The International Cessna^® 170 Association

Me and my Father just rebuild the c-145-2h in our 170a and we are finding that it only gets 2100 RPM static. The numbers I once saw for the 170a was 2250-2350 static. (Now for the list) It has 6 new millemium cyl., new slick mags, 1a170 7653 prop., new plugs.

We have checked and changed the timing, and tried a few things, I think it has come down the the carb. In my OH manual it calls for marvel-schebler part# 10-2848 or 8-4439, but I have a 10-4439. Any help would be great.

Thanks
9156A 49
Jay

Have you checked your RPM with an optical tach to make sure your panel tach is accurate? I had the same problem with the prop not making min RPM and it turned out to be just the tach was off at that RPM.

My 170A manual calls for 2230-2330 static RPM

I have changed tachs and they read the same 2100 RPM

I was in somewhat the same situation about 2 hrs ago (also 6 new Milleniums). I think your carb is ok, as when I spoke with Precision, they told me that one of the flavors of O-300A carbs was a 104439, so I think you're ok there. Some things to check:

- you still need to use a digital tach. Borrow one. I had just installed a very nice Horizion electronic tach, so that wasn't an issue for me.

- what is your prop pitch? not what is it marked. I ended up pulling my old prop off and putting the one we just had o/h'd on, so then I really knew the pitch. rpm came up some.

- silly as it sounds, check your throttle arm. I had the carb o/h'd and they repositioned the throttle arm, so it hit the engine mount before it fully opened. Hopefully you have replaced the old ball/socket throttle linkage with the Cessna prescribed rod end type. If you havent, do so. But watch the clearance from the engine mount. And remember that the engine twists in the mount.

After these things, I just barely made rpm. After an hour of flight, it came up another 30 rpm or so. You really do need a digital tach, or to borrow one. Don't spend a lot of time on the ground screwing around running it before you get those new cylinders in the air. After all you just paid for those jugs, buy a digital portable tach, if you can't borrow one (but your mechanic probably has one you can borrow).

Good luck. You going to pull the oil out and cut open the filter after the first hour? I recommend it.

-Glenn

p.s. I agree with the 2230-2330 range.

I just did a few static rpm checks a few days ago. Both cold,and hot after an hour's flight. It reads just over 2100 rpm. Several years ago,I checked my tach at 2400 indicated & it was about 50 low.So assuming the same error now, I figure I'm actually getting 2160 or so. The book sex 2230-2330. I'm running a DM 7651 prop,so I ought to be getting near the upper end of that range--say 2300 or so. I'm pretty sure the throttle linkage is OK,I was pretty careful when I hooked it up. The last little bit of throttle doesn't seem to do anything to speak of for the static rpm. Neither does leaning (I'm at sea level). I can get 2700 indicated (estimated 2750 actual) in level flight. Takeoff's are OK,nothing too spectacular,but no worse now than they've ever been. I've never done a full-power run-up to check the static rpm before in the 5 years I've had this 170. I had the engine overhauled about 2 years ago,had the prop OH'd too so I assume the pitch is 51 like it's marked. Checked the timing,it's right on at 26/28.
I do plan to check the accuracy of my tach with a handheld optical unit,but I assume that the error is gonna be close to the same at 2100 & at 2700. Unless it's reading low at 2100 and high at 2700?
Any comments?

Eric

Bela,do you or Jeff have one of those optical handheld tach-checkers?


Eric

Another friend at Crest has one (John Tomlinson, he used to
own a C-170B, now owns a '54 180) but it's "out on loan" to
yet another friend who keeps his C-180 at Paine. I'm probably going
to see the latter fella on Wednesday, so I'll ask him if he's done
with it.

My tach was 10-20 or so rpm low (according to the digital/strobe
doo-hickey). As a frame of reference, my 75.75 inch 51 pitch
prop gets 2400 rpm static and bumps up to 2450 50 feet into
the takeoff roll.

I agree, that with 6 new cylinders, go fly it for awhile and check
it later. Don't do (relatively) high power ground runs trying to
determine static rpm with brand new jugs.... 5 or 10 seconds
is enough (lined up on the runway, brakes locked, tach strobe
in hand, shove the throttle forward wait for the tach strobe to
top out and note the RPM, let go of the brakes). Helps if you
have a buddy in the right seat (he can do the tach strobe
reading / note taking while you fly the airplane).

Also agree that just because the prop is stamped as a 51 pitch
(or whatever), doesn't mean that's what it really is. Upon putting
one of my props in the shop for a re-pitch, they noted that one blade
was pitched 1 degree more than the other.

