What Octane Gas to use?

[SkyDirtWaterguy]

Ok RDP braintrust. I know this has probably been discussed here before but want opinions on which gas to use in my boat. The motor is a 2014 Mercury 540.
Some say 87 others say 91?

 

[Boat 405]

87 Pump gas is R+M/2. If you know that large portions of the fuel will sit between usage use 91. If you plan on burning most of it upon every fill up run 87

 

[boataholic]

Stock 540, I would run what MERC recommends. My question has always been around gas brands. I have always been advised to use a "Top Tier", station. Does it make that big of a difference, as the prices of some of the other stations are so tempting.

Sent from my SM-G930V using Tapatalk

 

[RiverDave]

87 if you run 91 you will likely end up cleaning more soot off your transom

 

[GETBOATS]

Run what the manufacturer recommends, run a name brand fuel, buy fuel from a busy station (turnover), keep it fresh, go boating a lot, if not run it a least once a month. in an engine designed to run on 87 will make less power on 91, remember, octane is a fuel resistance to burning. meaning it is more stable as heat and compression rises. that's why higher compression designs need higher octane. there's a chemical engineer on here, don't remember name. he'd be full of information.

 

[SKIDMARC]

I've always run 91 on my turtle. Never any issues. Been 10 years. Not partial to any brands. Usually just go to whichever is close to the house. Currently Alliance.

 

[ElAzul]

87 if you run 91 you will likely end up cleaning more soot off your transom

And eating spark plugs ask me how I know

 

[OldSchoolBoats]

Merc says to run 87 in the 520. Anything higher offers no better protection or performance. This is a good link to reference.

Don't Knock It!

So, what exactly is engine knock? Well, put on your engineering hats for a moment, as we’re going to get a little technical here. Simply put, engine knock (aka “detonation”) is an undesirable phenomenon that occurs when a “left over” pocket of air-fuel mixture in the combustion chamber ignites aft

www.mercuryracing.com

 

[robby dmax]

I've always run 91 on my turtle. Never any issues. Been 10 years. Not partial to any brands. Usually just go to whichever is close to the house. Currently Alliance.

But the question is what motor are you running?

 

[TexasJet]

87 plus Staybill marine stabilizer.

 

[Sleek-Jet]

All of it...

 

[SKIDMARC]

But the question is what motor are you running?

Merc 4.3 I/O

 

[Blackmagic94]

87

 

[copterzach]

Avgas


Sent from my iPhone using Tapatalk

 

[DaveH]

that article is a pretty poor and inaccurate definition of knock. besides that, the article specifically states the use of 91 is recommended.

its erroneously describes "leftover" fuel that ignites AFTER cylinder top dead center to cause it. doesnt work that way.

so what really is "knock" and what causes it? knock, or piston "rattling" is caused by pre-igntion, or when combustion starts BEFORE the spark event well before the piston reaches anywhere near top dead center (NOT after). the piston is effectively fighting the mechanical upward travel and trying to force the motor to run backwards. the piston rocks back and forth and side to side as it fights early combustion pressure which cause it hit the bore which is why we have all seen scuffed cylinders, broken ring lands and holes in the pistons and bearings pounded out if this is left unchecked.

so why would the fuel ignite BEFORE the spark event? first, excessive cylinder temperature. imagine the piston at top dead center (TDC) at the start of the intake stroke. the intake valve opens, highly atomized and combustible fuel is drawn in as the piston travels down, and then the piston reaches BOTTOM dead center (BDC) and the valve closes. the piston now travels UP (compression stroke) and tightly compresses that hot mixture ( and yet we still expect the mixture not to light off even though the mixture is extremely volatile at this point) and waits for the spark event to begin combustion.

OCTANE is the fuels RESISTANCE to start combustion in this very volatile environment. that is why high compression, high output engines need a higher octane fuel to survive compared to lower output engines.

temperature is only one factor, load on the engine is a factor, lean or "weak" fuel mixtures as well and other lesser known issues... all cause pre-ignition leading to knock.

we all remember "back in the day" when you would drive your car around and it would run fine and then pulling a heavy load up a grade or a really hot day you could hear the thing rattling. that was the effect of temp and load on an engine that needed a higher octane fuel.

highly tuned modern compuetr controlled engines have knock sensors and the ability to do all sorts of things to reduce/eliminate knock. our fairly crudely setup marine engines do not.

so does it hurt to run 91 if the book says 87. no. IMO its cheap insurance towards the longevity and protection of your expensive motor. some people claim you will make more power on 87 then 91. I challenge anyone here to share real world back to back dyno test results of runnnign the same engine under the same identical operating conditions and report back that 87 made more power.

some people have the misguided understanding that octane is the "rate" and how fast the fuel burns....which is why you run different timing for 87 vs 91. re-read what i posted above.....octane is the RESISTANCE to starting combustion and has nothing to do with how "fast" it burns. the chemical properties of the fuel dictate this...not the octane.

you dont see any real racers buying 87 and dropping in a bunch of octane "booster" do you? nope.....real high compression, forced induction or heavily modified racing engines run real race fuel which has entirely different properties then pump gas and not just the octane rating.

if i had a merc motor.....it would be 91 only.

