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Technical Question - High Compression Pistons in O-320

Bob Breeden

Registered User
Alaska
Guys,

Perhaps you expert mechanics have developed an opinion on this. I'd appreciate your thoughts.

The cylinders are removed from my O-320 right now and being sent to a cylinder shop to reface the exhaust ports. My mechanic says the cylinders look real good otherwise. 1570 hours total time on a 1977 engine from a 1977 Cub. We looked at the cam and tappets, and they look like new.

Questions: Now that the cylinders are off, is is advisable from a practical standpoint, or legal with an STC to replace the pistons with high compression pistons? Which pistons are recommended? I use 87 octane car gas all the time except when on trips, when of course the engine burns 100LL. Would I lose my cargas STC, or cause damage with 87 octane fuel? Is there a perceptable gain in efficiency getting more bang for the gallon of gas? Does leaning become touchy?

There is nothing like my trusty old 150 horse engine. She will pull stumps out of the ground. She takes off shorter than she will land in any given condition except super high density altitudes, even on fairly soft ground.

But there is nothing like horsepower to spin that 82 x 41 prop a bit faster on takeoff. I am not looking to cruise any faster, already cruise at less than 1/2 throttle, 95 miles an hour.

Thanks,
Bob Breeden
 
Bob - not a total answer here but if you do put in the higher comp pistons (yes there's an STC - you'll also need stronger wrist pins) and turn your A2B into a B2B 160 hp, you can only legally run 91 octane or higher unleaded car gas.

(In the real world guys will still run 87 from time to time as that's all that may be available in the bush to get you home. The engine will tend to diesel at shut down.)

Brad
 
Bob the changes that need to be made are more than just the pistons. Front main bearing has to be changed, if it's a narrow deck,longer studs for hold down plates. Just a quick and dirty overveiw.

Matt
 
Great, thanks Diggler, Matt and Brad,

Can you feel the reported extra 10 horsepower, when coupled to the same 41 pitch prop? And who sells the STC's?

Bob
 
Diggler no I'm not talking about changing to a -27. Lycoming A&E series engines run 4 individual main bearing shells up front and the B&D series run 2 bearing shells rotated 90 Deg to the case halves. (A&E series engines are low compression 150 HP and B&D series are high compression 160 HP.)

Bob you would have to split the case and have the front main anti rotation pins modified to accecpt the larger front main bearing. Make sure your Crankshaft will comply with AD 98-02-08, which basically means to comply with Lycoming SB 505 and 530 for pitting inside the front main on the crank. You may have to run a different P/N carb also. I don't have that exact info in front of me. (Steve P. MD, TJ, Dave C.).

Yes you can tell the extra HP.

I'll weigh in on the Peterson STC. Personally I don't think it is worth the paper it's written on and here's why. Basically no airport is going to have it available for use. They have 87 but not 91 in car gas. If 100 is available just burn that. If you can store 100 or so gallons of 91 to fuel out of for local flights then fine but hauling 5 gallons at a time frome town is a pain.( Yes I Know you guys in Alaska do it all the time.)

Matt
 
diggler said:
I just overhauled a 0320A2B wide deck engine and it had 4 individual main bearings and a 2 main bearing shells in the front pn 13884. It was a low comp Lyc engine. I installed the pistons etc per the stc. Didnt have to change any bearings.

I'll second that! Just did Five complete overhauls O-320-A2B's, over the last 6 months, with 3 being -27A wide decks and two being -27 narrows. All five had the long two piece (1 set) front bearings and the other four individual mains (2 sets). New ones installed were pt#SL13884 and pt#SL11020 respectively. The certified engine shop I use turned all three wide decks into B2B's by putting in high compression pistons pt#75089 and heavy wrist pins pt#SL13444-1 and not much else changed from the 150hp parts, to my knowledge (or the bills I have in front of me!). Carb remains the same correct 10-3678-32 for both 150's and 160's and my shop has a water dyno and they all ran to the + side of 163HP on the dyno after overhaul to B2B's. One engine ran 165.57hp @ 2671RPM. Two engines were done for homebuilt usage and the third was done certified for a PA-12, with new Superior Millenium cylinder assemblies Pt # SL32006W-A20P. The narrow decks are being left at 150HP as my guy doesn't like using the crank with the lightening holes in the prop flange for 160's, but I think the main concern is cost when you start adding new thru bolts/studs/hold down plates etc that you'd need extra to do a narrow deck.

