What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

mogas in possible high comp engine?

wadem

Member
Hi guys, with regards to detonation what exactly would happen to an engine that has higher compression piston in it if you ran regular octane mogas in it?
The story: I have an RV4 with a O-320 E2D in it. This engine only has the low comp pistons in it (maybe) resulting in 150hp. The engine was rebuilt at sometime a few owners back but the paper work is sketchy at best. The last owner only ran avgas in it because he thought that maybe when the motor was redone they upped the pistons to the high comp ones. Again, this was just his theory.
I only have about 12 hours on the plane since i bought it and it has ran perfectly on avgas. Today on a whim, i added about 7 gallons of mogas (reg octane) to each tank to top them up. I was planning on running this plane on mogas (150hp) but thought i would ease into it to see how the engine reacted.
I'm not sure if it was my imagination or not but i thought i got around 100rpm less static rpm on takeoff although it was a little colder then normal and I have not established a solid base line for this. Also the engine seemed to have a slight miss at around the 2300 rpm setting in cruise, again not there before. It was fine at higher rpm's. Finally, i have a climb prop on it and in the past was always able to push the rpm beyond redline at max power, today i was lucky to achieve 2700 rpm.
Could all of these or some of these issues be a result of a roughly 50/50 mix of avgas and mogas in a high compression piston engine? Or again, just my imagination? what exactly does detonation sound like? Is there anyway of figuring what compression pistons your engine has? Again no paperwork.
Even in a low comp engine would you see these issues just because you ran car gas instead on Avgas?
So basically i'm after what i should be looking for or seeing if i'm indeed running regular octane car gas in a high compression O-320.

Cheers
Wade
 
Depends on what you're calling High Compression. I ran tons of 93 Octane Mogas thru a 180 Lyc which has IIRC 8.4 to 1 and never a problem. The 150 also IIRC has 7 to 1 compression so the 180 is "high compression" and I seriously doubt there would be a problem with straight 93 octane at that ratio, based on my previous experience. There shouldn't be a problem with 100/87 at 50/50 either. All that with no ethanol and 8.4 to 1. With ethanol and/or some hotrod compression ratio I wouldn't run anything but avgas.
 
First things first

Wade,
Find out first exactly what you have on the business end of your new toy.
Compression, Fuel system, timing, some of that is listed on the engine data plate.
After you know all that you can experiment and it would be best to try mogas in one tank first so you have something to compare in flight.
I seriously doubt that you noticed any difference in performance with a few gallons mogas mixed into avgas.
Let us know exactly what drives your RV4 and as many specs on the engine and you will get a mountain of opinions on what you can do with mogas:D
 
Most likely if they did the HP increase it would now be running 8.5:1 compression which is fine for 91 octane. 7:1 is fine for 87. It's possible to put even higher compression cylinders on but it's not common. I'm having my E2A increased to 160 and will run 91. I don't know how to find out what you're engine has unless it's in the documentation....I'm certainly no expert.
 
Almost certainly if the timing is within specs, you can't get detonation with a 50/50 87/100LL mix even with 8.5 CR pistons. I doubt you'd be able to hear detonation in your average unmuffled RV. Sounds something like ball bearings rattling in a metal paint can.
 
It blows me away that some new cars like the Scion FR-S have a 12.5:1 compression ratio on pump gas. Clearly a different configuration but still instanely high by traditional standards.

Seems to me that even 8.5 would not be a problem.
But running a mixture keeps some of the anti-spoiling benefits of 100LL?

Tj
 
It blows me away that some new cars like the Scion FR-S have a 12.5:1 compression ratio on pump gas. Clearly a different configuration but still instanely high by traditional standards.
Seems to me that even 8.5 would not be a problem.
But running a mixture keeps some of the anti-spoiling benefits of 100LL?
Tj

These late model cars are running small bore engines at much higher RPM. The very large bore and low rpm torque of the Lycoming will contribute to detonation on lower octane fuel.
 
IF you look up the TCDS (Type Certificate Data Sheet) for a Lycoming O-320, (there are all the same TCDS 1E12 Rev 9 except for the H2AD engines) it lists 91/96 aviation gasoline as minimum fuel grade requirement.

Now if you go look at the Superior Vanguard engine TCDS E00001SC Rev 2, you will find that the Superior 360 (same bore, stroke, and compression ratio as the Lycoming parallel valve 360 engines) was certificated on 91/98 lead optional aviation gasoline or Motor Gasoline (R+M/2) Octane 91 (no alcohol) (See Note 7).

