Hi all,

I'm in the process of topping my PPONK 470-50 and am exploring the option of putting in high compression 8.5:1 pistons. They're not part of the original STC, but I know there are guys out there who have done it with good results.

Has anyone on here done it? Was it accomplished via 337?

Any help and info would be greatly appreciated.

Thanks,

Chris

Chris:

I did the same thing and replaced my P-Ponk pistons with higher compression pistons.

Not sure I can honestly say that I experienced any better performance, but the gas usage certainly did climb some. In retrospect, not sure I would have bothered. I didn't do any real detailed before and after testing, but just casual observation didn't result in any noticeable difference.

You may want to call Steve K and discuss with him about expected changes. He could probably give you a more well thought and precise answer as to what to expect in performance.

I will look in my log books and see, but I do think it was a 337. I believe that Steve sent us a copy of other 337s that he had obtained.

Do you want me to pass along a copy of the paperwork?

L

I'd be interested in hearing what Knopp has to say about it.
What's the standard compression ration for that conversion?
Dunno if Knopp's STC approves mogas use, but the 7:1 compression used on the stock 470's (other than the U engine) is within the limit for 87 octane regular--
bumping to 8.5:1 might require 91 octane premium grade.
FWIW I burn mogas almost exclusively, my 470K runs great on it and it's only about $2.75 a gallon right now vs $5.10 for 100LL at my airport. So about $25 an hour savings, plus no lead fouling to boot.

Not sure I can honestly say that I experienced any better performance, but the gas usage certainly did climb some. In retrospect, not sure I would have bothered. I didn't do any real detailed before and after testing, but just casual observation didn't result in any noticeable difference.

You may want to call Steve K and discuss with him about expected changes. He could probably give you a more well thought and precise answer as to what to expect in performance.

I will look in my log books and see, but I do think it was a 337. I believe that Steve sent us a copy of other 337s that he had obtained.

Do you want me to pass along a copy of the paperwork?

Thanks for the info!

Do you recall your before and after fuel consumption? Doing the math suggests only a 5% performance boost over regular pistons (ie: 270hp to 285 hp = about 5%), is that about the fuel consumption difference you saw?

I'd like to take you up on your offer of the paperwork; that would be really helpful.

Thanks,

Chris

hotrod180 wrote:I'd be interested in hearing what Knopp has to say about it.
What's the standard compression ration for that conversion?
Dunno if Knopp's STC approves mogas use, but the 7:1 compression used on the stock 470's (other than the U engine) is within the limit for 87 octane regular--
bumping to 8.5:1 might require 91 octane premium grade.
FWIW I burn mogas almost exclusively, my 470K runs great on it and it's only about $2.75 a gallon right now vs $5.10 for 100LL at my airport. So about $25 an hour savings, plus no lead fouling to boot.

The normal pistons in the P.Ponk engine are 7.5:1, which result in approximately 270hp. Although there is no official approval for mogas, you CAN use it without damaging anything. However, if you install the 8.5:1 pistons, you're restricted to avgas only. So for you, the 'upgrade' really wouldn't be a viable option if you want to stick with mogas.

Chris

I think having the flexibility to use mogas is worth a lot, even giving up a possible 5% power increase.
I know guys who run 150hp 320 Lycomings instead of 160hp, because they want to be able to run 87 octane gas. You can run 91 mogas with the 160, but sometimes its hard to find ethanol-free 91.

airChris wrote:

hotrod180 wrote:I'd be interested in hearing what Knopp has to say about it.
What's the standard compression ration for that conversion?
Dunno if Knopp's STC approves mogas use, but the 7:1 compression used on the stock 470's (other than the U engine) is within the limit for 87 octane regular--
bumping to 8.5:1 might require 91 octane premium grade.
FWIW I burn mogas almost exclusively, my 470K runs great on it and it's only about $2.75 a gallon right now vs $5.10 for 100LL at my airport. So about $25 an hour savings, plus no lead fouling to boot.

