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Converting O-320 from 150 HP to 160 HP

WWhunter

Member
Was talking to a fellow airplane builder the other day and he is converting his 150Hp O-320 E2D to 160HP by installing the higher compression pistons. He mentioned that he was going to take the rings off the low compression pistons and install them on the higher compression pistons. I asked why not just buy new ring sets. From what I gather, the current engine, which only has a couple hundred hours runs great and uses very little oil, hence his reasoning for using the same rings.

Since I am the type to always install new when rebuilding, his idea seemed a bit foreign to me. Have others done this? Is it something acceptable? I'm guessing if they are installed on the same cylinders in the same position, they should work fine. Opinions?

Always learning!!!
 
It's generally not a difficult mod, and been discussed quite a bit over the years. Suggest searching for, "320 150 160," or similar.
 
DEFINATLY need new rings and a hone. The higher compression pistons put the the rings in different spots (will travel further up the cyl wall), so re-use is out of the question. The upgrade also requires the heavy walled piston pins. Cracking the top ring is a real possibility, depending on whether there is a step at the top of the old ring travel on the cyl wall.
 
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Don't do it unless you're rebuilding the engine already in my opinion. Even then I would consider staying at a 150 horsepower. My RV4 ran a 150 horsepower 320 and I was as fast as most 160 horsepower RV's. It's a lot more fun to me to try and reduce drag than add 10 more horsepower which really does very little to your top speed but does gain you a slight increase in initial climb rate. That gain in climb really comes into play if you're rocking a constant speed prop. You want better climb performance and have a fixed prop then put q climb prop on.

Also with a 150 horsepower (CR 7.0:1) you have the option of running lower octane mogas. 160HP (CR 8.5:1) is not crazy but less margin for low octane fuel. I'm not a big fan of Mogas, but the option is nice. I have an O360 180 horsepower (CR 8.5:1) so the 160 HP is the same. So don't misunderstand a 160 horsepower is an awesome engine. If you're just spending all the money to get 10 more horsepower save it and buy more gas and have fun.

I did a few official sanctioned XC races and unofficial side by side races in my 150HP RV4. I finished first and second for two races, in the 160 HP class. Airport buddies with 160HP RV's I could match or beat. BTW My O320 had a constant speed prop. A few pulled away slowly. 10HP is a knot or two with more fuel burn. Look at Van's old 160 vs 180 HP speeds. Not a big difference for 20HP. Yet there is a HP advantage in climb and top speed apples to apples. Yes but with a fuel burn increase and less fuel flexibility.

PS reuse of rings? No. New pistons new rings was what I was taught. Who is doing engine mod? No reputable engine shop would do this IMHO. If you're going through all the trouble to pull the jugs off you should go through the valves the seats the guides, hone bore, IRAN. Look at the cylinder bores and consider reconditioning.
 
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When doing 160HP you should use a hardened choked cylinder bore surface not plain straight steel. On a 150 HP engine it is permissible to use a hardened choked barrel or a straight, plain steel barrel. So some of the cylinders that are used on a 150 HP engine could be used on a 160HP engine. On the other hand, many 150 HP O-320's have plain steel straight bore barrels, these are not recommended for 160HP.
Once the cylinders are off, you need to have them honed.
You may also have to open out the main jet in the carb by up to 0.007" to get the motor to run rich enough.
Only undo one cylinder's worth of hold down through bolts at once, otherwise the case halves may fret once its all back together.
If you let the torque off all the through bolts at the same time bad things can happen to the main bearings and/or case halves.
And you may need to re-pitch the prop.

