The Continental C145 and O-300 engines were used in Cessna 170s, early Cessna 172s, and a few other birds. The six-cylinder C145 morphed into the O-300, making 145 hp at 2700 rpm and is known for its smooth operation. With its 7.0:1 compression ratio, it was designed for 80 octane aviation fuel. Since 80 octane has not been available for some time, many operators have purchased auto fuel STCs. This helps reduce the nagging spark plug fouling but still leaves the aircraft it pulls needing a bit more power.
With over 19,000 Cessna 170s and 172s manufactured since 1945 with the smaller Continental six-cylinder engine, David Cox at Alabama-based Airworx Aviation realized there may be a sizable market for a simple power upgrade for these engines.
Cox has been working on an STC to allow for the installation of the cylinder and pistons from the certified Continental O-200D engine on to the C145 and O-300 crank cases. The new pistons and cylinders increase the compression ratio, allowing for the use of 100LL aviation fuel, which might reduce the risk of lead fouling. However, the best thing this change does is increase the power output to 170 hp.
Additionally, the STC will utilize certified Continental parts and does not require any custom machining. Additional changes will require re-jetting the carburetor to handle the increased fuel the additional power output will require. However, no propeller and/or exhaust change is required.
Airworx expects the STC to be available in late 2026, with the candidate engines having fewer than 500 hours SMOH and less than six years SMOH. Engine exceeding those limitations will require a complete overhaul. “Above all else we are prioritizing safety with these conditions,” Cox said.
The STC will be available to local engine rebuilders, so customers preferred engine shops can perform the upgrade under the Airworx STC.
Needs to be certified to run on unleaded fuels. Otherwise this STC will be useless if certified for 100LL only. Would love the extra power but will wait until the 100 octane unleaded fuels situation gets resolved. With the O-300 in my plane now I can run on auto fuel if needed as mentioned on this article.
O300s were designed for 80. Anything that was approved for 80 can be run on Swift UL94 without an STC. I presume UL100 will be the same.
The Peterson auto fuel STC that I have also covers the various UL92 and UL94 fuels. My point is that certifying the STC with 100LL is going to be a waste of time if 100LL is actually banned in 2030, or if the manufacturer of the lead additive stops producing it. So far the only unleaded 100 octane fuel that is FAA approved is GAMI’s GU100.
GAMI G100UL
Absolutely! I run my O-300 Skyhawk on UL91, and it runs much better than on 100LL, which is mildly toxic to the engine (although despite the wording in the article, it’s OK). Converting the engine to “require” 100LL isn’t an improvement – it is ruining one of the best features, and seems curious in the current climate where 100 LL is discouraged.
But the real question is, will this upgrade stop it from leaking?
With a stock propeller the big advantage would be that the static rpm would be somewhat higher meaning more power for takeoff, but without a coarser pitch propeller I doubt you’ll do any better in cruise. My stock O-300A with a stock 76/53 McCauley fixed-pitch propeller is quite happy turning 2600-2650 rpm in cruise, and does that at less than full throttle below 5000 feet. And, the math says that a propeller that absorbs 145 hp at 2700 rpm would have to turn about 2850 to absorb 170 hp.
Agree. I have a later ’75 C172M which had a O-320-E2D 150hp engine and 75/53 prop. I converted it under STC to a O-320-D2G 160hp engine which required a pitch change to a 75/57 prop for that 10hp gain. I don’t see how it’d be possible to not require a prop pitch change for a gain of 25hp? Further, my conversion required a METO RPM rating of 2600rpm. A 170hp engine would be able to turn far above that rpm … not good.
Beyond that, requiring any engine over 500smoh or 6 years old sorta defeats the purpose of trying to save money by overhauling a still usable engine. And if a teardown showed a worn crank shaft beyond limits that you didn’t know about, look out below money wise.
I don’t see this as much of a useful idea unless you’re a lover of the smoothness of that 6 cyl engine over a higher HP 4 cylinder.
From a performance standpoint it is always a good idea to bring up the compression ratio to the capability of the fuel we are using. It is best to really know what you are doing when you make major engine changes:
If the engineer on this program does not understand that this increase in CR does NOT require readjustment of the carburetor, that it DOES require a different prop and that it DOES require different ignition timing and possibly different spark plugs, then there will be plenty of problems with this mod.
Oh, and make sure you measure the torsional deflection of the crankshaft at the increased HP!
For the reasons you mentioned and more, Avworx is going through the full STC process with rhe FAA rather than doing the upgrade via a 337 field approval.
Once the STC is approved, all you concerns will be addressed.
If you need detailed answers, please reach out to Avworx, their contact info is in the article.
You are correct, Mr. Smith, about increased efficiency when raising compression ratios. But no matter how the engine uses heat generated from a given amount of fuel and air, BTUs are BTUs, and increasing compression alone will net only a small increase in power in a more efficient use of those BTUs.
Interfering in this slight power increase is a retarded shift in peak combustion chamber pressure which, from a practical standpoint, nullifies any HP gains achieved by shifting the optimum peak pressure crank angle away from a more advantageous position.
