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M20K Power Settings..?


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Understand, I am not endorsing the position the author takes.  I am simply putting it out for discussion (as he has). I think most people who have experience with the intercooler setup would agree that the 252 settings or the STC manufacturers’ adjustment recommendations are a place to start, but they underpower the engine.  Why?  Here is what the author (Paul Millner) says:

”If you actually put a turbocharged engine on a test stand and then observed the effect of the intercooler, you’d find that, say, 32 inches of MP and 200 degrees of induction air input deliver almost the indentical horsepower of 32 inches of MP and 120 degrees of induction air.

”So unlike obeying the restriction of the STC and reducing manifold pressure so many inches per 10 degreess of cooling, leaving the manifold setting alone delivers the same power, with safer-for-the-engine conditions. How?

”Remember when I explained how the intercooler imposes an insertion loss on induction air pressure, but no worries, the turbo control system will compensate? There are no free lunches. It turns out that the addional back pressure on the engine exhaust from the wastegate closing just a little bit to maintain induction pressure through the intercooler reduces engine power output just enough to offset the increase that would otherwise occur from the denser charge, It may not be obvious why such a conservation-of-engine output effect would occur but for efficient intercoolers, that’s the relationship.”

He is obviously writing with an automatic wastegate in mind, not the semi-manual Merlyn, but the point is still the same.  The point is real, and to translate a little, what he is talking about is that sufficient drag occurs in passing the induction air through the intercooler (its a radiator with fins) to cause back pressure in the exhaust, which reduces the horsepower output of the engine at a given MP and fuel setting.  To achieve the same power requires adjusting MP up, basically back to the setting you started with before cooling the induction air.  (I am talking ROP ops here.). This is, in fact, the great fallacy of turbo engineering, that it is “free” to put a turbo in the exhaust, or in this case to put an intercooler in the induction air stream. Exhaust back pressure reduces power output.  The turbo oviously still helps, it adds to power output, but it is not a 100% efficient, free lunch. Nor is the intercooler.

The gist of the article is that aftermarket intercooler designs are not very well tested.  The adjustment tables are set up making assumptions that are not true, particularly the one above, that the insertion of the intercooler in the induction air stream has no negative effect on power output and only has the positive effect of lower induction air temp.

One little hint that applying 252 charts is not correct is something I learned from an exchange with Paul Kortopates on this site a couple of years ago.  The 252 chart uses 36” for max power, at least that’s what I recall from looking at one a decade ago.  However, the STC for the Turboplus comes with special fuel flow set up instructions.  The max MP used to set up full power fuel flow varies a little depending on OAT, but essentiallly, the document uses 37” for max power, not 36”.

Now, the other issue it seems to me, is that the engine obviously is working harder to put out a given BHP than it was before you inserted the intercooler.  I don’t think it would be smart to simply not adjust MP downward and the author does not make that recommendation.  What he is saying in the article is that intercoolers, especially aftermarket intercoolers, are not quite as nifty as their advertising says.

 

PS if anyone did not follow it, the mechanism by which increased drag in the intercooler creates back pressure in the exhaust, is that in order to overcome the drag, the turbo must work harder.  In a fully automatic wastegate system, where the MP has been set to, say, 32, the system compensates by closing the wastegate a little and making the turbo work harder in order to generate the same 32" of pressure, and the turbo working harder, creates more backpressure in the exhaust, requiring the engine to work harder to push the exhaust out.  In the semi-automatic system (the Merlyn), the same thing happens.  If you manually set the MP to 32", the wastegate will close a little more in a system with an intercooler, than in a system without an intercooler, in order to make that 32".

Edited by jlunseth
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6 hours ago, jackn said:

To put a little more flesh on the bones, say you are flying ROP at 26” or 30” LOP to get the same power (speed), the turbo must boost some amount over an additional 4” MP? 

Apples to oranges.  On the ROP side, power is determined by MP and fuel flow, but mainly by MP.  On the LOP side power is determined by fuel flow and not by MP.  I don’t quite follow what you are asking.  

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It really depends on the efficiency and rated airflow of the intercooler core. It also depends on the amount of cooling air being supplied to the core. For example, a stock Cessna 414 uses a NACA inlet for the intercooler. A RAM VI adds a pitot style inlet for the core. I have a fair amount of time in both setups and to compare the two, the stock airplane performs like you are running carb heat.

