Auto PSRU's Philosophy

  1. Preface
  2. Don't be tempted to build a hot rod engine
  3. Torque and Horsepower Charts
  4. Auto engines VS. certified aircraft engines
  5. Belts, chains, and planetary gears
  6. Use the auto engine as it was intended to be used
  7. Resolve torsional vibrations by using a centrifugal clutch
  8. Engines require different lubrication than the PSRU
  9. Be sensible, honor others, and have integrity


The following is a synopsis of Auto PSRU's philosophy based on Bud Warren’s personal opinions, first published in Geared Drives website, about the auto engine conversion industry, and some of the concepts that are in use regarding automotive conversion technologies. Many of these opinions are shared by me and will continue to be the basis of Auto PSRU's future. The updates below include some changes while respecting Bud's work. As you read about this philosophy, please remember that it is not intended to offend anyone. This philosophy is based on Bud's many years of personal experience with auto engines, experiences with certified engines in aircraft as an A & P, his knowledge and experience as a job machine shop owner and operator for thirty years which has given him a unique perspective based on actual knowledge, and my own experiences as an engineer. Bud as well as I feel that this perspective is based on real experience, as well as sound engineering design. This may represent what you might consider to be some rather strong opinions regarding some issues and designs that you might disagree with. Auto PSRU's does not wish to alienate anyone or make any enemies. It is very important to us that you know that we do not advocate what we know is dangerous, based on personal experiences and sound engineering. Doing so goes against our values and concern for our fellow man. So, be advised that the following will speak honestly and without reserve regarding this philosophy.

Don't be tempted to build a hot rod engine
Race car engines are not the best for aircraft use so don’t make the mistake of building a race car engine for your airplane. Use a mild engine at a conservative power setting. For instance, a stock Chevrolet LS1 crate engine has a nice flat torque curve in relationship to HP. Ideal auto conversion engines will have a low lift and low duration cam in combination with heads that have very small air chambers. This combination allows for a stronger power stroke and increased torque, much like one would want for use in a truck or a tractor. For example, the torque curve of this engine shows maximum torque between 3000 and 4000 RPM. Now, add a prop reduction drive utilizing straight cut spur gears with a mild reduction ratio of 1.562:1.00 and the result is a conservatively built engine, operating at a conservative engine RPM, using less fuel, creating less engine heat, and at the same time amplifying torque to the prop. Performance being one of the goals, our automotive conversion aircraft will fly circles around the same airplane with a certified aircraft engine, due to amplification of torque to the propeller through our Auto PSRU's PSRU. For instance, 350 ft lbs of torque to the crankshaft at takeoff results in 546 ft lbs of torque to propeller. With a constant speed prop and the Auto PSRU's PSRU, you will not believe the climb rates that you will have at your disposal. Believe me, when you need it one day, you will be glad you have it.

Torque and Horsepower Charts-
Click on the links below to view the graphs. Once you open the graph, move your mouse to the lower right of the image and click on the small box with four arrows on it to expand each graph to full size for easier viewing. Click your browser's back button to return to the previous page.

LS1 Horsepower vs. Torque at crankshaft
Prop RPM vs. Engine RPM Through the Auto PSRU's PSRU, Comparing Reduction Ratios 1.562 : 1 and 1.72 : 1
Torque to Prop as multiplied through the Auto PSRU's PSRU, Comparing Reduction Ratios of 1.562 : 1 and 1.72 : 1

Auto engines VS. certified aircraft engines

Automotive engine technology is the best it has ever been, however, as an A & P, Bud had seen the inner workings of certified engines on a consistent basis. Stuart has also seen the engineering developments that went into the latest generation of automotive engines. It seems that the FAA has made it so costly and difficult to have any new advancements for certified engines approved for use that the industry has found it more realistic to just stick to the status quo when it comes to engines.

After all, new aircraft are still being sold, and there is no real reason to change anything up in the area of the engine technology. Advances such as a spiffy new paint scheme, leather interior and a new flat panel instrument set are about all it takes to make a sale. Certified aircraft engines utilize virtually the same technology that they have used for the last fifty years or more.

Today’s automotive engines have technological advances that in many cases render them virtually ideal for use in our experimental aircraft, due to their compact size, fuel injection systems that automatically adjust for altitude and a variety of horsepower ranges available.

