Percent Horsepower Calculation

[dstates]

I'm getting ready to break in my new cylinder that was just installed. The break in instructions discuss running between 75% and 65% horsepower. I'm using the charts in the O-300 operators manual to try to find what rpm that will give me those settings with assumed altitudes and temps. However, I don't have a manifold pressure gauge and that is one of the inputs into the chart. Any suggestions on how to come up with a manifold pressure to go along with the RPM? Any rules of thumb?

Thanks,
Doug

[cessna170bdriver]

Full throttle is about 30 inches at sea level and, in round numbers, drops 1 inch per 1000 feet. Do a full throttle climb until you get to whatever altitude gives you the manifold pressure you’re looking for and leave the throttle wide open. Or you can just set 2400 to 2450 rpm with a standard prop and you’ll be in the ball park.

A little off topic, but I recommend installation of a manifold pressure gauge, even with a fixed pitch prop. I put one in when I installed a standby vacuum system, but have found it to quite useful as a carburetor ice detector. As long as you’re at a constant altitude, you’ll see a drop in MP with carb ice before you see it on the tach.

[TFA170]

A little off topic, but I recommend installation of a manifold pressure gauge, even with a fixed pitch prop. I put one in when I installed a standby vacuum system, but have found it to quite useful as a carburetor ice detector. As long as you’re at a constant altitude, you’ll see a drop in MP with carb ice before you see it on the tach.

Continuing off topic, I agree with the MP gauge. Mine had one installed when I got it and I've found it useful simply for setting power at cruise to get repeatable settings and good starting points for settings.

[hilltop170]

You are trying to seat the rings and high manifold pressure is what keeps them against the cylinder wall. So more MP the better in my opinion. The engine is good for 2700rpm continuous. I would limit climb to a minimum to reduce CHT in the new cylinder and run at least 2600rpm until broken in. It won't hurt the engine and you only get one chance to do it right. Don't baby the engine during break-in.

[GAHorn]

Agree! In fact, lower altitudes will not only provide higher MP and better ring/cyl break-in pressures.... it will also provide denser/better cooling-air to fight high cyl temp.

[ghostflyer]

I was given a EI digital MP gauge and didn’t fit it thinking it would be relatively useless as I had a fixed prop , I ended up fitting it to fill a hole in the instrument panel and find it’s one of the best instruments around. It’s a very valuable tool in engine diagnostics. Makes engine management in cruise a breeze.

[dstates]

Thanks for the info everyone. A manifold gauge is on my list, but it all comes down to time and money sometimes. I’m looking forward to getting my 170 back in the air after being down for over a month with my engine issue.

Anyone else have cylinder break in tips?

[Bruce Fenstermacher]

My tip is to follow the cylinder manufactures instructions to a T so you don't void their warranty. That will include not only how you fly it but also the engine oil you use. None of us are warranting your cylinders.

Look in the handy Owners manual for the power chart to figure out at what rpm and altitude you would be running 65- 75%. I love my MP gauge put unfortunitly the manual gives no MP vs HP settings

[GAHorn]

28 inches MP at 2300 RPM equal 82%...(122HP).

Where/How did I do that?

I looked at pages 12/13 of the Continental Engine Operators Manual for this engine, and compared it to the static RPM specification of the TCDS. This is the power produced on a standard day using the TCDS approved propellers for this airplane at take-off power (static RPM).

Math whizzes can interpolate additional data from those pages.

(I’m no math whiz. I count on my fingers. But the pages mentioned indicate that for break-in purposes you should use Full Throttle and remain Near Sea Level).

Assoc’n Members may obtain a free copy of the Continental Engine Operators Manual at http://cessna170.org/forums/membersOnly ... op_man.pdf

[hilltop170]

One thing that hasn't been mentioned so far is to limit your low power operation/time on the ground to a minimum. After a normal start to do a short leak check run, shut down and confirm everything is ok. Start up, taxi with no delay, do a normal run-up, then take off at full power and stay as low as practical and run whatever high power setting you choose.

On fixed pitch engines, I orbit the airport at pattern altitude and high power leaned for conditions for a couple of hours while monitoring CHT, oil pressure, and oil temp. Make adjustments as required to keep parameters in the green or land and investigate if unable to control. I usually do another hour or two around the airport before I leave the airport area. It gets boring but you don't want to have to make an off-airport landing just because of some easy to fix problem if you had stayed in the pattern.

[GAHorn]

I don’t lean when breaking in new cylinders to provide maximum cooling for valves and keep CHTs down.

[ghostflyer]

I was running in a new lycoming in a cardinal rg and orbited the airport for about a hour and engine was operating great so I decided to fly along a beach as it was about 60 miles long and about 500 yds wide. Have often landed there . There is no population in this area. So the book says fly low and wide open throttle . Ok so I was flying in ground effect and flat out . I was having fun. The realestate was disappearing very quickly and just ahead was a four wheel drive parked . I thought I would just skim over the top and as I got within 100 yards it had a sign on it RANGER ,oops . But parked in front of it in a beach gully was another vehicle and it had the sign on it POLICE. Oops again . [BIG oops] These 2 must have pulled up together for a chat. I cleared both by about 6 ft . Then it was a very shallow turn into the island side so they couldn’t see the identifying letters. Tracked towards the mountains and about 1 hour later landed and then pushed it into the hangar . Haven’t heard a thing since and that was 6 years ago. It’s great being crazy.