100 octane gasoline

[fishinworm]

Is their a problem with top end speed running 100 ll av gas in the yamato 102????

[Yellowjacket]

100 octane doesn't buy you anything if you don't need it. I don't think the 80 has enough compression ratio to need it. No harm, but no good either.

100 LL actually has a good bit of lead in it. It is only low lead relative to the old 100 octane avgas. If you've ever done an annual inspection on an airplane engine you'll remember picking balls of lead out of the lower spark plugs.. The lead forms on the plug electrodes and eventually melts off and accumulates in the bottom of lower plug... Better to use unleaded racing fuel if you need the octane.

Your plugs will thank you

[Ram4x4]

100LL, or any fuel with tetraethyl lead is not necessary and has potential to be bad for the engine and bad for the environment. The only real reason it is still used in aviation has more to do with the size of the market, lack of pressure to change it, the gobs of red tape it would take to change the aviation regulations and manufacturer's lack of willingness to derate or recertify their engines to run on lead free alternatives.

Some will argue that old engines need it because of the relatively soft metals used in them and the cushioning or lubrication properties the lead provides, but I've never seen any real science to back up that claim.

The only thing the lead does is increase octane, and you only need higher octane when you increase compression. Higher octane does not make your engine more powerful, but some people will swear it does.

If you aren't getting engine knock or detonation, then you don't need higher octane.

[Smitty]

When lead was to be taken out of gasoline, the auto industry responded by induction-hardening valve seats. If rebuilding an older engine, the solution was/is to have stellite inserts installed. I found a pretty good explanation (especially of how valve/seat recession actually occurs) at www.mossmotors.com/forum/forums/thread/9723.aspx

That aside, Worm, what they're telling you is correct; octane beyond what your engine needs to stay out of detonation will give you no benefits. And also as they said, many people are very sure that av-gas makes their cars really go!! Of course, many people believe in Bigfoot, Nessie, alien visitations, etc., etc..

Well, now I have to say that I think we are all assuming your 102 is a stocker. If you have raised the compression or increased spark advance, or done things with stacks, bigger carbs, porting, or anything that increases cylinder filling, then possibly you do need more octane than the factory called for . . .

[ZUL8TR]

As stated above engine compression ratio and other engine design elements will determine what octane you need to prevent knock and produce power. Octane rating of Research (RON or R) and Motor (MON or M) are expressed as an anti knock index of (R+M)/2 of gasoline. Regular fuel like 87 (R+M)/2, High test 91 (R+M)/2 as well as 100LL have essentially the same chemical energy (BTU/gallon) for making power just different additives like low lead in 100LL. Engine design, timing etc will determine what octane is needed to convert that chemical energy into power. However some fuel blends will burn more completely than others and that will affect power output, ideal is to have it to burn 100% at the right time. You only need the octane fuel that will prevent knock during the compression stroke, any more octane is not needed. Higher compression will result in greater mean pressure on the piston during the compression stroke which will produce more power (without knock) if the correct octane is used for the compression and other features built into the engine like Smitty states. A stock Yamato 102 needs 86 min octane according to the manual. Is your Yamato stock compression? I would stay away from the 100LL stuff for reasons stated above and just use the non Ethanol fuel of the octane needed.

Ref: Energy ratings of fuels, note the BTU/US gallon column:

http://www.appropedia.org/Energy_content_of_fuels

[dwhitford]

Bottom line is: USE THE LOWEST OCTANE YOUR ENGINE CAN TOLERATE because it provides more BTU per gallon than higher octane fuels and hence more power.

I learned most of what I know about mixing fuel from Bud Wiget when I worked at his Lakeland Marine Supply during 1966-'67. Bud had then retired from Shell Oil in California as a fuel expert to buy and run his boat store, a Johnson dealership. During the 1960s, Bud was annually contracted by an Indy Car team to mix their potent nitro-enhanced methanol qualifying fuel for the Indianapolis 500.

Bud once mixed a batch of fuel for a co-worker, Cookie Fernandez, who raced a modified Wizard-Merc in AOH. This was before we had a Modified category with its rigorous fuel rules, so Cookie could race any liquid fuel he wanted to in an alky class. Bud's gasoline-based fuel mix for Cookie contained 20% kerosene, specifically to LOWER the octane. Cookie reported that the fuel produced noticably quicker acceleration than the fuel he previously used.

If you link to ZUL8TR's fuel chart (above) and compare the BTU/gallon numbers of various fuels, you'll quickly see why using the lowest octane you can get away with is best.

