Experience with Subaru at 358 Hours

[rv6ejguy]

Originally Posted by rocketbob

Looks to me like the hone on the cylinders wasn't done very well. The angles look way too shallow. You might want to talk to your ring manufacturer and see what Ra (avg) Rpk (peak roughness) and Rvk (valley roughness) numbers to shoot for on the hone and check with a profilometer.

And are those push on fittings for the oil cooler??

I have a recipe for cylinder finish and ring types that has worked flawlessly for 35 years. This one had the same treatment, 220 grit honed on a CK-10. Checked with bore gauge and final miked in 3 spots on each bore. This is a typical CK-10 pattern, very normal in the automotive world.

Barb fittings, Hey it's a car engine! - yep, again never had one leak, fail or fracture in 35 years and a lot of race cars. OEMs use them on EFI fuel lines, trillions of hours, very reliable. Just a PITA when you have to take them off, have to cut the hose because you can't pull them off once the clamp is removed, but after 9 years, I want to replace them anyway. Now, I have seen braided stuff and AN elbows break on race cars. Vibration, heavy, stiff braided hose, aluminum elbows, sometimes not a good combination.

[rv6ejguy]

Originally Posted by DanH

When you think about the unusual rod bearing damage, consider how hard the crank is oscillating those rod throws during your 1100 to 1600 RPM resonant period.

The bearing damage would appear to be classic (but rare) cavitation damage. The other 2 bearings were so mint, they could have been reused. The flake from the big pit was still inside the pit when I split the rod caps, polished from rubbing around in the hole. I have built and torn down a lot of engines in my time but never seen this before. The mechanism is apparently bubbles in the oil are collapsed by firing pressure, this only occurs on the top shells and usually within the TDC to the 20 degrees after location.

Why only on 2 bearings and what really precipitates it? Could be something like you mention. Diesel engines sometimes have severe cavitation damage to the cylinder liners because of the hard shock of the firing impulses...

I ran Aeroshell in the engine for about 40 hours just for the heck of it, the rest of the time it had 5 or 15W50 Mobil 1. Oil pressure is always 70 psi except hot idle, have around 45. Never starved or run low on oil and no aerobatics.

I need to get another 350 hours on the rebuild to see what it looks like inside next time!

[BillL]

+1 on bore polishing for your oil consumption increase. Heavy duty engines, will have an increase in top land deposits and with the temps and piston motion can polish the bores. It looks like it is mostly on the thrust sides of the bore.

Also, the higher temps for the top ring when running at a high load steady state (i.e. cruise) the ring gap may need to be increased. Suggest checking the gap for butting evidence under high magnification.

Was the rod bearing cavitation on all rods? Have you considered all aluminum bearings? They have much higher fatigue resistance than overlaid bearings, but require a very clean oil during early life, and after oil changes. Also, a one factor for cavitation is aeration of the lube oil. Is there any particular churning of the oil that may be occurring? Like crank dipping during climbs etc? You know the engine best.

Just some thoughts from my database.

Thanks for sharing your results in such detail, this could really help make some key modifications to make an non-traditional engine last longer.

[DanH]

Originally Posted by rv6ejguy

The bearing damage would appear to be classic (but rare) cavitation damage. The other 2 bearings were so mint, they could have been reused.

So which ones were damaged? Close to the flywheel or most distant?

[rv6ejguy]

Originally Posted by BillL

+1 on bore polishing for your oil consumption increase. Heavy duty engines, will have an increase in top land deposits and with the temps and piston motion can polish the bores. It looks like it is mostly on the thrust sides of the bore.

Also, the higher temps for the top ring when running at a high load steady state (i.e. cruise) the ring gap may need to be increased. Suggest checking the gap for butting evidence under high magnification.

Was the rod bearing cavitation on all rods? Have you considered all aluminum bearings? They have much higher fatigue resistance than overlaid bearings, but require a very clean oil during early life, and after oil changes. Also, a one factor for cavitation is aeration of the lube oil. Is there any particular churning of the oil that may be occurring? Like crank dipping during climbs etc? You know the engine best.

Just some thoughts from my database.

Thanks for sharing your results in such detail, this could really help make some key modifications to make an non-traditional engine last longer.

Glad it is interesting!