This situation is remedied by increasing the octane rating of the fuel and adding more fuel to the air/fuel charge in order to slow the flame-front across the combustion chamber and shifting the peak pressure crank angle back to a point that, by engine design, will provide the greatest push on the crank.
You are also correct about using care with any of these modifications. A bumping compression from 7.0:1 to 8.5:1 compression carries very little risk, but detonation margins, fuel consumption, propeller/engine torque curves, timing requirements and heat considerations, among others, will need to be evaluated.
Lastly, the O-200s and O-300s do not shed heat very well. When operating at 7.0:1 with 80 octane fuel, it’s happy enough, but as we have found with operators of higher compression O-200s burning 100LL, exhaust valves, guides, and seats take a beating. In the case of this pending STC, it would be helpful to fully vet out fuel options in order to allow owners the option of other, more suitable fuels.
Thank you for that Mr. Brevard. I think we are together on most of these issues but let me point out that when increasing CR, the flame speed (initiated by the spark) becomes faster, thus the timing needs to be retarded to position the peak pressure over the piston whenever the geometry of the engine can best convert the pressure into force. You can fish for peak torque timing. I do that with two potentiometers.
WRT 0-200/0-300 cylinders, the finning is indeed marginal but there is less heat loss/HP when the CR is increased (absent any preignition or detonation). The excessive wear you are referring to in racing the 0-200 comes from excessive rpm and rules limited CR (Reno).
At 10:1 and cruising at 2650 rpm (racing at 3000 max) The wear can be almost normal.
There is a similar STC floating around out there to install O470 cylinders on the old E225 engine. You are limited to the original 225 HP engine rating and TBO is reduced.
Something not mentioned here. The STC is being required because of a turn over in management of the FAA in that department. That was from someone within the FAA that wporked on the project. Was not due to Airworx. Next, the engineering order for the upgrade is only for the engine work. This is no approval to mount it on the airframe. There is only paperwork for the O-200D cylinder installation and nothing more. We have one of the 170 motors still in a crate for almost a year. For increase in HP with respect to the TCDS and mounting it requires a STC and nobody has ever applied for one. Don’t believe me then look it up on line (if you increase the HP. ) There are IAs that say it is legal but when we talked with several they changed their minds. The 145 mentioned on the TCDS is a limit , not a reference. Another thing. Check the HP limits for props listed for the O-300. The limit is 165 and this motor produces more than 170. Our 337 paperwork states “horsepower increased to 170 IAW as per Engineering Order…” . In the end, a great idea but no permission to install on either a 170 or a 172 that had a O-300 on it. Right now we own a paper weight until we fix the problem and we aren’t certain how that will happen. Any body mount and signs it off is risking an attack at some point by the FAA. Is an IA license and getting grounded with it? Not for us but we are still trying to figure this out.
Very good point Ron. A lot of STC’s in the past were approved with limited engineering and test data to support them. In the case of the E225 cylinder upgrade discussed above, the limitation is the ability of the propeller to absorb more power and the question as to how the extra power affects vibration within the engine. Without a full, instrumented vibration analysis on a test stand, there is just no way to know. This is why the 0470 cylinder upgrade on the E225 engine limits you to the certified 225HP and reduced TBO to 1000 hours. Owners of that STC are essentially doing the engine testing!
There are a couple STCs out there for more power instead of trying to increase the HP of an O-300. You can choose between a Lycoming 180HP, a Continental IO-360 from TLGW and a Titan which is from Stoots in Alaska. Nothing is cheap if you go that way but the motor will be mounted without any problems from the FAA. I know there are at least two or three 170HP Airworx motors flying around. Those mechanics and owners should be worried because not only was the installation not approved but neither the re-installation of the propellor. We were told that it was cleared to mount on the airframe but when we received the motor and the papework we found out differently. There was nothing there to indicate that. I don’t know how long it will take to get ours figured out. Just for your information, the upgrade is an additional $10,000 over a standard rebuild. If Airworx gets the STC for the cylinder exchange approved using O-200-D cylinders then that really doesn’t help an owner since there is still NO STC that premits the installation on the airframe. Same with the STC for a field installlation. You can put the O-200D cylinders on but you can’t fly it. The 170 HP upgrade is a great thing. 145 HP is enough for the most part but we all know what 25 extra HP can do. I hope Airworx takes note of the problem and has the airframe installation included with the cylinder upgrade STC. They will be so busy selling 170 HP O-300s that they will not be able to keep up.
Maybe the FAA should have a pathway to change an Aircraft “Standard” Airworthiness Certificate to a “Special” Airworthiness Certificate. Then STCs wouldn’t be necessary just engineering reports.
“Maybe the FAA” is wishful thinking. We thought about Experimental then go do anything we wanted but that doesn’t work for a club plane.
It sounds like a plan, but what will this do to the TBO of the 0-300. I expect it will take a hit about the same as the geared 0-300 on the Cessna 175’s because of the higher turning RPM’s causing increased wear on that little engine.
Can’t answer that but it would sure be fun finding out.