P337 Intercooler Flight Test Comparison

[SkyKing]

Here's the American Aviation Intercooler flight test comparison data for a 1973 P337. Notice the difference in the servo inlet temperature with and without, as well as a nice increase in speed. Ours is a 1977 model and experience with the AA Intercoolers dictates the speeds are fairly close.

SkyKing

[Bob Cook]

Skyking

I am now a believer in magic... The intercoolers provide additional airspeed with no other drawbacks...... Right ?

(what is missing with the data Skyking is the Fuel Flow differences running similiar ROPs. None of the Intercooler mfgrs bothered to include these when selling intercoolers).

Basically it reduces the Upper deck air temp creating additional cooling and subsequent O2. But what about fuel flow figures.

If I try and run the AC using "book" figures it doesn't work. You are at least 1 gph richer / engine to run with same EGT/CHT. So where is the savings?

My feeling is they provide additional power on TO and climb, more reserve..........but..but.but ..... the rear engine oil temp or CHT is going to altimately create the limit.

It is kind of like that little coil that went into the top of the distributor (sold at carnivals).. you should remember eh DON?

Note for the newbies; Don't ever go by the POH or you will "cook" your engines!

Basically you are reducing density altitude... Anyone else want to jump in with a different thought ? <G>

Bob

[SkyKing]

Bob...

Were you able to successfully open the attachment.... I wasn't able to! Why is it that some items open within the message box and on others you have to click?

In any event, I'm going to try adding one additional item that might help you analyze the situation.

Regarding oil & cyl head temps... everything is much COOLER with the intercoolers.

BTW, does your P337 have intercoolers, or not?

SkyKing

[SkyKing]

Things seem to be hit and miss tonight... will try again to attach the fuel flow/power data for American Aviation Intercoolers. Hope this is enlightening Bob...

SkyKing

[SkyKing]

Things seem to be hit and miss tonight... will try again to attach the fuel flow/power data for American Aviation Intercoolers. Hope this is enlightening Bob...

SkyKing

[Bob Cook]

Yup

It has Riley intercoolers. ....... and no attachment. What kind of file are you trying to attach?

I seem to creep to 200 deg on the oil temp and I end up reducing the rate of climb. I usually am pulling 33/2500 to 2600 on the climb with approx 240 lbs/hr total fuel burn (full rich).


Bob

[Kevin McDole]

Skyking, I was able to download the chart you posted. I have not heard of the term "Servo Inlet Temp" previously. I assume they are referring to Induction Air Temp (IAT), which is the temp of the air entering the upper deck (after it's been intercooled).

I notice that even with medium power settings up high, you're getting a 100 degree F cooling benefit from the intercoolers - which is going to directly relate to a higher % HP output for the same RPM/MP. Some installations say that you should reduce the MP 1" for every 10C of intercooler effect. In your situation, that would be about a 5" to 6" reduction in MP to keep the same percentage HP.

How do you compensate for this?

[Bob Cook]

Kevin

HP = FF < losses (fricitional and otherwise).

Fuel flow is the only way to determine actual hp unless you work backwards knowing the Inlet air temp and using that to calc density altitude. Even that is tricky.

You can only assume you are accurate at two points; 1) full throttle take off, and cruise leaned to peak or thereabouts and then resorting to the engine mfg. charts.

The fact you are cooling the inlet air temp does NOT give you more hp, it provides you with better combustion characteristics. You still need the correct fuel/air mixture to maximize efficiency.

Get out the sliderule......... errr..... calculator! or, you run the engine within its temperature limits avoiding pre-ignition or detonation.

During world war II they used water and/or water methane injection to get emergency hp.
Limits ? strickly temps.
How much additonal hp ? > was theoretical based solely on density altitude from charts.

Compare the POH fuel flow leaned to peak and work backwards, using the correct rpm scale, you can see where the MP would be if there was no intercoolers. The difference is what the intercooler is contributing the performance. The amount of hp (%) remains the same for the given fuel flow.

GMAS ?? comments

Bob

[kevin]

FWIW, the advice I have been given (and have been following) is to reduce MP (or RPM I suppose, although that is not what I do) to get the book fuel flow. If you use book MP/RPM settings with intercoolers, you will be developing more HP, including higher cylinder pressures, than the engine was designed for. 'Course, if you use 65% power MP and get 75% power fuel flow, you are OK, you are still at or below 75% power.