--- insert Copy / Paste Note 7 ---
NOTE 7. Experience has shown that there is a higher probability of vapor locking on aircraft, especially on those equipped with fuel injected reciprocating engines when operating with high volatility fuels such as motor gasoline. Aircraft fuel system designs for the powerplant installation of these engines may need to incorporate special design features or enhanced cooling to accommodate operation with high volatility fuels such as motor gasoline. The aircraft fuel system hot weather testing requirements of FAR 23.961 must be successfully accomplished for each aircraft powerplant installation design of these engines (both carbureted and fuel injected) to obtain approval for operation with motor gasoline, reference AC 23.1521-1B.
--- end insert ---

I will let you draw your own conclusion. The links will take you to the FAA web site and documents where I got the info.

I only use 100LL in my airplane because that is what they sell at all airports that I have landed at in my airplane over the past 15-years. I am too lasy to truck auto gas to my airplane to save a few dollars. If you do your own fueling, be careful about static electricity causing a spark and setting your airplane on fire. I remember seeing a video (I think it was YouTube) of an RV-4 being refueled from a plastic container that created a spark and set the wing on fire. Fire was put out quickly and it appeared that there was no damage or injuries.
 
How about 9:1, fuel injection and mogas??? Would that combo work OK if using high octane?
 
Take a look at the TCDS E-274 for the Lycoming O-320 H2AD engine that has 9:1 compression. It calls out 100 or 100LL* aviation gasoline. The * tells you to see the latest copy of Lycoming Service Instruction 1070 for alternate fuel grades.

I will let you look at SI 1070 to see what substitute fuels are acceptable to Lycoming. Some are only available in the C.I.S. and have LOTS more lead then what we use in North America. All the substitutes are aviation fuels and not motor gas.

As you know, you can put something in your airplane that is not recommended but then you are the one that may or may not have consequences to face.

I presently have 9:1 pistons in my O-320. I will need to cross the bridge on a substitute fuel when the time comes. The only saving grace that I have is that I have a set of NEW 8.5:1 pistons sitting in a box in my hangar with cylinder gasket sets. Having all the tools necessary to install the pistons, I only need to purchase rings, hone the cylinders, and do the work.
 
Almost certainly if the timing is within specs, you can't get detonation with a 50/50 87/100LL mix even with 8.5 CR pistons.

Ross, any qualifications on that? The available detonation survey data is limited, but what is available says you can detonate an 8.7 CR angle valve with only moderately dumb control inputs, on 100LL.
 
>>>>The available detonation survey data is limited, but what is available says you can detonate an 8.7 CR angle valve with only moderately dumb control inputs, on 100LL.[/QUOTE]<<<<

Sorry but what does the "moderately dumb control inputs" mean? Too lean? The angle valves I've flown never gave any detonation problems even when unmercifully abused in an aerobatic biplane in the hot summer time. But that's just my experience with them.

Another reason our motors require so much more octane than much higher compression car motors(even the old ones w/o an ECU monitoring things) is that they're run at much higher % power settings and they're air cooled both of which makes hot spots more likely under maybe marginal but common operating conditions.

Sorry. I messed up the quote.
 
Ross, any qualifications on that? The available detonation survey data is limited, but what is available says you can detonate an 8.7 CR angle valve with only moderately dumb control inputs, on 100LL.

Various sources list D910 spec of 100LL at a MON of 99.5 minimum to about 101.

87 octane pump gas has a MON of 82-83

92 octane pump gas has a MON of 87-88

We don't know how the base stock unleaded will react to the 1.2-2 grams of lead in the 100LL but generally there is a more pronounced effect for the first small percentage of TEL. But for conservative estimating let's say there is no synergistic effect and we have a linear relationship when mixing the two fuels and we use the lowest MON rating for both fuels.

99.5 + 82/ 2= 90.75 This is noticeably higher than the MON of 92 octane pump gas which many people are running successfully on Lycomings with CRs of 8.5 or higher without detonation issues.

I'll qualify my earlier post and say 8.5 TRUE CR. You should CC your heads and do a calc to know what your actual CR is.

I've tuned a lot of turbocharged engines on pump gas, 100LL, 100/130, special brews with heavy aromatics such as Xylene and Toulene as well as Methanol blends. Inducing detonation is relatively easy on turbo engines via boost adjustments so you can play around and measure the relative performance of different fuels easly on the dyno.