The normal pistons in the P.Ponk engine are 7.5:1, which result in approximately 270hp. Although there is no official approval for mogas, you CAN use it without damaging anything. However, if you install the 8.5:1 pistons, you're restricted to avgas only. So for you, the 'upgrade' really wouldn't be a viable option if you want to stick with mogas.

Chris

So will Knopp warranty it if you've been running strictly moGas and something happens to your engine?

I know my plane will pull 2700-2750 static with a 82" two blade prop. I know a 185 or 206 with the same injected engine will make 2850 static with the 82" two blade. I'm sure a carburetor 520 with the Higher compression pistons would as well assuming it was rich enough. That's gota take a bit more ponies to do that... I wish someone had some hard performance numbers for the two different compression ratios on the same engine but I would bet most installations are rebel high compression pponk motors with no real approval basis causing owners to be reluctant to share numbers on a forum....

P Ponk has a service letter forbidding use of the higher compression Pistons in the 0-470 case based engines..

http://www.pponk.com/Service_Letters/SIL002.doc

Says nothing about using them in the 520 case based engines... Best to call Steve and ask if you have one of those..

A1Skinner wrote:

airChris wrote:

hotrod180 wrote:I'd be interested in hearing what Knopp has to say about it.
What's the standard compression ration for that conversion?
Dunno if Knopp's STC approves mogas use, but the 7:1 compression used on the stock 470's (other than the U engine) is within the limit for 87 octane regular--
bumping to 8.5:1 might require 91 octane premium grade.
FWIW I burn mogas almost exclusively, my 470K runs great on it and it's only about $2.75 a gallon right now vs $5.10 for 100LL at my airport. So about $25 an hour savings, plus no lead fouling to boot.

The normal pistons in the P.Ponk engine are 7.5:1, which result in approximately 270hp. Although there is no official approval for mogas, you CAN use it without damaging anything. However, if you install the 8.5:1 pistons, you're restricted to avgas only. So for you, the 'upgrade' really wouldn't be a viable option if you want to stick with mogas.

Chris

So will Knopp warranty it if you've been running strictly moGas and something happens to your engine?

Pponk engine isn't approved for mogas. So Knopp most likely won't warranty it methinks...

Halestorm wrote:

A1Skinner wrote:

airChris wrote:[quote="hotrod180"]I'd be interested in hearing what Knopp has to say about it.
What's the standard compression ration for that conversion?
Dunno if Knopp's STC approves mogas use, but the 7:1 compression used on the stock 470's (other than the U engine) is within the limit for 87 octane regular--
bumping to 8.5:1 might require 91 octane premium grade.
FWIW I burn mogas almost exclusively, my 470K runs great on it and it's only about $2.75 a gallon right now vs $5.10 for 100LL at my airport. So about $25 an hour savings, plus no lead fouling to boot.

The normal pistons in the P.Ponk engine are 7.5:1, which result in approximately 270hp. Although there is no official approval for mogas, you CAN use it without damaging anything. However, if you install the 8.5:1 pistons, you're restricted to avgas only. So for you, the 'upgrade' really wouldn't be a viable option if you want to stick with mogas.

Chris

So will Knopp warranty it if you've been running strictly moGas and something happens to your engine?

Pponk engine isn't approved for mogas. So Knopp most likely won't warranty it methinks...[/quote]
My thinking as well. So burning moGas in a $30k engine doesn't seem very smart to me...

I believe Knopp would stand behind engine if run on mogas.

When he was working on the STC it was to include allowing mogas. Feds held up the process due to the mogas addition so he dropped that part to get the STC. At that point he had a engine that had been run for approximately 400 hours on mogas with no issues. A year or so after he had the STC he went back to feds to get mogas added to STC. They said he would have to start from scratch to get it added. There just wasn't enough value in pursuing it at that point.

Thats the story I've been told by a very reliable source.

I'd be more worried about what the insurance company would say if I stacked it up.

