Engines and Conversion Kits.

General information.


Interest in 4-stroke auto-moto engines converting for Light Sport and Ultralight aircraft is growing all over the world. We know that popular 2-stroke Rotax 503 and 582 are not certified aviation engines but variations of snowmobile engines with all 2-strokers' problems. Prices of certified 4-stroke aviation engines are quite high. The active development of aviation piston engines has stopped more than 60 years ago with the jet age beginning. The same time, cars become more and more perfect. Modern auto engines are light, have high reliability and life time. Electronic ignition and fuel injection with microprocessor control system, low gas consumption and flight hour value, simple and inexpensive maintenance with attractive unit price, these features usually distinguish current-technology converted auto engine from its "real aviation" brother.

I started to use 4-stroke Auto-Moto engines for aircraft in early 90-th. These engines have never been alternative for me. Many people interpret that to mean weird, more complicated or lesser way for builders who cannot afford to buy good & proven Rotax 912S". I never considered them only as "cheap" way of aircraft engine development. Automotive companies spent hundreds of million dollars for their new engines. We all know about serious competition in this industry for the last decades. As a result, even being less expensive because of mass production, modern auto engine is way more advanced. It is a pleasure for me to design new conversion systems and fly them.
There is a lot of experience in converting of Subaru, Honda, Chevy, Nissan, Kawasaki, BMW and other engines but light and small GEO-Suzuki G10, G13 and G16 are the most interesting engines for me as Weight-Shift control aircraft (trike) designer, builder and pilot. G10 with stock fuel injection has about the same HP as popular Rotax 582 model. Trikes with G10 have good flight performances with significantly lower flight hour cost. G10 Turbo, G13BA and G16B have about the same power as Rotax 912, 912S and 914. But my favourite engine for trikes for the last several years is G13BB. Different trikes were built and supplied to customers with these engines. I test fly every one by myself. Years and hundreds of flying hours without major problems.
The GEO-Suzuki has its advantages, as does the Rotax 912. My opinion as an aviation engineer and pilot, and the opinion of many builders and pilots who have tried both Air Trikes conversions and 912 engines is that our GEO-Suzuki conversion in many aspects is the preferable choice.

This is a small video of Apollo Delta Jet trike flying with G13BB. Similar trike with our engine was flying at European 2013 and World 2014 FAI championships. Our engines are used for PPG, Airplanes, Auto gyros, Airboats, Hovercrafts etc. Air Trikes Enterprises supply ready to install engines with gearboxes, propellers, motormounts, control systems, mufflers etc. We also supply gearbox conversion kits and parts for different (Suzuki, Subaru, Honda, BMW) engines. More information below.


Suzuki Power Group (SPG)-3 and 4 gearbox conversion kits:


These gearbox conversion systems were designed as modifications of a popular SPG-2 system (was in production from 2000). They are intended for aviation conversion of 2 and 4-stroke engines and using them with propellers for NOT SERTIFIED experimental light aircraft, hovercraft, airboats etc. SPG-3 has the same gear pair as SPG-2 but lighter and easier to install-remove. Recommended for up to 130HP engines. The available reduction ratios are 1.8, 2.03, 2.34, 2.5. SPG-4 has higher gear module and width, a little bigger body, about 2LB heavier than SPG-3, RR up to 3.16, recommended for up to 180HP, high torque engines. There is a German (BMW) rubber damper between the gear box and the engine. The propeller moment of inertia up to 9,000 kg/cm2. Rotax-style propeller hub, hollow propeller shaft so it can be used with variable pitch prop or for glider towing. It is possible to use these gearboxes for both (tractor and pusher) layouts. The gearboxes can be installed up, down and any degrees left or right on the bellhousing. It allows to install the engine not only straight up but inclined or laid flat.

SPG3 for GEO and Subaru

The body consists of 2 bolted down parts:
1) gearbox with the gear and oil inside + damper housing
2) mount plate (bellhousing) and crank-damper coupler.
BMW damper and hardware included. Bellhousing and coupler are different for different (Suzuki, Subaru, Nissan, Honda etc.) You can see SPG-3 sets for GEO (left) and Subaru (right)

The gearbox life-time is 1000-3000 hours (depends on the engine power and the prop moment of inertia). High reliability and efficiency, simple maintenance. Mechanical part of the engine conversion is simple. SPG-2 instruction with step-by-step conversion pictures HERE. Click and look - nothing difficult to convert your auto engine using this gearbox bolt-on conversion system. SPG-3(4) system installation is even easier because of studs used instead of bolts for gearbox-damper connection.

