The International Cessna 170 Association

Hi all,
I read somewhere that I should restrict cruise RPM's in my 170A's C-145 engine to 2450. But I forget where I saw that.
I also lost track of the only power setting chart available for the 170A.
So at sea level and up to modest altitudes like 4000 feet I use 2400 RPM.
But... flew a long cross country trip recently at 6500 to 7500 feet, and at those heights I feel I should be able to use full throttle and accept higher RPM's, maybe up to 2700..
Who can offer some advice?
Is there really a cruise RPM which should not be exceeded?
Thanks.
Jim

Don't know where you saw that but it's WRONG.

At sea level at 2700 RPM with a lean mixture your only developing 99% of the engines rated power. The engine can run at 2700 all day long. At 2500 feet at 2700 RM the engine is only developing 85% of its rated power. It DEFINITELY CAN run there ALL DAY LONG.

Look at the cruise performance chart found on page 40 of the B model owners manual. It's also in the '48 and A model book.

If you baby your engine it will not last longer. In fact it may not last as long.

And there is an RPM that you should never exceed at cruise. It's 2700 RPM

I cruise at 2600 RPM everywhere I go no mater what altitude I'm at and as a helicopter pilot I'm RARLY more that 2000 ft agl.

Bruce is correct. The Type Certificate states 2700 rpm for all operations. However the owners manual states:

Cruising: Any cruising r.p.m.
between 2200-2450 (green arc on
tachometer) may be selected.
The recommended cruising
r.p.m. is 2450, which will supply
the most practical cruise
performance when such factors
as cruising speed, miles per gallon,
fuel consumption, engine
efficiency, and engine life are
considered.

Ah, but about leaning near sea level at 2700 rpm...can't be good, can it

Yes it's fine.

Look at the manual. It says the chart is based on a lean mixture.

When cruising around lightweight (solo or 1 passenger) I usually set 2450 and lean it for that altitude & RPM. This usually gives me a no-wind cruising speed of 98-100 knots.

If I'm heavy and/or forced to cruise at higher altitudes (crossing Utah, Idaho, etc.) I will set 2550 RPM. Over time I have noticed that my engine will burn/spit out a little more oil at the higher setting and require a small wipe-down of the belly just aft of the fire wall (verrrrry small wipe-down, basically just dampening one third of one paper towel). Also, the fuel burn will increase (duh) if I'm below about 7000 feet.

The reason I will increase the RPM if heavy (at/near gross weight) is that the airplane will just "plow" along at 92-95 knots if it's heavy with RPM @ 2450. If I increase it just 100 RPM to 2550 I can get it up to 97-101 knots (no-wind).

Once I flew at 2550-2600 RPM and low altitude because it was fun and the air was smooth. At the end of the flight I discovered I was burning about 9.5 gph. I didn't like that burn rate (but I was getting about 110 knots no-wind) so I haven't done that again. I typically see just under 8 gph (7.5-7. when I set 2450 RPM. For flight planning I use 9 gph for the first hour then 8 gph after that and 98-100 knots across the ground.

I'm not an old-timer, but I've had the airplane for just over 3 years and about 400 hours. That's my experience, hope this helps.

Fly safe,

Looking at the SEA LEVEL PERFORMANCE CURVE for the 0-300-D, in my engine operator’s manual, the max horsepower and prop load is at 2700 rpm. I cruise around at 2450 rpm because that is the power setting recommended in the C170 operator’s manual.

Looking at the 0-300 performance curve, at 2450 rpm the engine has a lot of horsepower not being used. I really like the low fuel consumption at 2450 rpm but will definitely try cruising at 2600 rpm to monitor the differences in fuel consumption and cruise speed.

I agree with Bruce that lower rpm settings to baby the engine will not extend engine life. But why did Cessna put the yellow arc at 2500 rpm

At what rpm do the rest of you guys and gals cruise at?

Bill

While cruising around at higher altitudes and spinning the revs way up IS ok according to the book, be ready to have an engine that has never burned oil before............to burn oil at those rev speeds. I like my engine to not burn oil. I like to burn 7.5 gph and not 10gph. Just my personal feelings. I'm sure my feelings will have to change when I buy a 195.

theduckhunter wrote: I like to burn 7.5 gph and not 10gph. Just my personal feelings. I'm sure my feelings will have to change when I buy a 195.

