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  #1  
Old 11-03-2008, 07:51 AM
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Sam Buchanan Sam Buchanan is offline
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Default Piston/compression/fuel combinations for O-360 and O-320

This info was originally submitted by Mahlon Russell:

Quote:
7:1 compression ratio, in an O-360, is had by installing 75413 or equivalent pistons and it is generally accepted to burn mogas with 87 octane and above with that compression ratio. Take off Hp is in the range of 168 with those pistons.

8.5:1 is the standard compression ratio for the O-360 and it is attained by use of 75089 or equivalent pistons. It is generally accepted to run mogas of 91 octane and above with this compression ratio. Rated power is 180 HP with those pistons

9:1 compression is attained by use of the LW-15357 or equivalent, in the O-360, and it is generally accepted that you should use 100LL r above with this engine. Normally see around 185 HP at take off, with those pistons.

The 320 and the 360 have the same bore, only the stroke is longer on the 360 to get the extra displacement. So you use the same pistons to get the same compression ratios for the 320 as with the 360.

7:1 compression ratio, in an O-320, is had by installing 75413 or equivalent pistons and it is generally accepted to burn mogas with 87 octane and above with that compression ratio. Take off Hp is in the range of 150 with those pistons.

8.5:1 is the high compression ratio for the O-320 and it is attained by use of 75089 or equivalent pistons. It is generally accepted to run mogas of 91 octane and above with this compression ratio. Rated power is 160 HP with those pistons

9:1 compression is attained by use of the LW-15357 or equivalent, in the O-320, and it is generally accepted that you should use 100LL above with this engine. You should see around 164 HP at take off, with those pistons.

Good Luck,
Mahlon

“The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
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Last edited by Sam Buchanan : 11-04-2008 at 05:51 PM. Reason: added link to Mahlon's bio
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  #2  
Old 11-04-2008, 11:17 AM
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Default Thanx, Sam!

And all this time I thought the difference between the -320 and -360 was in the bore size. Amazing how much you can learn browsing these forums.
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  #3  
Old 11-04-2008, 12:05 PM
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Default Mahlon

For those who wonder who this guy Mahlon is, and "why should I pay attention to what he says??", well, see for yourself.

Click Tech Advice on left.
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  #4  
Old 11-04-2008, 03:36 PM
Rick RV-4 Rick RV-4 is offline
 
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Default Mogas in a normal O-360?

So if I read this right, 91 Octane MOGAS "should" be okay with a stock (180 HP) O-360 with 8.5:1 pistons.

How about seals in the carburetor, fuel tanks, etc? From my limited knowledge of auto fuels, the big issue is to make sure it does not contain any ethanol or alcohol solvents. These will eat into some of the seals used throughout the fuel system, at least that's the rumor.

As for the tanks, I think I've read auto fuel is okay if you didn't slosh the tanks . . .

Curious what everyone else thinks about MOGAS in a Lycoming. I know this has been discussed before, but there seems to be more support these days than in the past.

Any thoughts?

Rick
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  #5  
Old 11-04-2008, 04:26 PM
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Default

Quote:
Originally Posted by Rick RV-4 View Post
So if I read this right, 91 Octane MOGAS "should" be okay with a stock (180 HP) O-360 with 8.5:1 pistons.

How about seals in the carburetor, fuel tanks, etc? From my limited knowledge of auto fuels, the big issue is to make sure it does not contain any ethanol or alcohol solvents. These will eat into some of the seals used throughout the fuel system, at least that's the rumor.

As for the tanks, I think I've read auto fuel is okay if you didn't slosh the tanks . . .

Curious what everyone else thinks about MOGAS in a Lycoming. I know this has been discussed before, but there seems to be more support these days than in the past.

Any thoughts?

Rick
It is understood (I assume) that we are talking about premium fuel not fuel enhanced with ethanol.
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  #6  
Old 11-04-2008, 04:27 PM
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Also, a X-540 can burn 92 octane mogas also if running 8.5-1 pistons.
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  #7  
Old 11-04-2008, 05:36 PM
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I hope I'm not diverting/cluttering this thread... if so dump me off in a new one.

I'm rebuilding my h2ad, which has 9:1 pistons (15357). So I take it I *can* switch to 75089 pistons and still maintain 160hp?

Same pins and plugs?

Does anyone know which rings to use with these pistons, or is it the same as the 15357 rings?

Thanks!
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  #8  
Old 11-04-2008, 06:14 PM
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Default No free lunch.

Sorry but if you lower compression from 9:1 to 8.5:1, you will not maintain the same hp.
The H2AD is a totally different engine.
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  #9  
Old 11-04-2008, 07:47 PM
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How many have flown the same airplane with the same engine only to have different piston compression ratios?

