The International Cessna^® 170 Association

I currently installed an EI fuel flow indicator and was wondering what other members fuel flow readings are at full rich on climb out? I think my carb may be a little lean. I have the mighty C-145 installed.

Fuel flow depends on the Density altitude and what RPMs your turning. 145 Hp is at 2700 RPMs at Sea Level. Anything different than that and the developed Hp is different and your Fuel Flow should reflect that change. Reductions in "back pressure" with altitude notwithstanding.

I see around 11 GPH (depending on the DA) in my '59 172 at 1000' from seal level and climbing out at 2400 RPMs. As a data point my static RPMs are in the 2250 RPMs range.

Rule of thumb for fuel flow and MAX power with engines in our compression ratio range is around .9-1.0 gallons per 10 Hp. For example: making all 145 Hp your target FF would be around 13.6-14.5 GPH. In my case, at 1000 feet DA and turning 2400 RPM's my 0300D is making "around" 124 Hp. 2400/2700 RPMs * 27.5'/28.5' MP * 145hp = @ 124.35 Hp . And extrapolating from that we get a target FF range from 11.1-12.4 GPH. So while mine is on the lower end of the ROT it is, nonetheless, still pretty close. But I have issues too and I'll explain later what I think is happening but, I digress.

Why do you think you have a lean condition?

Thanks for the response. My fuel flow at takeoff is below 11 GPH at a density alt. around 6000 ft. And a rpm of 2400. I talked to a tech at Marvel Schebler and he thought it should be around 13 GPH. My number two cylinder is running around 420-450 CHT in a climb. The other cylinders are at or below 400 CHT. The tech suggested that I could ream the main power jet .003-.004 to get more fuel to the cylinders. I can get cylinder two to run lean of peak at cruise and the cylinder temp drops to 380 or so with the others in the same range. I have concluded that the intakes on this engine are not the best. I know this is not new knowledge to many of you. I have checked the timing and replaced the baffling with new.

twlareau wrote:Thanks for the response. My fuel flow at takeoff is below 11 GPH at a density alt. around 6000 ft. And a rpm of 2400. I talked to a tech at Marvel Schebler and he thought it should be around 13 GPH. My number two cylinder is running around 420-450 CHT in a climb. The other cylinders are at or below 400 CHT. The tech suggested that I could ream the main power jet .003-.004 to get more fuel to the cylinders. I can get cylinder two to run lean of peak at cruise and the cylinder temp drops to 380 or so with the others in the same range. I have concluded that the intakes on this engine are not the best. I know this is not new knowledge to many of you. I have checked the timing and replaced the baffling with new.

I have found that reducing the throttle (MP) early in the climb is PROBABLY the best OVERALL option for most people and the 0300.

Here's why! Both 2&5 share the same shape, mirror image, intake runner of the other. They should be your leanest, and 1&6 your richest.The intakes seemed to be "TUNED" for cruise flight. Makes sense, you'll be there for the longest period of time, why not take advantage of it. So, pulling the throttle(MP), early you can "tune" the fuel distribution in the intakes between individual cylinders, and get to that sweet spot sooner, with better fuel distribution between one side of the engine and the other. Of course, that's at the expense of total power but, if your battling an overheating issue, climbing with impunity, at a somewhat reduced rate seems like a better option than waiting until your CHTs overheat and you r are forced to ultimately reduce the power anyway. IMO

Ive tried adding carb heat ( works better in the 182 vs the 172, don't know why), and while it works, it creates other issues, like fouling plugs prematurely and clogging the ring grooves and causing all kinds of secondary Mx issues from globby rich mixtures..

Going to one mag, the right one IIRC with the 0300, anyway the one firing the TOP plugs, killing the bottom plugs(works remarkably well). I'll concede, going to one mag is not for everyone, doesn't really bother me but, if it make you uncomfortable, I get it............ Switching to the left mag, the bottom plugs, has NO effect whatsoever, at least for me. Even retarding the timing on the bottom plugs, equalizing the staggered timing or retarding them further still doesn't totally negate the effect. It's still there, possibly to a lesser degree but, there seems to be something going on that I cant quite explain.Right on the cusp of detonation! Any, and and I mean any additional "stress", tip's the scales.

I guess maybe I should ask the obvious though, what airspeed are you climbing out at, Vx, Vy, Vcc?

