Home | Question for Bob Steward regarding an engine swap: I met a Musketeer owner at Midland Barstow Airport this morning and we got to talking about upgrading the engine in my C-23 to the 200 hp. He said that you had recently completed such a conversion a

Question for Bob Steward regarding an engine swap: I met a Musketeer owner at Midland Barstow Airport this morning and we got to talking about upgrading the engine in my C-23 to the 200 hp. He said that you had recently completed such a conversion a

Question for Bob Steward regarding an engine swap:
I met a Musketeer owner at Midland Barstow Airport this morning and we got to talking about upgrading the engine in my C-23 to the 200 hp. He said that you had recently completed such a conversion and that he read about it in this group. The only posting on the subject that I could find in the past couple of months (#22341) was not answered by you, nor did it reference you. Could you enlighten me? Or, if you’re tired of answering this question, could somebody direct me to any thread which discusses this often-debated topic?
Jim Gorman

Credit to Bob Steward, A&P-IA:

I completed the paperwork and FAA inspections, to approve an IO-360 installation in an A23, earlier this year. The plane was signed off with ACO and FSDO support, for a field approval of the installed engine and prop.

It was basically an A23-24 firewall-forward, bolted back onto an A23 that previously had Continental IO-346A power. Instruments had to be re-marked, and placards added in the cabin, to conform the installation to that of an A23-24. About 40 pages of the AFM had to be re-written, to correct the procedures which originally referenced Continental data, so that it would have the Lycoming data and performance charts for the A23-24.

No changes in the airframe limitations were requested or granted. The gross weight remained the same, as did the V speeds. However, the engine operation data was changed to match the Lycoming IO-360, and some of the performance data was also changed.

Was this a viable conversion for any old Musketeer? Probably not. Was it a useful way to get a different engine on an IO-346A Continental-powered A23 (or A23A)? Yes, it was a good match for someone wanting to leave the orphaned IO-346A behind. I’d say that the obstacles to duplicating it would be the availability of the complete engine, mount, and prop from an A23-24 (with all the baffles and accessories); and someone with a fair amount of time and patience. It took more than 6 months of plodding along at the FAA’s pace of operations, and 3 different inspectors being assigned to the project, before it was finally approved.

I do think the FSDO would let me conform another one. I have the data approved, and have shown them that the conversion works. Nonetheless, the lack of suitable firewall-forward components will stop most owners who are dreaming of 200 HP. Buying all those needed parts individually would be cost prohibitive from Raytheon, and most difficult to locate in the salvage yards 1 piece at a time.


Several Musketeer family aircraft have been converted to different engines. Most on field approvals and the rest on One Time STCs.

We have frequently discussed larger engines and many people have done the conversions. Even the factory converted some for Purdue University (or was it University of Illinois at U-C?) to 180 HP from 150.

Beginning just under a year ago I undertook the project with another list member to get his plane approved with the IO-360 in it. The plane had been built as an A23 with the Continental IO-346A, and at some time 4-5 years earlier some shop in Florida installed the IO-360, but never filed the paperwork (337) with the FAA, even though it LOOKED like it had been approved and a copy of the BOGUS 337 was included in the aircraft records. The current owner didn’t have an adequate pre-buy inspection done
and was later surprised to find that the FAA had no record of his plane being converted, and it had been flying illegally through 3 owners and 400+ hours. The answer was to show them that it “conformed” to the A23-24 configuration, and to develop the missing data for the Approved Flight Manual. This was actually the MOST challenging part, and it took weeks of writing and re-typing pages in the POH/AFM to get every emergency procedure and performance chart to match the new configuration. The only work to the plane that needed to be done, since the conversion was all bolt-on, was to document the correct parts were installed and to get the cockpit instruments re-marked; they were still showing the CONTINENTAL limitations!

The plane needed a few placards to be legal (even without the engine swap, the placards were still needed), and after 3 inspections by the FSDO the paperwork was signed off last Spring.

Today that plane is legal and has tremendous performance, because it has a 200 HP engine in a Musketeer. As with most high powered planes the performance is obtained by using the superior climb to quickly get into the low teens and use the increased TAS to give some impressive cruise speeds.

I’m sure the owner hopes to never have to go through that again, but the next person to do such a swap should plan it out in advance and get the FAA to approve all the paperwork BEFORE the plane is taken out of service. That makes it a mater of unbolting one engine, bolting in another, and swapping the needed accessories and getting the instruments re-marked. Something that can be done in a matter of days, rather than months, and you know going in that you already have the signed approval of the data in your pocket, so there are no sleepless nights wondering IF the FAA WILL approve it.

Ly-Con has an STC to Upgrade to a 160hp engine. If you let them do the
engine overhaul, They can Port and Polish the cylinders, and do a flow match of each cylinder. My Test Cell results said that at 2700 RPM my engine produces 181hp. I can send you a copy of the Test Cell results if you would like.