Bela P. Havasreti
'54 C-170B N170BP

Eric, there may be several factors at work in your situation.
Firstly, an error at one rpm on your tach may not be linear throughout it's range. In fact, mechanical tachs are made to be most accurate within their calibrated cruising range. (The correct calibration rpm/model for a 170 is 2300 rpm on an AC RT-7)
Secondly, although at sea level, ...what was your density altitude?
Thirdly, were you pointing upwind, downwind, or zero wind?
Lastly, did you check your actual propeller diameter? It must be between 74.5 and 76 inches.
Did you digitally check accuracy at the observed static rpm?

In any case, the 2230-2330 limit referred to in the TCDS does not refer to propeller pitch. It applies to all standard pitches of the McCauley 1A170 model.

George:
1) Tach error: I assumed (wrongly,I guess) that the error would be constant or linear.
2) I didn't note the density altitude. It was less than 59 degrees,so DA shoulda been lower than true altitude (107').
3) I was pointed upwind for 2 of the checks,and kinda crosswind for the third (hot temp) check. Didn't seem to make any noticable difference. There wasn't much wind any of the three times I did the check.
4) As I recall from when it was OH'd 2 years ago,the prop diameter is 75.25" or 75.5",way above minimums.
5) Like I said,I plan to check the results with a digital/optical unit,but I haven't yet.
6) About a week ago,George,you posted on one of the forums that the rpm range/limits in question (2230-2330) applied only to the standard (7653) Mac prop. Now you say it's all of them. Which is it? I assume that it applies to all pitches,as no pitch information accompanies the rpm limitations. But a 51 pitch should produce close to the top of the range.
Bela: I know JT,I assume that he wouldn't have a problem with me borrowing that thing. I was down your way last Sunday,flew over Jeff's and it didn't look like anybody was around and the wind was pretty blustery so I didn't even land. It was out of the south when I arrived overhead,then quickly changed around to outa the west. At RNT it was variable from 150 to 230 gusting to 15 or 18. I noticed that the winds aloft changed quite a bit as I flew south and then again as I flew back north,so the old convergence zone thing was going on bigtime.
Maybe I'll try to get down there this weekend,if the weather isn't too terrible.

Eric

According to the McCauley info sheet in their overhaul/mx manual it applies to the 1A170 (DM) 7653 prop. In this msg thread, I point out that according to the TCDS, prop pitch is not mentioned. In theory the TCDS would prevail. In the Irish pub I frequent, it depends on whom I'm arguing with and how much the bet is worth.
The real question, in my mind, is whether the TCDS limit applies to the prop or the engine. In other words, ...is this a test of engine output? Or prop performance?
I lean toward the argument it is mostly an indicator of prop performance, since the TCDS contains the info in the prop section. If so, making a judgment about engine h.p. output based solely upon static rpm may be misleading. Certainly, at every take-off I make certain my engine is turning 2230-2330 as a minimum. It it's noticeably less than that I would assume there's a problem. (The lower density altitude you had during your tests should give a lower rpm result than usually experienced.)
But you may be assured that the standard engine is not producing 145 h.p. unless it is turning 2700 rpm.

I too have been dealing with the above mentioned low static RPM concerns. I believe HP falls off @~2.5%/1000ft. I operate at a DA of 5000' and usually greater. I too was seeing 2100 RPM with a 7653 McCauley prop. (I too verified tach, timming and comp.) I had it repitched to a 49 and the RPM went up to 2200, which agrees with the prop people claiming 25 RPM per in. of pitch. The TCDS numbers are for SDSL and I believe RPM falls off with altitude. If I ever leave MT and head for the coast I will report back with SL static RPM numbers. Sensenich told me there was a Piper SB or SL giving an equation on how to calculate what static RPM should be at any given altitude. Has anyone heard of this? I believe static RPM may vary with the cubed root of the air density ratio.

Pete

I fly a lot of formation and I'll tell you another check. Fly almost a trail position, just offset enough to keep you out of the wake. Now watch the other propellor through yours. The strobing effect allows you to adjust your RPM until you stop the other prop. Now the two of you can compare what your tachs read. You aren't gauging it by anything calibrated, but it is a comparison and it tends to be fun. Be careful, and play nice with others.

Kelly

If you have lower than normal rpm it could be several things. One of them is the lifters.If the engine is just overhauled it takes about an hour of flight to get the lifters pumped up to open the valves all the way. If it is an engine that has sat a lot it could be that the lifters are pumped all the way up and don't do the blead off to keep the valve lash adjusted correctly.You will not find the latter on compression test or any other way because it mostly happens when the engine is running. When you shut down they leak enough to give a good compression. You have to make sure there is nothing else. I don't understand all I know but I know what I speak of.This is something to look into.