P337 Intercooler Flight Test Comparison

[MikeZ]

Hi folks: read threads with interest. My P337 has Riley intercoolers, been flying it for 13 years. Installed Riley AC two years ago. I NEVER have been able to exceed 205 knots true at max altitude, but I'm SO careful not to overstress the engines so I almost never run more than 32" and 2400 and burn 13.5 gph/side. Since I've been running richer I haven't burned a cylinder out. I used to go through them regularly.

I'd love to go faster at altitude. Any suggestions as to what settings to achieve better speed? I noted 213 kts from Skyking above.... is that real?

[Kevin McDole]

SkyKing, Something doesn't add up in all of these charts.

In the standard Cessna charts, notice that any given %HP at any given altitude is a constant PPH usage. For instance, 65% HP is 140 PPH - regardless of altitude, RPM, and MP. And whether it is above or below standard conditions makes no difference. The constant is that the leaning must be done the same in every example.

Conversely, for any given PPH burn, you can use the chart to identify the %HP regardless of altitude, RPM, and MP. Again, the leaning procedure must be a constant for this to work.

An intercooler has the same effect as changing the conditions from 20C above standard to 20C below standard.

Cessna’s POH says 158 PPH will give you 75% HP. Yet your chart says 176 PPH will be 75%. Doing the simple math of 176/158 = 1.113, and 1.113 * 75% = 83.4% HP. If you believe the Cessna charts, then you're actually running at 83.4% HP when you think you're at 75%.

You also report an increase in airspeed for the same constant %HP. How is this possible? To increase airspeed, you have to either reduce drag or increase HP. Since intercoolers do not reduce drag, they must be running at an increased HP.

The fact that you are burning enough fuel to equal 83% HP - and you are getting the expected airspeed increase of extra HP, this should cast doubt on the claim that you’re still running at 75% HP.

Try finding a power setting that gives you a fuel burn of 158 PPH (50 ROP). I'll bet you get the Cessna airspeeds for the 75% HP setting at that particular altitude.

[Bob Cook]

Kevin

Ding Ding Ding. You win the brass ring.........
......

Laws of physics are rather constant. Your observations are correct.

given fuel flow = given hp (less losses / frictional and less wasted fuel)

peak hp = peak egt = best efficiency = A Constant.

from this point you back off and increase fuel flow (waste fuel) to save engines. 50 rop / 100 rop/ full throttle. your choice in between. Above 70% we agree there is not much choice due to detonation potential. Below 65% we have all the choices including running at peak or lean of peak.

Bottom line > the intercoolers do exactly what you say. they cool the incoming air or change the density depending on airspeed, altitude, OAT, and intercooler efficiency. They do not defy the rules of physics!

I consider intercoolers were another "carnie" thing that does add performance under certain conditions, mostly under full power climb or hot and heavy departures, even then, they are not as efficient due to lack of increased air flow due to lack forward speed. They sure help when OAT is ISA +++++

So why would anyone reduce the mp on takeoff as it has been suggested? 100 % power is 100% power with 37 inches! The engine doesn't care about outside temp, just over or under boost and FUEL FLOW. Riley gives you the MINIMIM FF for a given RPM (2800) and they suggest reducing the mp after a given altitude ( based on book fuel flows). So much fuel gives you so much fire.

Assuming you had a switch to turn the intercoolers on and off you should see the engine running hotter OR your ROP value (drop) has changed due to improvement in air/fuel mixture (oxygen).

In either case, regardless of intercoolers or NOT, your HP should be calculated at a given book fuel flow and RPM, then MP becomes the variable. If you set your engines to PEAK then richen the mixture does not give you more horsepower. It is a "throw the fuel overboard lever" that contributes to fuel cooling. Therefore, yell if I am wrong, but the true measurement of hp is at the peak lean. Read the fuel flow and work backwards with given rpm. Voila...... hp! You have set the proper stoimetric power for the given conditions (atmospheric, with or without intercoolers).

What we are trying to do is save our engines by setting the proper mixture level.

Mike is correct. 32" and 2400 and burn 13.5 gph/side. Look at the POH for 32/2400 and the difference between POH value and the 13.5 is fuel cooling or the amount of fuel sacrificed to save the engines.