Good luck
 
Don't do it unless you're rebuilding the engine already. Even then I would consider staying at a 150 horsepower. My RV4 ran a 150 horsepower and I was as fast as a 160 horsepower R v's. It's a lot more fun to me to try and reduce drag than add 10 more horsepower which really does very little to your top speed but does gain you a slight increase in initial climb rate. And that game really only comes into play if you're rocking a constant speed prop. You want better climb performance and have a fixed prop then put up Is climb drop on. Also with a 150 horsepower you have the option of running lower octane mogas. I'm not a big fan of that and like a 100 low lead and it remains to be seen with this new U L1 100 no lead fuel is like. Whatever it's like I'm sure it's gonna be very expensive. So I have an 0 3 60 180 horsepower which again is lower compression like the 150 horsepower O320. Don't misunderstand a 160 horsepower is an awesome engine. But if you're just spending all that money to get 10 more horsepower save it and buy more gas and have fun. Like I said I did some offical sanctioned XC races and unofficial side by side races. I'd finished first or second in the 160 HP class. Airport buddies with 160HP RV's I matched or beat. A few could pull away slowly. 10HP us a few knots top speed. Look at Van's old 160 vs 180 HP speeds. Not a big difference. Yet ther is a hp advatage in climb and top spesd it also comes at cost of mir GPH.

PS reuse of rings? No. New pistons new rings. Who is doing engine mod? No reputable engine shop would do this IMHO. If you're going through all the trouble to pull the jugs off you should go through the valves the seats the guides, IRAN. The pistons the rings new. Look at the cylinder bores and consider reconditioning or OS if needed.

I couldn't agree more. Not worth the time, expense, or hassle. I try and resist the "always need more, more, more" mindset. I often remind myself, I have 95% of the performance at 50% the cost. That kind of value always puts another sort of "grin" on my face.
 
Question for lr172

Just for my education, and perhaps others. If the rings move up and the stroke (ie. displacement ) remains the same, I assume the wrist pin moves down in the piston ? If so, is tail skirt increased any to normalize the "rocking moments on the piston.
 
What Larry said!

DEFINITELY need new rings and a hone. The higher compression pistons put the the rings in different spots (will travel further up the cyl wall), so re-use is out of the question. The upgrade also requires the heavy walled piston pins. Cracking the top ring is a real possibility, depending on whether there is a step at the top of the old ring travel on the cyl wall.

With only a couple of hundred hours, it might be tempting not to bother to hone the cylinders. But even after that short time, a small ridge forms at the top of the ring travel. It is a virtual certainty that the top rings will break from being slid up into that ridge.

In this case the re-hone is less about rejuvenating the cross-hatch (which is still a good idea too) and more about removing the ridge.
Once this is done, one MIGHT get away with re-using the rings. But why bother? The risk of a broken ring scoring a cylinder requiring a new cylinder is just not worth it.

Presumably the correct wrist pins come with the new pistons?

The STC for the -E2D upgrade should say whether the standard cylinders on an -E2D are appropriate (tapered) or not.
 
Just for my education, and perhaps others. If the rings move up and the stroke (ie. displacement ) remains the same, I assume the wrist pin moves down in the piston ? If so, is tail skirt increased any to normalize the "rocking moments on the piston.

Yes, the pin moves down on the piston and this forces the piston to travel higher into the cyl.

Yes, the stroke is the same and the stroke is what sets displacement, along with bore. HOWEVER, in order to get the increased compression ratio, The piston is moved up, toward the head, relative to the pin. This decreases the compressed volume at TDC and therefore a greater ratio of compression The piston is also higher at BDC, due to this, so stroke remains the same. So the amount of air drawn in is mostly determined by the stroke, but because the piston is higher, the compressed volume is much smaller, hence a greater ratio. This is what changes the compression ratio (volume at BDC vs volume at TDC). There will be a ridge where the old rings stopped their upward movement. The new piston will be driving the new rings further up, past this point and that is bad with a capital B.

skirt is unchanged and rocking is not an issue. Only have about .004" between piston and wall and therefore don't need much length at all to avoid rocking.
 
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And IIRC from the Lycoming Course, new rings are recommended any time a cylinder is removed. The rings never seat in the same place after removal.
Mike
 
Thanks all for the replies!

The plane is experimental so no STC required.

Understood, but if there is an applicable STC it would be wise to read it and understand the information and engineering that goes into the modification. There might be other considerations besides just changing pistons (as noted above).

My O-320-E2D engine has the 8.5:1 compression piston upgrade, and the cylinders and everything was upgraded at the overhaul. I left the upgrade to the repair station that did the overhaul, so I don't know if anything else was required. I also converted the accessory case for a fuel pump, and changed to AFP fuel injection. The engine has performed great! I have yet to run any UL fuel, but it should be able to run the 93 octane.
 
