We have a Texas Skyways O-520 in our 180. With their STC you have the option of HC or LC pistons. We opted for HC as there is no Mogas STC’s allowed down here and Mogas is only slightly cheaper anyway, so no advantage for LC pistons on that front. In theory HC should be more efficient as well as produce more HP. Ours is 285 HP at 2700 with 2500 hr TBO. Any pull test data I have seen showed that there was higher static thrust at 2700 than 2850 anyway. So the extra revs while producing more HP didn’t necessarily convert into more thrust, just made more noise. It certainly was a different beast after the conversion, especially when loaded. Also added a 3 blade MT.

hotrod180 wrote:What cam does the Ponk 470-50 use, vs the cam in the IO-520D?
That might account for the different RPM limits.

The P.Ponk conversion doesn't touch the innards of the engine, only the cylinders and/or pistons (and removes the FI in the case of an IO-520). That means the cam is whatever was installed in the engine you started with. If you started with an IO-520D, that's the cam you have. If you started with a TSIO-520C, you keep that cam, and so on.

The original STC to build a PPonk used only a 470 or TSIO-520C. Neither of those was rated at 2850RPM, so one can assume the STC wouldn't be able to offer 2850 as an increased limitation. The option to build the PPonk from the IO-520D/F was added to the STC at a later date, and my guess is that adding 2850RPM as a limitation would mean a whole new certification process.

Chris

airChris wrote:...and my guess is that adding 2850RPM as a limitation would mean a whole new certification process.

Quite likely yes. However keeping the IO-520 pistons as-is also deviates from STC and requires a 337/field approval. Ironic isn't it? To not take the engine apart and install non-original parts requires extra approval. Now the 8.5 FA is considered easy to get (or at least was prior to the recent FSDO shakeup, I have not heard results since that happened but am about to try), so maybe the FA can also simply state the engine can be run up to the maximum RPM designed by the manufacturer and within recommended operating time limits, blah blah and so on. I dunno, it's something I want to inquire about. In this case it's literally just running the engine stock, other than converting FI to carb and a couple other minor mods.

Do we need 2850? Absolutely not. In fact in all but summer days it pushes an 84" prop like an MT past mach 0.92 so you're losing tip efficiency rapidly after that. But dial back to maybe 2800, and you're still 0.92 or better down to 50F OAT. Higher RPM would give some extra oomph on hot days without busting mach speeds, and could always be run slower in cold temps if desired. Of course if we could get our hands on prop performance charts at various temps, it'd basically tell us everything we need to know. But those are hard to come by.

colopilot wrote:Now the 8.5 FA is considered easy to get (or at least was prior to the recent FSDO shakeup, I have not heard results since that happened but am about to try).

The folks at P.Ponk will refer you to Bob Minnis of AvPower http://www.avpower.net/intro.html to do the 337 for 8.5:1 pistons. I presume he has a relationship with the FAA and has done several.

Chris

airChris wrote:The folks at P.Ponk will refer you to Bob Minnis... I presume he has a relationship with the FAA and has done several.

Bob is an awesome dude and a valuable resource. (He is the one who got the STC done for Flight Resources)

I had him do the approval for my 8.5 pistons in my O520F. He is a DER and can provide the basis for installation (for a fee.) This route is different (and immensely easier) than getting a Field Approval from the FSDO.

They did in fact refer me to Bob, I just haven't spoken to him yet. I didn't realize he was using a different process, I thought he was just providing whatever the FSDO needed to do their inspection properly. Even better if I can skip that step entirely. I'll buy the STC soon and will be chatting with him.

colopilot wrote: Even better if I can skip that step entirely.

Yep. No muss no fuss. Send him a check and he will send the paperwork. Done...

Bigrenna wrote:

colopilot wrote: Even better if I can skip that step entirely.

Yep. No muss no fuss. Send him a check and he will send the paperwork. Done...

That pretty much just made my afternoon.

colopilot wrote:That pretty much just made my afternoon.

For sure... There are very few things that go that easy these days.

In reading all this, a 8.5 Pponk and a Texas Skyways O 520 are basically the same?????

180Marty wrote:In reading all this, a 8.5 Pponk and a Texas Skyways O 520 are basically the same?????

Mostly. I think the Skyways may have a couple other minor changes, and also incorporates their fancy oil sump (basically a drain on the very bottom) to increase TBO. They don't sell the sump STC outside of their engine build, I asked.