To intercool or not to intercool?

[T. Peterson]

I’m replacing my engine with an overhauled unit from Western Skyways in January. My 1979 231 K model has been equipped with the Merlyn waste gate, but not the intercooler. Other than just hearing that intercoolers are good, I have no clue as to the quantifiable benefits. Yes, I do realize that cooler operations are good, but I am looking for a little more than that.

 I cruise almost always at 11000 or 12000 feet. I don’t fly lop because I don’t have Gami’s, and I don’t want to fly slower than 160 knots true. As it is, it takes 12.5 to 12.7 gallons an hour to make 160 knots. I keep my cylinder temperatures at 380 or less and my TIT at 1580 or less. I will allow 1590 if I need it to keep 160 kts. It annoys me when TAS drops into the 150’s.

 I would very much like to get at least 165 kts burning no more than 12.0 gallons an hour, but I don’t know if that is possible without an intercooler to keep the TIT cooler. Hopefully replacing a 30 year old engine will be of some advantage, but the question is will adding an intercooler give me a significant performance benefit?

Your opinions are invited, and from past experience invaluable. Thank you.

[dkkim73]

In theory you will get more oxygen so should be able to add fuel and get more power, thus speed. Might even be about to do it LOP. The practical answer would come from someone who's done this exact thing... Did the vendor give you any claims or reference points?

[T. Peterson]

42 minutes ago, dkkim73 said:

In theory you will get more oxygen so should be able to add fuel and get more power, thus speed. Might even be about to do it LOP. The practical answer would come from someone who's done this exact thing... Did the vendor give you any claims or reference points?

I have not talked to a vendor. 
I can add fuel now and get power, but I cannot lean to less than 12.5 gallons an hour or TIT goes high. I’m temperature limited not power limited. I could probably do 170 knots but the fuel flow would be 15 gallons or more an hour.

[LANCECASPER]

paging @Jeff Shapiro from TurboPlus (http://www.turboplus.com/)

http://www.turboplus.com/mooney231-intercooler.asp

Nice article starting on page 19: https://themooneyflyer.com/issues/2019-MayTMF.pdf

 

My first Mooney was a 231 without a Merlyn or an intercooler and was very much temperature limited. Once you fly with an automatic wastegate and an intercooler, you'll never go back. Thankfully you already have the Merlyn.

[Will.iam]

Come on Torrey what’s an extra 6k in the grand scheme of an overhaul? Course the intercooler might be more than 6k now a days.

 

[dzeleski]

I don’t have any specifics for airplane engines but I can give some for autos.

10 degrees of IAT generally equals about 1% of HP. This isn’t a perfect scale but it’s generally close enough.

Engines running a turbo and no intercooler I’ve typically seen run well over 200 degrees and some even more then 300, while that same setup with an intercooler should be around 100 degrees under the same conditions. Most ECUs/tunes will pull timing above 130 or so unless a record is trying to be broken.

I don’t know what the charge temps look like on these engines but considering they run at fairly high power settings I’m going to guess that an intercooler is going to make a fairly decent impact in power.

[A64Pilot]

The purpose of a turbo is to increase charge density of course, but you also increase charge density by cooling, so the effect of an inter cooler is very similar to an increase in boost, except of course there is a point of diminishing returns by just increasing boost, because compression adds heat and you get to a point that your adding so much heat that even with higher pressure you lose density.

So long story short, an inter cooler will increase performance under most conditions, especially steady state like aircraft fly but remember it takes fuel to make HP, so if everything else is the same, fuel burn will increase with the  HP.

TANSTAAFL

We can draw similarities with Autos, but Auto’s we run lots of boost to significantly increase HP, and an auto turbo can increase efficiency by utilizing otherwise waste heat.

But aircraft we aren’t so much running high boost as we are maintaining HP as air density decreases and as we aren’t really running much boost there isn’t enough efficiency increase to overcome the decrease in efficiency from lower compression ratio.