[Yellowjacket]

If you look at the chart that ZUL8TR's chart carefully you'll see that btu/gallon isn't related to octane, it's related to fuel weight per gallon. That is, what are generally referred to as "heavy" fuels have more weight and more carbon molecules per gallon. It's that simple. Gasoline weighs about 6 pounds per gallon and jet and diesel fuels weigh close to 7 pounds per gallon. More weight equals more energy, it's that simple. These fuels have poor octane ratings because they are primarily blended for diesel or jet engine applications.

Btu/gallon is also unimportant since you need to jet your engine correctly. That is, if you're using a light fuel like methanol, you need to open your jet sizes to get the proper mixture. If you were using heavier fuels you would need to close down your jetting to get the right fuel/air ratio. Fuel/air ratio is by mass, not by volume. If you don't adjust your jetting to match your fuel weight you won't get the optimum performance from any fuel.

Octane requirement is heavily dependent on combustion chamber shape and size and engine speed. That is, the bigger your combustion chamber is and the shape of "squish areas" and spark plug location can create situations where the engine will detonate. Older aircraft engines are very "octane hungry" because the combustion chambers are big and open, and they run at relatively low speeds and high manifold pressures. For that reason it is critical that they get fuel that his a high octane rating in order to not have a detonation problem. To get that high an octane along with the correct fuel vapor pressure (so the fuel doesn't boil at altitude) isn't as easy as it sounds. Auto fuels are blended for the locality and the fuel that is used in the upper midwest in the winter would boil off easily in the summer at 20,000 feet. If it was easy to make a totally unleaded 100 octane avgas it would have been done a long time ago.

[Ram4x4]

Actually, a lot of newer small aircraft coming on the market are being certified for "mogas"...basically, regular old pump gas without ethanol. A lot of older piston aircraft can get certified for pump gas for a price. I think it's $500 for some inspector to slap a sticker on the aircraft.

Ethanol is a serious issue for aircraft. One of the primary reasons 100LL is still around has more to do with a lack of an octane booster to replace lead. Ethanol isn't suitable, so what else is there short of mixing your own fuels?

Back when I was a young man learning to fly, I remember 100/130 (dual rated octane equivalent...100 for lean mixture, 130 at rich mixture) being available at the FBO as well as 100LL. 100/130 had about double the lead content as 100LL. I know 115/145 is still made once a year for the Reno air races.

The removal of 110/130 and the once a year availability of 115/145 for a specific purpose, has left owner's of old war birds to use 100LL. The problem is, they can't run the old radial engines at full power anymore without damaging the engine. They have to use reduced power settings.

[bh/]

AV fuel I believe must be stable for 2 years.
Pump gas today is lucky to lite up after 2 months!

[Yellowjacket]

Most of the engines that can be run on mogas are the engines that ran on the old 80 octane avgas. These are generally lower performance engines. Most engines much over 200 hp require 100 LL and can't be run on mogas. The STC that you can get for mogas covers a number of engines. The mogas you can use is premium, not regular, and there are limitations as to how high you can fly when using it since mogas doesn't have the controls on Reid vapor pressure that apply to avgas. It's fine for touring around the patch, but it isn't for any serious cross country work.

Good gas is getting harder to come by, that's for sure. The EPA is screwing it all up...

[jeff55v]

Years ago, a snowmobile racing friend mixed a special batch of his 100+ octane racing fuel for me to try in my ASR. When I tried it, I lost 3mph instantly, when compared to standard pump gas. Lesson learned.

[bh/]

Jeff; -I wonder if the power loss was due to the 100 octane. Could he have 'juiced' the fuel
with ethanol/methanol etc to raise the octane, thereby requiring a richer fuel setting to get max power ?

[goose]

What does 90 octane Rec gas do. I run 112 octane with rec gas in a very well built race sled @ 1 quart race gas to 3 quarts Rec gas @ 50/1 oil mix. Recommended by the engine builder.

[ZUL8TR]

Based on a volume proportion mix 1 gt of race gas @112 + 3 qt rec gas @ 90 would yield 95.5 octane. What would it do, nothing if your engine doesn't need 95.5 octane. Detonation would occur if the engine needs more than 95.5 octane for the conditions you run it at. There are a lot of other unknown conditions at play here.

[Smitty]

Worm, the funny thing about those guys who think that av-gas gives their cars a lot more pep is that 99% of the time, they don't even need the LOW-octane pump gas. When you are cruising in your car along a flat stretch of freeway at 60mph, your engine might only be putting out 30hp, and could probably run just fine on the kind of real low octane hair-oil they had for gasoline ninety years ago. But you have to have the manufacturer's recommended octane for those times you are, for an extreme example, accelerating hard up a long steep hill with a full load on a hot day. It's a worst-case thing.

