av gas

[strokeracevt]

Thanks to all that replied. Before I rolled the dice, I spoke with a good friend that is an airplane mech and owns his own repair business. He said go for it and set me straight on some of the myths surrounding the use of av gas in all the different types of planes. To say that all airplanes only run 3k rpm is false for one. I put 5 hours on the meter today and the engine ran perfect at all rpm's. When I got home i pulled the plugs and they were no different from the last time I looked at them a couple weeks ago. I will keep checking them for sure. For the extra $3 a gal I will continue to run it as extra insurance that I will not have to screw with the water in my fuel.

[rodbolt17]

who said ALL aircraft engines run below 3000 RPM?
MOST do.
its also why MOST gear cases on outboards turn the prop BELOW about 2500 RPM.
its actaual common sense.
but believe who you wish and don't look at the PHYSICS of what the ell I typed.
physics will win everytime.
hint, google surface feet per minute.

[almetelo]

Back in 1995 when I bought my 30DMO/30MLH x3 I used for a few years leaded fuel (not AV gas) until it was forbidden due to environmental issues.
At the time didn't had a tach or speedometer but couldn't notice any diference when moved to unleaded fuel 95 or 98 octane (I've read somewhere here that the leaded fuel of that time had 97 or 98 octane)

But my question is:

The octane choise have anything to do with compression ratio?

I've read somewhere that higher is the compression ratio higher should be the octane.

[TownsendsFJR1300]

But my question is:
The octane choise have anything to do with compression ratio?

I've read somewhere that higher is the compression ratio higher should be the octane.

Compression ratio indeed makes a difference. Genereally, IME, the higher the compression ratio, the more likely you'll need a higher Octane fuel.

The higher octane, as Rodbolt posted, allows the fuel to burn SLOWER. The slower burning helps stop knocking/pinging, pre-ignition(in severe situations, dieseling).

With that said, follow the manufacturers recommendations for fuel grade. If your getting the pinging, etc, upgrade to premium to prevent the pre-ignition/knocking.

My motorcycle, a Yamaha FZ600cc (I4, 95 HP, 14,000 red line) has a 12.2 compression ratio. (water cooler, oil cooled) The recommended fuel is regular. Never had an issue with knocking, etc.

Now my friends BMW's, almost all of them, similar compression ratio's, the manufacturer recomends premium fuel. Is this for preventive, possible pre-igniton? I can't say, he's never tried regular...

Running premium, when NOT required, your wasting your money. Should you experiance, pinging, etc, get premium in there asap as its very bad for the engine..

My above mentioned bike;

[rodbolt17]

trust me, at a true 11/1 you wont run regular for long.
9/1 is about all you can run and still use 87 octane.
its also why most modern autos and quite a few inboards run knock detect systems.
the higher the octane rating the slower the fuel burns.

like I said, its kinda like the difference in black powder and smokeless.
black powder is listed as an explosive and can only be shipped and stored in limited quantities and some places have even more stringent limits on it due to its an explosive.
goes BANG all at once.

smokeless powder is listed as a propellant, kinda burns if you place a cap full on the floor and light it with a match.

you have to remember MOST aircraft engines have to use a fuel that can burn con*****ably from se*****l to about 20,000 ft.
if the engine is supercharged and the pilots use oxygen they can go a bit higher.
the reed vapor pressure of aircraft fuel is a bit different as well due to the altitude change.
most boats don't change altitude that much or very rapidly.

the reason for the RPM at the shaft is very simple.
as prop shaft speed increases so does the speed of the tips of the prop.
on a boat with say a 13" prop that means the tips are swinging in an arc 6.5" from the shaft.
the surface feet per minute is a lot faster than shaft speed.
on an aircraft prop that's say 6' in diameter the surface speed at the tip can approach and pass the speed of sound.
as it passes the speed of sound bad things can and do happen.

can you turn a boat prop faster than about 2500-3000 RPM? yes but the standard non-surface piercing prop starts losing efficiency.

so is AV gas worth the money? most likely not.
last we used it it was a pain in the rear and we had to sign a non-highway use affidavit.

its also why ethanol and methanol are arbitrarily given an octane rating of 100.
you simply cannot compress it enough to make it detonate.
which is why we ran methanol motors with ratios of 12.5/1 up to about 14/1.

