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Originally Posted by 61DX
And don't forget: Vapor Pressure. RV's are low wing aircraft, and most of the fuel system is under negative pressure. Ethanol increases the possibility of 'vapor lock' especially at higher altitudes.
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Thanks for mentioning it. MOgas discussions can get into heated debates. Autogas proponents defend the use vigirously, but tend to not mention the down side. There are both good and bad.
I did a search of "vapor lock" and also "automotive fuel" in the NTSB. I got about 250 hits, about 50 with experimental. Some hits where unexplained loss of power with out mention of vapor lock, but the signs where there, erratic fuel pressure, power surge and high ambient temperatures. IN many cases the FAA just came out and said vapor lock, which is hard to prove, since the evidence is not there later when things cool down. It's like carb ice, the ice melts there is no evidence.
You'll hear stories of someone's buddy who has flown 20 years in a 150 HP Piper Cherokee on auto fuel. No doubt true and he is happy, but this is low compression engine, large cowl, may be flown in temperate climates and not at high altitude. All effect vapor lock. A high compression, tight cowled RV flown at high altitudes and hot temperatures may not do as well.
Yes the EAA has done great work on the subject, and lead the development of STC's for autofuel, but I have my doubts RV's with the tight cowls, wrap around cross over exhaust pipes would pass a formal STC approval out of the box or at all, even after fuel system modification. I don't know that is a guess and don't think anyone has done formal flight test with fuel temp monitoring.
Before any one tells me we don't need a STC, I know. However lets be realistic. Consider this from Petersen:
Q. Why isn't the Mooney M-20-C (or Piper Comanche) approved?
A. The Mooney and Comanche both experienced vapor lock problems when they were tested. We solved the vapor lock problem, but could not overcome pneumatic lock. Pneumatic lock takes place when the fuel boils as it enters the carb. The engine then dies due to an over rich mixture. This is just the opposite of a vapor lock where the engine quits or runs poorly due to a lean mixture. The better an airplane performs, the more difficult it is to get it through the flight test program.
The above is from experts in the field. They did have vapor lock but needed to modify the fuel system. They could not solve the other issue. I'm not anti-autofuel just be careful. The Mooney with a 180HP carb engine and constant speed prop is a pretty close match to the RV. In fact my RV-7 engine and prop are identical to a Mooney-C model. So why would I not suffer the same issues if I used auto fuel.
On a related topic, Octane. The Octane on auto pump gas is about 5 points less than if it was sold as AVgas. Low compress in engines (in the low/mid 7.0:1CR range) where certified for 80/87 octane. Higher compression engines in the mid 8.0:1CR range are certified for 91/96 octane. The two numbers are for lean / rich octane respectively. This is unique to AVgas ratings. Car gas has a combined single rating. Bottom line if you buy premium auto gas rated at 92 octane it is really like 89 octane AVgas. Will your 160/180 Lyc engine certified for 91/96 octane run on 87. May be? I don't know. You might be able to retard the timing a degree or two. I know the combo of retarded timing and lower octane means less power. At worst it could produce detonation and engine damage.
I try to be straightforward with the facts and not prejudicial. I am a little immune to testimonials. I did fly a C182 over 10 years ago. We (a group owner ship) decided to stop using autogas, despite having a for it STC. Carb ice was worse with autofuel. Also the engine ran fine on 100/100LL and was easy to buy. The price difference at the time was less than 0.50?, sometimes less. To use auto fuel for some where MOgas is not pumped at the field, you have to haul it in cans to your plane. This opens a whole can of whoop a** and possible ways to contaminate your fuel, not to mention the hazard of handling fuel. It can be done safely but look at the NTSB reports, they are full of fuel contamination accidents related to autogas and poor handling and transfer.
Last is my pet peeve is cost savings. Yes it is true it MIGHT BE cheaper. However if you look up the Min/Max of 100/100LL AVgas, MOgas (sold at airports) and premium auto gas sold at your local corner gas station you will find
AVgas can be cheaper than MOgas. Also the price differntial between AVgas and premium auto gas can be a $1 or a few dimes. So the average savings my be in the $4.00/hr range? Is it worth the risk and hassle?
There are folks that live in a part of the country that can get ethanol / alcohol free cheep premium autogas and have low compression engines in certified planes with an auto gas STC. I understand why they use auto gas. I would also use auto gas in that situation. However, I have a high compression experimental with a tight cowl, exhaust pipes that go next to the carb, fuel pump, gascolator and all the fuel lines. Yes I could shield, insulate and air blast tube everything, but I would rather use AVgas. I also like to fly high cross country. Not all airports carry MOgas. Besides low octane MOgas is not suited for high compression engines (160/180HP Lycs).
Clearly the work the EAA has done is great. The fact is those 80 octane engines don't run well on 100/100LL. They NEED auto fuel. 100/100LL has a lot of lead in it, so you hear stories of how much better an engine runs on auto gas, it's true. Low compression engines that used 80 octane AVgas where made for basiclly unleaded gas. This is not the case with high compression engines, However unleaded fuel (UL) is coming for EPA reasons. Lead is a way to get the octane up, but it is not needed as an ingreadent in its self. The future of AVfuel is likely an unleaded version of 100/100LL for a UL95, like they have in Europe. They need to do more tests and figure out ways to keep the octane up. Also UL95 will (may?) not work for the highest compression engines, but it will work for our little 320/360's.
As far as autogas please be careful. Here is a source for new fuel pump cooling shroud (select products, scroll to bottom):
http://www.showplanes.com/index_1024.htm
Vapor lock can even happen with AVgas, but have gas has a Vapor pressure 3 times lower and tightly controlled. Auto gas especially in the winter when they blend it for higher vapor pressure for easy starting in cold weather can be a real pain. Just keep that in mind.
Gary
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