I like the fact that you are not afraid to experiment with your engine. I'm of the same thought proces as you. I have also tried some of your experiments and found similar results. The number two cylinder is the leanest and hottest in climb and that makes sense to me with the placement of that cylinder and the length of the intake run. I have been able to find a sweet spot with my fuel flow indicator and engine monitor in cruise. The number two runs lean of peak wile the rest of the cylinders are running rich of peak. The results seem to even out cylinder temperatures more than running all cylinders rich of peak. The engine does run slightly rougher in this configuration with a fuel flow of 5.5 GPH at 2500 RPM, DA 7500. I have not been able to acomplish the Same in climb. The best i can do is lean for the initial takeoff and then go full rich in the climb to keep the number two cylinder at a reasnable CHT. I have tried pulling back the throttle in the climb and it does not seem to help. I have studied the carb manual and by pulling back the throttle you would be cutting of the enrichment circuit of the carb and hence a leaner mixture. My A & P and I are going to take some readings tomorrow and will probably take the carb off and drill out the power jet a few thousandths. You never know unless you try. I typically climb in the 80-85 mph range.

6000' DA, 22.5/28.5 * 24000/2700 = 101.74 Hp

FF should be between @ 9.15- 10.17 GPH

At first glance, your FF seems more than adequate.

Climb at a higher airspeed and/or check your baffles.

twlareau wrote:I like the fact that you are not afraid to experiment with your engine. I'm of the same thought proces as you. I have also tried some of your experiments and found similar results. The number two cylinder is the leanest and hottest in climb and that makes sense to me with the placement of that cylinder and the length of the intake run. I have been able to find a sweet spot with my fuel flow indicator and engine monitor in cruise. The number two runs lean of peak wile the rest of the cylinders are running rich of peak. The results seem to even out cylinder temperatures more than running all cylinders rich of peak. The engine does run slightly rougher in this configuration with a fuel flow of 5.5 GPH at 2500 RPM, DA 7500. I have not been able to acomplish the Same in climb. The best i can do is lean for the initial takeoff and then go full rich in the climb to keep the number two cylinder at a reasnable CHT. I have tried pulling back the throttle in the climb and it does not seem to help. I have studied the carb manual and by pulling back the throttle you would be cutting of the enrichment circuit of the carb and hence a leaner mixture. My A & P and I are going to take some readings tomorrow and will probably take the carb off and drill out the power jet a few thousandths. You never know unless you try. I typically climb in the 80-85 mph range.

Do you have a multi-point engine monitor? If so, which one, fast probes, slow probes?

I have the JPI Classic scanner and the EI fuel flow indicator.

10-11gph at full throttle is what I always see.

Thad

twlareau wrote:I have the JPI Classic scanner and the EI fuel flow indicator.

Good, I'm familar, that's what I have.

What is your EGT for number 2 (your leanest) at take-off? Are you familiar with the concept of leaning to a target EGT while in the climb? Makes leaning for take off at any altitude almost a "no brainer" for those with a mult-point engine monitor.

For those who aren't, here's a link that has a tutorial on the technique of leaning to a "Target EGT" along with some others.

http://www.advancedpilot.com/tech.html

Well...this is likely to enflame you guys, but I promise that is not my intention...
The carb runs slightly richer at full throttle by design, for a reason..... improved cooling. (Reducing throttle from full on takeoff or climb drastically changes the mixture along with the Manifold Pressure (MP). There is NO advantage to doing this over manually leaning unless one wishes to reduce power, especially if you are "battling an overheating issue." Rather than reducing fuel-flow, one should reduce angle-of-attack (increase forward speed).
The reason adding carb heat might help engine temps is due to the enrichment which occurs (at the reduction of performance and spark plug mx associated with that.)
The CHT on this engine, and the EGT on this engine...will be highest on Cyl. No. 2. This is explained in the operating manuals for this engine and installation. (It's a well-known matter and doesn't require extensive study or instrumentation.)
Operating on ONE MAG is the most incorrect thing one might do. It dramatically increases EGT (proven by looking at an EGT gauge, if you have one) because it mimics a 'retardation' of spark.....more fuel is burned OUTSIDE the cylinder, in the exhaust where it does you no good but harms your exhaust valve and exhaust system, than when operating on both mags.
The intake manifolds on this engine is not "tuned for cruise". It is designed for simplicity and reliability, and within reason, is designed for maximum power at full throttle. Not reduced throttle (which is obtained by shutting a flapper valve/throttle plate in the throat of the carburetor thereby creating a restriction to air-flow, entirely prior to and outside the intake manifolds.
If you want to know what is normal fuel flow for your airplane, the worker at the carb mfr (who was likely born long after this carb went out of production) is not likely the most knowledgeable source. Why? Because he does not likely know a thing about INSTALLATION configurations of various aircraft. The intakes of your engine is influenced by each, individual, installation design. This engine was intended for several different installations, all with differing cowl designs and other subtle differences which influence engine parameters. That is a major cause of variations among output power ratings of the same engine in different aircraft models. (Aside from prop differences. The spinner alone, can make large influences. You guys have the same spinner?) It is an area of considerable engineering attention during the design phase of aircraft.
While this experimentation may be fascinating to you, it is largely erroneous, not only because you have no "control" aircraft, but also because you have such a small sampling, uncalibrated test instrumentation (what you two have is production instrumentation...not flight test instrumentation) and you also do not have a controlled test environment. The factory had all those things and made their data available in the Operator's Manual for the engine (which makes the precise same observations I'm regurgitating about installation errors), where one will find the correct fuel flow versus developed-power versus propeller loading charts which they published.
The "ROT" offered is interesting, but not an actual "rule". It's actually a "WAG".
(Sorry if this seems to throw water on your dogs....but, in the interest of offering good input, I believe I'm offering a more realistic view of this discussion topic. The TCM Operators Manual for this engine can be downloaded from the Members Only areas.)

