IGSO-540 vs IO-720

[JimC]

I love my 500B. In the two years I've had it, we've been to 3 corners of the US - Alaska, Southern CA, and Vermont. I don't know if it'll get to Florida this fall, but maybe. We've taken it into ridiculously short gravel strips, the longest unpaved strip in the US, the highest unpaved strip in the US, the lowest strip in the US, grass strips at DAs of over 10,000 feet, and dozens of other "normal" places to stop and look around.

We're reaching a point where we've decided we want more. A little more range, a little more speed, and a lot more space. We're willing to give up a chunk of the wallet for this, so obviously, I've decided I need to upgrade to a 680FL. That's bad decision #1.

Now the question is which engines. Here comes bad decision #2.
When a 680FL goes up for sale, it's about a coin flip to see which engines it has - the IGSO-540s or the IO-720 Mr RPM conversions.

What I hear about the IGSO-540:
It pulls like a team of mules
It's the worst engine ever made
The fuel controller will make you quit aviation
The fuel controller is fine if you get it overhauled at Central Cylinder
It drinks 100LL more than any engine since WWII (literally)

What I hear about the IO-720s:
Replacing the rear cylinders sucks
You never have to replace cylinders
It has the wrong prop and needs 4,0000 feet of runway
They'll go faster than the IGSO on less fuel
They'll go 2,000 hours without a hiccup
They're six figures to overhaul when it hiccups
They're about 90 lbs more than the IGSOs.

We're interested in runway performance and we're willing to trade fuel efficiency for it. If I could swap props on the 720s, I'd go for it. Scrappy (Mike Patey's plane with a bored IO-720) can drag a 1/2 ton pickup with the brakes locked, so I know it's not the engine - it's the prop. There are a lot of high-thrust props that have been mated to 720s, but obviously it would be a massive uphill battle to get a new prop on this plane when there is zero market for even a second customer.

I think I can put my hands on several IGSO-540 engines for "cheap." Two are zero time overhauled and pickled. If I can buy a 680FL with four extra engines for the same price as a 680FL with 720s, would I be nuts? I mean, more nuts than normal. I figure that means I don't worry about full overhaul costs for as long as I own the plane.

BTW, I've read and reread Glenn's posts about the exhaustion of keeping his IGSOs running. The IGSOs on the plane I'm looking at have been averaging 50-100 hours a year for a while except for the most recent year. The logbooks don't look miserable.

I expect to climb the plane around 75% power and cruise at about 55%. That's how we've been running the 500B on most trips. The book says with IGSOs I can expect to burn 15.5 gph a side at 55% getting 183 kts, although the "55% max range" graph works if you burn 17 a side. Are these numbers realistic? I haven't found performance charts for the 720 STC. (note: there is a typo in the manual on page A-8. Fuel burns for 55% and 60% are switched.)

So...can I get comments and feedback? Any input from people who have owned or flown both would be amazing.

[donv]

Just curious-- why a 680FL over a 690A (or a decent straight 690, if you could find one)? I can't think of anything the 680FL will do better, and while you will spend a bit more to buy a 690A, you will get your money back when you sell-- and unlike a 680FL, you will actually be able to sell it.

If your answer is that you love big pistons and want to have one for that experience, I can completely understand that (after all, I own a bunch of old German and Italian cars)-- essentially like owning a warbird.

[JimC]

Acquisition cost.

The cheapest 690A needs HSIs soon. Total first year cost will be near $500k vs $200k for a 680FL.

It might be cheaper to own over 15 years (Yes, I assume I'll be the last flying owner of the 680FL), but I'll also have an extra $300k tied up in a non-productive asset.

Trust me, a 840 is very much part of the plan but it's not in the cards now. We decided that investing in upgrades to the 500B wasn't something we'd like to do, either, since we want a larger airframe as soon as we can find one.

It doesn't make sense as an answer to "What is the cheapest Commander for the next 15 years?" but it does make sense as an answer to "What is the cheapest Commander for the next 5 years that preserves capital so we can get an 840 in 5 years?"

[donv]

As you yourself posted, an IO-720 is over $200k to overhaul. Yes, most of them are low time, but because they have been sitting, they will probably need some work, possibly even including an overhaul. So not that much cheaper than a -5. Furthermore, you may be able to rent a set of -5s, or buy a used set of mid-time engines.

A cheap 690A is going to cost some money to get going, no question about that, but so will a 680FL, and I wouldn't expect a comparable 680FL to be any less expensive to make reliable than a 690.