 

[HST4ME]

The 540's were 8.5:1. 87 octane.

 

[Blackmagic94]

that article is a pretty poor and inaccurate definition of knock. besides that, the article specifically states the use of 91 is recommended.

its erroneously describes "leftover" fuel that ignites AFTER cylinder top dead center to cause it. doesnt work that way.

so what really is "knock" and what causes it? knock, or piston "rattling" is caused by pre-igntion, or when combustion starts BEFORE the spark event well before the piston reaches anywhere near top dead center (NOT after). the piston is effectively fighting the mechanical upward travel and trying to force the motor to run backwards. the piston rocks back and forth and side to side as it fights early combustion pressure which cause it hit the bore which is why we have all seen scuffed cylinders, broken ring lands and holes in the pistons and bearings pounded out if this is left unchecked.

so why would the fuel ignite BEFORE the spark event? first, excessive cylinder temperature. imagine the piston at top dead center (TDC) at the start of the intake stroke. the intake valve opens, highly atomized and combustible fuel is drawn in as the piston travels down, and then the piston reaches BOTTOM dead center (BDC) and the valve closes. the piston now travels UP (compression stroke) and tightly compresses that hot mixture ( and yet we still expect the mixture not to light off even though the mixture is extremely volatile at this point) and waits for the spark event to begin combustion.

OCTANE is the fuels RESISTANCE to start combustion in this very volatile environment. that is why high compression, high output engines need a higher octane fuel to survive compared to lower output engines.

temperature is only one factor, load on the engine is a factor, lean or "weak" fuel mixtures as well and other lesser known issues... all cause pre-ignition leading to knock.

we all remember "back in the day" when you would drive your car around and it would run fine and then pulling a heavy load up a grade or a really hot day you could hear the thing rattling. that was the effect of temp and load on an engine that needed a higher octane fuel.

highly tuned modern compuetr controlled engines have knock sensors and the ability to do all sorts of things to reduce/eliminate knock. our fairly crudely setup marine engines do not.

so does it hurt to run 91 if the book says 87. no. IMO its cheap insurance towards the longevity and protection of your expensive motor. some people claim you will make more power on 87 then 91. I challenge anyone here to share real world back to back dyno test results of runnnign the same engine under the same identical operating conditions and report back that 87 made more power.

some people have the misguided understanding that octane is the "rate" and how fast the fuel burns....which is why you run different timing for 87 vs 91. re-read what i posted above.....octane is the RESISTANCE to starting combustion and has nothing to do with how "fast" it burns. the chemical properties of the fuel dictate this...not the octane.

you dont see any real racers buying 87 and dropping in a bunch of octane "booster" do you? nope.....real high compression, forced induction or heavily modified racing engines run real race fuel which has entirely different properties then pump gas and not just the octane rating.

if i had a merc motor.....it would be 91 only.

there are many dyno pulls proving you make more power on less octane with the same timing as long as you don’t detonate

My gsxr1000 was faster at the track on 89 then 91 in fact

 

[Justfishing]

Run what the manufacturer recommends, run a name brand fuel, buy fuel from a busy station (turnover), keep it fresh, go boating a lot, if not run it a least once a month. in an engine designed to run on 87 will make less power on 91, remember, octane is a fuel resistance to burning. meaning it is more stable as heat and compression rises. that's why higher compression designs need higher octane. there's a chemical engineer on here, don't remember name. he'd be full of information.

Good points.

A higher compression engine will have more timimg advance. The fuel will have a longer time to burn. The slower burn of high octane will produce an extended push of the piston versus a sudden surge and engine knock.

A standard low compression has a shorter stroke , less advance. It has less time to burn the fuel. In this case 87 matches the duration for max power. 91 octane will burn to slow

 

[robby dmax]

Good points.

A higher compression engine will have more timimg advance. The fuel will have a longer time to burn. The slower burn of high octane will produce an extended push of the piston versus a sudden surge and engine knock.

A standard low compression has a shorter stroke , less advance. It has less time to burn the fuel. In this case 87 matches the duration for max power. 91 octane will burn to slow

well said!!!

 

[DaveH]

Good points.

A higher compression engine will have more timimg advance. The fuel will have a longer time to burn. The slower burn of high octane will produce an extended push of the piston versus a sudden surge and engine knock.

A standard low compression has a shorter stroke , less advance. It has less time to burn the fuel. In this case 87 matches the duration for max power. 91 octane will burn to slow

you are kidding right?

as compression ratio goes up....the need for ignition advance goes DOWN. why? in simple terms the tighter you compress the mixture the faster it burns. its NO DIFFERENT with a super charger (effectively a mechanical device that raises cylinder pressure which equates to higher compression) do you run more timing with boost? absolutely NOT.

when compression is reduced, you need MORE advance to compensate for the slower burn rate of the less dense charge. its no different where you run radically more timing at light engine loads due to part throttle reducing the charge density of the engine.

someone needs to read up on basic engine fundamentals.

 

[Racey]

Good points.

A higher compression engine will have more timimg advance. The fuel will have a longer time to burn. The slower burn of high octane will produce an extended push of the piston versus a sudden surge and engine knock.

