Cherokee 140/160 Climb RPM

[Lloyd Read]

I have a 72' PA28-140 with a RAM 160 STC and Powerflow exhaust. It was installed by the previous owner but he did not replace with carburetor with the RAM recommended one and did not repitch the prop from 58 to 60.

The issue I run into is on departure. I cannot hold Vy for more than a couple of minutes before CHTs are over 400 and I start to lower the nose. 100mph IAS seems to keep them steady at 400 but my RPM is 2680-2700 as measure by my JPI. This seems to happen when OAT is 80 degrees or more @ SL and I see about 500fpm when pitching for 100mph. WOT in cruise would easily blow past redline. I see about 2450RPM on takeoff roll. I can't recall what the static runup RPM is but I want to say around 2410.


I've had a few instances when I am climbing out and im faced with lowering the nose past 100mph to stop the CHTs climbing and also pull power or to completely level off and pull power to cool off the engine all before reaching 2000 AGL.


Baffling is in good condition and engine appears to be healthy.


Would repitching the prop help lower the RPMs and in turn help lower CHTs? I asked RAM about the carburetor but they did not know the difference between the carbs or why the say to swap it out...

 

[dmspilot]

Repitching the prop from 58 to 60 will lower the RPM. My understanding is that the procedure is not terribly expensive. If you're getting redline in a Vy climb I would do it.

On the other hand, you could just reduce the power and/or climb at a higher airspeed. But then you're not getting much advantage from the engine upgrade.

 

[Bob Noel]

lowering the nose also helps forward visibility.

 

[Dan Thomas]

And if the carb and prop weren't corrected as per STC instructions, the airplane is not legally airworthy. An STC is not a buffet where you pick what you want and reject the rest. The mechanic can't sign the job off as "done in accordance with STC #1234."

 

[edessa]

Our club is installing this same STC on our clubs PA28-140. We had to replace the carb and the fuel pump needs to be a certain part number. The fuel pump we had was already installed at the last overhaul.
Your aircraft is not legal until the carb and fuel pump are per the STC requirements.

 

[George Mohr]

Yeah and it sounds like it might be lean due to this incomplete conversion. 100kts should not be required for a 160HP O-320 to keep reasonable CHT's.

 

[Magman]

How is Fuel Pressure during climb?

Neighbor had issue with 140 when nose was raised during climb out.

One day engine quit.

Turned a fitting was loose aft of Firewall and sucking air.

Must have caused a Lean Mixture when sucking air.

I’m thinking Fuel Press likely dropped which would show on gage.

 

[gacoon]

I used own one and climbs on hot days were a problem sometimes. Couple of things, make sure winterization plate is off the oil cooler, check baffling carefully to make sure its good. There was a good article on this by Mike Busch in the Dec 2020 AOPA mag. If it heats up above 380 level off and cruise a while to cool it down, then head up again, repeat as required. (Step climb). Find a copy of the article, it will give you some good ideas to look for.

 

[RyanB]

There was a good article on this by Mike Busch in the Dec 2020 AOPA mag. If it heats up above 380 level off and cruise a while to cool it down, then head up again, repeat as required. (Step climb).

I prefer to just climb at a shallower rate than do a step climb, but I realize that may not be possible out west, in the middle of summer, in a normally aspirated 4-banger. I wouldn’t be concerned with 380F in climb though and really wouldn’t worry about 400F either. It’s not an ideal temp range, but according to Lycoming, an occasional CHT in the high 400s is not a problem.

 

[brcase]

My observation is that 160hp o-320’s are a bit challenging to keep all the cylinders below 400. 420 is pretty common in a climb. Also observed that generally the higher the compression the harder it is to keep the CHT’s down. But most o-320’s don’t have CHT gages, let alone 4 channel gages, so most don’t know that they run that hot.
Agree good to try to keep them cooler, but I don’t start getting concerned on o-320’s until they start going over about 420. I don’t think repitching the prop will change your CHT temps much, but might allow you to run a higher power setting in cruise, ie. Fly a bit faster at the cost of a bit of climb performance. For best climb performance the 2700rpm you are getting is just about perfect, but probably not ideal for cruise.

 

[bluerooster]

We had a '73 140, when the powerflow was installed the prop had to be re-pitched to prevent exceeding max static rpm.