Am I the only one to have used 8.5:1, 10:1, and 9:1 pistons in the same airframe and engine?

Flew an hour today for the first time in 2 months and 4 days with 9:1 pistons for the first time.

Seat of the pants, an RV-6 O-320 with constant speed prop feels the same in cruise with the 8.5:1 and the 10:1 pistons. ROC may be slightly better with the 10:1. Unable to put a hard test number on the ROC improvements. In other words, I was not able to measure any improvement in ROC. Fuel burn was less with the 10:1 if you lean aggressively. Most engine shops that install 10:1 do not recommend leaning agressively.
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  #10  
Old 11-05-2008, 01:22 PM
rv620mr rv620mr is offline
 
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Cool Same plane, different pistons, choosing a CR

I have flown the same airplane with the same engine, with the same fixed pitch prop, but with different piston compression ratios. My RV-6 had 50 SMOH on a parallel valve IO-360 with 7:1 pistons when I swapped for 9.5:1 pistons.

My experience seems different from yours, Sam. My RV-6 has never had a constant speed prop. I only flew with the 9.5:1’s for about 2 hours before it became clear to me that I needed to replace that beautiful Warnke fixed pitch prop. My hazy recollection is that the new pistons added about 200 RPM, at WOT, in level cruise, at the same altitude, in similar atmospheric conditions. The testing was not scientific, precise, or repeated. It was, however, consistent with a rule of thumb used by Ed, at Sensenich, when I purchased a new 72FM with 85” of pitch. FYI, my particular application is NOT approved by either Ed or Sensenich.

This thread is “Piston/compression/fuel combinations for O-360 and O-320.” My understanding is that the purpose of the thread is to help others, perhaps like Andy, select an appropriate compression ratio for their engines. Sam, quite accurately I think, reports that “Most engine shops that install 10:1 do not recommend leaning aggressively.” That’s information I wanted to know when I was choosing a compression ratio for my engine. But, when I was making my choice, I also wanted to consider real world performance information. I wanted to know, “How much difference will it likely make if I do, against the engine shops’ recommendations, lean aggressively?”

My operating practices seem to differ from the engine shops’ recommendations. Maybe there’s an important distinction between my 9.5:1 pistons and those engine shops’ 10:1 pistons. I’m not sure. Maybe I’m just lucky. I’m not sure. I am sure that with the 9.5:1’s, I do lean “aggressively.”

My RV-6 burns only 100LL. It’s equipped with EI’s UBG16. And, I really do use it and monitor it. The aircraft is also equipped with GAMI injectors, one Slick mag timed at 25 deg BTDC, one Lightspeed III retarded 5 degree in accordance with Klaus’ recommendation for high compression piston applications, and Vetterman's crossover exhaust. With all of that out of the way, here are a couple of data points.

Example of my RV-6 with “aggressive” mixture leaning to a repeatable power setting yielding repeatable performance: 6,500 ft MSL, WOT, GAMI’s “big pull” on the mixture knob, 2550 RPM, mid-summer CHTs all less than 330 deg, oil temp at 180 deg, making 165 KTAS, burning 8.5 gph.

Exactly how many degrees lean of peak is that power setting? I dunno. It’s probably about 100 degrees lean of peak. Honestly, I’m a little afraid to check. I installed the EGT probes too far from the exhaust flange. They are slow to respond. At WOT, at 6,500 MSL, with the fixed pitch Sensenich, I estimate this engine will spin at least 2,850 RPM while I’m trying to precisely determine the peak EGT. It’s not worth knowing . . .

I think of this as my “flatland-headwind-go-fast” power setting. “Flatland” because my normal mission is 12,500 ft MSL or higher, on a 650 nm flight, over the Rockies, from Scottsbluff, NE to Las Vegas, NV. “Headwind” because, without that factor, I’d be flying at 12,500 ft MSL--even over the flatlands. And, “go-fast” simply because it’s WOT.

Comparison to my RV-6 with “traditional” mixture leaning to a repeatable power setting yielding repeatable performance: 8,500 ft MSL, 50 deg ROP, 2550 RPM, mid-summer CHTs all less than 380 deg, oil temp at 210 deg, making 172 KTAS, burning 10.5 gph. These numbers are reliable for my RV-6, but I almost never use this power setting. I think of it as unnecessarily hot and inefficient.

Note that the Example is at 6,500 ft MSL while the Comparison is at 8,500 MSL.

Hopefully, these data points--in conjunction with the nearly illegible Lycoming power charts (or Kevin Horton’s excellent engine power spreadsheet), prop manufacturers’ recommendations, and reports of the impending demise of 100LL--will be helpful to someone who is choosing the compression ratio that is right for his or her engine.
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