Several companies have STCs to upgrade the Lycoming O-320 engine from 150 HP, by replacing the pistons with Lycoming #75089 pistons to bring the engine up to 160 HP (via higher compression). To add emphasis, this STC authorizes the change to the certified engine, modifying that specific engine’s airworthiness Type Certificate by means of the Supplemental Type Certificate. What is lacking in most cases is the STC to then install the modified engine in the AIRFRAME. (Ed. note: In fact, without the accompanying AIRFRAME STC or Field Approval, installing the modified engine in the airframe makes the airframe legally unairworthy. Consider your insurance status, for example.)

Airframe approval currently has to be obtained through a field approval, on a case by case basis. Since Beech produced the Model 23 with the O-320-D2B, obtaining the field approval can be pretty straight forward. (Ed. note: as with Bob’s earlier advice, you’ll lose a lot less sleep if you obtain the approval first, before having your engine modified)

I would also question the wisdom of running 180 HP though a prop that is
designed for 165 or less HP according to its Type Certificate; P-886 – SENSENICH Model 74D (Ed. note: This means that any actual engine swap to 180 HP or 200 HP, if pursued, will have to include a certified compatible prop. The chances of getting FAA approval without the correct prop will be nil).

Addendum by Jeff Bryant:
Sensenich had no problem with using the 74DM6 Propeller with the Ly-Con 160 HP O-320 STC, and neither did the FAA at Van Nuys. The FAA did want me to change the pitch, though. The current propeller is a 54″ pitch, and the ASI thought that maybe a 56″ to 58″ pitch would be better. They left that option open, and told me that if I wanted to go up in pitch it would require another field approval. They also said that the ASI would not have any heartburn with that, as long I used the 74DM6 propeller (used on the B-19 from S/N 289 – 460).
The limiting factor on the prop is the bolt size. The 74DM6 uses 3/8″ diameter (dash six) bolts. Sensenich felt this would be fine, but not to exceed 200 HP. They DID, however, suggest getting a new prop with the 7/16 (dash seven) bolts, and to change the inserts on the prop flange to match. Ly-Con said that if I do take that route, they will change the inserts for me.

Commentary from Jeff Bryant:

Even though the Sundowner has a 180HP engine, there is no STC at this time to replace the Sport’s 150HP with 180HP, which is a shame. Per the FAA guidelines for a field approval (8300.10) if you increase the horsepower MORE than 10 percent, it will require an STC. The current mod for our Model 19 aircraft takes the power from 150 to 160, which is only a 7 percent increase and falls under the guidelines of a field approval.

You will find that the 160HP upgrade is great for our aircraft. I have loaded my Sport 200lbs over gross, and at different stations, to verify the CG loading and climb performance, and the aircraft performs like a dream with this additional power. It really makes a difference, and is worth the money. Beech should never have installed the 150HP engine, and should have either stayed with the 165HP Continental or the Lycoming 160HP 0-320.

As far as the 180HP STC goes, I have been working off and on in that direction. The 0-360 has different installation considerations than the 0-320, and these differences have to be addressed. The folks at the FAA whom I have talked to have said that because the 180HP was installed in the Musketeer, they would work with me in trying to get an STC going; but even they pointed out that the modification might be so expensive that it will most likely be cost prohibitive.

Here’s why: The 0-360 will require a different mount, which may not be too much of a problem but would have to be addressed anyway for the STC. You would not want to go through all this trouble and not put on an overhauled engine; this will be approximately $15,000 (not including any extra charge for a non-matching core). Guess what, the prop currently on the Sport is only certified to 165HP, so now you need a new prop for the 180HP, which will cost another $3,000 to $4,500, and which may need an S5 spacer to clear the cowling. Oh, and by the way, the new prop will also need an STC because the Sport was never certified with a 76″ prop, so add another cost for the STC.

So you have a 180HP Sport, for around an additional $20,000 to $25,000. Now what? Well, you have the ability to lift over 2400lbs in a 4 place airplane with no baggage compartment. If you are thinking that you will get additional speed, guess again. I have done Drag Polers on my Sport which say that the faster the airframe goes, the drag REALLY INCREASES. In fact, according to the data, to get an additional 12 mph speed I would have to install a 200HP engine, because the induced drag climbs so fast due to the airframe design. Ask anyone who owns a Sundowner, and see just how fast they really go at cruise, and you will see my point.
(Editor’s note: A rough rule of thumb, for modest power changes, is that it will require four additional horsepower for each knot of added speed. It is far better to work on drag reduction.)

The Sport is certified to 2150lbs. So to really be able to use all this new power, you will need to apply for a gross weight increase, which will again require another STC, which will most likely be another $2,000 to $4,000 in cost.

So here is what you end up with. A Sundowner with no baggage compartment, for around $2,000 additional cost over the cost of a Sundowner. The moral here is that if you want 180HP and what it brings with it, BUY A SUNDOWNER and save yourself all the trouble, money and time. You will go about 7 mph faster, lift 2400lbs legally, and have a baggage compartment all for around the same price of all these STC’s.

I love my Sport now with the 160HP increase; and I know that if I ever need to load it beyond the 2150lbs, it can handle it and then some. As far as bags go, the kids are fine with the bags between them. They set their game boys on the bags like a table and after about an hour they are asleep anyway.

Thank you for adding to the resources available for your Fellow BAC Members.