Throttle position vs RPM

[Andre_C]

This is my first post so I would like to start by saying hello to everyone. You guys have posted a wealth of knowledge on our beloved aircraft and it has been instrumental in my decision to buy my first aircraft. A 1953 C170B.

I have noticed in flight that my engine produces 2400-2500 RPM at full throttle. When I level off at only 3000 MSL however, I notice that it takes full throttle to achieve 2500 in level flight. My prop is a DM 7653 and the engine has about 20 SMOH.

I have not tried a static run on it yet but it feels like its pulling like it should on takeoff. I see 115-120 mph indicated at 2450 so the engine seems to be producing the correct cruise power.

Am I just optimistic that there should be a little more power there? Or am I just paranoid?

Any input would be greatly appreciated. Thanks again guys.

[mit]

First step, check accuracy of tach.

[GAHorn]

Yes, check for tach accuracy first. Then check static RPM per the TCDS (2230-2330 for the DM prop) on a standard day.

Not achieving 2700 RPM during cruise is not uncommon. Remember that if you reach 2700 RPM in cruise that you've likely got a (climb) prop that is underpitched and needs more "bite" to achieve a cruise setting. If your prop truly is a DM 7653, then your RPM is not out of line.

[Andre_C]

Thanks for the tips guys.

I checked my logs and it does say its a DM7653 prop. Ill confirm that when I pull the spinner.
It just seems like i shouldnt be running out of manifold pressure until about 5000 ft or so. But I will check on the tach and prop soon and let you know what I find.

Thanks again,
Dre

[GAHorn]

Thanks for the tips guys.

I checked my logs and it does say its a DM7653 prop. Ill confirm that when I pull the spinner.
It just seems like i shouldnt be running out of manifold pressure until about 5000 ft or so. But I will check on the tach and prop soon and let you know what I find.

Thanks again,
Dre

Manifold Pressure??? You didn't mention what MP you're experiencing at 5,000 but on a typical day it should be around 24 inches (+or-) I doubt you're "running out" of MP.

[Andre_C]

Well not an exact number per se. As I climb in other aircraft, the throttle has to adjusted as altitude increases to maintain a set RPM. Usually the full throttle is required at about 5000 ft or so. I just want to make sure my engine is performing as it should. She is way too much fun to fly so I dont want to pull her down for maintenance.

[Bruce Fenstermacher]

First question is you have about 20 SMOH. Who broke in the engine? How did they do it? And the most important question is, is the engine broke in? 20 hours, or about, is awfully close to a fresh engine to be pulling the throttle back until your sure the engine is broke in.

As Tim suspects, I also suspect your tach is not accurate at the upper end if all you can get is 2500 rpm on the tack. I think and expect about 2600 rpm at 3000 msl or less.

But I also wouldn't be alarmed at this point as it sounds like your engine is performing fine. Check the tack and check the prop marking assuming it is accurate.