100LL is pretty good stuff compared to 92 pump gas. We were able to run engines up to a BMEP of around 450psi on straight 100LL before encountering detonation. A Lycoming at power peak makes around 145 psi. Unless the chamber is extremely poor on the Lycoming (possible) there is no way it is going to ever detonate on 100LL at 1/3rd the BMEP of our race engines unless the CHTs, IAT or timing were WAY out of limits.

I believe from this, it is also fairly safe to say, given the same conditions, that a Lycoming with a true CR of 8.5 or less is not going to detonate on 90.75 MON fuel, especially when many people have been running these engines on roughly 87 MON fuel successfully for years.

To really verify, someone could load one up on a dyno and try it. :) That would be the true test.
 
Last edited:
Your post on the percent power aircraft engines are run at is a good point many tend to overlook. About 11 years ago Malibu boats started offering the Corvette LS series motors in their boats. On paper and in tests the motor was fantastic. The mid range torque was so strong that waterski magazine could not complete their 25 to 36 mph test with a ski drone. The boat kept snapping the tow rope! They quickly sold 80 boats with the LS series engines then announced the motor was off the market. It turned out then in the first year of ownership 25 percent of the engines came apart. After lots of additional testing it was determined that the engine could not handle sustained high power demands and would start to detonate. Normally you would think a 400 HP engine out of a Corvette would be fine in a 20 foot boat but since boat motors also operate at very high sustained power demands like aircraft it did not work out. Malibu exchanged the engines out with a more traditional V8. Changes were made and the engine eventually came back into their line for a while but always seemed to be problematic.

George
 
Your post on the percent power aircraft engines are run at is a good point many tend to overlook. About 11 years ago Malibu boats started offering the Corvette LS series motors in their boats. On paper and in tests the motor was fantastic. The mid range torque was so strong that waterski magazine could not complete their 25 to 36 mph test with a ski drone. The boat kept snapping the tow rope! They quickly sold 80 boats with the LS series engines then announced the motor was off the market. It turned out then in the first year of ownership 25 percent of the engines came apart. After lots of additional testing it was determined that the engine could not handle sustained high power demands and would start to detonate. Normally you would think a 400 HP engine out of a Corvette would be fine in a 20 foot boat but since boat motors also operate at very high sustained power demands like aircraft it did not work out. Malibu exchanged the engines out with a more traditional V8. Changes were made and the engine eventually came back into their line for a while but always seemed to be problematic.

George

What ECU and calibration were these folks using on the LS engines? The OEM units go sick rich (11-11.5 AFR) after about 30-40 seconds of WOT above 4000 rpm- the reason for this is the pistons would die otherwise and fail the WOT validation tests. In factory form, the LS engine was tested and validated for over 200 hours continuously at FULL power. The later engines undergo even more than this- about 800 hours of WOT TP to PP rpm.

Many people who have not read or understood the findings of the development engineers on these engines have hurt pistons when tuning with different ECUs. The latest 450 hp DI LS uses oil squirters to keep the pistons alive without going super rich. saves fuel and emissions.
 
How about 9:1, fuel injection and mogas??? Would that combo work OK if using high octane?

That is my compression and mattituck built it to the highest compression for 91 Mogas. They have a reputation for knowing what they were doing.
 
That is my compression and mattituck built it to the highest compression for 91 Mogas. They have a reputation for knowing what they were doing.

No argument about Mattituck.
I can't give you the scientific answer like some of these guys but can give you the benefit of 50 years experience fooling around with these things. I would be OK with 9:1 and Premium Mogas if I knew for sure it was no ethanol and fresh and that my cowling install was optimum and my CHT ran consistently no more than about 400. But with the stuff we got for mogas now I would be wary; especially about Premium No Ethanol. Around here it's a slow seller and likely to be old; if you can find it. I bought 100 gallons for an half VW U/L and a Rotax 100 HP Zenith 701 I got here at the strip. Neither would run right on it. Talked to the bulk plant and found it was old when I bought it. They picked it up and gave me a full refund for the entire 100 gallons($569 several years ago). I would consider 9:1 and our questionable mogas in a tightly cowled RV to be potentially marginal. (And if you happen to be running an Ellison TBI from personal experience it will vapor lock on premuim mogas even in a loosely cowled plane, not so on the Marvel Schebler/same 180 Lyc).
I would have to run avgas. My opinion...
 