George, I'm glad you chimed in and gave your opinion on this matter. I'm not sure why you would think that it would inflame anyone. I like to hear many opinions on subjects and then make the most informed decision with the information presented.
I think we all relize that we are not doing any of this in a test cell under controlled conditions. These are just our findings from our aircraft under non controlled conditions. Do they have any meaning to a anybody else? That's for the individual to decide.
I do like to experiment with my aircraft within the limits of FAA guidelines and my IA's guidence. I think most of us try to make our individual airplanes the best we can. I do know on my airplane that I can typically get the fuel flow down to 5.7-6.1 at a density altitude of 7200 RPM 2580 with a spread of 52 deg. Between the coldest and the hottest cylinder. (Coldest 322 hottest 374) Does this mean anything or does anyone care about what I found on my airplane? The excersie is not to say this works for everybody all the time but to start a dialoge on what different aircraft are experiencing in different configurations.
You are absolutely correct that it's a WAG at best and I wouldn't even consider it a rule of thumb because it may not apply to another aircraft. I also recall telling many students for many years to run their engines 50 deg rich of peak because
That's what the POH said to do. Many very respected folks in the aviation community have come to the conclusion that's the worst place to run an engine. Do we have room to improve our antiquated engines? That's for the individual to decide.

gahorn wrote:Well...this is likely to enflame you guys, but I promise that is not my intention...
The carb runs slightly richer at full throttle by design, for a reason..... improved cooling. (Reducing throttle from full on takeoff or climb drastically changes the mixture along with the Manifold Pressure (MP). There is NO advantage to doing this over manually leaning unless one wishes to reduce power, especially if you are "battling an overheating issue." Rather than reducing fuel-flow, one should reduce angle-of-attack (increase forward speed).

There are so many errors, half truths and old wives tales in what you posted, this is going to be like, " How do you eat an Elephant"? "One bite at a time"!

Of course the carbs runs richer at full throttle by design, who said it didn't?

George your missing the point. It's not a problem of the fuel flow (Volume) as much as it's one of fuel distribution. 2 &B 5 are too lean, 1 & 6 are too rich and 3&4 are just right. There's enough fuel flow there, it's just not going to all the cylinders in sufficient quantities. And Running wide open, straight and level , with maximum cooling airflow, won't solve the issue. SO much for "by design".

And furthermore, reducing the throttle(Manifold Pressure) has several benefits that help control the CHT's. Reducing the MP lower the internal cylinder pressures, that CAUSE HIGH CHT's. And by moving throttle plate, with the benefit of a multipoint engine monitor, one can see instantly, that you CAN improve the fuel distribution dramatically between ALL of the cylinders and more easily control your CHT's in the climb.

Don't believe me, believe the data. And why reducing MP can help in "battling an overheating issue."

http://www.savvyanalysis.com/articles/c ... tion-event

The reason adding carb heat might help engine temps is due to the enrichment which occurs (at the reduction of performance and spark plug mx associated with that.)

Well, of course but, thanks for re-stating the obvious. However there's more to it than that. Adding full carb heat in my 182 increases my fuel flow by over a gallon, plain as day right there on the digital fuel flow. Clearly there's more going on there then simply enrichening the mixture with the introduction of hotter, less dense air.

The CHT on this engine, and the EGT on this engine...will be highest on Cyl. No. 2. This is explained in the operating manuals for this engine and installation. (It's a well-known matter and doesn't require extensive study or instrumentation.)

