Turbo Subie Passes 250 Hours on the Hobbs

[rv6ejguy]

My Dad and I just finished a 2.1 hour cross country flight up North yesterday and on shutdown, the Hobbs clicked over to 250.0 hours- a small but important milestone.

How has the experience been?

The first two years seemed to be spent making countless modifications to the cooling system. Last year all I really changed was the type of coolant going from Evans NPG+ back to EGW (70% water, 30% Prestone EG and a touch of Redline Water Wetter). Since that last mod, the cooling system performance has very good. Temperatures in the climb rarely exceeds 180F making it one of the coolest running Subes flying.

I did add a new turbocharger scavenge pump this spring going from a diaphragm type to a gear type which also saved 3 pounds.

The core of the engine has never been touched in this time despite a forced landing due to my fault in not noticing a bum alternator and one time losing all the coolant and flying for 11 minutes. Compression and leakdown were good so I continued to fly it.

The engine was hard on spark plugs initially and I had to go to platinum types to solve this.

I had to remove the oil pan last year to fix a persistent seep from an unused turbo drain fitting.

A couple years ago, I removed the turbocharger to install a larger compressor wheel and housing for better altitude performance. The hot section parts and bearings are original.

I've had zero problems with the Marcotte M-300 PSRU and zero problems with the IVO Magnum prop. I get about 50 hours between brush changes on the prop ($7.50/ set).

Lately, oil consumption has been creeping up and compression creeping down. I'm attributing this to a steady diet of 100LL which has been slowly sticking up the rings in the grooves (much tighter tolerances than a Lycoming). I've lately been feeding the engine a dose of Marvel Mystery Oil at each fuel fill as this has been shown by others to free up lead bound rings on Subarus. I hope this works.

Performance wise, we see around 1000 fpm climb at 1750 lbs. gross at +20C and 4000 MSL and 2000+ fpm solo/ 1/3rd fuel and at around 0C, again at 4000 MSL. Due to fuel prices today, I generally cruise only at 25 inches now and 4400-4600 rpm. Leaned to 1400F EGT, this gives us a fuel flow of just over 7 US Gal./hr. Speeds range from about 135 knots TAS at 6000-7500 MSL to around 140 knots TAS at 9-10000 MSL. The airplane will cruise at 160-165 knots true above 14,000 feet using 30 inches and about 8.8- 9 gal./hr.

The engine package has been remarkably reliable in the last two years and we are having a lot of fun flying it.

I plan to revise the radiator system in the future to reduce drag, weight and complexity over the present system.

I've learned a lot, especially in the early years and it has been satisfying (and sometimes frustrating) developing this unique propulsion package from scratch.

[Mike S]

Ross, thanks for the report.

Even though I have no intent to put a Subie in my plane, I really do enjoy the learning experience from following the various threads on this subject.

Thanks again,

[David-aviator]

Good report, Ross.

I'm all out of ideas with the H6 and the original small EGG radiators designed for the 2.5 engine. Yesterday after the second take off coolant temp hit 220 before reaching 2500' so it was level off and increase prop pitch as usual. The cowl has more than adequate exit area so the only thing left to do is modify the front of the cowl to provide for direct ram air to the radiator surface. At present I am guessing the Van's cowl inlet is about 60% of the boxed radiator area. Air has to flow around the inlet opening and find its way to the radiator and there probably is much turbulence. Straight in would be better as Jan has found out with a new cowl. I can make a cold engine take off and climb OK but with a heat soaked engine, forget it. The H6 needs more radiator area and that's why current engines have it. The EGG factory recommends cowl flaps but with the small rads there is inadequate cooling with a fixed opening and closing it down would only make it worse.

I could change from NPG to water and EG anti freeze as per your set up. But that would necessitate increasing coolant system pressure. At present it is 7. At what pressure are you running the cooling system? Will changing to water and EG net 10 degrees better cooling? I guess I could try it and find out.

Cruise speeds relative to fuel flow are similar. I have been using 8 gph and 143 KTAS in the past but it may be a couple knots less since opening the cowl for more air flow plus the new internal muffler may have a bit excess back pressure. There is an EGG recommendation to drill out some of the internal baffles to correct it.