With the advancement of superior aluminum casting techniques within the automotive industry and the availability of even lighter complete engine packages, auto engines can now compete with, and in many cases are even lighter weight than the certified engines that they would replace, offering a superior horsepower to weight ratio.

We have observed that a lot of resistance to auto engines in aircraft is a direct result of the lack of a dependable way to convert horsepower and torque to the propeller. Belt and chain drives have achieved marginal success, but marginal success is not what we are willing to settle for. No offense to anyone, because we could have developed a belt or chain drive as well, but the plan was to set out to build a better mousetrap.

Belts, chains and planetary drives

From our perspective, the inherent danger with a belt drive is the belt itself. Anyone that has been to the drag races or watched Speed TV has seen the sudden loss of power due to a belt catapulting off into the atmosphere.

This might be an accepted risk in a race car since you have a chance to come back next round and start over, but in aircraft there might not be another opportunity. We are not willing to stake our life or anyone else’s on any belt of any kind.

Bud had personal experience with chain drives as well, and said he did not like the idea that chains are only as strong as their weakest link. Chains fail too, and there are too many horror stories regarding slack in the chain of a PSRU, whose builder had dismissed as “normal”.

These chains can crawl off the sprockets at an inopportune time, leaving the pilot with no choice but to put the aircraft down wherever he happens to be and pray for the best. We are not resistant to taking a calculated risk, as there is always risk, but this kind of risk is not acceptable to us when there are more safe and dependable options that we have the experience and knowledge to develop.

Being a former drag racer for many years, Bud had hands-on experience with planetary gears which do well in situations that require high RPM for a very brief period, but are not designed to operate under consistent and continuous use. They were designed to simply transition from one gear to another.

This ultra high RPM due to the gears being so small is not conducive to long life of a PSRU. We are strongly opposed to using planetary gears in something as vital to safety as a PSRU, especially when the straight cut spur gear is the superior option and has been incorporated into a superior redrive, such as in the Auto PSRU's designs.

Use the auto engine as it was intended to be used

Bud’s challenge was to engineer a PSRU that would allow an aviator to use an auto engine in an airplane in exactly the same way that it was designed to be used in a car. Therefore, This design transfers power to the prop from a series of spur gears driven off the crankshaft on the transmission end of the engine, and through an input shaft just like an automotive transmission.

In doing this, there are no side or thrust loads on the crankshaft from the propeller, so you can expect long engine life, dependability, and similar service as you would obtain from your family car. In addition, by reducing RPM’s from the engine to the prop through sturdy spur gears, torque is multiplied to the prop. Actually, the Auto PSRU's PSRU is probably more accurately described as a single speed transmission in its design and function.

Another important consideration of the design of Bud's PSRU is that it must maintain the propeller thrust line while allowing the engine to be placed in a lower position within the cowl.

Driving the prop directly off the crankshaft of an auto engine, is simply a major failure waiting to happen. Some builders are doing it and appear to be doing it successfully, but there are too many differences between automotive engines and aircraft engines to advocate a direct drive from an automotive crankshaft.

You can do just about anything for some period of time before suffering a catastrophic failure. Direct drive off an auto crankshaft is not wise because crank bearings on auto engines are narrow and comparatively offer perhaps 20% the bearing surface for support of the crankshaft than that of a certified aircraft engine. Auto engines were not designed to take the side and thrust loads that the propeller will apply to the engine if directly driven off the crank. With this in mind , a PSRU was developed that would allow the engine to drive the prop the same way it would drive a transmission.

Our PSRU's do not apply any loads to the engine that it was not designed to accommodate. Therefore the result is longer engine life with a higher safety factor and greater dependability.

Resolve torsional vibrations by using a centrifugal clutch

The Auto PSRU's designs incorporate a rather ingenious custom design for any automatic centrifugal clutch and flywheel. The centrifugal clutch assembly keeps the starter from pulling the inertia of the prop just long enough for the engine to start, which will contribute to longer starter life. The clutch also acts as a damper, eliminating harmful harmonic vibrations and offers smooth operation.

Once the engine is started, the prop will begin to spin due to inertia. At around 900-1000 engine RPM, the counter weights engage, capturing the clutch disc between the flywheel and pressure place, fully engaging the propeller.

The harder the engine runs, the harder the clutch is engaged, therefore, the more firmly the prop is engaged. The prop will remain engaged as long as the engine is spinning. As a result, when the engine is turned off, the propeller spins down to a stop-similar to a turbine engine aircraft. This prevents the propeller from forcing the engine through its high compression strokes until it stops, also known as "prop kick-back", eliminating the resulting vibration loads from going into the aircraft frame and delicate systems.

