The International Cessna 170 Association

My engine monitor shows the best fuel distribution at 7500 feet wide open throttle at 2400 rpm. Burn was around 7.5 when at lower altitudes its usually around 9

c170b53 wrote:My engine monitor shows the best fuel distribution at 7500 feet wide open throttle at 2400 rpm.................

More evidence our intakes are "tuned for cruise flight"!

I'm kidding of course.

Yep, Ive experienced the same thing, sometimes. Others times, cocking the throttle helps, some times a lot. Jim I get your point, at least I think I do. I could have been more precise in that last paragraph. I'm reluctant to edit the original post to reflect that change now that you've posted.

How about, ........."Having said that, anyone with a multi-point engine monitor can plainly see, again and again, that under certain conditions, sometimes you can achieve better, more balanced fuel distribution................

Results, I had hoped that enlarging the power jet .003 would have helped my number two cylinder cool a bit in the climb but to no avail. It seems that it did not make a difference at all on the temperature in the climb. It cools nicely in cruise but in clim it's still over 400CHT. I haven't given up and will post my results as I find answers.

Sorry to hear that.

Did the EGT on #2 change at all?

And how many degrees ROP is it?

Nothing changed much but I did have to lean on takeoff to get rated power out of the engine. I was able to takeoff at 7000 DA with full rich and get 2400-2500 RPM, that didn't seem quite right to me. I tried that at full rich with the power jet enlarged and was only able to get 2200 RPM's, that seems more normal to me. The fuel flow went up about .8 GPH. Once we were in the air and at a safe altitude I went full rich again and it made no difference on the CHT on the number two. Like your findings, once I pulled the throttle back a bit it did make a difference in the CHT's. in cruise the number two drops to 380 very quickly.

Thad

That more or less mirrors my results at sea level too. Enlarging the jet increased total FF but, even distribution was still lacking. Clearly there is a "sweet spot", where the fuel distribution is better balanced and all of the CHT's are under control, below 400. And for whatever reason, be it just the right intake temperature, just the right intake pressure, just the right swirling/mixing of the fuel and air from cocking the the throttle plate, or some combination of any or all of them, backing off WOT can help.

WSHIII wrote:[...George your missing the point. It's not a problem of the fuel flow (Volume) as much as it's one of fuel distribution. ...

gahorn wrote: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".

I never disagreed that induction system distribution is crude in these engines. I don't know why you assumed otherwise.
In fact, it's SO crude, that it can be reasonably argued that sophisticated instrumentation such as is being discussed in this thread not only proves it deficient....the same instrumentation also demonstrates it's own limitations to cure the problem. The simplicity of these engines and their wide operating limitiations is quite adequately served by simple instrumentaion....which causes some of us to think multi-probe instrumentation to be tits-on-a-boar, (which should answer at least one of your questions. Have you ever heard of measuring something with a micrometer, marking it with a grease-pencil, and cutting it with a hatchet?)

Perhaps it's only my perception, but you seem to be more interested is appearing to be expert at playing "stump the chump" than to engage in enlightening and cordial discussion.
Firstly, the trend in these engines for no. 2 to run leaner/hotter is a mfr'r documented matter and is so stated by the mfr'r. I place a bit more credibility in their knowlege and their publications of this specific installation than the references you posted about large bore engine designs and cams and valve-timing that haven't been identified OR related to this design/installation of O-300's. (I thought it obvious the comment I made regarding fuel burning "outside" the cylinder was figurative for illustration purposes, but apparently that was too cavalier and perhaps you wanted to impress us with "statistics". In any case, as a former production-assembler I have a fairly clear understanding about how 4-stroke engines work and I'm not baffled by BS or references that don't apply specifically to the discussion at hand. My intent was to discourage anything approaching long-term operation on one magneto for equally important reasons.)

Also, I'm hopeful you'll agree, your individual experience with your no. 5 cylinder hardly makes a "standard" for comparison for the rest of the fleet. While your NO. 5 may indicate higher with your instrumentation in your installation, that doesn't create a norm for the rest of the thousands of these things in the field. Your No. 5 cylinder may suffer from several maladys which cause it to differ in symptoms from the rest of the fleet.....that hardly makes it a "standard" by which to judge the rest.