I think this hocus pokus came from Riley and American Intercoolers trying to sell intercoolers------- any people bought it , hook line and sinker!

Want more horsepower then go to IO-540s or burn more fuel.

Sorry to disagree with you Skyking.

Bob

[SkyKing]

Bob, et. al.,

No need to be "sorry"... you're not disagreeing with me, as I am simply relying on the published materials/manuals from American Aviation, Inc., who manufactured and flight tested the intercoolers that came with the airplane.

Insofar as the AA charts are concerned, my limited three years of experience with this equipment in a variety of flight regimes indicates that the fuel flows, fuel burns and airspeeds associated with the published parameters is right on target.

Bob, I'm not sure how you obtain "peak hp = peak egt = best efficiency = A Constant". While it is true that peak EGT will get you best efficiiency, i.e., best fuel economy, if "peak hp = peak EGT", how do you explain the Cessna POH wherein it states that, "...beste economy mixture settings (peak EGT) results in approximately 8% greater range than shown in this handbook accompanied by approximately 4 knots decrease in airspeed?" It seems to me that if you were obtaining peak hp, you would also be obtaining max airspeed, no?

Well, perhaps all of these technical gymnastics have moved us far afield of the end result, which is why some of us have the interccolers to begin with, and that is, to provide cooler and denser air from the compressor section of the turbo before it enters the induction system and throttle servo which equals better efficiency of the engine. That would also appear to be the reason for limiting the takeoff to 35.3" MAP at sea-level ISA, as the cooler, more densely packed air molecules would tend to create an overboost situation if you run the engines at 37" like a stock airplane... but I'm not in a position to argue that technical point, as AA has the STC and their FAA authorized placard says to limit the takeoff power to 35.3" MAP. I'm sure that American Aviation, Inc. had to provide certified flight test results to the FAA in order to obtain the STC... they wouldn't arbitrarily limit the takeoff MAP without substantive reasoning.

BTW, with respect to Mike's comments about burning cylinders on a regular basis, I wonder what operating parameters caused this? Our AA intercoolers were installed in 1978, a year after our airplane was manufactured, and reviewing the logs I don't find any evidence of a cylinder burnout in the 25-year logbook history.

SkyKing

SkyKing

[Kevin McDole]

Originally posted by SkyKing … which is why some of us have the intercoolers to begin with ... better efficiency of the engine.

I think all will agree that intercoolers improve engine efficiency, but I'm not sure we all agree on what "efficiency" means.

In this case, I’d define efficiency as the fact you are getting more HP for the same RPM & MP due to the denser intake air. Or in other words, you can achieve the same HP output using a lower RPM and/or MP.

I think you’re saying that the efficiency comes from getting more airspeed for the same given HP. This defies physics. For a 225 HP engine, 75% power is 169 HP with or without intercoolers. The airspeed you achieve at 75% HP is not related to intercoolers.

You should be suspicious of the intercooler charts. If you’re getting more airspeed, then you have to be creating more HP.

[MikeZ]

yeah, I used to burn 12.5 gph per side. We'd probably change them before some other shops would, but I tend to be cautious with my maintenance. its just a matter of caution with the fuel burn as well.

[Bob Cook]

SkyKing

I guess I did not respond to your last comments.

Since you have numerous variables such as OAT and/or density altitude that affects performance, the lower the density altitude the better the performance of any aircraft.

If there is such a constant it is in the TCM manual and curves that indicate hp vrs fuel flow for this engine at a given rpm. As a comparison it would be peak egt (POH) with a given fuel flow.

It APPEARS that I am running higher HP based on fuel flow and 60 ROP settings. I can get book figures by backing off MP but fuel flow drops due to the fuel controller............Hmmmm. The fuel controller DOES NOT SEE the "intercooler" it sees OAT or density altitude as calibrated by the factory.

So.... I work backwards.
set mp and RPM, then find peak and go for 60 ROP if I am above 68% hp. Under 68 I will go LOP or peak as long as the temps are holding true. I never worry about climb as I go full RICH where the highest risk of burning a cylinder exists.

