Low RPM's during run up/take off

[wldlndfirefghtr]

Question for the group:

O 300D, EM 76/53 prop, SL about 40° F, at gross weight.

Static run up 2175ish, high speed taxi's (was at full throttle), getting up around 2200, but not much over. Wasn't paying attention to the ASI.

Mech looking at it, compressions are OK not great. Lower 70's/upper 60's. Has sat for a bit. Mags were retimed, cleaned and gapped spark plugs.

RPM's Seem low.

Sound same to you guys? What am I not considering?

Thank you in advance

Jason

[DaveF]

Tach error?

[wldlndfirefghtr]

Nope, used a digital Tach and both were very close

[Bruce Fenstermacher]

That RPM is in the low end but I don't think low enough to indicate there is a problem. Book numbers are 2230 to 2350 static.

I'd look over the engine for obvious issues which it seems you have.

Do a proper lean static test with a digital tach. It's hard to see the difference of 30 RPM rolling down the runway.

How do you know your prop is exactly a 53 pitch?

[wldlndfirefghtr]

Bruce,

I do not know if it is exactly 53" prop, just going off the prop log book.

It has sat since March, and consensus is after going through the engine, is to run 'hard' per se, and oil change, and see where we sit.

Some mentioned the O300D being a "doggy" engine, 1st I heard of that.

Hopefully a getting oil running through it will help out a bunch

[wldlndfirefghtr]

Aryana,

Thank you for the Data point. I figured it should be around that setting as well.

Jason

[Bruce Fenstermacher]

Just a data point, I'm getting 2350 static with a digital tach at those same conditions at SL with a MDM 7653 prop and O-300-A

MDM is a different prop. Static numbers are 2250 t0 2350

[wldlndfirefghtr]

What I could find for the EM 76/53 was a max static RPM of 2360.

Consensus is to get up and fly and run the engine.

[Bruce Fenstermacher]

Look at the 172 TCDS 3A12

II. Model 172A, Model 172B (cont'd)
Propeller and 1. Propeller
Propeller Limits (a) McCauley 1C172/EM 7652, 53, or 55
Static rpm at maximum permissible throttle setting:
Not over 2350, not under 2230
No additional tolerance permitted
Diameter: not over 76 in., not under 74.5 in.

This is with the 0-300 D.

[cessna170bdriver]

Nope, used a digital Tach and both were very close

This is just a pet peeve from an old retired instrumentation engineer, but the tach being digital doesn't necessarily make it accurate; precise maybe, but not accurate. If it's optical, check it against a fluorescent light, which should read 3600 RPM if the tach is set for a two-blade prop.

[wldlndfirefghtr]

Nope, used a digital Tach and both were very close

This is just a pet peeve from an old retired instrumentation engineer, but the tach being digital doesn't necessarily make it accurate; precise maybe, but not accurate. If it's optical, check it against a fluorescent light, which should read 3600 RPM if the tach is set for a two-blade prop.

Good point, I will check again, good info.

[GAHorn]

"doggy engine"...???? Wha....??

It only holds the record for longevity! (Check it out!) It puts out the horsepower it claims at the RPM it's rated. It has a reputation for being durable, resistant to failure, and reliable in the extreme. It often gets the plane to an airport despite serious damage. (ask our Historian Steve about that!)

I'd fly behind a 145 hp Continental places I'd never consider some other brands/models. (Now to be fair...they're not known for lack of oil leaks.... but "doggy"...??? ... I don't THINK SO!)

[wldlndfirefghtr]

Gahorn,

That's good to hear, I heard good things about it being a smooth running engine. Like I said first I heard of the O300D being called 'doggy'.

I figured due to it sitting for a bit, is the most probable cause of the lack of HP, plus I need to double check the pitch of he prop. Being at gross weight probably didn't help much.

I don't have any experience in this aircraft yet, was just noticing the RPMs doing a full power run up and high speed taxi and comparing that to the 2400+ I get doing the same on my 150.

Really do appreciate all the feed back from everyone

[Bruce Fenstermacher]

You can not compare two completely different aircraft in this area. It is even hard to compare two aircraft of the same make and model.

[edbooth]

You can not compare two completely different aircraft in this area. It is even hard to compare two aircraft of the same make and model.

Yeah,.....what he said !