Prop Question

[pif_sonic]

I know this has probably been answered several hundred times on this site. I have a 76/51 prop, a friend of mine has a 76/53 prop. Is there really a big difference in the two? Is there a 76/52 prop and would that be a nice in between of the two. I know a 76/51 is a climb prop and a 76/53 was the standard for most 170’s. What do you guys have on your plane and if you were going to replace the prop what would you get?

[ghostflyer]

PROP WASH
I had the same problem as you, after extracting a little more power from the O-300 and wanting to put it to good use. The 53 inch prop was repitched another inch and it completed the job as expected. I was also given a seaplane prop and tried that one also. What a rocket ship ,climb but shocking cruise revs. Fuel consumption was up also. So after swapping props around , I came to the conclusion " What is my mission " when flying. Landing/takeoff on beaches and other non paved areas or long distance cruising. The 54in prop came in first but I still love landing on beaches. Its a great way to go fishing.

[bsdunek]

My 170 has always had a 76-51, as Dad flew out of our farm strip. I like the way the plane takes off and climbs out, and don't really miss the few miles/hour. I fly because I like to fly, not just to go fast.
If they were 100% efficient, at 2450, the 51in. prop would give you 118mph, the 53in. 123, or 5mph difference. Not much if your typical leg is one - two hours.

[4stripes]

I have the 76/51 and find the performance just right. Decent T/O performance and cruise speeds from 90 -110 Kts (depending on how much fuel you wish to burn). The climb and cruise RPM is also reasonable. I wouldn't change a thing (especially if your prop is servicable). Take the money you save and go flying!
Cheers Eric

[GAHorn]

My 7655 is the reason I'm gray. I sure get tired of seeing the 20' power lines barely slide under my wheels after 3500 feet of struggling to get aloft at gross wt. The climb to altitude during summer trips to convention are also less-than-stellar. The throttle is wide open in cruise at anything above 5K to get 2450-2500, although the speed is good and the fuel burn good at 120/7.8 I'd sure prefer some better takeoff/climb. I'm always on the lookout for a deal on a EM7653 (6-bolt).

[blueldr]

ghostflyer,

What did you do to "extract a little more power" from your O-300? C-85 pistons, maybe?

[WWhunter]

I'd be interested in what you did also to extract some extra power from an O-300!

On a side note...Why are the props One inch different in pitch for the 172 vs 170 with the same engine. I noticed it when I was checking my prop. The ones for an O-300 pwered 172 seem to be even number for pitch vs odd number for the 170. An EM7652 is climb prop for the O-300 powered 172's vs an EM7651 for the 170's Anyone know why this is? Just curious.

WW

[Robert Eilers]

I have the 7653 and have been happy with it - for the most part. I have considered having repitched to 7654 - just have'nt gotten around to it. Anyway lately I am pulling the throttle back to 2300 or 2200 in an effort to reduce fuel burn. Interesting comparison - at 2450 KLVK to O69 total time roundtrip is about 60 minutes + or - at 3500 MSL, fuel burn is ordinarily 7.9 gph. My last trip I used 2300 at 3500 LVK to O69 and 2200 (due to tail wind) at 5500 O69 to LVK. The total time roundtrip was within a minute or two of what it has been at 2450, however my fuel consumption was just over half of what it has been, i.e., 5.3 gals vs. 7.9 gals. Fuel out here is over $5.00 a gallon or damn near at most airports.

[4stripes]

My 7655 is the reason I'm gray. I sure get tired of seeing the 20' power lines barely slide under my wheels after 3500 feet of struggling to get aloft at gross wt. The climb to altitude during summer trips to convention are also less-than-stellar. The throttle is wide open in cruise at anything above 5K to get 2450-2500, although the speed is good and the fuel burn good at 120/7.8 I'd sure prefer some better takeoff/climb. I'm always on the lookout for a deal on a EM7653 (6-bolt).

Why not just get it re-pitched?
Cheers Eric

[GAHorn]

On a side note...Why are the props One inch different in pitch for the 172 vs 170 with the same engine. I noticed it when I was checking my prop. The ones for an O-300 pwered 172 seem to be even number for pitch vs odd number for the 170. An EM7652 is climb prop for the O-300 powered 172's vs an EM7651 for the 170's Anyone know why this is? Just curious.

WW

The 172 airplanes you're thinking about most likely used a 6-bolt EM series of prop rather than the 170's original 8 bolt DM or MDM series.
The EM series used a thinner blade which improved efficiency, so they could be pitched slightly coarser for slightly better fuel economy. That's likely my problem... it's probably off a 172 and it's pitched too much for my tastes.

...Why not just get it re-pitched? Cheers Eric

I don't think it'd pass inspection. Any shop that was tasked with repitching it would also have to recertify it and I suspect it's too thin to pass. I can keep flying it as long as I don't have it overhauled/repaired/repitched/inspected per McCauley's manual.