Same deal with airplanes. My dad flew Lockheed Constellation propliners for many years; four Wright 3350 twin-row radials putting out 2500hp apiece on 100/130 . . . but only on takeoff and climb-out. At cruising altitude, even leaned-out to reduce the fuel burn, those engines probably would have done fine on 80/87.

But here's one way that raceboats differ from, say, roadracing motorcycles. The bikes run much of a lap at less than full throttle. Boats run flat-out almost the whole time . . . so whatever octane is called for really is needed the whole time.

(Of course, any PRO guy who happens on to this thread is thinking, "Methanol is for racing, nitro is for winning (or breaking), gasoline is for cleaning parts.")

[dwhitford]

Smitty, I'm a PRO guy LOL about gasoline for cleaning parts. RIGHT ON!

[1100-one]

Smitty - That busted me up on your last portion of your reply. I was into it until then lol. Not trying to be smart azz but we use a mix of gas to fog with just sayin. In all honesty Worm you have been given good advice. Yes I tried av gas years ago in a mod motor when I ran FER. Didnt know what I was doing lol but had left over parts. The one that always puzzled me was when I ran regular pump gas and added marvel mystery oil just to keep it clean. Thrown out as then it moved the old GT meter I think it was called. If it where me follow the guys who run this motor which sounds like regular gas at a certain octane needed

[deeougee]

So for example if I have a Yamato 80 putting out 175 lbs of compression on each cylinder what octane should be run? What's the formulation for figuring out what octane to use for any motor?

[Ram4x4]

It's not that easy. There isn't a magic formula you can use. Manufacturers determine the rating after designing and building the engine through testing with decreasing octane until knock is detected. That's why most discussions on octane tend toward "just use what the manufacturer recommends".

However....there are general rules of thumb if you know the compression ratio (carb'd engine, no automatic controls) to get you in the ball park:

Compression Ratio: Octane Number:
5:1 - 72
6:1 - 81
7:1 - 87
8:1 - 92
9:1 - 96
10:1 - 100
11:1 - 104
12:1 - 108

Other than that, there are too many factors involved to use a single formula.

Just changing the timing will affect the octane requirement, etc, so if you are modding the engine, then that will affect the needed octane over what the manufacturer calls for.


p.s. there's no magic equation for converting cylinder pressure in PSI to compression ratio. We know that in terms of cylinder volume we can say that when the piston is at Bottom Dead Center (BDC) the volume is at maximum. When the piston is at Top Dead Center (TDC) it is at minimum, so the comparison of max and min volume can give us the ratio....but, piston rings don't seal perfectly, and 2-strokes have a tendency to shove some of the incoming fuel charge out the exhaust port (i.e. lost compression), also why tuned pipes are great for increasing power on a 2-stroke.

[Yellowjacket]

So for example if I have a Yamato 80 putting out 175 lbs of compression on each cylinder what octane should be run? What's the formulation for figuring out what octane to use for any motor?

Static compression pressure isn't everything. You need to know the compression ratio when the motor is running and if the engine has some sort of pipe on it that would increase the dynamic compression. Also how hot are you running the engine. Cooler cylinder walls reduce the tendency to detonate. Finally what does the combustion chamber look like. Does it have "squish" areas or is it more open. Ignition advance, how much and are you being greedy???

All of the above plus the engine speed are factors that go into it. Best to learn from folks who've nuked engines and destroyed motors to try to find out what the limits are.

As a practical matter if you're running a modified engine the best thing to do is get the best gas you can get, and run the coldest plugs you can get away with without having fouling problems and then try to keep raising the compression until you get to the point of detonation, and then back off a bit.

One other thing is that there are detonation control systems that you can get for more modern electronic ignition engines and we use one on our single cylinder heavy fuel test engine. We are running a small engine on jet fuel with a lower compression ratio and an advanced fuel injection system. We added a piggy back detonation warning/control system that retards the ignition and tells the data system how much it cuts back the spark. Our system is running on a single cylinder engine, but with the "coil on cylinder" systems becoming commonplace I would think that it wouldn't be too tough to set up a system like we have on a boat motor. We got our system from J&S electronics and they modified an existing system to work on our engine for a very reasonable cost. You'd need a small battery to use it on a boat that didn't already have an alternator, but the amount of power consumed is minimal. If you had a system like that you could figure out how much advance you can get away with and not slag down your motor

If you're looking for that last few horsepower that's where you could find it

[Ram4x4]

You're running a spark ignition engine...at a low compression ratio...on kerosene???