Next....

WSHIII wrote:[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"! ...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.....".

Lowering his MP is not a solution to his problem when he's already stated he's operating at DAs of 7500' . His problem (if indeed he has one) is reduced cooling due to high DA/low cooling airflow, and a contributing factor may be too lean mixture (one so lean as to approach peak pressures/temps instead of one sufficiently rich to provide fuel-cooling.)

You direct us to a Busch article, writing: "Don't believe me, believe the data. And why reducing MP can help in "battling an overheating issue." ...an article which has NO discussion about overheating relative to CHTs...but discusses high-output larger engines in Bonanzas and Barons (all of which are fuel-injected and quite different animals than our engines) and how MP vs RPM affect peak pressures and EGTs. You lead us to believe that article proves it has a profound effect on the CHT "problem" in this discussion, yet that article does not even mention CHTs much less support the argument you claim it does as regards our O-300's. It was misleading on your part. Large bore, fuel injected (and turboed) engines in Bonanzas/Barons have different operating procedures. It's as different at tri-cycle versus taildragger ops.

I take offense at the manner in which you attempt to change the meaning and intent of my prevous post. I'd ask you to refrain from it and I'll do likewise. Example:

WSHIII wrote:

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....

I wasn't offering any solution to such a "detonation/pre-ignition suspicion. Did you simply make that up? At the time, I didn't recall any suspicion or symptom of detonation or pre-ignition. I was only commenting on the increase in EGT experienced when switching one mag off...in a suggestion that it would not offer any useful help to a CHT indication which, in any case, is incorrectly being perceived as "high". (See the limitations of this engine for the permissable CHT indications. The 400-450 range being discussed is no where near violating the actual 550 limitation. Larger engines might benefit from it, but they also usually have cowl flaps to help control that ...not the crude induction systems and no cowl flaps we have.) The suggestion that magnetos which are properly timed to the mfr's specifications are somehow "wrong" and advising someone to switch one of them "off" as a method to improve otherwise normal engine operations is ridiculous and patently incorrect advice and I believe, the mfr's operating manuals support that view. I'm sorry if you were "stunned" by it. I challenge you to find an engine mfr's document which recommends that as a regular operational procedure or technique to further reduce CHT's from normal by use of single-mag operations and produce it for us to read for ourselves.

I'm very interested in how you went about getting your modified engine baffles approved. There are many people who would like to modify their cooling baffles using a cut-and-fly-it method, but the certification folks seem to think it's more complicated than that, and perhaps some of those owners might then be "stunned" when they find a condition-tag on their airplanes.

I am cautiously reponding to this thread as it seems to be a hot topic - no pun intended. I am experiencing this #2 hot cylinder issue after a recent carb exchange. Never had any issue of CHT alarms on climb out until installing an freshly overhauled exchange. I have just completed annual and took the time to make sure all intake/exhaust gaskets have been changed to give me the best chance at sealed systems. I have done a simple ground run and everything appears to be good on the seal part - I will now fly it and see if the results are different. I believe that my new carb is too lean across the board - if I continue to run hot on #2 - I have enough confidence in my baffling - that it will lead me to pulling the carb and sending it out to have the next size jet installed. With the old carb I was leaning all the time - so going back to that and curing the hot cylinder would not bother me in the least. Also I read a very interesting article on AVWEB - The Savvy Aviator #59: EGT, CHT and Leaning - caution is thrown into the wind with the spark plug gasket thermal couple versus the threaded boss probe on cylinders - the spark plug gasket tends to read 40 deg hotter than the normal threaded probe on the same cylinder. Makes you wonder what you should truly believe.
Bottom line is I have read all of the thread - does anyone have a solution other than my increase in jet size?

You say you have a "exchange" carb. Do you mean by that, you have a "different" carb which has been recently rebuilt?
Or is it one that was rebuilt a long time ago, sat on the shelf, and now sold to you?

