and increasingly not so free, time. Eventually, growing demands on my time forced me to cut back on the long hours the racing program consumed. After a while, I found myself dreaming of a more conventional power plant. One that I could actually BUY parts for, without having to spend endless hours modifying before I could actually use them.

After some damage from a ripped off oil cooler line, the 4 rotor was replaced with a 700hp alky burn'n small block Chevy, which proved more than competitive, and consumed way less time.

Was it fun? doing over 100mph on a short dirt track, tires blazing, rubbing fenders with the competition, all with a 5' high steel wall sometimes less than inches away? fun just doesn't describe it.

The series only lasted a season longer before being cancelled, so the car was eventually sold. Last I heard, it was somewhere in Oregon.


an addition featuring a description of the coupling making process, and a few of the other key components, including some dimensions taken from some old notes from the build in the early 1990's (before I had a computer or digital camera)...

CLICK HERE to check out the old scratch paper notes!!!


From here can seen the aluminum adapter plate between the two engines, which features the lifting eye that you see sticking out of the top/center of the engine. The external cooling plumbing can also be seen entering and leaving the center housings, as can the dual oil filter setup, which serves as a distribution manifold for supplying oil to the rear engine. The huge airbox filters out the dirt clods using a reinforced furnace filter (hey, I was on a budget). A Chevy power steering pump assists a Ford power rack & pinion steering gear. Unseen behind the firewall is a custom made planetary "quick steer" box, which speeds things up quite a bit, resulting in 1 turn lock to lock steering. No need to take your hands off the wheel, no matter how out of shape you get!

The car I built for the 4 rotor

Of the few rules the class had, one was that the front-end had to be stock appearing. We said the nose looked like a 1st gen RX-7. The tech inspector bought it.

Stock car racing on dirt is a contact sport, and body panel replacement is just part of the game. Big reason dirt Stock car bodys are made of flat sheet aluminum? - you get tired of building fancy panels.

The transmission is a hand fabricated unit, featuring only direct drive and neutral. A small multi-disc motorcycle clutch in the countershaft is used only to get the car rolling. Engaging dog ring on the mainshaft allows power to pass directly thru, bypassing the clutch. An aluminum flex plate, mounted on two bearings and one-way clutch, supports the ring gear. The one way clutch allows the starter to start the engine, but after the engine starts, it is no longer required to spin the flex plate. The result is the car could start and drive onto the track as per the rules, but didn't have to pay the penalty of the extra rotating mass of a conventional multi-disc clutch and flywheel. They never did figure out that one.

From here, you can see what can be a somewhat puzzling rear suspension. The basic design is a 4 link, in conjunction with an adjustable watts linkage, but here's where the dirt cars can get different. The 4 link brackets are not welded to the axle tubes, but allowed to rotate on greasable sleeves. This allows the rear axle to rotate in each sleeve independantly, eliminating the binds from their linkage. The axle housing rotation is controlled by a torque control rod attached to the aluminum plates atop the Quick Change centersection. The other end of the torque rod is attached to a spring and shock controlled "reaction arm", anchored to the chassis just to the right of the steering wheel. Barely visable is the blue specially made minature coil-over shock that controls the reaction arm. The reaction arm controls the wind-up of the rear axle, providing a degree of cushioning to help maintain traction over the rough and sometimes slick dirt track surfaces. The initial angle of the rod produces chassis lift to plant the rear tires on accelleration, but as the miniature coil-over is compressed, the angle levels out, reducing the vertical bind that an angled rod might otherwise have on the suspension travel. It works in reverse during de-celleration, controlling the initial pitch when the rod is closer to horizontal, then gradually increasing in angle to add squat to the rear.
The brake calipers are also mounted on floaters (the RR control rod can seen just inside the RR tire). These allow the individual brake reaction forces to be seperately tuned, which can really help when you are at the limits of your traction on a rough dirt track.
This all worked quite well, helping to get the most out of the sissy 14" wide hard tires we were forced to run by the rules (hey, the Sprint Cars were lighter and got to run soft 18" rubber). The suspension and steering featured 35 rod ends (not counting the throttle & brake balance bar linkages), each of which had to be cleaned and lubricated every week to keep things working freely.
Because of the low torque/hi-RPM characteristics of the rotary engine, we had to use the steepest change gears we could find. Typical rear axle ratio used was 8.50 & up.


Although the scrub radius of this geometry is huge, it can actually be benifical on a dirt track, producing an automatic jacking effect that changes the rear tire loading as the steering wheel is moved from side to side. The spindle and control arm geometry was all special to compliment the Ford steering rack.

Here's another unusual Dirt Late Model I built back in the '80s...

This one was built to beat an engine setback rule. The entire BB Mopar engine was reversed in the chassis, placing it's water pump, distributor, timing gears and oil pump on the "rear" of the engine, and only a lightweight ring gear and starter on the front. The net effect was like an additional 4" of engine setback. The mandated setback was 10-1/2" maximum from the upper balljoint centerline to the center line of the #1 sparkplug hole. Some argued that the #1 sparkplug was now at the rear of the engine, but luckily for us, the stock BB Mopar intake manifold is reversable (no water passages) and has it's runners numbered. We bolted it on and clearly pointed out the factory numbering sequence, with #1 being up front. Nobody said we had to keep the stock firing order, so we marked it as 1563487 instead of 18436572 because of the changes.
As a bonus, the reversed engine gave us a reverse-rotation drivetrain. The quick-change rear had it's centersection modified to allow flipping the diff carrier over, placing the ring gear on the other side of the pinion, allowing the rear tires to spin in the right direction.

Here's a closer look at the little home-made transmission that's used in the backwards engine car, same one that was later used in the 4 Rotor car. Tha aluminum cover on the right of the pic is the hydraulic cyl that was used to apply the motorcycle clutch that was located in the countershaft. The long rod extending from the right side is the spring loaded shift rod, which is basically moving a dog ring slider to engage direct, bypassing the countershaft clutch for racing. The input shaft was driven off of a splined hub that was bolted to the BB Mopar's balancer. The Balancer got 2 more "keys" added between it and the crank snout to prevent shearing. A small fabricated bellhousing mounted the transmission to the front (or rear?) of the engine in the picture above, but it was originally mounted back 18" farther to the chassis and used a really long input shaft.