Bud started developing his PSRU in the early 1990’s. He had many exciting victories, and moments of pure let down when he realized that the theory did not result in success. The result was the PSRU being redesigned multiple times, each time resulting in a better unit with fewer issues. It had to be smaller, lighter, stronger, more dependable, and simplify the installation of an automotive engine within the cowling. After years of development Bud was happy with the performance and longevity. We are pleased to offer it to other aviators.

Bud said that all of the experiences in his life have lead to the development of this PSRU. Without these experiences, the development of this unit would not have been possible. It is the mission of Auto PSRU's to continue with the development and improvements to keep them the best available to our customers.

Engines require different lubrication than the PSRU

Gears require different lubricant than does the engine. Basic engineering standards were developed for the proper lubrication of a set of gears to assure a long service life. Separate lubrication systems are required for safety and for proper engine and gear function. Sharing engine oil with the PSRU is asking for engine and/or PSRU failure in advance.

The Auto PSRU's design has its own reservoir, forced and filtered lubrication by means of a dedicated oil pump, and utilizes the proper weight and type of gear lubricant that ensures long life of the unit. Bud felt that synthetic lubricants certainly sound great, but they do not perform well in a gear box.

Bud tested multiple synthetic lubricants and each failed to provide adequate cooling and eliminate noise. Bud recommended the use of all mineral 85-140 Gear Lube, of a type containing anti foaming properties, which is indicated for fill up, not just top off.

For now the use of any lubricant other than what is recommended could void the Auto PSRU's warranty. The results of additional testing of synthetic lubricants, and improvements in the lubrication system, will be announced if they are found to be an improvement.

Be sensible, honor others, and have integrity

Bud said that he developed his PSRU from the sweat of his brow and from his own personal earnings. Though he said he had many offers, he resisted taking on any investors or partners. “I have a huge personal commitment to this project” said Bud, just as each and every kit or scratch builder has. Bud flew his PSRU in his test aircraft for nearly 400 hours before he was prepared to offer it to other aircraft owners at Oshkosh in 2006. “We flew our Geared Drives PSRU on a 383 Chevrolet engine in our Wheeler Express with great success” Bud commented, “I do not know of any other PSRU builder that has actually owned and flown his own invention in an aircraft for any period of time. Until we lost our Wheeler in October 2008 due to a failed fuel line and resulting fire, we had flown just under 700 hours in approximately three years, including trips to Oshkosh and Sun N Fun.”

Bud had a lot of empathy for those that he had met that expressed to him that they had been taken to the cleaners by someone who came before. Bud said “I feel strongly that no designer or builder should finance the research and development of his design concept from the funds of fellow aircraft owners. ” Many of us have met some of the victims of this kind of activity and found it deplorable. Doing business like that creates bad will in the aviation community and the end result is just downright dishonest.

If you are going to do business this way, just go ahead and collect money from folks and call them investors, not customers, so at least they will know that there is a chance that they will lose their money in advance, and that they never receive the product that they have ordered and paid for. At least that would be honest.

Bud said "My opinion is that nothing should be offered for sale until it is tried and true and the designer can ship it out with confidence that the customer will enjoy a good flying experience with the product.” We all acknowledge that experimental aviation is a risky endeavor. We constantly strive to implement new changes and modifications to make our units even more dependable.

Safety being our first consideration, we do not advocate the use of belts, chains or planetary gears in something as important as a propeller drive. Our experienced opinion is that belts break, chains stretch, and planetary gears are simply not designed to operate at high RPM and under such a great and constant load as they are under while in use in an airplane.

Straight cut spur gears, as those used in our PSRU on the other hand, are designed to safely operate at a high RPM and under continuous use. Noisy? No. Many will argue with that statement. Using fine quality gears (these are not farm tractor gears) setting the unit up with the correct lash and clearances, and keeping the gears cool and well lubricated is the key.

We chose to use what we know to be the best technology for this application because our objective was to do what makes sense in terms of safety, reliability, and ease of maintenance.

Thank you for taking the time to read about these opinions.

Stuart Davis

Bud's opinions above have been modified by Stuart Davis to reflect the current situation and the transition to Auto PSRU's. The dedication to improvement through testing, safety, high quality, and support to all customers will continue.

E-Mail your questions to Stuart!