Airborne_Again wrote:
Ask Cessna – I have no idea. . That’s how the Cessna 172R was designed. In the Cessna 172S, which has the same engine, the rating was increased to 180 hp @ 2700 rpm.
Probably just marketing 
I am guessing they are very similar in performance, with “lower horsepower” version outperforming in some parts of the envelope, because it comes down to what speed range the fixed propeller performs best at.
What RPM to do see on the runway and then at typical climb speeds, and is it always full throttle in the climb for both types? My logbook only goes up to the P model.
Bathman wrote:
The 160HP O-360 is by all accounts the most reliable of the cont/lycoming engines out there. If I am correct all you need to do is reprop it to get 180HP.
Not quite, as the auxiliary stuff (alternator etc.) are geared for max 2400 rpm.
“Should we have gone for an overhaul already when the second cylinder broke at 2245 hours?”
No. In fact I would of replaced/overhauled the 4th clyinder and then flew it to 3000 hours assuming everything else was ok.
3000 hours wasn’t really an option as that would likely have been in the middle of the summer season 2022. We don’t want the aircraft to be grounded for an extended period of time then… We had planned for an overhaul around december-january at 2700-2800 hours.
Ted wrote:
I am guessing they are very similar in performance, with “lower horsepower” version outperforming in some parts of the envelope, because it comes down to what speed range the fixed propeller performs best at.
Yes, they’re very similar. The 172S has 46 kg higher MTOM and slightly better performance (particularly at higher altitudes) than the 172R at the MTOM of each.
What RPM to do see on the runway and then at typical climb speeds, and is it always full throttle in the climb for both types? My logbook only goes up to the P model.
Yes full throttle in climb for both types. The full throttle static rpm is given as 2065-2165 for the R model and 2300-2400 for the S model.
The Q remains why all four cracked. This is not typical “post-TBO” behaviour, not seen earlier.
Nothing to do with RPM.
Peter wrote:
Nothing to do with RPM.
Why would it not? We have to start by saying that this is excellent overall reliability. But that is not answering the OP question.
Cracking is directly connected to thermal profiles and peak ICP (Internal cylinder pressure). We have to remember that these engines are fixed ignition timing in terms of ignition advance angle, not in terms of actual time advance vs TDC. Therefore the ICP timeline and peak ICP are different at different RPM, even for the same BMEP (torque). Torque is the same at max power on the two aircraft types , peak ICP is higher at lower RPM due to higher time advance for ignition for same angle advance.
I love alternative theories…
Could the reduced noise and lower tacho readings contribute to pilots using higher power settings than they might otherwise? i.e. more throttle to get similar rpm to what they are used to, not everyone reads the flight manual Is there any significant difference in fuel usage?
I flew a great many hours in a 172R out of Prescott, Az, including several times coast to coast. For me, it was perfect, because the instructors didn’t like to fly it off our 5000’ elev runway with average size students. So it was put aside for touring and came to be regarded as ‘my’ airplane. It was always regarded as high maintenance compared with the SP’s, but it never let me down. Eventually the owners bought an upgrade kit to convert it to a 180 like the SP’s and that involved a lot more than the rev limiter – I recall new cylinders, accessories and more. I was told the compression ratio was different, don’t know if that’s right.
As an ‘R’, it was a terrific touring plane for a solo pilot – quiet, smooth, and very long legs. Especially as they didn’t care when I brought it back.
As converted, I felt it was a bit of dog, not smooth and fast climbing like an SP. And it continued it’s high maintenance lifestyle. Eventually they sold it to a guy in Israel who flew it home. Regrettably I didn’t get the schedule in time and wasn’t able to meet ‘my’ airplane one last time as it transited UK.
Where are you now, N9549Q?
Ted wrote:
Could the reduced noise and lower tacho readings contribute to pilots using higher power settings than they might otherwise? i.e. more throttle to get similar rpm to what they are used to, not everyone reads the flight manual Is there any significant difference in fuel usage?
That’s a good point. Possibly.
Our 172S (180 hp) has an average fuel consumption about 10% higher than then 172R (160 hp) which pretty well agrees with the difference in rated power (12%).
