%HP by fuel flow not matching charts… what am i missing?

[Will.iam]

I understand the high power charts are skewed as your TIT is limiting but on the 60% piwer charts and below by what everyone says you can set the mixture to what ever power you want and can not hurt the engine, i assume that the mooney power chart set peak tit as it is written for the 60% power chart. The formula i have heard for turbo tsio-360 is a compression ratio of 13.7 so ffx13.7/210=%hp. My mooney chart for 252 at 60% hp has a ff at 2500 rpm of 10.3 but when i use 10.3 x 13.7 / 210 i get 67% not 60%.  I know the charts were for 50 degrees ROP at higher setting but would not peak tit be at zero ROP / 0 LOP once you got down to a power level that would not exceed tit of 1650 per the note on the chart? so why doesn’t the formula work? Or is the 13.7 compression number wrong for my tsio-360? Working backwards it would be 12.23 to make the %hp correct to the charts. 
even the 35% power chart when using their ff of 6.1 which is the same value across all rpm settings still shows 39.7% hp on the hp by ff formula. I’m sure the chart would be right as they had torq instruments setup on the test airplane to get there data. So I’m just wondering if i have the wrong ff formula for figuring horsepower. 

[Shadrach]

Two things at play. First isn’t FF to calculate power is a rough “back of the envelope calculation. Two, all cylinders must be on the lean side of peak EGT, using the factory’s leaning procedure, some cylinders may be close to 50ROP. Every cylinder must be on the lean side, not scattered around peak EGT. 

[Will.iam]

39 minutes ago, Shadrach said:

Two things at play. First isn’t FF to calculate power is a rough “back of the envelope calculation. Two, all cylinders must be on the lean side of peak EGT, using the factory’s leaning procedure, some cylinders may be close to 50ROP. Every cylinder must be on the lean side, not scattered around peak EGT. 

True but they always say ROP is more power than LOP. So the Mooney chart for 60% power is coming out at 67% LOP. Or to put it another way the amount of fuel charted for peak tit even if slightly ROP on the Mooney chart if I go LOP with same amount of fuel flow I will be producing 7% more power? Now that’s  not right so either my compression number of 13.7 is wrong (closer to 12.15 works mathematically) or there is something else not making the math equal the chart. 

[Will.iam]

Just looking at max recommended cruise power at 2500 rpm is 12.8 ff and Mooney got 78.6% power but this formula says if I go LOP at 12:8ff I will get 83.5% power! If that was the case Mooney charts would all be lop instead of 50 ROP.  Since we know that is not true the only other variable is the the formula is the compression ratio 13.7 must not be what in the tsio-360.  I could see 1 or 2 percent off but 5 to 7% a lot bigger error than I thought it would be. 

[Fly Boomer]

3 hours ago, Will.iam said:

Or is the 13.7 compression number wrong for my tsio-360? Working backwards it would be 12.23 to make the %hp correct to the charts. 

I believe the compression ratio for all versions of the TSIO-360s is 7.5:1.

[Shadrach]

1 hour ago, Fly Boomer said:

I believe the compression ratio for all versions of the TSIO-360s is 7.5:1.

It is. The multiplier for that is the 13.7 number he referenced. It was awkwardly worded.

[Shadrach]

7 hours ago, Will.iam said:

True but they always say ROP is more power than LOP. So the Mooney chart for 60% power is coming out at 67% LOP. Or to put it another way the amount of fuel charted for peak tit even if slightly ROP on the Mooney chart if I go LOP with same amount of fuel flow I will be producing 7% more power? Now that’s  not right so either my compression number of 13.7 is wrong (closer to 12.15 works mathematically) or there is something else not making the math equal the chart. 

You’re really confusing things. ROP is always more power for a given MP because all of that air (MP) in the intake charge is used for power. LOP is alway more power for a given FF because all of the fuel in the intake charge is used for power. That’s why best BSFC is on the lean side of peak. It makes more power per volume of fuel.  
 

A fuel burn of 10.3 gph on the lean side will require additional air (mp) when compared to a fuel burn 10.3 on the rich side because ROP mixtures do not combust all of the fuel in the intake charge. Furthermore, there is a margin of error for both book numbers and LOP FF power calculations. You’re dealing with two fuzzy numbers (especially at peak TIT). Two fuzzy numbers don’t make for precise comparisons. I would be interested to know if Peak TIT FF is different when leaning from the lean side.

The correct multiplier for your TSIO360 is indeed 13.7

[Will.iam]

Ok thanks. I was really hoping to be able to verify my tit probe / guage by comparing it to the chart values but with that much fuzziness I’m not sure how valid the check would be. 
i guess assuming peak tit would be consuming all the air and all the fuel but apparently it’s not or the cylinders are not balanced enough in the test plane thus there must have been some extra fuel not being burnt in those chart published settings. 

[Marc_B]

@Shadrach that looks like a pretty hand spreadsheet.  One you put together?

[Shadrach]

37 minutes ago, Marc_B said:

@Shadrach that looks like a pretty hand spreadsheet.  One you put together?

Nope. It’s been around for a decade or more. It’s available in the downloads section.

 

[Shadrach]

1 hour ago, Will.iam said:

Ok thanks. I was really hoping to be able to verify my tit probe / guage by comparing it to the chart values but with that much fuzziness I’m not sure how valid the check would be. 
i guess assuming peak tit would be consuming all the air and all the fuel but apparently it’s not or the cylinders are not balanced enough in the test plane thus there must have been some extra fuel not being burnt in those chart published settings. 

This is the reason N/A operators use leanest cylinder for ROP ops and richest cylinder for LOP ops. All cylinders are operating within the same mixture spectrum to some degree. Peak TIT is an aggregate of all EGTs. While it loosely correlates to the mixture in cylinders, at peak TIT! they are likely clustered around peak EGT on the rich and the lean side.

[Fly Boomer]

5 hours ago, Shadrach said:

It is. The multiplier for that is the 13.7 number he referenced. It was awkwardly worded.

I see.  Thanks.  Where did that spreadsheet come from?