Compared to an auto at 10 PSI or more boost 40” MP is not much. My motorhome Diesel regularly runs 30 or more PSI and that’s what pushing 100” MP for example? 

Edited Wednesday at 01:02 PM by A64Pilot

[Pinecone]

You don't need GAMIs to run LOP.  Do a GAMI mixture sweep and see what your spread is.

My TSIO-360-SB doesn't have GAMIs either.  In fact, they will not sell me a set.  Because my GAMI spread is only 0.3 gph.

You can go fast LOP also.   NA guys are limited by the air density, so at higher altitudes they can't make enough power LOP to go faster.  You have a turbo, so that is not a limit.

I run 175 KTAS on 10.1 gph on my 252/Encore at 15000 - 17000.  Lower, a bit less. 

[T. Peterson]

13 minutes ago, Pinecone said:

You don't need GAMIs to run LOP.  Do a GAMI mixture sweep and see what your spread is.

My TSIO-360-SB doesn't have GAMIs either.  In fact, they will not sell me a set.  Because my GAMI spread is only 0.3 gph.

You can go fast LOP also.   NA guys are limited by the air density, so at higher altitudes they can't make enough power LOP to go faster.  You have a turbo, so that is not a limit.

I run 175 KTAS on 10.1 gph on my 252/Encore at 15000 - 17000.  Lower, a bit less. 

That’s impressive! Thanks for the tip.

[Will.iam]

5 hours ago, T. Peterson said:

That’s impressive! Thanks for the tip.

Torrey we need to go up in my plane sometime so i can show you what LOP can do for you. You will definitely need gami injectors because unlike Pinecone and my aircraft that are the MB engines your engine is the LB and cylinders 1 and 2 will be too rich compared to 3,4,5,6. Once you get them balanced, you can really push up the manifold pressure to stay deep LOP and adjust fuel flow to get 75% up to maybe 78 or 79% power depending on your comfort level of how hot your TIT is running as that will be your new limiting temp factor not your CHT’s but that would also need to have the intercooler installed to do that. Without the intercooler I’m guessing you might hit the cht limit before the TIT limit. The only good news is that at 12k the air is more dense and thus more cooling efficiency than if you went up to say 14k or 16k. 
 

[Geoff]

Definitely add the intercooler.  The 231 installation runs hot and anything that reduces overall engine temps will be beneficial for both longevity and performance.

 

There is a reason the 252, Bravo, Acclaim, and Ultra all have intercoolers.

Edited Wednesday at 04:00 PM by Geoff

[dkkim73]

FWIW it is not hard to figure out what your GAMI spread is. See a very thorough method here: https://www.savvyaviation.com/wp-content/uploads/savvy_pdf/Savvy-Flight-Test-Profile-Expanded.pdf

 

[Jim F]

14 hours ago, T. Peterson said:

I’m replacing my engine with an overhauled unit from Western Skyways in January. My 1979 231 K model has been equipped with the Merlyn waste gate, but not the intercooler. Other than just hearing that intercoolers are good, I have no clue as to the quantifiable benefits. Yes, I do realize that cooler operations are good, but I am looking for a little more than that.

 I cruise almost always at 11000 or 12000 feet. I don’t fly lop because I don’t have Gami’s, and I don’t want to fly slower than 160 knots true. As it is, it takes 12.5 to 12.7 gallons an hour to make 160 knots. I keep my cylinder temperatures at 380 or less and my TIT at 1580 or less. I will allow 1590 if I need it to keep 160 kts. It annoys me when TAS drops into the 150’s.

 I would very much like to get at least 165 kts burning no more than 12.0 gallons an hour, but I don’t know if that is possible without an intercooler to keep the TIT cooler. Hopefully replacing a 30 year old engine will be of some advantage, but the question is will adding an intercooler give me a significant performance benefit?

Your opinions are invited, and from past experience invaluable. Thank you.