Last edited:
Thanks for all the info and replys....some good stuff there. I was up today for about 1.5 with the same fuel as yesterday and seemed to run fine. Like i said earlier, it was probably just my imagination.
I'm 95% certain that my E2D is as published, low comp pistons with 150hp.
I may try the compression/formula/transmission fluid in cylinder check if it keeps bothering me.
I think what i will do is fill one tank up with avgas and the other with straight regular octane (ethanol free) and then set out for a flight and observe the differences from tank to tank......stay tuned!

Cheers

Wade
 
Thanks for all the info and replys....some good stuff there. I was up today for about 1.5 with the same fuel as yesterday and seemed to run fine. Like i said earlier, it was probably just my imagination.
I'm 95% certain that my E2D is as published, low comp pistons with 150hp.
I may try the compression/formula/transmission fluid in cylinder check if it keeps bothering me.
I think what i will do is fill one tank up with avgas and the other with straight regular octane (ethanol free) and then set out for a flight and observe the differences from tank to tank......stay tuned!

Cheers

Wade

If it's a 150 you can't kill it. Well, almost.
I used to run exclusively 87 mogas no ethanol in my old Apache with 150s...55% and way lean of peak. 6 GPH down low. I finally decided it would go dead before I damaged it with the mixture
 
Eating Crow

Ok, some solid information about this topic has come to my attention from a very reliable source and it shows that you can indeed detonate a 8.75 CR IO-360 even on 100LL with a combination of lean mixtures, advanced timing, high IATs, high OTs and high CHTs, even with seriously reduced ignition advance.

:eek:So, I eat my previous words and urge caution to those running a 50/50 mix of pump gas and 100LL and even more caution to those running straight 91 pump gas. It looks like you should keep the power below 65%, CHTs well below max, timing retarded and watch that lean pull to LOP if you are inclined to run that way. Most people already follow these guidelines I think and there are good reasons not to push these limits.

It would seem the chambers on a Lycoming ARE worse than I imagined they could be!:);)
 
Planes and Automobiles

As for your car analogy, smaller bores and higher RPMs are completely irrelevant. Modern cars use a constantly changing ignition advance. This is one of your key things that are different from automobiles and cars and is the first thing to remember. Compression is just that, compression. So we won't delve too far into that other than to know how it relates to fuel. So, higher compression ratio engines generally need higher octane gas. Hence the STC restrictions. That being said, higher octane ratings affect the flash front of your fuel. The fuel being not just petrol but also air as well. The stoichiometric ratio is 14.7:1. This is why we have to lean as air density decreases with altitude. So...higher octane in gas at proper ratio equals slower flash front which means less chance of detonation or possible more timing advance if different from what your airplane's engine is rated for and what RPM range is specified by your given engine, hence you needing the numbers. The reasons that cars get away with all of this and have such a large RPM range? Here is why the car bore and RPMs are irrelevant...completely different ignition system. Not only can cars precisely meter their fuel, they do it based off of measuring the incoming air charge so there is no need to lean with altitude on an injected engine (there are no new engines produced that have carbs-take heed Piper). The computer automatically advances the ignition timing for ANY given RPM until the det/knock sensor sense a slight ping and then retards the timing just slightly. Technological wonder! But then again, if it fails you can just pull over. And there are no redundancies.
We generally don't use this system in aircraft because we operate in a narrow set of parameters as set forth by our props, governors, etc. Also, if both magnetos were doing this, they would fight each other and have separate ignitions at different times due to variances in the system. So we have to stick to our tried, tested and proven parameters. Use the 100LL, or use 90 or above octane, buy a tester from Spruce so you don't have ANY ethanol in your system at all and use ROP procedures whilst monitoring your temps. We won't go into the cooling effects of lead....lol.
Btw, I use mogas in my Cessna and have the STC for it. Look at some of the STCs and you might see better info than mine.
 
In the auto racing world, common belief is that necessary octane is also altitude dependent. Common rule of thumb is 1 less octane point per 1000 feet elevation. I say belief because I have not personally done the research and I don't want others to take my word as fact.

What I can state as factual is that in Florida if I put less than 91 octane on the street or 104 on the track in my turbo track toy, the detonation sensor will cut the boost and retard the timing. Here in Denver, 87 on the street and 100 on the track have never bothered the detonation sensor. So, I'm a believer. That says nothing other than the higher you operate, the less octane is necessary to prevent the dreaded detonation. YMMV
 
Last edited:
Back
Top