PAMR MX wrote:I know my plane will pull 2700-2750 static with a 82" two blade prop. I know a 185 or 206 with the same injected engine will make 2850 static with the 82" two blade. I'm sure a carburetor 520 with the Higher compression pistons would as well assuming it was rich enough. That's gota take a bit more ponies to do that... I wish someone had some hard performance numbers for the two different compression ratios on the same engine but I would bet most installations are rebel high compression pponk motors with no real approval basis causing owners to be reluctant to share numbers on a forum....

On an aircraft with a constant speed prop like these the rpm is limited by how low the pitch on your prop will go. This is typically limited by the governor unless the internal low pitch stops on the prop are set too high. If it is set up properly the governor will allow the engine to spool up to its maximum recommend rpm which is typically where you reach your maximum hp. Static rpm on a fixed pitch prop can be a good indicator of hp but with a constant speed prop hp has very little to do with it. For example my plane has a Continental IO-360 engine 210hp and is designed to achieve that hp at 2800rpm so the governor was adjusted to reach that rpm. But this engine makes way less hp than a IO-520 300hp motor that has the governor adjusted to 2850 where it achieves its 300hp, yet mine is only 50 rpm less. Hopefully this helps a little.
Shawn

BTV wrote:I believe Knopp would stand behind engine if run on mogas.

When he was working on the STC it was to include allowing mogas. Feds held up the process due to the mogas addition so he dropped that part to get the STC. At that point he had a engine that had been run for approximately 400 hours on mogas with no issues. A year or so after he had the STC he went back to feds to get mogas added to STC. They said he would have to start from scratch to get it added. There just wasn't enough value in pursuing it at that point.

Thats the story I've been told by a very reliable source.

I'd be more worried about what the insurance company would say if I stacked it up.

I wonder if it would make more sense for Petersen or EAA to add it to their mogas STC? They seem to cover almost every variation of the Continental engines.

BTV wrote:When he was working on the STC it was to include allowing mogas. Feds held up the process due to the mogas addition so he dropped that part to get the STC. At that point he had a engine that had been run for approximately 400 hours on mogas with no issues. A year or so after he had the STC he went back to feds to get mogas added to STC. They said he would have to start from scratch to get it added. There just wasn't enough value in pursuing it at that point.

I've heard that, too.

As far as warranty is concerned, it'd be up to the engine builder, not Steve Knopp. I think most people don't realize that many of the P.Ponk engines out there are not actually built by Steve. He owns the STC, but there are many builders licensed to do the conversion. So whoever builds your engine is the place to check.

I wish someone had some hard performance numbers for the two different compression ratios on the same engine but I would bet most installations are rebel high compression pponk motors with no real approval basis causing owners to be reluctant to share numbers on a forum....

I agree those numbers would be nice to have. I don't know if fuel injection vs carburetor makes any horsepower difference. If not, you can almost be assured that a P.Ponk with high compression pistons makes 285hp since that's what any run-of-the-mill IO-520 (which come stock with 8.5:1 pistons) does at 2700rpm, and a P.Ponk is pretty much just a fancy name for a carb'd 520.

Chris

ak2711c wrote:

On an aircraft with a constant speed prop like these the rpm is limited by how low the pitch on your prop will go. This is typically limited by the governor unless the internal low pitch stops on the prop are set too high. If it is set up properly the governor will allow the engine to spool up to its maximum recommend rpm which is typically where you reach your maximum hp. Static rpm on a fixed pitch prop can be a good indicator of hp but with a constant speed prop hp has very little to do with it. For example my plane has a Continental IO-360 engine 210hp and is designed to achieve that hp at 2800rpm so the governor was adjusted to reach that rpm. But this engine makes way less hp than a IO-520 300hp motor that has the governor adjusted to 2850 where it achieves its 300hp, yet mine is only 50 rpm less. Hopefully this helps a little.
Shawn[/quote]

Right you are for the most part.
The low pitch setting for a 2 blade c66 propeller on the 470-50 is 9.5 degrees. The same c66 propeller on a C185 with a IO-520D is 9.5 degrees. The low compression io 470-50 will not make 2850 even with the governor set for higher. A IO-520d will make 2850 static no problem. It's very much the same as a fixed pitch prop as the propeller rides the low pitch stop until the governor starts to crank in more pitch to limit RPM.
That's all well and good way to say one probably needs a dyno or pull test to see the fine details between the two. I would bet ya a beer that the high compression o520 will make 2850 just fine.