you wanna see a pease poor running engine? try 100% methanol with 8/1 pistons and 2.32 gears at the back.

octane rating is simply a measure of how fast and con*****ed the burn rate is.
the higher the rating the slower it burns.
not good in a short stroked small bore cylinder at high piston speeds.
remember that piston at 5000 RPM moves maybe 2"-3" on most two strokes.
that piston goes from dead stop at TDC to dead stop at bottom dead center.
it MUST close off the intake and ex ports BEFORE it can start compressing and firing the mix.
so now we have maybe 1.5 inchs of upward piston travel to compress and ignite the mix,we have about the same from TDC until the ports are uncovered and the EX is vented.
not much time at 5000 RPM to ignite,expand and exhaust.

if you don't belive me look up the Yamaha tech bulliten about performance issues using octane rated fuels above 89 in the old 225 excel.

so will that high octane fuel work, seat of da pants you most likely wont tell.
the increase of internal deposits may bite ya.

[TownsendsFJR1300]

Rodbolt; trust me, at a true 11/1 you wont run regular for long.
9/1 is about all you can run and still use 87 octane.


Was this reference towards my post above? Or?

[boscoe99]

Running premium, when NOT required, your wasting your money and the machine is not running quite up to its full, designed potential.. Should you experiance, pinging, etc, get premium in there asap as its very bad for the engine..

Why do you say this?

As noted, I tested three different four stroke Yamaha's using first 87 octane gasoline and then 93 octane gasoline. There was no measurable performance difference between the two gasoline grades.

Exhaust gas temperatures were the same, fuel/air ratios remained the same, WOT RPM and maximum boat speed were the same.

Even though I got the gasoline from two different sources do you think it is possible that I got the same octane gasoline even though it was posted at the pump as being different?

Let's look at engine timing for a moment.

At WOT the ignition timing is set to occur at some number of degrees before top dead center. This timing is established by the engineers to get maximum HP from the motor while maintaining a certain margin of safety against the onset of detonation, with the use of a fuel grade as published by the engine maker. For the F150 it is 86 octane.

The point of ignition timing is structured so that peak cylinder pressure in the combustion chamber will be at its maximum value at XY degrees after top dead center. This imparts the maximum possible downward force on the piston which in turn creates the most amount of torque.

If the advance timing is retarded somewhat then the peak cylinder pressure is going to occur at a less favorable point and less power will be created. Do you agree? And if so, and if the higher the octane rating the slower the fuel/air mix burns, then wouldn't this cause the point of peak cylinder pressure to occur later (same advance timing point but using two different octane grades) and also result in less power being developed? Theoretically, I would suspect so.

But, this is not what is seen in performance tests. If the higher octane gasoline burns slower, which would reduce the HP created, then why isn't this seen in performance tests?

We got fuel suppliers saying that octane has nothing whatsoever to do with gasoline burn rates. We got engines producing the same power using different grades of gasoline. We got authoritative sources saying that octane is merely a measure of a fuels ability to resist detonation, nothing more. We got fuel suppliers saying that high octane gasoline can be formulated to actually burn faster than a low octane gasoline because the engines RPM so fast that the fuel needs to burn faster.

We got folks on the internet saying that the higher octane gasoline burns slower. Which, based on what I have observed, is not the case.

Hope we can keep this discussion on a level keel without personal attacks occurring. Let's see.

[TownsendsFJR1300]

No its not personal at all, an exchange of experiances, etc as far as I'm concerned..

In your post, (if I read it correctly) you mentioned several different scenerio's with the same fuel (towards the end). Can't be all of them, agreed?

I was most probably premature (wrong) to say you'll make less power on higher Octane as I don't know (and will edit it now). It does burn slower, to stop the detonation (please read on).

IME, the higher the performance engine (higher compression ratios, hotter engine-say an air cooled Harley) they need high octane to prevent the pinging or knocking. A, say, Toyota Corolla, with a 8 or 9:1 compression ratio, doesn't need high test. Sure you can run it, won't hurt anything but your not gaining anythng either..

I can personanly attest to this for example; years ago, when I was a cop (I am retireed and rode that bike for about a year and a half) on Police Motors (Harleys), someone left the bike on fumes (we usually fueled up at the marina with with higher grade octane).

