I'm looking for some input on an engine upgrade for my 108-3. I'm giving up on the Franklin 165's.

I've run the numbers and I can put either an o360 or o540 on my -3 for less money than moving up to a C180, while still having more useful load than a 180, as I can't fit a wing x'd 180 in my hangar, and I don't care for the later models with the 185 vertical stabilizer all that much, which is purely a cosmetic issue to me. I just like the classic tube and fabric Stinson look. Mogas is a must.

I really like the Stinson, and don't mind putting more money into it than I can get out of it.

As it currently sits, the Stinson weighs 1361lbs with the Franklin 165 with McCauley 7649 metal prop. Gross on wheels is 2400lbs, or 2500 on floats.

I can get a MT Prop for either the o360 or o540 coversion, the installed weight is 46lbs. My fixed pitch McCauley is 36lbs. So the constant speed prop itself, minus governor, is only adding 10lbs.

I don't have any lightweight accessories on the Franklin, so for simplicity sake, we'll use TCDS numbers to compare the engine weights:

Franklin 165: 280lbs
Lyc o360 A1AD 180hp: 257lbs
Lyc o540 E4C5 260hp: 370lbs

I would be sending either Lycoming engine to Lycon for cylinder porting and any other certified mods they can do to it. They claim they can get 200hp out of the o360, and 290 out of the o540. At the same time I will add all the lightweight accessories I can to either engine.

Weight is a concern of mine, as I like my large useful load. If I go the o360 route, I can likely reduce my empty weight from where it currently is, giving me a very generous useful load, and better hauling ability than I currently have with the added hp, and nearly 6" larger diameter constant speed prop.

If I go with the o540, I am likely going to be somewhere around 1475-1500lbs empty weight, but I'd have enough power to operate like a rotary wing...

I have nothing to offer to your question, but I like where you’re going with this. Good luck!


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Since you mention Mogas, the STC for 170’s specifically says that 0-360 conversions are excluded. I don’t know if that’s the case for Stinson, but just because the airframe and engine each are mogas compatible, the combination may not be (at least from a paperwork perspective).

The STC's don't mention anything about mogas, Petersen issues an STC for the airframe, and then another one for the engine, so I'll be covered for mogas with either engine.

Why not a 0-470? I work on a local 108-3 with a 470 on it. He actually weighed 540s and the 470 was lighter every time. It seems that the Lycoming weights are pretty bare bones, where as the 470 is with accessories.
290 hp out of a low compression mogas burning 540 seems high, but if they can do it then that's pretty sweet! Bigger is always better isn't it? Haha.

A1Skinner wrote:Why not a 0-470? I work on a local 108-3 with a 470 on it. He actually weighed 540s and the 470 was lighter every time. It seems that the Lycoming weights are pretty bare bones, where as the 470 is with accessories.
290 hp out of a low compression mogas burning 540 seems high, but if they can do it then that's pretty sweet! Bigger is always better isn't it? Haha.

What is his empty weight? Every o470 108-3 I have seen is at least 30-50lbs heavier than an o540 equipped 108-3, not to mention they are 30hp less, and overall regarded as less likely to make it to TBO without needing cylinders than an o540.

I think the popularity of the o470 conversion is price. A used mid time o470 is half the price, or less, than an o540.

The e4c5 o540 is 8.5:1 compression, and is 260hp stock. The b series is lower compression but only 230hp.

I say run the CG numbers of both. See what the 360 and 540 are compared to the Franklin. CG is something to consider. Too light on the nose limits baggage, to heavy on the nose flys like crap when light and forward cg. CG with either may not be a factor but then again, it may. Just another thing to consider.

Too bad 108’s, Sedan’s and 170’s did not have the 0-360 available during the design and manufacture stage, those would have been incredible airplanes if originally built with the 0-360.

Kurt

His is all metal so its higher weight. With a big mccauley C/S it's just over 1700 lbs. The fabric mate to his that is setup with the same engine/prop combo but cleaned up panel and fabric is at 1575.
There was a very nice one built up here with a 360 and it was a beauty. Good performer as well. It got flipped over in the wind at a fly in last year unfortunately.

Good call on running the CG #'s, I'm working on getting that data from the STC holders. I'll share the numbers here when I have it.

I'm assuming the o540 isn't too bad CG wise as the o470 is heavier, and that is a very popular conversion. The 1575lb example A1skinner is talking about is the lightest I have heard of, I haven't seen one under 1600lbs yet, but I don't doubt it is possible. Most o540 conversions seem to be 1490-1600lbs, but none of the examples I have seen have made an effort to keep things light.

I'm curious to hear from people flying other aircraft at similar weights:

180-200hp at 1350lbs empty weight.
260-290hp at 1475lbs empty weight.

I looked into the 108-3 at one point. One thing no one has mentioned is the fuel tanks are a little small for a 540. In the -3 you only have 46 gallons usable... take out your vfr reserves and you are at.. what 38-39 gallons?

corefile wrote:I looked into the 108-3 at one point. One thing no one has mentioned is the fuel tanks are a little small for a 540. In the -3 you only have 46 gallons usable... take out your vfr reserves and you are at.. what 38-39 gallons?

Good point.

I'll be adding HardtailJohns 13gallon aux tanks. Hopefully he will chime in with what the useable is on those, but I'd guess around 12 gallons?

That would provide 58gallons useable.

The Stinson is pretty draggy. Currently 120mph is about 9gph, 130mph is about 10gph. I don't think I'll be cruising any faster with either Lycoming, the main advantage is getting the weight off the ground sooner, and having a better rate of climb.

