You get what you pay for
prkaye said:
A related question - it has been suggested that you can run MOGAS in a 160hp O-320. Is this also true for a 118hp O-235?
MOGAS in high compression engines? You can't use the MOgas at the airport. You need buy premium at a gas station (w/o alcohol, ethanol), hauled it to your aircraft in jugs, pour it into your tank. Prem national avg is $3.31, AVgas is $4.91, so looking at max min of each, you could save may be 50 cent min to a $1.50 max per gallon, avg about $1. So flying at 8g/h you save $8 an hour for all the hassle and potential dangers (read below). Of course premium is not available on the road since the Auto gas at the airfield is only 82 octane. Also the availability is not great. In fact some FBO's will not sell it to you unless you have your STC on file that you can use it.
Remember many 235's, 320 (160HP), 360 (180, 200) HP engines are certified for 91/96 octane. The pumped autogas at an airport (MOgas) is about 82 octane. Premium auto gas may say 94 octane but is really 89 octane on the AVgas scale. Will the engine run? Yes, but what are the draw backs. (read on).
The MOgas for use with early STC's for factory planes was squarely for low compression engines (in the 7's to 1 ratio). Here is a list of approx CR's for Lyc's.
O235 (115HP) 6.75:1 80 octane
O235 (115HP) 6.75:1 80 octane
O235 (116HP) 8.10:1 80 octane
O235 (125HP) 9.50:1 91/96 octane (now 100/100LL)
O320 (150HP) 7.00:1 80 octane
O320 (160HP) 8.50:1 91/96 octane (now 100/100LL)
O360 (180HP) 8.50:1 91/96 octane (now 100/100LL)
IO360 (200HP) 8.70:1 91/96 octane (now 100/100LL)
Mogas at airports (82 octane) is great for low compression engines but not so much for high compression engines. Also RV's have tight hot cowls with tight exhaust. All of these things are not good for using autogas. Just saying be careful.
Not all aircraft that are tested for autofuel STC's pass. The Mooney M-20-C and Piper Comanche with the 180HP, O-360-A1A (my engine), could not pass. They got pass the vapor lock problem, but the fuel kept boiling in the carb. Quote Petersen:
"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." (Note, the last line. This is from a group that does autogas STC's.)
So your custom RV is an unknown. If there is a STC out there for your engine, it's valid and tested only for the installation. Of course we don't need a STC, but it sure is an indication of caution when a plane can't get the STC due to a tight cowl and exhaust.
Vapor lock is caused by not only high temps of the fuel but altitude. If you intend on flying autogas high and hot, consider the following:
-Insulate/firesleeve all your fuel lines especially fwd of the firewall
-Heat shield / air blast tube on the gascolator and mech fuel pump
-Vapor return line from just before the carb back to the gas tank
Some people laugh and say, "Ahwww you don't need all that". OK. There are about 250 NTSB reports where autogas was on-board and vapor lock was named or suspected, about 80 reports in experimentals. Vapor lock like carb ice is hard to prove, since the evidence either melts (carb ice) or condenses as in the case of vapor lock. The description of erratic and low fuel pressure and loss of power are in many more reports without the words vapor lock mentioned. BTW, carb ice is thought to be more likely with autogas. That is my experience.
Vapor lock affects factory planes, experimental's and even planes using AVgas alike. So vapor lock can happen to anyone.
Autogas no matter how you paint it has much lower resistance to vapor lock than AVgas. Bottom line auto gas has much higher vapor pressure than AVgas, especially when car gas goes to the "winter blend". The winter blend has even high vapor pressure to assure better starting of cars in cold weather. Of course you need to test (worry) about alcohol and less than 10% ethanol which is becoming an issue.
Also lower octane means lower detonation margins. That is a fact without dispute. Are you running electronic ignition with timing advance? If you want to risk detonation or retard your timing (at loss of power) you can do that. It is experimental after all.
My advice or comment is engines that are designed for 81/86 octane are better suited for autogas. If you want to mess around with buying premium fuel at the corner gas station and hauling it to your 91/96 octane engine, go a head, but read about all the hazards of fuel contamination and fire danger.
NTSB reports that involved AUTO GAS, many accidents involved fuel contamination due to miss-handling of the fuel.
Octane at the Q-mart gas pump is 5 points lower than the AVgas equivalent, so "Super Duper Premium" that's labeled 94 octane is really 89 octane in AVgas land. When you fly cross country you will not able to get premium fuel from a car gas station, since it's unlikely you'll be able to haul it. The MOgas available at airports is only about 82 octane. That's great for engines certified for 80 octane, by not high compression engines.
You get what you pay for. Just be careful. It is serious business. The easy way and safer in my opinion is use AVgas and pay the extra $4 to $8 an hour. If you go for lower cost fuel, educate yourself and talk to folks that have do it, especially in a RV. I had experience with MOgas long ago in a C-182 and I was not impressed. As gas prices rise further, the incentive is great enough to use Mogas. Each to their own. Safety first, cheapness second is my motto.
No where did I say autogas is bad, but there are real issues and you are messing with your "margins of safety".