Hi T.Peterson,

I am in a similar position and am planning an overhaul mid 2025.  I have an 81 231 and apparently the only 231 left without a Merlyn or intercooler.  I am planning to install a Merlyn wastegate and a Turboplus intercooler before I do the overhaul so that I can do a PIRP on both.  My position on the Merlyn is that a higher critical altitude would be helpful and allow me to fly 75% power in the flight levels at 2500RPM.  Currently I need to be >2600RPM to get enough MP out of the turbo for 75% power.  On the intercooler, my thought process is why did Mooney not put an intercooler on the 231 and why did Mooney add an intercooler to the 252/Encore (I know the engine is TCM).   The 231 was a low-cost entry into the turbo market and in general the industry considers the 231 a hot running engine.  After market feedback Mooney decided that the addition of an automatic wastegate and intercooler was what customers wanted, and I do think that was a great decision.  Everyone with a 252 or Encore raves about their Mooney being the best option for their mission.  On our 231s the compressor discharge pressure can get very warm, and that warm air charge can really push the CHTs up in the 400s on hot days with a slow climb.  I look at any aftermarket add-on to see if the manufacture released a similar solution in later years.  With the wastegate the fixed wastegate is a pain to deal with and has a low critical altitude, so the automatic wastegate makes sense.  On the intercooler, the hot running 231 when you add an intercooler it has a much better CHT margin from red line.  For me these are reliability decisions, the Merlyn will not be spinning the turbo at as high of an RPM so in theory it should last longer.  The intercooler will have a cooler air charge and should lower the CHTs ~20d and the cylinders should last longer when running cooler.   Remember after the engine change and intercooler add, if your aircraft is faster, you are producing more horsepower.  Also, make sure the fuel system is set correctly once the intercooler is installed.  The intercooler cools the air charge and at that lower Max MP(~36in) you need about the same FF as MP 40in on your non intercooled engine.   Refer to the Merlyn and intercooler documents.

I fly 75%HP, 100d rich of peak at 17.5/16.5ft, 13.9FF, ~30.0in MP, 1550TIT and I see 181Kn.  After I added fine wire plugs, I am now able to run lean of peak, but I don't very often.  Also, I love the fact that I can pick up ~2kn TAS per thousand feet, so I cruise high almost always.  At 17.5 65%HP, 12.4FF I get 170kn.  Fly fast, Jim

[T. Peterson]

2 hours ago, Jim F said:

Hi T.Peterson,

I am in a similar position and am planning an overhaul mid 2025.  I have an 81 231 and apparently the only 231 left without a Merlyn or intercooler.  I am planning to install a Merlyn wastegate and a Turboplus intercooler before I do the overhaul so that I can do a PIRP on both.  My position on the Merlyn is that a higher critical altitude would be helpful and allow me to fly 75% power in the flight levels at 2500RPM.  Currently I need to be >2600RPM to get enough MP out of the turbo for 75% power.  On the intercooler, my thought process is why did Mooney not put an intercooler on the 231 and why did Mooney add an intercooler to the 252/Encore (I know the engine is TCM).   The 231 was a low-cost entry into the turbo market and in general the industry considers the 231 a hot running engine.  After market feedback Mooney decided that the addition of an automatic wastegate and intercooler was what customers wanted, and I do think that was a great decision.  Everyone with a 252 or Encore raves about their Mooney being the best option for their mission.  On our 231s the compressor discharge pressure can get very warm, and that warm air charge can really push the CHTs up in the 400s on hot days with a slow climb.  I look at any aftermarket add-on to see if the manufacture released a similar solution in later years.  With the wastegate the fixed wastegate is a pain to deal with and has a low critical altitude, so the automatic wastegate makes sense.  On the intercooler, the hot running 231 when you add an intercooler it has a much better CHT margin from red line.  For me these are reliability decisions, the Merlyn will not be spinning the turbo at as high of an RPM so in theory it should last longer.  The intercooler will have a cooler air charge and should lower the CHTs ~20d and the cylinders should last longer when running cooler.   Remember after the engine change and intercooler add, if your aircraft is faster, you are producing more horsepower.  Also, make sure the fuel system is set correctly once the intercooler is installed.  The intercooler cools the air charge and at that lower Max MP(~36in) you need about the same FF as MP 40in on your non intercooled engine.   Refer to the Merlyn and intercooler documents.