I agree. At 2600 rpm your burning 9+gph for a very small increase in speed . On a 500 mile cross country you would spend about $20 more (@4.25/gal) to get there just 15 minutes earlier.

When I am flying around, just to be flying (can't do that much anymore with these rediculous fuel prices ) I usually turn my 0-300B at 2450-2500. If I am trying to keep up with most 170's I will turn 2600 rpm and do not find any abnormal oil consumption on my engine w/ a little over 500 hrs on it. Going to Kelowna, there was one leg where I checked the fuel burn and it was 8.3 gph and that was turning 2550-2600. Didn't check it on the other legs. I would say that out of 38.8 total hours on the trip that more than 90% were at 2500-2600 rpms. I burned or leaked some oil (I still have a minor oil leak around the tach drive housing ), but that isn't a bad oil consumption for that many hours. Works out to about 1 qt per 7.75 hours or so. I used to have a friend, now deceased, who spent many, many hours flying everything from AirCampers (his first plane) to high performance jets (first flight of F102, F106 & F-111). He indicated that the military had them to run the engines at more or less full bore most of the time. I suppose if fuel was running low they may not have done so, but I don't recall what his answer was to that situation. I don't know if other military aviators had the same experience or not, but their theory, according to him, was that the engine was designed to run at a maximum rpm (this was recip's, not jets) and it made sense to run them that way and get the mission/trip over quicker even though the fuel consumption was higher----might not have been higher overall if a longer engine operating time was considered. Of course we know who was paying the fuel bill

After I rebuilt my 0300A I ran it between 2500 & 2700 rpm for the first 10 hrs.and then headed for Kelowna. On the trip I averaged 6.87 gph @ 2400 rpm @ between 6000 to 8500 asl and used just over 1 qt of oil in 38 hours round trip. Greg

I am new to the aviation world but I believe an engine is an engine. Find what RPM your engine runs the best (Smoothest) at and run it there. After I put new cylinders on my O-300, and broke them in, I tried several different RPM settings to see how it ran. I ran the engine pretty hard while seating the rings. I tried 2350, 2400, 2450, 2500, and 2550 at cruise. I found my engine runs the smoothest at 2550. I run that RPM and lean for what altitude I’m flying at. I am usually cruising between 7500 and 9500. My fuel burn is 8.0 gph. That is counting from the time I start the engine to the time I shut it off.

I think the RPM argument is like the argument of which pickup is better, Ford, Chevy, or Dodge. What ever you like is what you are going to buy, and it will be the best.

pif_sonic wrote:I am new to the aviation world but I believe an engine is an engine. Find what RPM your engine runs the best (Smoothest) at and run it there. After I put new cylinders on my O-300, and broke them in, I tried several different RPM settings to see how it ran. I ran the engine pretty hard while seating the rings. I tried 2350, 2400, 2450, 2500, and 2550 at cruise. I found my engine runs the smoothest at 2550. I run that RPM and lean for what altitude I’m flying at. I am usually cruising between 7500 and 9500. My fuel burn is 8.0 gph. That is counting from the time I start the engine to the time I shut it off.

I think the RPM argument is like the argument of which pickup is better, Ford, Chevy, or Dodge. What ever you like is what you are going to buy, and it will be the best.

An opinion is an individual right...but I'd respectfully disagree with that logic on this matter. The selection of cruising rpm should be made based upon the desired power setting versus fuel consumption/speed as depicted in the performance charts. If your engine is rough at the recommended cruise rpm ranges the designers built into this engine, then it might be a good idea to have the propeller dynamically balanced. (Ford pickups are best anyway!)

(Ford pickups are best anyway!)

Unless its a clasic Chevy

Gahorn, No disrespect taken. But I find it very hard to believe that every 170 that has an O-300 in it runs identical at the same RPM just because a book that was written 50 years ago says so. My prop is just fine it was checked about 6 months ago. I have flown in four different 170’s. They all feel different and they all fly a little different but I bet the pilot hand books for each plane say the same thing.

I’m not saying my engine runs rough at 2450 I just said it runs better at 2550. And I will bet every 170 driver who really pays attention to their engine will have a slightly different RPM setting where their engine runs better.

And I agree Ford Pickups are the Best.

I hope I don’t offend anybody with my remarks about the 170.