Definitely a worthwhile change for me, done at overhaul, need 3" more pitch on the Sensenich propeller to bring the max rpm to a sensible number.
 
Front Main Bearing Upgrade

If memory serves, some of the earlier 150 HP models used a 4 piece front main bearing, while the 160 HP models used a long [4 inches] 2 piece bearing shell. I believe that this longer 2 piece front main bearing became the standard for all models.
 
Yes, the pin moves down on the piston and this forces the piston to travel higher into the cyl.

Yes, the stroke is the same and the stroke is what sets displacement, along with bore. HOWEVER, in order to get the increased compression ratio, The piston is moved up, toward the head, relative to the pin. This decreases the compressed volume at TDC and therefore a greater ratio of compression The piston is also higher at BDC, due to this, so stroke remains the same. So the amount of air drawn in is mostly determined by the stroke, but because the piston is higher, the compressed volume is much smaller, hence a greater ratio. This is what changes the compression ratio (volume at BDC vs volume at TDC). There will be a ridge where the old rings stopped their upward movement. The new piston will be driving the new rings further up, past this point and that is bad with a capital B.

skirt is unchanged and rocking is not an issue. Only have about .004" between piston and wall and therefore don't need much length at all to avoid rocking.

Larry, I think you may have stated this incorrectly.

“**Compression Height is measured from center of pin hole to top of crown” - From Combustion Technology.

The difference in height between a 7:1 and a 8.5:1 piston is .138”. I don’t believe the wrist pin is moved relative to the rings. Rather, the crown of the piston has more material left on the top of the piston and the relationship between the wrist pin and the rings is the same. The higher compression piston tops are raised and the edge is chamfered more to clear the combustion chamber.
 
Steve is correct: the ring lands to piston pin relationship remains the same with 8.5:1 pistons. Ring travel does not go further up the cylinder!
 
The ring lands are in the same location on all the parallel valve pistons. The dimension above the top ring gets larger with a chamfer to clear the head on the 8.5 and the 10:1.
May not work for everybody, but I reused the last set as they had little time on them. Each from the same jug, same ring groove without a hone job and no ill effects. Runs like a striped @ss ape!
 
0 320

The earliest 0 320's were 150 hp engines. The E series engines came much later and were attempts to provide cheaper engines. 0 235 front main bearings and plain steel cylinders. The 235 mains did not support constant speed props.
Some E series have different sump/intake tube configurations.
 
Where did the OWT come from that says Striped @ss Apes can run faster than Plain @ss Apes? Inquiring minds want to know! :rolleyes:

I remember reading somewhere that Checkerboard @ss Apes are not quite as fast, but are aerobatic! i.e. They can swing from trees upside-down.
 
Mr Hunt, I may be off the mark but stripes are always fast, especially when applied with a belt. At least I was fast when being chased by the belt!
Mel, I always liked checkerboard!!
 
Larry, I think you may have stated this incorrectly.

“**Compression Height is measured from center of pin hole to top of crown” - From Combustion Technology.

The difference in height between a 7:1 and a 8.5:1 piston is .138”. I don’t believe the wrist pin is moved relative to the rings. Rather, the crown of the piston has more material left on the top of the piston and the relationship between the wrist pin and the rings is the same. The higher compression piston tops are raised and the edge is chamfered more to clear the combustion chamber.

You are probably correct. I have never compared different CR pistons for lycomings. Only experience is with auto engines, where the ring lands typically maintain their distance with the crowns and therefore move up with higher CR pistons. Did not mean to sound as an authoritative figure on the subject, only using related experience to help someone from creating a serious issue.

I still wouldn't re use rings without a hone in this case. DOn't know the tolerances used by Lyc and others here, but if the bottom of the ring land on the new piston is just a few thou higher, relative to the pin center, than the original piston, you will get cracked rings.
 
There will be a ridge where the old rings stopped their upward movement. The new piston will be driving the new rings further up, past this point and that is bad with a capital B.
Great point. Besides cylinder choke mentioned in this thread the ridge may cause a capital B bad result. My soap box was this is more than throwing in pistons. It may require a total top end. Pre 2020 before mass inflation and WW panorama I'd say get a 180hp 360 and sell the existing engine. Now I say run what you bring.
 
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