The cowl exit was at 5x17 but I closed it down to 4x17 after initial cooling results were not much better and there was some added drag. The cowl exit has been modified three times and I hope to fill and finish it this summer so it can be painted again. The new muffler has not affected oil temperature one bit even though it is within .75" of the pan. It is quieter, the 2 torpedo external mufflers are gone, and the bottom skin no longer vibrates.

[rv6ejguy]

David, I run a 13 psi cap and saw a 10-12F reduction in temps just switching the coolant with no other changes. My opinion is that the 2.5 rad setup is not adequate on the H6 for extended climbs on a hot day.

I wanted to be able to climb from my field elevation (4000 MSL) directly to 10,000 at 85 knots (my best climb speed) and +25C OAT without the coolant temps exceeding 200F. This is no problem now.

[TSwezey]

The fun or trial and error and educated guesses!

[dlomheim]

At present I am guessing the Van's cowl inlet is about 60% of the boxed radiator area. Air has to flow around the inlet opening and find its way to the radiator and there probably is much turbulence.

David: It doesn't sound like you have attempted to place a "trumpet" shaped fiberglass fairning from the cowling inlet to the radiator face without leakage? Our rotary guys have found over the years that this is very beneficial to improved cooling.

Doug Lomheim
RV-9A, 13B FWF

[frankh]

Thanks Ross,

This power plant has obviously been the tinkerer's dream...And you don't have to worry about a potential $3600 hit to your pocket at 350 hours plus as us ECi drivers now do..

Good job. I had an EA81 soob conversion in my Zodiac and despite the fact it tried to kill me three times I really enjoyed the proces of finally turning it into a reliable package.

Cheers

Frank

[rv6ejguy]

Here I've been telling people who don't want a Subaru to get a Lycoming clone and then these cylinder things come along. Looks like a "real" Lycoming might have been a better choice. Arrrgghh. I feel for anyone who has to lay out money for things like this.

I guess we all have our burdens to bear. If my rings don't unstick in the next 50 hours or so, I may be pulling some stuff apart too.

[RV8RIVETER]

ROSS

Congratulations on your milestone!


DAVE

Exit flow and shape would be my guess as the best way to improve your cooling. I say this because I assume you are just dumping the outlet of the box radiators into the lower cowl. If I am wrong my apologies.

I don't know if you have read the posts on RV Cooling from a traditional engine perspective, but there is good info and links for reading that would pertain to your water cool engine. You also have an -A which has caused some exit area cooling flow difficulties. A picture has been posted which shows flow into the exit.
Completely controlling the outflow of the radiators all the way to the cowl exit and smoothing the cowl exit would provide a substantial improvement. A good "trumpet" inlet is also recommended as non optimal shapes have been shown to leave the edges of the radiator devoid of airflow.

My 0.02 anyway.

[David-aviator]

Originally Posted by RV8RIVETER

ROSS

Congratulations on your milestone!


DAVE

Exit flow and shape would be my guess as the best way to improve your cooling. I say this because I assume you are just dumping the outlet of the box radiators into the lower cowl. If I am wrong my apologies.

I don't know if you have read the posts on RV Cooling from a traditional engine perspective, but there is good info and links for reading that would pertain to your water cool engine. You also have an -A which has caused some exit area cooling flow difficulties. A picture has been posted which shows flow into the exit.
Completely controlling the outflow of the radiators all the way to the cowl exit and smoothing the cowl exit would provide a substantial improvement. A good "trumpet" inlet is also recommended as non optimal shapes have been shown to leave the edges of the radiator devoid of airflow.

My 0.02 anyway.

Thanks, Wade, what you suggest is very true.

A smooth plenum would probably make a world of difference as was well established as long ago as WWII. I think the diffuser angle is something like 12-14 degrees to and from the radiator to provide for no separated air flow.

The problem creating a plenum with this installation is the radiators are located quite close to the engine block, there isn't room for a plenum. The air coming through the radiator hits the engine and has to make a 90 degree turn to escape. The right radiator is about .75" from the block, the left a bit more. Air does find its way out of the compartment, but it is not a smooth flow.