Subaru actually specifies a smaller top ring gap on the turbo EJ22 than the atmo version in an attempt to reduce blowby I'd guess. I ran a few thou more than they recommended because it was less than what I'd normally run in a turbocharged race engine. I will look for ring butting, never checked that. But never butted rings using standard .004 end gap per inch of bore, even on engines producing pretty sustained 225hp/L

Obviously this little 2.2L is working hard to do this job. The life is different from a road race engine where we might extract well over 400hp from it but only for 50-75 hours before teardown. I do have a daily driver shop Nissan 240SX (KA24) which has been turbocharged for the last 15 years and I beat it hard on the street and track, has about 3500 hours on it now and basically no oil consumption. Nothing special inside, built the same way as the Subaru but no forged pistons. Cruise at 3500 rpm on the highway usually and it's under boost... often

I've built many similar turbo street engines. Two friends drove their Toyota turbos for 10 years and 14 years respectively, never used oil, compression still fine after many thousands of hours of collective abuse, bottom ends never touched. But even these do not sustain the high gas loads and upper ring temps as the aircraft application where the peak loading is a lot lower but duration is a lot longer. I would add, that this engine has factory oil jets to cool the bottom of the piston crowns.

No, as I said, only 2 rod bearings had this damage, the other 2 were mint. The OE bearings are an aluminum 2 layer type similar in composition to a GM Morraine 400 type. I used these on many really high output race engines and never saw a problem but there seems to be no sense in putting the same thing back in the 2nd time around. Something is causing this but I would have expected damage to all top shells if the oil was aerated when reaching the bearings. This engine has a windage tray stock, also has about a 30cc reserve pocket in the oil pump discharge housing. The crank is far below the oil level even in the climb attitude. It is possible under really high alpha, the rear counterweights could dip into the oil. The sump and pickup are very deep on this engine, turbo oil was returned far away from the pickup to the left valve cover (this has blown up a few engines due to highly aerated turbo oil being sucked into into the oil pump).

I'd be happy if my non-traditional engine lasted as long as a traditional one...

You make some good points, I'll keep pondering.

[rv6ejguy]

Originally Posted by DanH

So which ones were damaged? Close to the flywheel or most distant?

Have to check my notes on that Dan. You could be on to something. Always appreciate your insights and thoughts!

[rv6ejguy]

As a further note on bore finish, when using moly faced rings in Corvair and Toyota engines, I'd specify a 400 grit finish which looks like a mirror almost. No problems with break in, life or oil consumption but the bores had to be really straight and round. Cross hatched from 25 to 45 degrees seemed to make no difference in break in time or life of the rings.

With chrome rings, used on most of my turbo engines, I use 180 to 220 grit stones. I break in engines hard, WOT, boost and 3000-5000 rpm about 30 times, on/off to shear the proud metal and then remove the gas loading to release the particles past the rings. Again, did this with every engine I built and no issues. This Subaru also had very low oil consumption for the first 200 hours and then it slowly started to progressively creep upwards along with the stain on the belly. For the first 200 hours, the belly was totally dry.

I expected the years of automotive engine experience to directly transfer in this case to another application with similar, predictable and successful results. However, I have learned at various times working with aircraft that there is always more and new things to learn and you are never as smart as you might think you are. I must say, the whole process of building the RV, designing and building the FF stuff and systems has been very rewarding and educational. Keeps life interesting and the noggin working. It's my flying laboratory.

[zav6a]

Did you happen to check the rig temper relative to new? I know it is hard too waste a set but it could be enlightening. If temper is gone, pretty clear that they get too hot in continuous high output ops. Maybe a different material would solve it or just do routine maintenance. Gaskets, rings and a hone every half a decade ain't so bad so long as it's predictable and non catastrophic.

Ceramic coatings on the piston tops might cut down on heat flux. Especially with unleaded fuel quick will leave fewer insulating deposits.

[rocketbob]

Personally I think mirror finish is a mistake, with the finish too smooth there is no oil retention.

[BillL]

Here is a 1989 paper on the 2.0, is it representative of the lube system?

http://www.surrealmirage.com/subaru/files/SAE_boxer.pdf

If so, are the cavitated rod bearings in #2-3 or #1-4 perhaps? #2-3 are fed from the center main bearing and #1-4 are fed from one main each. Cavitation typically occurs at a point on the bearing where the pressure drops following the combustion pressure point. A radial force diagram for the bearing relative to the rod would be the typical method of investigation. Most engines in the field never see this (cavitation) as it is cured in the development process.

Sooo, if it is on #2-3 rods, it could be improved with a combination of higher oil pressure, (relief valve spring) , stronger bearing material, or reducing aeration of the oil. The last would be the most difficult. Attention to oil passages may help too. You probably have already thought of these.

One last comment, racing engines, although high power density seldom run very long at peak power; turns, shifting etc affect their ability to reach peak stabilized temps, like juggling a hot potato and surface temps on ones hand. Even F1 are only about 50% load factor per lap. Oval tracks (Indy cars) are a different matter.

Good luck, really interesting stuff.