Right you are for the most part.
The low pitch setting for a 2 blade c66 propeller on the 470-50 is 9.5 degrees. The same c66 propeller on a C185 with a IO-520D is 9.5 degrees. The low compression io 470-50 will not make 2850 even with the governor set for higher. A IO-520d will make 2850 static no problem. It's very much the same as a fixed pitch prop as the propeller rides the low pitch stop until the governor starts to crank in more pitch to limit RPM.
That's all well and good way to say one probably needs a dyno or pull test to see the fine details between the two. I would bet ya a beer that the high compression o520 will make 2850 just fine.[/quote]

I see the angle you are looking at it now. If props were identical with the same low pitch stops and the governors backed off then max rpm would be a good indicator of hp. You are right though a pull test is the real tell all. I'd be curious to see a dyno report on the pponk to understand why they limited it to 2700 rpm. Was that where max hp and torque was reached or was that to make the FAA happy? Interestingly in the pull tests I have done on planes with constant speed props it is not uncommon for it to pull harder at 50-75 rpm less than max rpm. I suppose that is a factor of the torque curve or blade design or both.

So back to the original question.....


....there's only been one volunteer so far to weigh in with any actual information on having high compression pistons in a P. Ponk engine. I'm sure there are others who must have something to add?

Performance gains/fuel burn increases/etc ?

Chris

Resurrecting this thread. I found it in a search, and have some data to add.

ak2711c wrote:I'd be curious to see a dyno report on the pponk to understand why they limited it to 2700 rpm. Was that where max hp and torque was reached or was that to make the FAA happy? Interestingly in the pull tests I have done on planes with constant speed props it is not uncommon for it to pull harder at 50-75 rpm less than max rpm. I suppose that is a factor of the torque curve or blade design or both.

Probably some of all of the above. Looking at power/fuel curves for the IO-520-D/F models, as well as Continental's specs, we see the following:
- Max T/O RPM is 2850@300HP, but only for 5 minutes. After that, max rated continuous RPM is 2700@285HP. They also recommend a max of 2550 RPM for cruise, which gets us around 265HP at sea level.
- P-Ponk's STC covers the 520, but also covers conversion of an O-470 which was never meant for >2700RPM. The high limit could very well be based on the design parameters of the 470, not the 520.
- They had a practical limit of props on hand, and time and money to test things. For the same reason they didn't follow through with Mogas STC, they probably didn't verify the 2850 max RPM for the direct 520 replacement.
- Max HP is absolutely achieved at 2850RPM according to Continental.
- Curiously, brake-specific fuel consumption (fuel used per HP delivered) is identical at 2700 and 2850. It gets worse at all RPM's below 2700 fairly linearly. Best BSFC is at about 2770, but only marginally so over 2700/2850.
- I've anecdotally heard of aircraft coming apart from vibration due to engine changes. Particularly with metal props and balance potentially being out of spec, it's conceivable that running a 520 at over 2700 on a 182 might vibrate things in a way not originally intended. Now, it isn't like my plane designed in the 50's had any sort of computer analysis done to prove it, but it might explain FAA limitations. A composite prop likely makes this a nonissue, but resonance can be a bitch.

If you're 337'ing the 8.5:1 pistons as I will be doing, I suppose it's theoretically possible to also 337 the max RPM of 2850 and then comply with Continental's published limits. I don't know if anyone has officially done so, or might otherwise be running 2850 just fine but not talking about it in mixed company.

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