How many hours per year do you anticipate flying it? Keep in mind the 690A can travel the same distance in considerably fewer hours.

[JimC]

Speed isn't my priority for this plane. Faster is always nicer, but 180-200 kts is fast enough for this mission. The reason for me to jump to a -5 turbine would be reliability, not speed.

If you can point me to a place where replacement -5s are available, I'm interested. The plan for any airframe I get is to not overhaul or HSI the engines while I'm the owner.

I may not be able to get a 680FL as reliable as a 690 for less, but I'm starting from a MUCH lower price point, so there's room in the budget.

This will be a 100-150 hr/yr airframe.

[donv]

Bruce would probably know, but I would guess that it may be impossible to make a 680FL as reliable as a 690 under any circumstances. In fact, I'm pretty sure that was true when they were new, but it is much more likely now as the knowledge base about those old engines fades.

And, that's just the engines. I believe the also have the high pressure hydraulic system, etc.

In the end, I doubt practicality is what is driving your decision-- and that's okay. If you want one of those big pistons, go for it-- but I very much doubt it will be a rational answer for your transportation needs.

[JimC]

"I doubt practicality is what is driving your decision"

Bingo. There are plenty of good Barons for sale if I want to be practical.

A really good piston aircraft can be 90% as reliable as a bad turbine. This isn't a really good piston aircraft.

I'm 99% sure that the 680FLs have the low(er) pressure 1000 psi hydraulic system only. The 1000 psi gauge I saw, the hydraulic schematic I've seen & the wheel well I've seen look just like my 500B. They didn't have pressurization that required the crazy 3000 psi hydraulic pumps. I haven't bought a maintenance manual yet, but I can't see where it would use the 3000 psi system.

Engines aside (and that's a big aside), the one I saw seemed to have the systems and structure of a stretched 500. There was a lot of commonality.

If I can get into this swamp for $100k or less I think I will. Financially, that money will be unrecoverable. There are too many being parted out for the parts value of the plane to be very high.

[Popeye]

It seems to be a decision of OPEX vs CAPEX. Yes the piston engine will have lower operating cost, but the reliability will not be the same as a 690A/B. Getting into an FL, as you mentioned, its a 500 Streched. So why enter the limitations of a 500 streched, when you could jump into a better performing, less runway, higher flying, better safety, 690A. I currently fly the IGSO, and they are good engines if you operate them well, and they are guzzlers to 25-30GPH (2650RPM and 31.5MP 8-10K feet, 190-200kts/ 185MPH indicated) per engine, and dont try to operate them LOP, as they will chug away cylinders within 100 hours. Its a ROP engine. ON the hydraulics, the 3000 psi pump is for pressurized cabins. If the aircraft is pressurized you will run into that scarcity. Just ask Glenn, who has not been able to find that hydarulic pump. Back to OPEX vs CAPEX. With the FL, you will loos about 200-300K as you probably wont be able to sell later. With the 690, capex will be like you said-500-600, but in 5 years, trading up to an 840, will be a small jump (+/- 200K loss of delta or same as FL), and the power of your dollar will not be the same as today's. The operating cost of an FL is just about the same as a 690A/B. Being that you like adventures, I suggest you see the charts on both aircraft, as performance, is not going to be to your liking for short fields or high density. And yesss, Central Cylinder is the only place, but pricing keeps on creeping up. The fuel unit can last you 500 hours or more, but to overhaul them is going around at 10K. Finance it, keep your cash and the service on the loan can go from 3-6K and jump one step up.

[JimC]

I haven't found anyone who will finance a 690A/B. They're just too old now.

The 690A has more HP but has less wing, weighs over 1000# more empty, and the fuel for the same distance weighs more. It seems like it would be at least 1500# more for a given flight. I have a 680FL manual and a 690A manual, and I show the takeoff distances as being very similar for similar missions (I'm assuming the 680FL is carrying the same payload but 500# less fuel.) This is using the 50ft height/standard takeoff of the 680FL, not the short field performance section. That's at sea level - at higher altitudes, the 680FL uses *less* runway. These are numbers straight from the factory manuals.

I don't plan on running IGSO engines as hard as you listed. The book says at 10k feet, 55% would be 15.5 gph / 2400 RPM / 26.5". Is that a reasonable number? Your numbers are pretty close to book values for 75%.

I won't pay $200k to buy an IGSO 680FL. I don't think I have to, either. The market is soft.

[donv]

I regularly get emails from companies with 331s available.

I just got one from these guys:

https://teci.com

No idea if they are reputable, but they have a number of -5s available.