On my airplane, this results in book fuel flow at 63% power being 2" MP lower than book. This is operating 50 degrees rich of peak on the first cylinder to reach peak (#5 on my airplane, both engines). Fuel flow is verified by actual fuel consumption, not just fuel flow indicator. So I use 27"/2500 RPM for 63% power cruise.

Kevin

[Bob Cook]

Kevin et al ?

Question:

1) How do you know 2"s is appliceable ?
2) if OAT -10 deg. is 2" applicable at 5kft ? 10 or 19k ft?
3) what parameters does the POH use for fuel flow settings?
4) what density altitude does the engine see with turbos? without turbos? assuming std temp and pressure.
5) What fuel flow does the TCM manual say for same settings as the one you just mentioned?
6) is 50 ROP developing same HP as PEAK ?
7) what ROP settings does TCM give for there mean effective brake hp figures ?
8) would the 2" hold if the OAT was 105 deg F ?
9)So what is the common denominator for HP?
10)How does JPI calculate the % of HP on their engine guage ?

Do you have the TCM manual on the TSIO360C or CB?

What happens if your tach or MP guage is out by 10 or 20% ?



Interesting discussion.

BTW Kevin what is your actual fuel flow for this power setting?

Bob

[kevin]

Bob,

To answer some of your questions:

I developed the 2" based on experimentation at various altitudes and temperatures, and measuring actual fuel consumption (with a dip stick). So cockpit instrumentation is irrelevant.

I fly from 13 - 20,000 feet most of the time, I didn't experiment at the lower altitudes. For 13 - 20K, OAT from -35 to +20 C, I did not detect a signifcant difference in fuel consumption. I suspect there was one, but I think the difference is buried in the basic inaccuracy of my measurements, i.e. +/- 1 gallon per tank.

My MP gauges may be both out the same amount in the same direction, but I doubt it. One guage does seem to be about 1/2" off based on indication at sea level. But again, I don't care what the indication is, I just want to run my engines at 63-67% power.

TCM manual is at the hangar.

50 ROP is of course not the same horsepower as peak, and you know that, this must be one of your stalking horse questions.

Rear tach IS out by 50 RPM, checked using ground instrument. If either tach or MP gauge goes out of adjustment, I will detect the difference in unusual fuel flow, low or high, and investigate.

I agree, an interesting discussion:)

Kevin

[kevin]

Actual fuel flow is 23 gph at 27/2400 50 ROP.

Kevin

[SkyKing]

Kevin, Bob, et al.,

Okay, maybe the following chart will help in the discussion, which shows recommended cruise-climb as well as normal cruise power settings.

Bob, your oil temps don't sound right... we very rarely see them approach 180 degrees unless it's a ++ISA day, and the rear engine always runs slightly cooler than the front with everything buttoned up. Maybe the Riley type intercoolers don't offer the same amount of "inter" cooling - ha! I've not seen the Riley up close, but from pictures it appears the radiator surface is somewhat smaller than the American Aviation units.

Another chart I attempted to scan wouldn't go, but basically the MAX POWER limitation is 35.3" MAP @ 2800 RPM for NORMAL takeoffs. For Density Altitudes above ISA, the chart reflects the correct MAP for a given condition, but in any event max is 37". Today, for instance, our OAT is 90 degrees at sea-level and the chart shows a MAP for takeoff of 36.3".

We normally cruise at 27" & 2300 RPM, and as you can see by the chart, that equates to approximately 55% power with a fuel flow of 57PPH each end using peak EGT for Best Economy, or 64PPH each end for Best Power. At this low cruise power setting, we opt for peak EGT or right in the middle, around 60PPH each end, so Kevin... you're burning more fuel than you need to -- if you have the AA intercoolers. We have the standard Cessna gauges and the dual Economy Mixture Indicator (EGT), which because of the probe placement in the exhaust stream is actually reading TIT. And note what the POH says... "Continuous operation at peak EGT is authorized only at 65% power or less."

SkyKing

[Bob Cook]

I always run at 29/2450 or 31/2450 for cruise. Burn is about 27 gph at 50 rop for 68-70% hp (29").

re temps. rear engine does run warm based on OAT being isa++ with the air conditioning on. Normal front temp is 160 and rear is about 180. they would run a lot cooler if i ran at 55% power.

My humble opinion is try power is based on fuel flow at peak EGT then consulting the tcm manual and correlating to rpm curve.