We supply complete gearbox conversion kits for different engines Suzuki G (10, 13, 16) and K series, Subaru EA (71, 81, 82) EJ (18, 20, 22, 25), Honda D and Fit (L15), BMW R, Toyota, universal gearbox kits, bellhousings, spare parts. We also can design and make mount plates and adapters for other engine conversions.

Dimensions and weight data:

SPG-3 Metric Imperial
Offset (dimension between crank and prop shafts) 80mm 3.15"
Gearbox length (no bellhousing) 240mm 9.5"
Engine-prop flange dimension* 300mm 12"
Propeller hub Rotax standard**
Gearbox weight 7.5kg 16.5Lb
Full conversion kit weight* 11kg 24LB
*For Suzuki-Geo G10 and G13, can be a little different for other engines.

SPG-4 Metric Imperial
Offset (dimension between crank and prop shafts) 90mm 3.55"
Gearbox length (no bellhousing) 230mm 9"
Engine-prop flange dimension* 290mm 11.5"
Propeller hub Rotax standard**
Gearbox weight 8.5kg 18.7Lb
Full conversion kit weight 12kg 26.5LB
*For Subaru EJ, can be a little different for other engines.

**Adapters can be ordered for other shape of propellers
Conversion kit weight data provided for the complete bolt on kit consists of gearbox, damper, bellhousing and hardware. The data can vary slightly during the production and depends on the engine model.

GEO-Suzuki engines converted with SPG-3:

This is one of G13BB engines converted with SPG-3 and installed on a trike. The standard engine for our trikes from 2005 (Suzuki G10 were standard before). Trikes with our engines have the same flying performances as similar trikes with Rotax 912S engine, with better fuel efficiency and faster throttle response. Short takeoff, maximal airspeed with 14.5m2 wing up to 176kph (108+mph), climbing up to 1600 FPM. The average fuel consumption around 2 GPH, minimal can be as low as 1.5 GPH. We use 87 octane autogas. Average static thrust of G13BB with 2.5 ratio gearbox and recommended prop 500+ LB (around 370 LB for G10).

The engine is 16 valves, full injection, with double ignition coil pack and DENSO computerised injection and ignition control system. The mixture is prepared based on intake pressure sensor data, so altitude compensation is automatic and the mixture is always optimal. Throttle body heating is a standard feature so we never had problems with icing. The engine block is not modified; the stock water outlet with thermostat is used. This makes the engine very thermo stable. I heard a lot about overheating problem from people using conversions with modified water cooling system but have never had it even with a small radiator closed by BRS system. It is well balanced, low level of vibration at all range of RPM, from 800 to 6000. Soft and pleasant sound. The engine is a little heavier than Rotax 912. With all our improvements, the real difference between 912S and G13BB made by Air Trikes Enterprises is around 20LB. As we know, the new injection Rotax 912is is heavier than its carbureted version so the weight difference between these injection engines is even less. The inline G13BB engine (installed correctly) has smaller mid-section than the same HP opposed type engine like Rotax 912S. Its aerodynamic drag is less. I dont want to make a full comparison of these engines. Just my opinion based on my education as aviation engineer + my real experience as light aircraft builder and pilot it is possible to use correctly converted and installed Geo-Suzuki engines instead of Rotax 912 with a very good end result.

All conversion parts for Suzuki like mufflers, motormounts with rubber silent blocks, engine control systems, control gauges, light alternators and starters, light aluminum pulleys, radiators etc. are also available with our GEO-Suzuki engine conversion brochure. Every part of the conversion was redesigned and improved based on hundreds of flight hours. "Small" improvements are in reality not so small. For example a set of 3 light (crank, water pump and alternator) aluminum hard anodized pulleys is not only weight saving. Increased alternator pulley diameter makes alternator running slower. It allows to avoid alternator overwork (usually we fly higher RPM than drive cars). This is not only a little bit more of available power but (more important) less possibility of alternator bearing failure. Our light alternator set saves 2.5-5lb of weight comparatively with the stock alternators.