Cheers,
Wayne
 
SC37 - You need to come up to BRD some time. We have 91 octane unleaded car gas at this field. Use it all the time. Brad
 
Anyone ever use Lyc SI 1304J to up the comp in the 320? It allows changes to the SN suffix by stamping a "C" than the new suffix, like going from a -27A to -39A. I think maybe if you did a 337 on the engine referencing the SI, the overhaul and parts manuals as approved data, you could skip the STC for the engine.
 
T.J. Hinkle said:
The proper carb is determined by the P/N of the oil sump. The Lycoming Parts Book list 4 carbs. They then refer to the sump P/N. Check this thread "Sick Bay", then "Engine runs rich."

T.J. what I was pointing out was that there isn't any need to run out and get a different carb to go from 150 Hp to 160 hp. The orginal will flow enough air/fuel to get the job done. The correct carb for an A2B or B2B is the 10-3678-32 and they both should have straight bore risers in theirs sumps. Other O-320 model #'s have tapered rises and may use a different carb. The other carbs that work for O-320's are 10-5009's, another 10-3678 with the dash #11??(as opposed to -32) and the 10-5135 that is applicable to the O-320-E2D's

As for SuperCub MD's post reference restamping the data plate. That is exactly what my engine shop did with the A2B that we turned into a B2B for the PA-12. All the engine shop is allowed to do is stamp a "C" on the data plate beside the A2B (to show it's been converted to something else) and make entries in the engine log stating engine Converted to 160 HP- B2B per Lycoming service manual..... and he is not even supposed to change it to B2B (in Canada anyhow!) on the data plate nor is he allowed to touch the 150HP marking on the data plate. No "STC" required for the engine modification (in Canada anyhow), just for the installation into the airplane (and I don't know where the owner of the PA-12 is getting that).

Cheers.
Wayne
 
Sorry guys in my above post I was thinking of the conversion of an E series to a D series and not the A series to B series sorry about the confusion. ( I know I shouldn't babble on at 3 AM after work without the proper manuals in front of me :oops: )

Brad good to know BRD has 91 oct. available. Still think the availabilty is going to be very limited. ( No MD you taxing up to the gas station doesn't count.)

MD you get the cub back up and flying yet?

Matt
 
Taxing up to the gas station does work the best thou, just push the right button, and the spigot pumps out whatever you want.

After I pulled the S-Cub engine apart, everything was pretty much trashed. I've got a good sump..with a data plate on it. I hope to build a new WD up on top of it. I've flown and built a lot of ND's and never had a problem with one I've built, but having this one (it was mid time, built by a large engine shop) come apart in flight without warning really pisses me off. So for now, I'm just gathering up the nesessary new parts, but building my own engine wasn't exactly in this years budget. I haven't even got the whole plane back to the shop yet, the fuselage is safe in a shed, but I want to get it home. Lot's of down time to catch up on all that patch work I've been putting off.
 
SuperCub MD said:
Anyone ever use Lyc SI 1304J to up the comp in the 320? It allows changes to the SN suffix by stamping a "C" than the new suffix, like going from a -27A to -39A. I think maybe if you did a 337 on the engine referencing the SI, the overhaul and parts manuals as approved data, you could skip the STC for the engine.

Did that a few times. Just have to remember that the B series have choked not straight wall cylinders, so legally you have to have your cyls converted to the proper P/N or exchanged for choked cyls and use proper rings.
 
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