I fly 75%HP, 100d rich of peak at 17.5/16.5ft, 13.9FF, ~30.0in MP, 1550TIT and I see 181Kn.  After I added fine wire plugs, I am now able to run lean of peak, but I don't very often.  Also, I love the fact that I can pick up ~2kn TAS per thousand feet, so I cruise high almost always.  At 17.5 65%HP, 12.4FF I get 170kn.  Fly fast, Jim

Thank you for the detailed response. Very helpful and informative.

[1980Mooney]

19 hours ago, T. Peterson said:

 My 1979 231 K model has been equipped with the Merlyn waste gate, but not the intercooler. .... Yes, I do realize that cooler operations are good, but I am looking for a little more than that.

 I cruise almost always at 11000 or 12000 feet. ... It annoys me when TAS drops into the 150’s.... I would very much like to get at least 165 kts burning no more than 12.0 gallons an hour, but I don’t know if that is possible without an intercooler to keep the TIT cooler.

 

8 hours ago, A64Pilot said:

The purpose of a turbo is to increase charge density of course, but you also increase charge density by cooling, so the effect of an inter cooler is very similar to an increase in boost, except of course there is a point of diminishing returns by just increasing boost, because compression adds heat and you get to a point that your adding so much heat that even with higher pressure you lose density...So long story short, an inter cooler will increase performance under most conditions, especially steady state like aircraft fly but remember it takes fuel to make HP, so if everything else is the same, fuel burn will increase with the  HP.

TANSTAAFL'''We can draw similarities with Autos, but Auto’s we run lots of boost to significantly increase HP, and an auto turbo can increase efficiency by utilizing otherwise waste heat...But aircraft we aren’t so much running high boost as we are maintaining HP as air density decreases and as we aren’t really running much boost there isn’t enough efficiency increase to overcome the decrease in efficiency from lower compression ratio.

As @A64Pilot points out the turbo and intercooler increase charge density.  The intercooler has the added benefit of reducing charge temperature which helps control engine temperature, TIT and detonation.  But as he also points out more charge means more fuel. And while more charge density will produce more HP and will deliver a higher critical altitude, the OP wants only to fly at 11-12,000 ft.  And he wants a bit more speed (165 kn) while burning a bit less fuel (12.0 gph).  I am not sure that the added weight, cost and complexity of the intercoolers alone will actually deliver higher engine efficiency at 11,000 - 12,000 ft.  (he is getting an overhaul, discussing LOP which may in themselves deliver the efficiency - he won't be able to tell the effect of the intercooler on fuel consumption).

This might be a case of the wrong plane for the owner's primary mission.  @T. Peterson previously posted that an exchange engine from Western Skyways was $55,000.  When you add in shipping, engine mount possible repair, mounts, accessories not included in O/H like fuel pump, new fuel lines, prop O/H, installation, maybe tax plus the cost of intercooler kit and install plus cutting lower cowl and paint, I bet that you will be close to $100,000. You might be better served to sell your plane as is, take the proceeds and that additional $100,000 and buy an Ovation.

The Ovation will meet and exceed your requirements with greater simplicity and likely lower maintenance costs.  And you will get more space.

The original Ovation is perfect for what you want.  The Ovation2 is better.

Original Ovation

Ovation2

Edited Wednesday at 09:03 PM by 1980Mooney

[hubcap]

I had around 200 hours on my 231 with no intercooler and no Merlyn Black Magic pressure control when I added the intercooler. I noticed immediately that temperature control was much easier. I added the Merlyn and got rid of the bootstrapping and got the higher critical altitude. 
 

 In my opinion the addition of these along with a good EIS so you can see what’s going on makes the 231 much much easier to manage and improve your engine life significantly.  The lower temperatures alone are justification enough in my opinion, but I am really serious about not hurting my engine.