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brad walton

Well Known Member
I am interested in LOP and have been experimenting with it. At present I have about .6 gph GAMI spread and working to get it closer. But it occurs to me that the drawback of LOP is uneven power production between cylinders. ROP you are operating with excess fuel. My cylinders were flow matched to within about 2 or 3 percent, so depending on other air flow conditions being close, all cylinders are producing similar HP. The same may not be true LOP because there is in my case .6 gph difference in when cylinders hit peak cht. So LOP if all cylinders are flowing similar air rates but dissimilar fuel rates, the cylinders may be producing many hp different one from another. Where am I going wrong? Will this put inordinate strain on the engine, maybe the crankshaft for instance?
 
what were the flow comparisons of your exhaust pipes? how about the flow balance of your intake system? that's what you are missing.
 
Lop hp delta

.6 gph difference in fuel flow from one cylinder to another could be a difference of : .6 gph x 6 lbs per gallon divided by .4 gallons per hp per hour equals 9 hp difference between the first cylinder to peak egt and the last to reach peak egt. Am I missing something obvious? Or is this level of power difference between cylinders acceptable?
 
Flow comparisons

Brian, i am not sure what flow differences may exists in my exhaust system or intake system. I expect there is some. But the cylinders were flow matched, ported and polished to within less than 3 percent. So i guess i can expect there will always be some difference in power produced by each cylinder. Is 9 hp insignificant between cylinders rated at 45 hp. That is 20 percent! Probably more than exhaust or intake flow rate differences.
 
Since I am not familiar with the methods used to flow balance an aviation cylinder, I am curious if it includes the filter tract, intake manifold and tubes.

If not, how would you assume that their influence is zero and the cylinders are getting the same mass airflow?

I would make the opposite case that the injection system, spider and nozzles are likely to flow the same and the Gami spread results from airflow imbalance and the nozzle adjustment is a means to correct for airflow imbalance.
 
rzbill said:
the Gami spread results from airflow imbalance and the nozzle adjustment is a means to correct for airflow imbalance.
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Correct. It would be a simple thing to get each nozzle to flow the same on a bench test rig--------but that is not what is needed here, it is getting the air/fuel mixture the same at each cylinder.
 
you should be able to get the spread down...

Make sure that you are leaning very slowly with stable readings when you are considering changes to injectors... you should be able to get down to a spread of .2

I went through a couple of tuning tests and moved the peak curves around and routinely have spreads at 0.0 - 0.2. (I ended up using a +1 and a -1 restrictor to bring move cylinder EGT curves around to align the peaks). With mags the engine would run smooth to about 75 LOP... with PMags It will run smooth much farther (although this is not most efficient).

Generally I use MPG to optimize my leaning and have found that there is a very broad speed range that will yield a similar MPG.

For reference, here is the tuning guidance from Don Rivera from Airflow Performance, FI guru:

Nozzle Tuning Data

The basis of nozzle tuning is to get each of the cylinder EGT?s to peak at the same fuel flow. Your aircraft must be equipped with EGT information on each cylinder and fuel flow information. A digital flow meter is preferred.

To gather correct data for nozzle tuning, set a cruise power setting. Typically 24? MAP and 2400 RPM. Set the mixture to be 0.5 GPH richer than peak on any cylinder. At this setting record all the EGT?s for each cylinder. Lean the mixture 0.2 GPH and record all the EGT?s again. Lean the mixture an additional 0.2 GPH; record all the EGT?s again. Continue leaning the mixture 0.2 GPH and record the EGT?s until all the cylinders have peaked.

An alternative method although not as accurate is to lean each cylinder to peak and record the fuel flow at that point. You will get the same data, but since the EGT reacts slower than the leaning process you may go past the peak and not know it. This is especially true if an engine monitoring lean find function is used. We get more accurate data taking the EGT data manually. If you use an automatic data acquisition function, allow 30 seconds or so at each fuel flow setting so the EGT value can stable out.

After the data is taken, we determine which nozzles to change to get all the cylinders to peak at the same time. You will notice that the EGT number at peak may not be the same for each cylinder, THIS IS NOT IMPORTANT. The cylinders that peak first (higher fuel flow) are the lean ones; the cylinders that peak last (lower fuel flow) are the rich ones.
 
brad walton said:
........ I expect there is some. But the cylinders were flow matched, ported and polished to within less than 3 percent. ......
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Your cylinders maybe matched...on a bench test, but that doesn't mean you have equal air flow distribution when your intake system and exhaust are attached.
 
[email protected] said:
Generally I use MPG to optimize my leaning and have found that there is a very broad speed range that will yield a similar MPG.
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Same in our factory new IO-540 powered -10. Running smooth at 25-35 LOP. Usually 15 mpg plus or minus, depending on winds mainly. So, have not bothered balancing injectors.
 
Brad, if your GAMI spread is 0.6GPH it needs to be better. On a 6 cylinder we suggest 0.5 or better and in my opinion a 4 cylinder needs to be 0.3 or better.

Now if you have flow matched cylinders then once you get the F/A ratio's right they will be producing very similar HP. The problem is right now yours are not optimal.

The thing you fear is from not enough understanding. There is a very educational course in March in Oklahoma ;)
 
Thank you

Thank you all for your replies. So it seems to be the consensus that there is more likely uneven mass airflow between cylinders rather than uneven rate of fuel delivery and since it is not reasonable to adjust airflow to each cylinder individually, adjust the fuel flow by changing restrictor size until I am at or below 0.3 gph GAMI spread. And I assume that since I may have unequal mass airflow to each cylinder, they will produce unequal power from one to the next regardless of whether I have a low or high GAMI spread. So don't worry about unequal power production. Its probably worse in many carbureted engines. Correct take aways?
 
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If you get board, you can get a degree wheel and check lift and duration of each valve:D
 
Brad,
Even though you are correct that the overall mass flows will be different from cylinder to cylinder and the resultant power should be somewhat different, if your engine is like mine, it will run smooth as silk LOP after the nozzles are balanced. Even with Slicks..:cool:

I had about 0.6 to 0.8 to start, similar to yours. 0.1 afterwards.
 
brad walton said:
Thank you all for your replies. So it seems to be the consensus that there is more likely uneven mass airflow between cylinders rather than uneven rate of fuel delivery and since it is not reasonable to adjust airflow to each cylinder individually, adjust the fuel flow by changing restrictor size until I am at or below 0.3 gph GAMI spread. And I assume that since I may have unequal mass airflow to each cylinder, they will produce unequal power from one to the next regardless of whether I have a low or high GAMI spread. So don't worry unequal power production. Its probably worse in many carbureted engines. Correct take aways?
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I think you are getting it ;)

The power variation when LOP even with small variances in airflow is so small it is not an issue. With poor F/A ratio's this is why they run rough, big HP differences from cylinder to cylinder.

You are about where George Braly was in the early 90's?.then he got busy :)
 
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