So after climb and VFR reserves I should have over 4hrs of normal cruise available.

I always figure 13 gallons from the tanks.
I might be wrong, but....as far as I know, there's no STC for the 540. That might be a consideration. The ones I've seen personally, were all done with a field approval years ago, Ken Brock's being the first I think. I'd rather have that engine, but until they give us a catagory like the Owner Maintenance Catagory that Canada has, I'm not so sure I'd be happy.
The ultimate solution as far as I'm concerned, is to get the Univair 360 STC and then get friendly with your FAA guy and get it bumped up to an O-390. The weight is basically the same as the 360, but the actual horse power and torque is higher than the Continental, according to the guy I talked to, due to a different certification parameter. A guy I know is just finishing his setup that way, and he went with an MT prop as well, so he's figuring he'll be nicely forward on empty CG.
The last O-470 I did was heavy, but it had a huge pile of avionics, extra wood baggage area, heavier leading edges, etc. I remember we wound up with about 10# on the tail post to get it where we wanted CG wise. I think somewhere I still have a copy of the weight and balance. I'll look tomorrow and shoot figures here if I find it.
John

John's way sounds like a great idea. Problem with the OM I'm Canada is that nothing over 200HP and no C/S props allowed.
I like the 390 idea.

I'm in Canada and we have a STC here for the o540.

What is an o390? Stroked or different cylinders and pistons?

If there is a certified example of this engine I think I could get a one time stc, but a one time stc for the engine itself might be something else. I'd love to hear more about this engine regardless though!

It would be great if you could find the W&B for the o470 conversion. Have you ever done the o360 conversion? If so, do you have a W&B for that?

The new Denali Scouts all run O-390s. They are making 210hp. So they are out there in certified aircraft.
For the W&B, the one I did with the 470 was within limits empty and just got better with fuel and baggage.

I haven't received any W&B sheets yet but I'll keep working on that.

It looks like there isn't a certified version of the o390, just io390. It appears the 390 is a larger bore. I'd be very curious to hear from anyone that has made an o390 from a o360, even more so if they have done it on a certified aircraft!

I did hear from one guy with the 200hp io360, and he says he can climb out at 1400fpm, overgross, on a hot Florida day. I am skeptical of that, as that is more than double the rate with the Franklin 165 with climb prop.

I'd love to hear from anyone operating aircraft in these power ranges and empty and gross weights.

Have you ever thought about going OM with the plane? Then go with a stroked 0-360,the 0-375 and a whirlwind ground adjustable prop. I set a bushmaster up with that combo and it is an absolute blast. Pulls hard, climbs hard, and still decent in cruise. OM gives you more abilities to use Pmags and such. Just another option...

A1Skinner wrote:Have you ever thought about going OM with the plane? Then go with a stroked 0-360,the 0-375 and a whirlwind ground adjustable prop. I set a bushmaster up with that combo and it is an absolute blast. Pulls hard, climbs hard, and still decent in cruise. OM gives you more abilities to use Pmags and such. Just another option...

I live right on the border and cross into the US with no notice on a regular basis. With OM you have to get a permit for every crossing.

Originally I was pretty set on going with the o540, but after running the numbers the modified o360 is pretty tempting from a weight standpoint.

TheMachinist1 wrote:

A1Skinner wrote:Have you ever thought about going OM with the plane? Then go with a stroked 0-360,the 0-375 and a whirlwind ground adjustable prop. I set a bushmaster up with that combo and it is an absolute blast. Pulls hard, climbs hard, and still decent in cruise. OM gives you more abilities to use Pmags and such. Just another option...

I live right on the border and cross into the US with no notice on a regular basis. With OM you have to get a permit for every crossing.

Originally I was pretty set on going with the o540, but after running the numbers the modified o360 is pretty tempting from a weight standpoint.

Ok. Makes sense then for sure. I just dont know of any certified modified versions of the 0-360.

TheMachinist1 wrote:I haven't received any W&B sheets yet but I'll keep working on that.

It looks like there isn't a certified version of the o390, just io390. It appears the 390 is a larger bore. I'd be very curious to hear from anyone that has made an o390 from a o360, even more so if they have done it on a certified aircraft!

I did hear from one guy with the 200hp io360, and he says he can climb out at 1400fpm, overgross, on a hot Florida day. I am skeptical of that, as that is more than double the rate with the Franklin 165 with climb prop.

I'd love to hear from anyone operating aircraft in these power ranges and empty and gross weights.

Sorry, I got busy and didn't get to the shop.
You're right,.. IO-390. Don't know why I was blanking that, other than we'd talked about trying to get an STC to convert them to an O-390, kind of like the Kenmore mod on the 520's. It's been a long couple weeks here...haven't slept in a bed since last Thursday.

This was the last one I did and it was a pig. It's got everything you could throw on it and it's weight reflects that: .032 leading edges to the struts, .025 from there out, extra stringers in the belly, float fittings, VERY heavy interior (1/4 plywood with lots of epoxy varnish then foam then fabric over everything), extended baggage with heavy walls and sides, heavy fabric and heavy paint job (he wanted extra coats of Aerothane...that adds up really quick, as does the filling of heavy fabric, but the customer is right), full IFR panel, 120 amp alternator, Madras tips, my aux fuel tanks, cooling, heating and defrosting fans and ducting, etc. It's by far the heaviest one I've seen, but he says it's still a rocket.