Air Trikes Enterprises, our clients and partners have made a lot of different engine conversions for the last 14 years. We can state - Air Trikes Geo-Suzuki is not just an alternative engine or project but well proven and efficient powerplant for experimental aircraft and other air propeller driven craft. We provide not only conversion kits but also ready-to-install powerplants based on GEO-Suzuki engines + conversion kits + KOOLprops:

A) Throttle-body injection (TBI) G10 with DENSO Electronic Control Module (ECM) - 62-65HP
B) G10 Turbo with MPI, resonance intake, turbocharger up to 92HP (intercooler & knock sensor can be used)
C) G13BA 8 valves SONS naturally aspirated up to 80HP
D) G13BB 16 valves SONS about 100 HP naturally aspirated
E) G13GTI 16 valves DONS 110+HP naturally aspirated
F) G16 16 valves SONS NA about 115HP


Different GEO-Suzuki converted with SPG-2(3):

Air Trikes sells SPG-3 and SPG-4 conversion kits for Subaru EA and EJ:

GEO-Suzuki engine conversion brochure

Information brochure about converting Geo-Suzuki G series engines for Homebuilt aircraft and other air propeller driven craft is available for your order. The price is 99USD paid with PayPal to info@airtrikes.net E-mail address. This price included 4% PayPal fees, print and Canada Post regular mail cost. Do not forget to include your mail address. We send only printed version, no CD, no download. If you want more information about it, to see Content or read Introduction/Conclusion - CLICK ON THIS LINK.

If you can't use PayPal and wish to send 95USD by check, or have other questions about ordering this brochure, contact us.


Honda engine conversion

SPG3 + Honda Civic

Honda Civic engines are widely converted and used for aircraft and airboats. These engines are highest quality (as everything made by Honda). They have stock carburated version with a distributor. We have complete gearbox conversion kit for Honda Civic (D series) engines.

New, more compact and light but very powerful L-serie engines came to North America under the hood of subcompact Honda Fit in 2007. Now they are well known, not expensive and easy available with very affordable price.

L15A VTEC engine official data:
Displacement: 1,496 cc (91.3 cu in)
Bore x Stroke: 73.0 X 89.4 mm (2.87 X 3.52 in)
Compression Ratio: 10.4:1
Horsepower: 81 kW (110 PS; 109 hp) @ 5800 rpm, up to 117HP at 6600 RPM
Torque: 143 Nm (105 lbft) @ 4800 rpm

It is 16 valve SONS. This engine block is about 10kg (22lb) heavier, but 1.5L instead of 1.3 and has about 20HP+ If we compare it with G13BB. The same power as G16B.
Positive moments - timing chain instead of belt, 4 stock sparkplug-on coils (one for each cylinder).
Negative moments - more complicated motormount, bulky intake, more complicated stock control system (VTEC, electronic throttle control).
This engine is able to give about 300kg (660LB) static thrust on a trike (pusher config) comparatively with 240-250kg from G13BB. The gearbox conversion kit has been designed and tested in 2011. Available for your order.


BMW R serie engine conversions

What about motorcycle engines? BMW 2-cyl boxer looks attractive for aero conversion. Everybody knows highest BMW quality, but not all know BMW company was one of the pioneers in aviation engine industry. I and II World War fighters and bombers fly with BMW engines. Light and compact air-oil cooled BMW boxer looks much more "aviation" than most sertified steam-age piston engines. It is more expensive than Suzuki-GEO but easy to instal under a cowling of most tractor-layout planes. Takeoff company in Germany makes aviaconversions of BMW R1100 and 1150 engines. They are well known and popular in Europe. Regardless the high price (about the same as Rotax 912), hundreds of light panes fly with BMW conversions in Europe.

This is SPG-2 based conversion of BMW R1100S. 2.5 gear ratio. It is a little lighter, more compact and less expensive than German version. 99HP and only about 170LB included 40amp alternator, starter, radiator, ECM... This engine was running at my test trike. It is lighter than Subaru but has more vibration at low RPM than the same HP 4-cyl engines. Look at the pictures below.

After tests of BMW R conversion system based on SPG-2(3) 2.5RR gearbox, we received many questions if we can supply conversion systems with a clutch and higher than 2.5 ratio. This system was made based on a new reinforced SPG-4 gearbox with 3.0 ratio and centrifugal clutch. Look at the pictures below. There are also 2 pictures showing SPG-3 gearbox installed up and down at R1100 engine. We can supply gearbox conversion kits for BMW R series, as well as converted engines.

Can SPG gearboxes be used for other engines?

Yes they can be used. You see a Corvair engine on RagWing Stork RW-20 with installed SPG-2 and 73" KOOLprop. This conversion was done by Murray Green from Rosetown Sask. Canada. He noticed increasing of all performances comparatively with direct drive he used before. From his letter to me: "I couldn't be happier. The gearbox and prop seem to be made for the Corvair engine. A wonderful combination."

This is a Chinese variant of Suzuki engine converted in Florida USA in 2012. They used SPG-3 gearbox and installed it on a flat aluminum plate. The simplest way of adapting SPG-3(4) gearbox to a new engine.