That is sImply NOT true. Number 5 on my 0300 is has/had the highest EGT and CHT's. ( It originally "Had" the highest CHT's with the stock/factory baffles. I've since modified them for better cooling for #5) And I have hundred of hours of JPI data to prove it. Another "well-known matter" that apparently needs some more "extensive study".

gahorn wrote,
Operating on ONE MAG is the most incorrect thing one might do. It dramatically increases EGT (proven by looking at an EGT gauge, if you have one) because it mimics a 'retardation' of spark.....more fuel is burned OUTSIDE the cylinder, in the exhaust where it does you no good but harms your exhaust valve and exhaust system, than when operating on both mags.

Wow, that is so profoundly wrong and inaccurate, its stunning. Let's break it down.

"Operating on ONE MAG is the most incorrect thing one might do."

To the contrary, its actually one of the best things you can do if you suspect a detonation or pre-ignition event. Along with reducing the throttle (MP) and increasing the RPM's if you have a CS prop. Taking all these actions are helpful for the same reason, retarding the effective timing, reducing the MP, Increasing the RPM's, they ALL reduce internal cylinder pressure! And why going to one mag is so effective in controlling skyrocketing and problematic CHT's.

"It dramatically increases EGT (proven by looking at an EGT gauge, if you have one) because it mimics a 'retardation' of spark."

Well, yeah, of course it does buts that's precisely why you go to one mag! To retard the "effective" timing, moving the pressure pulse further away from TDC, reducing the Internal Cylinder Pressures, which are directly related to and control CHT's.

High EGT's are all but meaningless, it's the CHT's that are so important and what you should be trying to control. When you retard the timing, EGT's go up, CHT's go down.

Advancing the timing, of course, does just the opposite. And if total EGT's would matter, then why do the CHT'S go DOWN, when you retard the timing and EGT's go UP? Its simple, because its the Internal Cylinder Pressure that matters, that control CHT's, not EGT's.

.......more fuel is burned OUTSIDE the cylinder, in the exhaust where it does you no good but harms your exhaust valve and exhaust system, than when operating on both mags.

This statement shows such a fundamental lack of understanding of the combustion event, it's timing and how 4 stroke engines work, I'm almost speechless. At the RPM's we normally run our engines, the combustion event is finished burning around 30 degrees ATC, and certainly well before 40 degrees. 'Stick a fork in it, its done'. That's not my opinion, it's an accepted, known, scientific fact. Firing one or two spark plugs has very little effect on the overall length of the combustion event(for several reasons, which are beyond the scope of this exchange but for some clarity, after the first spark fires things are slow to get started, and it takes roughly about 10% of the total time of the combustion event to "get going". From there it's all chemical reaction and that timing is controlled by the fuel/air ratio. And its this "slow starting" why 1 or 2 sparks has a marginal effect on the total time of the combustion event.) However, it does change the location of the peak pressure pulse, away from TDC, profoundly reducing the ICP's. Again, that's not my opinion, the data, and known science confirms it. For further reference, you can find confirmation with the graph (1-12) and more results of the research done by C.F. Taylor in his 2nd volume of "The Internal Combustion Engine in Theory and Practice". Chapter 1 to be more specific.

So, according to Taylor's research the timing of fuel burn with 1 spark plug firing is complete around 30 degrees ATC, and certainly by 40 degrees ATC it's only a distant memory. That would mean the crank has to continue to rotate another 140 degrees! Or more than twice as much further than the distance/time as the original combustion event, before the piston reaches the bottom, then begin to change direction and start the exhaust stroke. And it is then, and only then, that the exhaust valve begin to open. Clearly, experiencing "after burning" on one mag doesn't pass the sanity check and is just an "old wives tale". And not even a remotely logical one at that. "After burning" and all it associated problems is only possible if there is NO spark at all. Allowing raw fuel from the unspent charge to enter the exhaust. It is not only improbable but all but impossible to experience this phenomenon from running on one mag.

The intake manifolds on this engine is not "tuned for cruise". It is designed for simplicity and reliability, and within reason, is designed for maximum power at full throttle. Not reduced throttle (which is obtained by shutting a flapper valve/throttle plate in the throat of the carburetor thereby creating a restriction to air-flow, entirely prior to and outside the intake manifolds.

"it is designed for simplicity and reliability, and within reason, is designed for maximum power at full throttle."

Well, of course it was but, you know they're not mutually exclusive with being designed and "tuned" for cruise flight either. Is that your personal opinion or do you have an actual reference for that claim?

Having said that, anyone with a multi-point engine monitor can plainly see, again and again, that you can achieve better, more balanced fuel distribution, to ALL of the cylinders by "cocking the throttle plate" ( yes,reducing MP), and better control the CHT's, rather than using WOT and relying on simply shoving the nose over and hope.

George, do you have a multi-point engine monitor in your airplane?