Hello again, Larry95!
While there are any number of engine manufacturers on the planet,
finding this unique combination of characteristics has proven rather
difficult. Of course you already knew this, or you wouldn't have
turned to us.
I have interpreted the power/weight ratio and horsepower range as the
more important factors in your question, and the rest as preferences.
I've included some engines above the 20hp range, as it seems you have
some flexibility, but if necessary they could likely be tweaked to
provide less power.
After viewing the websites of a great many manufacturers, I am unable
to locate a production four-stroke engine in the size and
power-to-weight ratio you want. If anybody has one, they're being
oddly secretive about it. Honda's miniscule GXH50/GXV50, for example,
puts out 2.5 horsepower, but at a weight greater than 5kg/11 lbs.
This holds true for engines of larger output as well. Honda's GX620
model, puts out 18hp at a weight of 41kg. While I've used Honda for
my examples, I've compared model-to-model with engines from
Robin/Subaru, Briggs and Stratten, and numerous other manufacturers
and found much the same result. The ratio seems to be in the range of
2kg/hp, which is roughly four times what you'd wanted. I will not
post links to these manufacturers here, as none of them offer what you
want, but will do so in a clarification if you need that info for
With two-stroke engines, the normal power to weight ratio is roughly
1kg/hp, or about twice what you are looking for. However, I have
found a few candidates for you. It would seem that the engines best
suited to your specification would be the lightweight, high output
models used in kart racing and ultralight aircraft.
The Rotax 100 KART PP-E model meets your 20hp limit, and just fails to
meet the 1:1 ratio of power/weight. At the upper end, the 100 KART
DS-F-L/LC version of this engine is over your horsepower limit
(roughly 28hp) but exceeds the 1:1 ratio. To view the list, click the
Note that Rotax engines are widely used in snowmobiles and personal
watercraft, so their parts & service network is excellent. They are
part of the Bombardier "empire".
The Hirth Aero F33 is slightly above your hp requirement at 28hp, but
weighs only 35 lbs with exhaust system.
Perhaps the best of the bunch, for power-to-weight, is the
Swedish-made "Raket 120". This engine is widely used both in karting
and ultralights, and at 14hp/15 lbs, it is remarkably close to that
1:1 ratio. For the Raket engine, look here:
And for a grid comparing the Raket, Hirth F33, and many other engines
for ultralights, look here:
For your convenience, I have tabulated these models for you. The
formatting in Google Answers is sometimes unpredictable, but this
should work if you cut-and-paste into your word processor, then choose
a monospaced font such as Courier.
Manufacturer Model HP Weight (Approx)
Rotax 100KART PP-E 20 12 kg/26 lbs
Rotax 100KART DSFLLC 28 12 KG/26 lbs
Hirth Aero F33 28 35 lbs
Raket Raket 120 14 15 lbs
For a somewhat less favourable power to weight ratio, but smaller
engines, you may wish to consider chainsaw engines. These typically
fall into the usual 1:2 power/weight ratio of two-stroke engines, but
are easily modified to reduce their weight. This is a common practice
within the scale-model community, since (as you've observed) the
engines manufactured for model aircraft are not especially robust.
Chainsaw engines, in contrast, are designed to operate for many hours
at a time with frequent revving.
I will not provide you with a comparison of major chainsaw makers,
because they all seem to be pretty close for power and weight.
However, for extensive discussion of the work involved in modifying
this class of engine, I will refer you to this page:
There are a few newer technologies on the horizon which I'll mention,
since you'd expressed an interest in prototypes.
The ball-piston engine is a proposed technology (it does not appear to
have reached the prototype stage as yet) which would use conventional
four-stroke technology in a manner more akin to a rotary engine. For
a detailed description of the design, look here:
The "Quasiturbine", or "Qurbine", is another high-efficiency engine
which appears to meet with more of your desired characteristics.
Research prototypes are available for rental in a variety of sizes,
including one aimed at the chainsaw market. Models intended for
hydrogen and natural gas are in the works, as well. If you wish to
bring a product based on this engine to market, it is possible that
either the Quebec or Canadian government might offer some incentives
to smooth the path. Their home page (ugly, but lots of good stuff) is
Finally, the Wankel heritage is being brought into the new century by
Freedom Motors, of Davis CA. Their designs are extremely
low-emission, and again can be hydrogen-fuelled. With only two moving
parts, their long-term reliability should be good. Although this
article at NASA's website describes a miniscule 10hp engine,
unfortunately that was a one-off test unit. For now, the company is
focused on their Rotapower 530 engine, which will go into small
recreational vehicles such as ATV's and Personal Water Craft.
However, in his reply to my e-mailed inquiry, general manager Bruce
Calkins indicated that they could readily produce an engine in the
size and power range you wish, if a suitable buyer should arrive at
their doorstep with financing in hand. Their website is here:
My initial search used these keywords, the latter two being added to
filter the results after some initial digging:
+lightweight +engine +gasoline -car -automobile +"4-stroke" -honda
Further searches used these keywords:
"high power to weight ratio"
I also searched on individual manufacturers, some of which turned up
in the course of my searching, and others (such as Honda and Briggs &
Stratten) that I already knew.
Thank you for an interesting question, and hopefully I'll have answers
for you on the other two within the next few days (awaiting return
e-mails at present).
If I've inadvertantly left anything out, or been unclear, please let
me know using the clarification function.