Anyway, I had to put some regular pump gas (we had pumps at the back of the station for the cars). After putting in just a gallon, (just to get me to the marina), that bike, pinged and knocked so bad on regular, it wasn't funny. Put the higher octane, ran fine.

Referring back to my bike for instance, the manual calls for regular (even with that high C/R-maybe as its water cooled, oil cooled-coolant also cools the oil-separate system). The bike runs fine. I've put high test in, and "seat of the pants" testing, idleing, acceleration, no difference, just less $ in my pocket.. And my bike does NOT have any knock sensors, it is fuel injected, has TPS, air temp sensor, water temp sensor, two CAT's in the exhaust, a radiator with an automatic fan that will kick on by self, a sensor there to read the exhaust and a ECU. I gather, all working together, the ECU is adjusting mixture, timing etc to prevent pinging on regular.

Running the higher octane is simply allowing the spark plug to ignite the mixture, vs say a hot spot on the piston, now acting as a diesel engine. , IE;

Years gone back, I came across many a GM car that would continue to run after turning off the ignition, literally running as a diesel engine would (sounded like it too) . A hot spot in the combustion chamber ignited the fuel (now it didn't have spark-ignition is turned off).

Didn't happen all the time, occassionally, as I re-call. The way to kill the engine was to leave it in gear (put some load on the engine) while still running normally, then turn off the key. That additional load from the auto transmission was enough to stop the engine..

Now, the marine fuel, at least down here(non-ethonol) is 91.

[rodbolt17]

the first generation F350 had no knock sensor, it used an Ionic detect.
it would fire the spark plug a second time and monitor the firing voltage to determine if combustion chamber conditions were condusive to detonation.

my 455 olds running CC'd heads,11/1 trw forged pistons, a luanati cheater cam,their w-30 version,crane triple valve springs an edlebrock torker intake and an 800 holley spread boare with mechanical secondaries in front of an M21 4 spd and 3.90 gears did NOT like anything less than 95 octane.
by playing with the jetting and backing off the advance curve it would run decent on 93,89 in a pinch if I did not load it but it hated 87.

as I took advantage of some of the local rich kids with there mustaangs and cameros with glass backs and big back tires I kept the idle speed at 1000 RPM.
this nessesitated slipping the clutch in gear with a foot on the brake at engine shut down.

what it did do was smooth out the ex idle tone running through 2 1/2 in pipes and z 28 mufflers.
if the kid was still hesitant the hooker was I would take off in 2nd gear.
even with the street master l60 15's you had to slip the tires.
really made no difference if I took of in 1st or 2nd I would still have to feather the throttle to get that 3300 pound beast moving.
occasionally I could sucker them in with a 20MPH rolling start .

I have seen detonation severe enough to blow off part of the piston,blow holes in the center of the piston and blow off the porcelain around the center plug electrode.

with decent fuel I could dial in about 40-45* total advance.
as the fuel quality fell I had to back down to around 30* or so.
I ran that 442 w30 car for about 22 years in various engines and engine configurations.
first engine lasted about 7 days.
that's when I found that 4 1/8 piston on that long stroke motor will separate at the wristpin much above 5000 RPM.
as a slow learner I actually did it twice.
I was 18 when I purchased that car from a junk yard and about 43 when I gave what was left of it to a friend.
the trip I made to maine while stationed at bath iron works was its death knell.

wish we would have had programmable EFI setups way back then .

[99yam40]

I think I remember looking at the owners manual of the old Ford PU (Think it was a 68) I bought that said to run regular leaded fuel ( 92 octane)in the 300 straight 6.
May have been even higher octane rating. Seems we forget how high the octane rating of regular gas was. Premium was over 100

I did buy a 65 2+2 Mustang with a bad motor in the early 70s for $275, had 2 pistons with hole in center of pistons. Always wondered if it was caused by wrong plugs, over heat, or something else.
Slapped 2 pistons out of a junk motor laying around just to see if it would run.
Did not even change rings or bearings, it ran for a few years until I lost both skirts one #1 piston and damaged head and block

[rodbolt17]

back in the day my 442 like super shell 98 octane with tetra ethyl lead.
not many years after the introduction of catalyst equipped motors the lead was phased out.
some of us had to replace the ex valves and seats due to we could not find leaded fuels anymore for street use.