[ghostflyer]

It was after I had pushed the send key I realised I was opening a Pandora's Box regarding the extra power out of a O-300. I am a Aircraft Engineer by profession and have been doing this sort of thing [passion] all my life. We found this 170A in bits in a hanger and we decided to rebuild it. It had a great history. The engine hadnt been started in nearly 20 years but was full of oil. The bores of the cylinders were badly corroded ,the engine needed TLC. The cylinders were bored 15 thou oversize, new pistons and rings fitted. TCM came to the party with some "lovely" pistons. We balanced each piston within 2 grams of each other[what a job] ,then each piston was paired with another piston of very close weight. New valveguides were fitted and the valves were triple cut on the seats,plus different rocker shalfs were fitted.The pots were then flow checked and ported and polished. The carby was a rubbish bin job but a friend at Precision took pity on me and swaped it for a new one [plus some cash]. I pulled the new carby apart and polished the throat and venturie . The carby was rejetted, I think it was the next size up in the main jet. The mags were over hauled[Bendix] everything was replaced. [Should have purchased new ones,cheaper] . The sump was removed and cleaned. The intake tubes were measured and matched to the intakes of the cylinder heads [proper aligment was important] Push rod tubes were modified so not to leak oil. The engine MUST run on 100ll or higher. Timing was set up to left 27 deg BTDC and right 26BTDC. The exhausts were thrown away [pancake style] and the Cessna 172 type fitted {O-300] [allowed under a E.O.] The exhaust mufflers were modified internally [susssh,I didnt say anything about this ] .The lower cowl had to have bulges made to fit the new exhaust .
The engine was dyno- ed and had a Knock sensor fitted with EGT and cylinder head temps amd a spin on oil filter with extra cooling to it .The prop and engine was then balanced together. The air box was modified so the air flow was smooth and a paper filter was fitted. The engine produced just under 160 hp. at 2750 rpm. That motor was so smooth at any revs.

[Robert Eilers]

ghostflyer,

What is a Knock sensor?

[blueldr]

ghostflyer,
You state that you're an "Aircraft Engineer". Are you in Canada where that term equates to our 'A & P" mechanic here in the states?

[GAHorn]

Please do not take my post to be critical of having fun tinkering with our airplanes. I'll bet the assembly of that engine was a lot of enjoyment. And I'm glad to see folks not feel like they have to be secretive about what they've done to enjoy our pastimes. (I don't want to support the idea that undocumented and unapproved modifications are acceptable, but like most of us, I also don't like the idea of the feds telling me what I can't do with my own property!

More inline with what I perceive as "my job" however, I feel compelled to at least point out that most of the tinkering described so far has only entertainment value, not real results which just anybody should spend any time and money on. Polishing/porting is not illegal, per se. (Although TCM and Lyc both, correctly in my opinion, poo-poo it in these type applications.) Just document the "clean and inspect" excersize.
The only thing more illegal than unapproved mods is undocumented work, in the eyes of the authorities.)

A "knock sensor" is a sophisticated microphone, usually mounted on the engine block or crankcase, which listens to internal engine noises to detect knock (usually caused by pre-ignition but sometimes caused by detonation of inferior fuel.) They are commonly used on modern automobiles which have computerized spark-advance and electronic throttle. The computer uses the data from the knock-sensor to finely-adjust spark advance/retard to avoid knock, and sometimes is used to (downshift) transmissions. Older designs used a "TSP"... a Transmission Controlled Spark advance system, which pre-set engine spark advance based upon which gear the transmission was in versus the speed of the vehicle. (Unless the engine being discussed has electronic, computer controlled magnetos then a knock sensor is a non-participant in the game, although if it has some form of cockpit data presentation the mixture might be tweaked a bit. I lack confidence in this engine's induction system and carburetor to believe that it'd be anything but another case of the micrometer/grease-pencil/hatchet job, tho'.)

Just for a reality check, any of these standard engines turning 2750 will produce "just under 160" hp. (See the TCM power charts for the engine on pg. 13 of the operators manual and project the prop-load graph up to 2750.) A lot of fun has probably been had by all that work and expense, but unless the prop is pitched to allow the over-speed rpm, then the hp will not be developed. The question I have is: What static rpm does the set-up achieve under standard conditions?

From a decidedly critical standpoint, the timing is set incorrectly (the left should be 28 degrees*) and the piston balancing act is nothing unusual, it's within the standard allowance that most reputable shops strive for by merely reading the ink-stamp beneath the pistons when making the selection, or alternatively, by weighing them. So the next question is: what pistons were used? Something other than C145/O300 pistons? (such as BluElder queried?)

* Is it possible that higher-than-std-compression pistons requires the retarded spark?

[ghostflyer]

It seems one of our colleagues is a traditionalist, which is great as we must preserve the heritage of the Cessna 170 for the future. I really respect him for this . But I also get worried about power lines and trees at the end of the runway and thus I have a need to modify to suit my needs of operation. Nothing was done illegal in the extensive modifications ,however the exhaust was a grey area and our rules didnt cover this so we raised a E.O.[engineering order] to cover ourselves.
In Australia ,the rules are a little different to the FAA rules. The pistons that were fitted raised the compression ratio to the point that I had concerns of pre-ignition and hot spots in the combustion chamber. TCM tried to array my fears by bombarding us with all sorts of data. At this point of time we were using TCM as consultants on another project [work] in building up a TSIO-520 engine The standard of casting of the heads need inprovement and the exhaust system is shocking. We did our best to clean up the restrictions both in and out. At the same time we tried different props as static rpm was a concern. "Power Flow"a company in the USA has some great ideas on exhaust systems, but no products for the Cessna170. Like I said originally a Pandora Box.