Look at 2 pictures I got from Brazil. Trike builders there are used SPG-2 to convert Honda engines they had. The last picture is a Russian automotive engine converted with SPG-2. I think it is possible to convert any auto-moto engine up to 200HP using SPG-3(4) kit with custom made or modified adaptor plate and crank coupler.

SPG systems, conversion components available and other useful pictures:

I decided to make a new page with addresses and feedbacks from some people using our products (including engines and SPG kits) for their aircraft. Look FEEDBACKS PAGE HERE

You also can see more pictures of airplanes with SPG converted engines at my "Simple" photo gallery. I hope they can help you to create your own design. CLICK HERE if you want to see them.

Air Trikes (AT) 160 gearbox conversion kit:


From summer 2012, we also use new conversion systems for our engine conversions. AT-160 gearbox can be used for tractor and pusher layouts.

AT-160 for Suzuki and Subaru

The gearbox set consists of:
1) 2 halves aluminum body with helical gears and bearings in oil inside, and 2 shafts drive spline shaft and driven propeller hollow shaft
2) Spline rubber damper adapter + 6 hole Mercedes damper (big or small)
3) Hardware
The gearbox conversion kits include bellhousings and crankshaft-dampener adapters (or flywheels machined together with dampener adapters) different for different engines. They can include other conversion parts, for example starters, modified oil pans etc.
Bellhousings and crank coupler are different for different (Suzuki, Subaru, BMW, Nissan, Honda etc.) You can see AT-160 sets for Suzuki-GEO and Subaru with 2 different size rubber dampers and adapters at the picture

Available gear ratio 2.0; 2.3; 2.5; 3.0 This gearbox is more compact than previous SPG models, easier to assemble (because of the spline shaft) and do preflight checking. TBO is more than 1000 hours. It is recommended to check and possibly change bearings and rubber dampener after 1000 running hours.

Dimensions and weight data:

AT-160 Metric Imperial
Offset (dimension between crank and prop shafts) 83mm 3.25"
Gearbox length (no bellhousing) 155mm 6"
Engine-prop flange dimension* 240mm 9.5"
Propeller hub Rotax standard**
Gearbox+spline adapter weight 7.9kg 17.4Lb
Full conversion kit weight* 11.6kg 25.5LB
*For Suzuki-Geo G10 and G13 short bellhousing, different for different bellhousings-engines.

**Adapters can be ordered for other shape of propellers
Conversion kit weight data provided for the complete bolt on kit consists of gearbox, damper, bellhousing and hardware. The data can vary slightly during the production and depends on the engine model.

AT-160 instruction with step-by-step conversion pictures HERE. Click and look AT system installation is even easier because of the spline gearbox-damper connection.
Pictures of G13BB engines converted with AT-160 gearbox and installed on our different trikes below. Click on them to see more details.


Two-Stroke Engines

General information.



Converted motorcycle and snowmobile engines are also very popular. It is not a secret for example that Rotax 2-stroke UL engines are conversions of Ski-Doo and Sea-Doo motors. A lot of ultralight or hovercraft builders all over the world use such conversions. A lot of redrives exist for them.



5-V redrive system is designed especially for snowmobile engine conversion. This classical system consists of two 6061 T6 aluminum pulleys for 5 V-belts, eccentric shaft with heavy-duty closed ball bearings, motormount plate and all machined parts, rubber elements and hardware necessary for engine installation. It can be made at most of machine shops. I used this redrive with licensed Rotax 640 from 1998 like a powerplant for my "Frigate" trike. It has been installed now with Rotax 503 on my "Eagle" trike. This system has a lot of advantages comparatively with other redrives:



a) Very reliable. Will be working with 4 or even 3 belts. You don't have to be so worried about your belt condition like with your single belt drive.
b) Very light and less expensive comparatively with gearboxes.
c) You can use not only provision-type Rotax but also non-provision. You don't need to convert the engine itself.
d) Spare belts are easy available and not expensive (Car Quest XL serie).
e) Reduction ratio can be easy and smoothly changed by changing the pulley diameter.
f) The engine can be installed head-up or down.
g) This system can be easy adjusted by changing some installation dimensions to almost any project.
h) Quick (a couple of minutes) and easy adjusted belt tension.
i) It is a whole conversion unit included a motormount plate and 4 metal-rubber silentblocks. You need nothing but muffler and prop for your engine installation on a trike or UL plane.

This system is available now as a set of drafts (12 sheets plus description).