I agree that some of the STC intercooler systems probably aren't tested as much as the factory systems. The core should be sized so there isn't an appreciable airflow restriction. Either way, I would much rather have any intercooler vs. none at all.

Sent from my Nexus 7 using Tapatalk

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On 8/31/2018 at 8:03 PM, jlunseth said:

Apples to oranges.  On the ROP side, power is determined by MP and fuel flow, but mainly by MP.  On the LOP side power is determined by fuel flow and not by MP.  I don’t quite follow what you are asking.  

Just ‘thinking out loud’. I’ve heard about the inefficiencies introduced by the inter cooler before, but was unable to quantify it. What I was inarticulately try to say is there are trade offs on everything. When I’m running ROP at a given MP and pull the mixture back to LOP,  I’ll get better fuel efficiency, but I’ll slow down. If I want to get that speed back, I’ll increase MP. Therefore my turbo will run harder. 

From what I read, the harder the tubo works, the more inefficiencies are introduced because of the inter cooling system, therefore, making the turbo work even harder. 

I run ROP & LOP depending on the situation. I just like to know the trade offs I’m making. 

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Turbos are built to work hard if you think about it.  They run constantly at RPMs up to 120,000 at an operating temp of 1600. I had mine rebuilt about 1100 ago, and it is still not making any complaints.

The LOP formula for percent horsepower in the TSIO 360 is fuel flow (GPH) times 13.7, which gives you the horsepower being produced then divide by rated horsepower (210 or 220 depending on your engine) to get percent horsepower.  According to the GAMI people, the only role MP plays according to the GAMI people, is to determine how lean the mixture is.  In other words, 32" or 34" MP would produce the same HP, but the 34" mixture would contain more air, so you would be more lean of peak than at 32".  In practice the MP does seem to make a little difference.  But on the ROP side it makes alot of difference, in fact for a given fuel flow and RPM, it determines what the power output is.

My standard LOP power setting if I did not mention it, is 11.0-11.1 GPH, 34" and 2450 RPMs for 71%HP.  I run 34" to make sure I am far enough lean of peak.  If you have the 220 HP engine the fuel flow would be a little higher.

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On 9/2/2018 at 11:03 AM, jackn said:

When I’m running ROP at a given MP and pull the mixture back to LOP,  I’ll get better fuel efficiency, but I’ll slow down. If I want to get that speed back, I’ll increase MP. Therefore my turbo will run harder. 

  This statement has me confused, if I understand correctly (which I may not) if LOP and you increase MP on a fixed fuel flow you are adding air therefor leaning the air-fuel mixture further which should not increase your speed, it will most likely slow you further.   I may be missing another variable that Jack is not mentioning or I may just need to go read some more!

 

Ron

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Yes, this is an issue with the 231 system discussed here before.  Move one knob, red or black, and the setting of the other item moves because of an interlink in the system.  To change MP and keep the fuel flow constant, you have to change MP, and then dial the fuel flow back to where you had it.  So if your fuel flow is 11.0 and your MP is 30", and you want to increase the amount of air in the mixture to make it leaner without increasing the power output of the engine (power output being determined by fuel flow on the lean of peak side), you would move the MP to, say, 34" and then you would have to use the red knob to bring the fuel flow back to 11.0.  If the moves are big enough you sometimes need to make adjustments a couple of times.  You would then be at a leaner mixture of 34" and 11.0 GPH, instead of the 30" and 11.0 GPH you started with.  

Lean of peak and rich of peak are air/fuel ratios.  You must change the ratio.  The interlink takes a given setting that you make and tries to keep the same ratio if you move just one of the knobs.  So, for example, it does not work to set your MP, say, at 30" with a fuel flow of, say, 13 GPH, and then dial just the fuel flow back, and then dial it back thinking you are going lean of peak.  It does not work because the interlink brings the MP back when you change the fuel flow in order to try to keep the fuel/air ratio right where it was when you started at 30" and 13.  You are still running the same fuel/air ratio (more or less) and you are still rich of peak, you are just at a lower power setting.

To go leaner, or to go richer, you must change the air/fuel ratio.  In the 231 that means you must work both knobs, red and black.

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