The reason I ask is, several years ago an AD came out mandating the original two piece venturi be changed to a one piece. (The AD was subsequently revised to alow the two piece to remain, but must be inspected for looseness every 100 hrs.) The "new" one piece ran richer than the original two piece types. A subsequent SB allowed a change to a "shotgun" type jet which had multiple ports, in order to address the excessively rich one-piece venturi.

I wonder if you have an old two-piece venturi in your "exchange" carb.... or if you have the correct PN carb for your engine. (The C125 used a similar carb which was jetted differently....how differently I cannot say, as it was revealed to me in a conversation with a carb rebuilder no longer with us, and I don't have the tech manual available.)

IN any case, the No 2 cylinder will be the hottest cylinder in the standard installation, and that is to be expected.
What, specifically, is the CHT observed? and by what type instrument/probe? (And the spark plug gasket type may indeed read higher than the bayonet probe, especially if installed per Cessna instructions, because 1) it is to be placed on the lower spark plug where it receives less cooling air and, 2) the heat-range of the plug can affect the reading since the thermocouple doubles duty as the transfer-gasket to cool the plug by passing heat to the cyl head.)

I exchanged my carb for an overhauled carb - reasoning behind this was my old carb was so worn with regards to the mixture shaft. I felt there was too much play - and was not sure even if they could re-bush it. I was told by the tech at the carb shop that "my old carb" had been preceded by two newer part numbers. I do not believe this carb was sitting on a shelf - but I have no way of knowing. Here is what I do know - never had this issue before with the old carb. I had rich looking plugs with the old - now I have plugs that look light brown with a hint of white. I never had climb out issues as far as temps are concerned with the old carb - now I have a CHT Alarm going off on my JPI analyzer - I have seen as high as 452 deg on number 2 during a climb. Once leveled off all cylinders play nice running around the mid to low 300s - exception 5&6 are high 200s. Also - i should mention - I have an A model with the inner baffling around the cylinders - not that it makes a difference to the issues at hand. Spark plug gasket probes installed on lower plugs per JPI and Cessna instructions........that is me.

Well, the CHT you're experiencing is 100 degrees BELOW the maximum for this engine, so I personally would not worry about it, and enjoy the clean plugs and low fuel burn.

I have read all the posts again and again on this issue and weighed all the pros and cons. Here is what I have done to band-aid the situation. If you read the last few posts on the "Fuel Flow" subject you will see that I never experienced this problem with a Hot #2 CHT until I exchanged carburetors. The following are my troubleshooting steps prior to calling the carb the culprit- first off I removed all cylinders and inspected the valves for hot spots or evidence of a true overheating - found nothing. While cylinders were off cleaned and inpected intake and exhaust ports/replaced all gaskets/seals. I wanted to make sure my problem was not a leaking gasket/seal issue. I then flew the airplane again - performing the same climb out of 750 FPM at full power - my fuel flow was reading 11.2 GPH - seemed a little lean to me. As usual once leveled out - #2 cylinder cooled to a reasonable temp. Time for a decision - came home - dropped the cowl and removed the carb. I sent the carb to Marvel Schebler with the instruction to richen the carb. After they had recieved the carburetor I recieved a phone call from a tech - we went over everything I have done and had been seeing in flight. We agreed that the carb should be richened - they completed the work and performed a flow check - setting the carb to between 12-12.5 GPH. I have re-installed the carb and found some improvement - my #2 cylinder now runs around 400 deg instead of the 450-460 that i had seen earlier during climb out. I can't say I am completely happy but - it is better than it was. I do believe it is a fuel distribution problem more than anything - let's face it #2 is the longest intake run. And though I love my JPI analyzer - sometimes more information can make you paranoid. So - this was my solution..........

It is really a shame that you guys are too young to have been flight engineers on the B-29s back in about 1944 or 1945. You could have had a field day every tine you flew on one of the bastards trying to figure what in hell the engine temperatures were doing.

My mechanic says those EGT/CHT probes are a mechanics dream. They usually result in all kinds of unnecessary work. I know from personal experience.

Much A-do (and $) about nothing....IMO