Has anyone found or obtained an STC to install a 150hp O-320 in a 172C?

Basically replace the mount, baffles, engine and cowl with the whole firewall forward from a 172I.

Yes I know a O-360 installation is the same amount of work but I have the O-320, prop and some of the other parts. Plus in the East the 172 stock performs fine with the O-320.

Same for a Stinson 108-1. I am looking to move away from the Franklins and just have one engine type I can get parts for between the 172 and two Stinson 108s
There was an STC for the Stinson but it has been inactive. Some talk about it on the Stinson FB groups but not much else.

I believe Penn Yan has the STC to convert the 172 to 180hp. I am not sure about the 0-320 conversion. The conversion is for 1968 and up models. Regardless, I know it can be done, one of our club planes had the 0-300 and was converted to 180hp.

https://www.pennyanaero.com/stc-approvals

Thanks. Yes a few ways for the O-360 but the cost soon exceeds the airplane cost.

I was flying a 160hp O-320 172I a few weeks ago and the performance was great here at low altitudes in the East.

The later 172s really benefit from the Penn Yan or Air Plains gross weight increase but the earlier 172C I have is not as heavy so we would be fine with the original gross and just a O-320.

Seems odd the conversion was not done more because it should be essentially a bolt on change.
O-300 case and sumps are becoming an issue due to corrosion.

I've been searching for an STC for 180hp for my inherited 172H for quite a while. Pen Yann, Air Plains and Stoots all have big money new engine options. As mentioned, the engines are worth more than the airplane. I did find one "paper STC" to put on a 180hp; not sure if they have anything else available. It might be worth an email. Here's the email I got back from them:


From: Joe Birkemeyer <joesaircraftmods@gmail.com>
Sent: Thursday, May 15, 2025 9:41 AM
Subject: 180 HP Engine upgrade for your Cessna 172H

Hello Ben,

Thank you for your inquiry about my 180 HP engine upgrade for your Cessna 172H. The STC paperwork package is an installation paperwork only package with no parts included and is for the installation of a Lycoming 180 HP engine with a fixed pitch Sensenich 76EM8 propeller. The engine models approved with the installation are the Lycoming O-360-A4A, A4M and A4N. STC SA564NE is approved on Cessna 172E thru P models and the paperwork package includes, STC Permission Statement, STC copies, Installation Instructions, Installation drawings and Flight Manual Supplement. The installation instructions also include drawings to make the airbox adapter plate, baffling modifications and the exhaust riser extensions. A Lycoming O-320-E2D engine mount can be used for the upgrade, it is modified with the Lord mounts from the Cessna 172Q series aircraft. Most of the rest of the items needed for the installation come from the Cessna 172Q parts catalog. The gross weight increase to 2500 pounds is applicable to the Cessna 172I thru 172L models and the 2550 gross weight increase is applicable to the Cessna 172M thru P models. There is no gross weight increase for the Cessna 172H model. The complete STC installation paperwork package sells for $1550 for the FAA STC package. There is an additional charge, if you require an EASA approval for international customers with aircraft located outside the USA, Canada or Mexico.

Your local A&P with IA authorization mechanic can install the conversion and sign off the FAA form 337 as installed as per the STC.

Please let me know if you need any additional information or if you would like to place an order.

Thank you.

Joe Birkemeyer
President
Joe's Aircraft Mods LLC
10294 S Oaks Road
Brainerd, MN 56401

That's a pretty good deal for that STC! It will allow you to scrounge for used parts (which for 172's are plentiful...people are parting them out all the time) and save a lot of money over buying an off the shelf kit with a new engine. It might actually make an 180hp upgrade on an early 172 financially viable.

I did a 210hp upgrade on a 172E, and I’m doing one on a 170A. The upgrade is fantastic for performance, but it’s quite a complex project. I’m glad it did it, but it’s not a budget upgrade.

Is anyone still installing a 215 HP Franklin engine? I flew one in 1968, and it was a rocketship!

Just because you have the engine…

I love the O-300 in early Cessnas. Build a nice one, now with the high compression mod out there, and enjoy the smooth ride and classic sound.

Yeah I am suspect of the Airworx High Compression mod and the MA-3 carb being able to flow enough fuel to support it. And how it was "Approved".

There is a reason Lycoming used the MA-4 on the 150hp O-320 and the dash-32 version on the 160 hp.

Then switched up to the MA4-5 on the 180 hp O-360. They are all very different carbs moving up in the HP ranges.

Franklin did the same with the 150hp engines using the MA3 but the 165hp using the MA4-5.

No engine ran the MA-3 on an application above 150hp. Except this "approved" mod.

Airworx is claiming 170 hp and at one time 180hp with the original MA-3 carb.

The real issue though is not performance. It is corrosion of the sump and accessory cases of the O-300 and lack of parts to overhaul.

It gets a little tricky also with Propeller hub ratings and bolt sizes if they are truly making those HP numbers. I wonder the same with the various exhaust mods on the Lycomings. If Lycoming felt 180hp needed larger prop bolts and hub many years ago why are they not needed with the same HP obtained by an STC?

Though I do also wonder why the O-300 and O-200 do not share the same magneto timing?
You would think they are the same engine just with two more cylinders.

The answer to this question is pretty simple. The longer crankshaft will have different resonances with approved props. Ignition timing plays a rather large role in exciting or taming these resonances. This is one of the reasons why switching to electronic ignition often puts you in the "test pilot" seat. I never would have believed ignition would have made such a difference but, after hearing a short dissertation from a Hartzell engineer, it seems the timing of the ignition impulse really does make a difference.

Is this STC what you are looking for - a 172C STC for an O-320-E2D?
https://drs.faa.gov/browse/STC/doctypeD ... r=SA1774WE

Yes That would work. It shows the STC was granted in 1968 but "Historical" now not active.