Bottom line / non technical/ my best guess/
fuel=hp using TCM book
high limits 60 ROP = high cruise
and Full throttle = for climb
and "who cares" below 65% hp = rop, lop, peak / whatever is comfortable and within the operating temperatures.
Intercoolers give you more efficiency (lower density alt) under CERTAIN CONDITIONS. They do not defy the laws of physics.

I would be far more conservative if it were not for the JPI, however, you could still be burning valves and not "see it". For this reason I believe in full rich and always make climbs at the
top of the green" while monitoring temps.

I have never obtained a straight answer to this question....
"Why not increase fuel flow on take-off (higher) to compensate for the intercoolers."
Consensus is that it does not apply to all conditions and you end up throwing fuel away.

So therein lies the dilema.

Bob

[JamesC]

Skyking very much for the intercooler information. Has AA published takeoff power recommendations ?
Do you have any other numbers from them other than the two tables you posted?
Thanks again

[Walter Atkinson]

Gentlemen:

Based on the hard data rather than people's opinions, there is a tremendous amount of misinformation in this thread concerning HP determination and intercoolers. I will try to address as many of the items as I can remember form the read:

1) HP when ROP is NOT affected to any meaningful degree by fuel flow in the usable mixture ranges. HP when ROP is a function ONLY of mass air flow. By definition, any ROP mixture has extra fuel already. Adding or subtracting fuel as long as one stays ROP does not change mass airflow, so FF has no effect on HP…. until one gets so rich as to slow the rate of combustion, changing the thetaPP and reducing HP to the crankshaft. The HP difference between being barely ROP and as much as 150-175dF ROP is extremely difficult to measure and would be within the error in reading an airspeed indicator. We can measure the differences on the test stand and they are less than a HP or two.

2) Best Power is found at 75-80dF ROP, not 50dF ROP… no matter what any POH might say. These are the laws of physics.

3) Best economy is not found at Peak EGT, no matter what any POH might say. This is not open for debate. This, again, is a function of the laws of physics. Best Economy is found at BSFC(min) between about 20 and 90dF LOP, depending on the power being produced. At very high power settings, Best Economy is at about 90dF LOP. At very low power settings it can be as close to peak as about 20dF LOP.

4) Intercoolers are very important and helpful in operating a TC engine. They reduce the IAT and widen detonation margins. This results in lower CHTs in many situations. The increase in O2 allows for more mass airflow when ROP and, therefore, more power, albeit at a higher FF at the same ROP mixture. Best Power with an intercooler will require more FF to match the increased mass airflow.

5) An intercooler when operating LOP will allow one to be either further LOP with cooler CHTs at the same FF or push more fuel LOP for more power with the same CHTs.

6) HP LOP is a function of FF ONLY. Mass air flow plays no part as long as the mixture is LOP. For the engines you are operating 13.75 x FF = HP. Period. MP and RPM play no part as long as the mixture is LOP. So, a setting of 26/2400/13gph produces the exact same HP as 28/2450/13gph as long as both mixtures are LOP.

7) Intercooler recommendations to reduce MP 2" with an intercooler installed are mathematically flawed. While it is true that the intercooler results in more O2 and therefore greater mass airflow and the resultant increased HP, what they forgot to include was the increased exhaust back pressure which negatively impacts the volumetric efficiency of the cylinders. These factor offset on another so closely as to be a wash. There is, therefore NO scientific reason to reduce MP with an intercooler.

I will post a paper by George Braly on Intercoolers in the next post. If anyone would like any of the above comments expanded upon, I will be happy to do so.

[Walter Atkinson]

From George Braly:

INTERCOOLER EXPLANATION:

For a year, I spent a good portion of my time re-engineering an intercooler installation and obtaining FAA approval for the changes. Very careful instrumentation and measurement. I have more recently done more of that with a different engine on the engine test stand (see www.engineteststand.com).

I learned a lot. I am a strong supporter of intercoolers, so some of what I am going to say may sound strange at first.

So, here goes: Frankly most of the conventional wisdom on this subject - - like a lot of the now famous LOP/ROP discussions - - is simply not true.

The practical application results in an outcome that is rather different than everybody anticipates. It is largely a myth that 30" of MP AFTER the intercooler gets you more HP than 30" of MP BEFORE adding the intercooler. In fact, a poorly executed intercooler installation can - - and often does -- result in LESS horsepower at sea level than the same engine without the intercooler.

