Myths and Choice of IO-360 Lycomings

[fodrv7]

Whilst extracting Cruise Performance data from Kevin Horton's excellent O-360 Engine Power Spreadsheets, I noticed that the Angle Valve IO-360 (Inclined Valves and Hemispherical Combustion Chamber, for better gas flow at high RPM) only has higher power output than the IO-360 Straight Valve engine when run higher than 2400 RPM.

It would appear that the Angle valve is more highly tuned and does not have as flat a Power Curve as it's Straight Valve brother and so the power output drops off more rapidly than the Straight Valve when RPM is reduced from 2700.

So on the cruise it would appear that there is very little advantage in the Angle Valve engine, particularly as it is around 30lb heavier. See picture of Spreadsheets below.

It begs the question as to why Lycoming made the Angle Valve engine.
Was it to provide obese Spam Cans with another 20BHP for Take-off.
If so, it has no applicability to RVs.

Look forward to some input on the matter from the Engineers amongst us.

Pete.

[Mel]

The angle valve engine is also more prone to cylinder cracking. Ask any Mooney owner.
Mel...DAR

[dan]

You don't want an angle valve IO-360, because everybody you fly with will be jealous of your superb climb, cruise, and economy. Better steer clear of those things!

I flew behind angle valve IO-360-A series engines in two Mooneys and now my RV-7. I have nearly 2000 hours behind the angle valve engine and I love it. Cracking? Mooney drivers? Dunno about that. Don't know how those particular Mooney drivers OPERATED their engines.

[osxuser]

The angle valve does have more cracking (both case and cylinder[) problems than the parallel. But the angle valve is really worth it if you use the IO-390 since they don't make a parallel version of that .

I'll be shopping for an angle valve if I can find one when I'm looking for an engine. Even with the 'problems'.

[gmcjetpilot]

Originally Posted by dan

You don't want an angle valve IO-360, because everybody you fly with will be jealous of your superb climb, cruise, and economy. Better steer clear of those things!

Don't forget about the extra cost of purchase and overhaul, plus the +30 lbs of extra weight!, ha ha ha I AM KIDDING.

http://www.lycoming.textron.com/main...Guide/360.html

The IO360 is an awesome engine. Cylinder cracks, never heard of them specifically on this engine either, but if its a Mooney thing, its from high flying and trying to get down, shock cooling. I taught at flight club/school which had 30-40 planes, including a few Mooney's. The key in checking members out was to get them to understand they needed to think about letting down as much as 60-80 miles out to allow a (warm) powered let down. Flying well above 10,000 feet in a fast retract requires more descent planning than flying slower C172 at 8,000 feet. Of course the bone heads would go , Doha! I am too high, chop, drop and causing thermal stress, aka shock cooling.


Yes Dan I am jellious.

G

[David Johnson]

Originally Posted by gmcjetpilot

Of course the bone heads would go , Doha! I am too high, chop, drop and causing thermal stress, aka shock cooling.

OWT. I say that very carefully and with much respect because I know you're a rather helpful and prolific poster hereabouts, but this is a topic I've studied. The greatest thermal stress your engine will endure is when you pull the mixture to idle cutoff before you back it into the hangar. Yes, aluminum accumulates fatigue cycles with heating and cooling, but pulling the power before heading downhill is not a significant source of thermal shock. You'll drop 10-20 degrees in CHT in the first several seconds, but that's not really significant. It will continue to cool, but the combustion happening on the other side of the cooling fins will keep them jugs toasty anyway, so the cooling profile is still gradual enough to not matter. In other words, the thermal cycle the cylinders experience- even in a crowbar descent, is shallow compared to that experienced when the engine is shut down.

First run cylinders aren't at much risk for cracking, no matter what you do to them. After about 3000 hours or so, the thermal cycles of startup and shutdown have accumulated to the point that the aluminum is weakened and more prone to cracking. For a real-world example, jump planes are the kings of the chop and drop descent, but their jugs usually make TBO.

Respectfully,
Dave

[dan]

I may be in the minority here, but I also think that the "advent of modern engine monitoring solutions" can only help to mitigate temperature-related issues of the past. That is, assuming the monkey with the stick in his hand knows how to interpret it!

[mlw450802]

Originally Posted by fodrv7

Whilst extracting Cruise Performance data from Kevin Horton's excellent O-360 Engine Power Spreadsheets, I noticed that the Angle Valve IO-360 (Inclined Valves and Hemispherical Combustion Chamber, for better gas flow at high RPM) only has higher power output than the IO-360 Straight Valve engine when run higher than 2400 RPM.

It would appear that the Angle valve is more highly tuned and does not have as flat a Power Curve as it's Straight Valve brother and so the power output drops off more rapidly than the Straight Valve when RPM is reduced from 2700.

So on the cruise it would appear that there is very little advantage in the Angle Valve engine, particularly as it is around 30lb heavier. See picture of Spreadsheets below.

It begs the question as to why Lycoming made the Angle Valve engine.
Was it to provide obese Spam Cans with another 20BHP for Take-off.
If so, it has no applicability to RVs.

Look forward to some input on the matter from the Engineers amongst us.

Pete.

This is entirely sensible behavior. If you think about the physics involved here, better cylinder filling depends on flow inertia and flow inertia is greater at higher velocities, which occurs at higer rpm.
If the engines operated at much lower speeds, smaller intake passages would probably have better filling characteristics than the larger passages and would probably have better power.

[N395V]

OWT. I say

Like Dave I am not a big believer in shock cooling being a major cause of cracking of cases or cylinders based upon my experience and watching CHTs during drastic power reductions, descents and when flying through heavy precip.
Shock Cooling Myth or Reality

or

Note the Graph and narrative



Of course there are no long term double crossover blinded prospective large sample studies to prove or disprove the case on either side I believe common sense and available data suggest that John is right. Keep it under 380degF to start with and you are unlikely to have a problem

[David Johnson]

Originally Posted by dan

I may be in the minority here, but I also think that the "advent of modern engine monitoring solutions" can only help to mitigate temperature-related issues of the past.

I agree completely. The factory CHT gauges in most spam cans are horribly imprecise. In my 182, 400 deg. CHTs put the needle in the middle of the normal range. With a multi-probe electronic engine monitor, I can either enrichen the mixture if I want to burn a bunch of gas, or lean it past peak EGT and watch the CHTs drop to 350 deg. and below. Either way, I pay a lot more attention to keeping the engine in a happy place than used to be possible.

Makes me wonder how often people were flying around in the old days pushing red line on their CHTs, completely oblivious. It's a well documented fact that the fatigue properties of aluminum start to change at a faster rate in the low 400 deg. temperature range. IMHO, this is a much more likely root cause for cylinder cracking than the shock cooling myth. Instrumentation and the ability to know what it's telling you is gooooood. Yup yup yup.

Dave