Kevin, I wasn't trying to bait you. my argument is cooling the inlet air can can help to run the engine cooler and develop more potential output based on hot and heavy climbs. Fuel flow still dictates % power regardless of OAT or intercooler temperature.

Skyking, I do not have my poh with me but what is the cessna spec pwr based on same RPM and FUEL FLOW in your chart. Must be a couple of inches difference ? just a guess.

Bob

[SkyKing]

Bob,

Sorry that chart was so BIG... guess I need to learn how to make it "fit the window".

If it wasn't readily apparent after looking at the AA Intercooler Power Settings, as you can discern, the basic idea is to remember three power settings for any altitude based on 55%, 65% or 75%... e.g., it's always going to be 33" and 2400 RPM for 75%; 31" and 2300 RPM for 65%; and, 27" and 2300 RPM for 55% -- and their corresponding fuel flows, i.e., 88, 76 and 64 for Best Power (50 ROP), and if at or below 65%, peak EGT's resulting in 67 or 57 PPH respectfully.

Now, with respect to Cessna's POH... the chart for a stock airplane seems to have finite differences in BHP and Total PPH for a given altitude, ISA. So, let's just pick 10,000' and 18,000' for comparison and arbitrarily use 2400 RPM and 33".

At 10,000', Cessna POH shows 72% Best Power (50 ROP) with a fuel burn of 153 Total PPH for 2400 RPM and 33", and a TAS of 184 Knots. With the intercoolers the chart shows this same power setting giving 75% Best Power (50 ROP) and a fuel burn of 176 Total PPH, or 23 additional PPH (3.83 gallons per hour). BUT, the airplane equipped with the intercoolers is giving 195 KTAS instead of 184 KTAS as in the POH.

AHhhh, but drop the intercooled airplane down to 65% at the same altitude (30"/2400 RPM) where you can run lean peak EGT and your fuel burn is now 134 Total PPH with no penalty in airspeed... your speed is the same as the stock airplane at its slightly higher power setting... 184 KTAS ... and beating the stock airplane fuel flow by 19 PPH, or 3.16 gallons per hour. This latter figure isn't quite comparing apples to apples, because if you compare to the stock airplane's comparative per cent of power setting, the difference is only about a gallon per hour. So, it depends on how you want to skew the numbers.

OK, for 18,000' the stock airplane Cessna POH shows 71% Best Power (50 ROP) being developed with 33" and 2400 RPM, with a fuel burn of 150 Total PPH, resulting in 198 KTAS. With the AA intercoolers for the same altitude, 2400 RPM and 33" nets out 75% power and a fuel burn of 176 Total PPH- but you might need an extra 50 RPM spool depending on ISA conditions to keep MP steady, turbos spooled and pressurization happy, so running 32.5" and 2450 nets the same 75%. Again fuel burn will be 176 Total PPH, but speed will be 211 KTAS instead of the stock airplane's 198 KTAS, a distinct 13 knot advantage. Dropping the intercooled airplane down to 65% power results in 202 KTAS, and a fuel burn of 134 Total PPH, which is still better than the stock airplane at 150 Total PPH for 71% in addition to a slight speed benefit.

We very rarely run 75%... and unless we're going long, it really isn't economical to climb to the teens... but we like the pressurization and the turbos for mountain flying, and so we usually run 55% to 65%. At these power settings we can run lean and at peak EGT, thereby grabbing the fuel flow savings and also gaining speed over the stock airplane, at least at the higher altitudes.

If we run around local, say 5000' for a hop, we generally run 55% power which gets us down into the 114 Total PPH range, or 19 Gallons per hour and generally gives us about 160-165 KTAS on ISA days. Besides, we like to enjoy the ride, and these lower power settings are easier on the pocketbook and the noise levels are reduced too.

I beleive the intercoolers were probably the best investment on the P337 as it enables the engines to run cooler and promotes longer engine life. And there's virtually no maintenance to this item, as long as the rubber connection hoses maintain their integrity. Above all, as with any add-on equipment, if someone was in the market for a P337, I'd strongly suggest getting one with the goodies that you want already installed, as aftermarket installs now would be QUITE expensive, almost to the point of being cost-prohibitive. (Same with boots, RSTOL, etc.) And speaking of RSTOL, last time we checked it was around $14K+ for Robertson.

Well, there you have my nickel.

SkyKing

[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]

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.


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.

[Walter Atkinson]

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?
Yes.

SThe AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.
Sounds like they may have forgotten to include the exhaust back pressure effects in their calculations. Not sure how they got their numbers.