I don't know who started the whole theory of de-rating an engine after an intercooler, but it sure as the devil was not somebody who was measuring the actual engine torque - accurately - - rather, they were just following the theory as you outlined it and making the calculations, etc. (And, yes, I have heard a story about one after market intercooler outfit that claims to have put a torque meter on before issuing the reduced MP instructions based on that result... but until I see the data, I will have a very hard time with that notion.)

The big problem is that in the euphoria over the large and very beneficial drop in Induction Air Temperatures (IAT) that one gets with an intercooler, the "engineers" forget all about something else: How much of the "good stuff" (ie, good, cold, high density air) you can get into the cylinder on each intake stroke - - also depends on how much of the BAD STUFF (ie,
exhaust products) you got out of the cylinder on the previous exhaust stroke.

The ratio of the new "good stuff" to the theoretical maximum "good stuff" is called the cylinder volumetric efficiency Ve. Normally aspirated engine values are up around 85 to 92% of the cylinder's displacement - - although with turbocharged engines, the number will substantially exceed 100%.

When you add an intercooler to a turbocharged engine - - and leave the MP constant - -, you place a restriction in the intake plumbing. That means that the compressor discharge pressure is now two or three (or 4 or 5 or 6 in one case) inches of Hg higher than the wide open throttle MP!!!

In order to generate that extra pressure, the compressor has to work harder. And that means that the turbo has to work harder. And that means that the wastegate is closed a bit more. And THAT means the exhaust back pressure increases and that reduces the cylinder volumetric efficiency.

Result? The improved number of molecules you get into the cylinder due to the denser air is just about perfectly offset by the reduced Ve. Thus, no net increase in useful airflow through the cylinder. And the result of that is no net increase in horsepower at the same MP.

Now... having said THAT - - in my view, it is almost criminal for anybody to operate a turbocharged engine without an intercooler.

There are excellent old SAE research papers that show a HUGE improvement in the detonation tolerance of these engines by use of even a modestly efficient intercooler. These improvements are not trivial. They are substantial.

Installing an intercooler means that you are very very much less likely to inadvertently cause detonation during a moment of inattention during a busy high power climb in an IFR environment - - when you accidentally fail to have the mixture rich enough. Lots of other benefits.

For the same reasons, it lowers peak cylinder pressures substantially and that seriously promotes reduced exhaust valve temperatures and greater valve and cylinder longevity.

Last, and this is something almost nobody appreciates, adding an intercooler results in much better fuel atomization by your fuel injectors during high power operation.

Like I said, the "conventional" wisdom on this subject is seriously flawed.

The benefits of intercoolers are sort of over promoted for the wrong reasons and vastly under promoted for the right reasons all at the same time.

They are generally a good investment.


Regards, George Braly - Tornado Alley Turbo & GAMI

[Walter Atkinson]

HP calculation ROP (mass airflow):

(100+(((RPM-maxRPM)/100*2.5)+((MP-maxMP)*3.5)))/100*maxHP.


HP calculation LOP (FF):

NA engines = 14.9 * FF = HP

TC engines = 13.75 * FF = HP

[CaptainRod]

Excellent posts, Walter. Thank you.

[JamesC]

Walter I really appreciate your input - LOP ops certainly become easier given the simpler relationship between fuel flow and HP in that regime. Much safer than choosing CHT targets particularly at cold altitudes.
For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?
Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

[Walter Atkinson]

Originally Posted by JamesC

For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?

The engine test stand data confirmed the math! This is a very good "rule of thumb" calculation that can be off a very small amount as other factors do affect the mass airflow. Those differences are insignificant operationally.

Originally Posted by JamesC

Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

Very good question. The answer is "no." Peak ICPs are controlled nicely. The additional FF provided by the fuel controller covers that problem in conjunction with the decreased volumetric efficiency. Mass airflow-type systems like Lycoming's make it a non-issue. The TCM fuel controller handles the increased O2 a different manner, but it works just as well.

[JamesC]

So then if AA placards t/o power MP = 35.3" then that is what we use in the equation for MaxMP when calculating cruise HPs, correct?
The AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.