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MoGas compression ratio

unitink72

Well Known Member
I know this has been beaten to death, but hoping for some input on my engine.

About to pull the trigger on a Lyc 540 Thunderbolt. Cold Air Intake, SDS Efi/Ignition. My important requirement is upper altitude efficiency. Don't care about max power. I want it to be MoGas compatible. 91E0 and 91E10 is available locally.

First thing I'm not familiar with - when you go to an airport with MoGas, what octane do they typically offer?

As for engine compression ratio. Since the RV-10 is a good higher-alt cruiser, a higher compression ratio should be able to squeeze a little more work from the fuel at those alts.

My first thought was lets try 9:1, as some have had success with that. Ross from SDS told me that would work, we would just reduce timing at high MAPs as a buffer against detonation. Others say 8.5 is good and should be rock solid with plenty of margin. I could also split the difference at 8.7, but what I don't know is the real improvement when bumping up these numbers. Its been said that past 9:1 the returns diminish.

Another factor I'm thinking about is the recent Kitplanes article says that Cold Air Intakes give you a little more margin against detonation.

So I'm torn. Since this is one of the single most expensive things I've ever purchased, I'd like to get it right the first time. Any thoughts or advice from the community?
 
I'm running an IO360 with the SDS, Superior cold air, and 8.7:1 compression. I retard my timing to 21 degrees at high (takeoff at 2700' MSL here) MAP, and run it up to a maximum of 29 degrees at altitude and LOP. During testing I discovered that at about 75+% power I was able to induce mild detonation pretty reliably at 31 degrees on 91E10 (Sams Club source) so I set my max timing at 29 and only use that at altitude and LOP.

I climb rich and full throttle at 21 degrees during takeoff up to 25 degrees within a few thousand feet more, and I hold that until cruise altitude then go LOP and add 4 more degrees of timing to 29.

I usually cruise in the oxygen altitudes in my 9A and the efficiency is terrific.
 

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Listen to Ross

Since the most critical and flexible part of your solution will come from Ross, I would listen to his advice.
 
High altitude requires power, power comes from displacement or CR.
I gave up a little efficiency for extra HP with 9.6CR (IO370), these days an extra gallon an hour is way less important to me than HP.
However I would never run mogas.
Pick your poison.
 
High altitude requires power, power comes from displacement or CR.
I gave up a little efficiency for extra HP with 9.6CR

I don't understand the statement about giving up efficiency in return for higher power. Is that solely based on the higher compression ratio? Mind explaining?
 
As a retired physicist, I like to remind people that ‘units matter’. Car gas octane (R+M/2) and AvGas octane (motor octane) are NOT the same units. Please be careful.
 
I don't understand the statement about giving up efficiency in return for higher power. Is that solely based on the higher compression ratio? Mind explaining?

For a given number of cubic inches displacement, and all else being equal, you can extract more horsepower from the fuel with a higher compression ratio - but you can't run mogas with the high compression, you'll be limited to 100LL.

I knew I wanted to run 91E10 in my airplane, and I knew that was going to cost me a few horsepower to set the engine up that way - so I installed an IO360 instead of the normal IO320 to add another 40 cubic inches in order to get that horsepower back. I still catch no end of noise from certain individuals on here that I'm going to die screaming by making that choice, even though I'm only running a few horsepower more than the theoretical magic limit.

Do you ever fill up on cross countries? What do you find for MoGas?

Sure - you can always burn a BETTER fuel - and 100LL is superior to both mogas and 91E10 from a fuel/detonation perspective. I can burn either one but I run 91E10 when flying from home due to the low cost.

As a retired physicist, I like to remind people that ‘units matter’. Car gas octane (R+M/2) and AvGas octane (motor octane) are NOT the same units. Please be careful.

He's right, ya know. The only way to know for sure is to test your own individual installation thoroughly, find out where the edges are, and stay away from them.
 
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I want it to be MoGas compatible. 91E0 and 91E10 is available locally.

My question is why?

30+ years ago back when I bought my first few airplanes (AA5, AA5-A) I ran around with two 20+ gal RV HDPE water tanks in my car and used them to xfer car gas. I was broke, had a **** car, and wanted to be as cheeeep as possible. Heck, I'd even xcountry with it in the plane and fill up on the way.

Pretty quickly that got old. Now I have nicer cars, a worse back, and the "red can" thing is just a pain. Car smells, you often end up pouring gas over the wing, mogas changes formulation throughout the year and across the country, and mogas isn't available many places on xcountry (at least where I fly, maybe more in farming country).

If I burned car gas 100% of the time it would still only save me under $1,500 a year, comparing (not the absolute cheapest) local 100LL, 100 hours a year, and 8.5 gph average burn to local premium unleaded prices. Realistically I think the maximum I could save would be under $1,000 a year. Is that really worth it when you weigh in the issues with car gas reliability, availability, and the pita of using it? Not to me by a long shot.
 
My question is why?

Its mostly about knowing myself. I'm used to throwing pallets of cash at this airplane while building. But once its flying, I'll think twice about flying sometimes based on gas price. Goal is to make it cheap as possible to fly, but without cutting any corners.

As for the downsides of Mogas, a 60-80gal tank with electric pump in my truck is probably *more* convenient than filling up on the ramp for local flights. As for availability - I know where they are and I'm comfortable with it.

I'll provision for MoGas with the knowledge that there's always a trusty backup if I can't find it - 100LL.
 
Greg has the best data points from actual flight testing with a similar setup and he's a smart guy on this stuff.

You could go to 9 to 1 and retard timing a couple more degrees at high MAP. The extra .3 CR might buy you 0.5% better fuel economy up high where you can advance timing a bit more with the EI.

Splitting hairs here really.
 
I don't understand the statement about giving up efficiency in return for higher power. Is that solely based on the higher compression ratio? Mind explaining?

Efficiency in relation to burning more fuel, I don't care about fuel burn, I want HP :D
 
Increased CR will give you more power as long as the octane rating of the fuel allows you to optimize timing for PCP at the optimal crank angle.

Higher CR is your friend up high with an atmo engine. The more you can squeeze the thin air up there, the more power you make. Dave Anders was running a 12 to 1 engine for a while. Was awesome up high.
 
My question is why?

Lead...it causes all kinds of issues and it's in the sights environmentalists.

Couple months ago Mike Busch had an EAA webinar that explained the reason for sticky valves, primarily in Lycomings...lead and improper scavenging.

If you want higher compression you can have one tank 100LL and the other mogas and burn 100LL during higher power operations. Not any different than having an ADI system.
 
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My question is why?...

Its nice to have the flexibility to run any gasoline, ranging from pure 100LL to the trash from the corner gas station, or any mixture of the two.

I built my engine and fuel system specifically to enable this flexibility.
 
What about a stock Van’s IO-390?

I’ll defer to the wisdom of those who know such things. What changes needed be made to safely run a stock IO-390 on:
- 94UL avgas
- 93 ethanol free mogas?

Thanks - looking to pros/cons between doing an RV-14A as they next plane or another RV-10.

Carl
 
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