No. Higher compression runs less timing at full load.

 

[rrrr]

Ok RDP braintrust. I know this has probably been discussed here before but want opinions on which gas to use in my boat. The motor is a 2014 Mercury 540.
Some say 87 others say 91?
View attachment 871173

You would think the manufacturer's data plate would include this information so users wouldn't have to rely on answers from SGOTI.

 

[rivermobster]

you are kidding right?

as compression ratio goes up....the need for ignition advance goes DOWN. why? in simple terms the tighter you compress the mixture the faster it burns. its NO DIFFERENT with a super charger (effectively a mechanical device that raises cylinder pressure which equates to higher compression) do you run more timing with boost? absolutely NOT.

when compression is reduced, you need MORE advance to compensate for the slower burn rate of the less dense charge. its no different where you run radically more timing at light engine loads due to part throttle reducing the charge density of the engine.

someone needs to read up on basic engine fundamentals.

He had to have been. Good gawd.

 

[Jed-O]

13:1 compression. I run 100LL

 

[farmo83]

I run 93 the first tank of the year as stated above octane degrades over time and run 87 the rest of the year. The 5-10 ish minutes extra time it takes to clean the transom is a price I will pay every year for additional piece of mind.

 

[fishing fool]

This picture says it all, why ask.........................

Ok RDP braintrust. I know this has probably been discussed here before but want opinions on which gas to use in my boat. The motor is a 2014 Mercury 540.
Some say 87 others say 91?
View attachment 871173

 

[SkyDirtWaterguy]

This picture says it all, why ask.........................

because it says 87 or 91 and based on the comments there are varying opinions as to which one to use wise guy.
We can talk about non boat related stuff here I guess instead

 

[LargeOrangeFont]

because it says 87 or 91 and based on the comments there are varying opinions as to which one to use wise guy.
We can talk about non boat related stuff here I guess instead

Just so you know, RON is a different measure than the (r+m)/2 method we use in the US. But I can see the confusion.

I know the 496 people get extra soot when running 91, but many other Merc engines do not, mine included.

 

[Lunatic Fringe]

because it says 87 or 91 and based on the comments there are varying opinions as to which one to use wise guy.
We can talk about non boat related stuff here I guess instead

You are looking at two different ratings RON International is European. If you decide to run your boat in London, go by that rating.
In the USA, use 87

 

[SkyDirtWaterguy]

thanks for your replies

 

[fishing fool]

This wise guy did not think you were going to run your boat on "international fuel"...... Sorry for the confusion.... Now back to boat relater stuff.

because it says 87 or 91 and based on the comments there are varying opinions as to which one to use wise guy.
We can talk about non boat related stuff here I guess instead

Just so you know, RON is a different measure than the (r+m)/2 method we use in the US. But I can see the confusion.

I know the 496 people get extra soot when running 91, but many other Merc engines do not, mine included.

You are looking at two different ratings RON International is European. If you decide to run your boat in London, go by that rating.
In the USA, use 87

 

[River Dirt 2]

I have run AV gas in my 496 HO since new (2007). I have to because of my gas tanks in the boat.
Runs fine, starts every time, 1 plug change in 13 years. No soot, although I run the exhaust thru the prop so maybe I would not see it if it’s there?
Also run AV in the kids bikes, quads and moms 2 stroke rototiller (love the smell of AV gas in the garden!)

 

[DaveH]

here is the deal with AVgas...

while it is called 100LL (100ocatne low lead) this is not based on the r+m/2 method our pump gas is rated it. using the r+m/2 method it really is only about 96 octane.

however, it does have lead in it which is a good thing. it also has no ethanol and does not degrade like modern pump gas does. that is why many choose to store their boats in the winter and put 100LL in it to keep from contaminating the fuel system when the pump gas goes stale.

typical air cooled aircraft engines are very low compression ( in the range of 6 or 7:1) yet 100LL is still the go-to fuel for them. wonder why?

as for soot on the transom, merc runs there fuel mixtures very rich and soot is a byproduct of this no matter what fuel you run. i think the reason you see it on some boats and not others is beacuse of the type of exhaust (outlet ports higher or lower to the water line at cruseing speeds) or swim steps and other things that produce more or less turblence at the back of the boat.

 

[oldschool]

You described what knock is and what causes it, here’s what it can look like when you rattle a motor.

 

[ka0tyk]

I'm curious if the additional ethanol in 87 fuel has impacts on preignition/detonation as the octane is one thing but the chemical makeup of gasoline changes throughout the years... 87 in the 90's is way different than 87 today.

 

[DaveH]

I'm curious if the additional ethanol in 87 fuel has impacts on preignition/detonation as the octane is one thing but the chemical makeup of gasoline changes throughout the years... 87 in the 90's is way different than 87 today.

ethanol, which is a fancy name for alcohol.......does offer additional knock resistance. mainly because alcohol has a greater "cooling effect" when mixing with air (think of pouring alcohol on your palm and its cold) and temperature is one of the main components to bring on pre-ignition. however, in our 87 pump gas it is usually about 10% of the fuel blend so the adnatages arent huge.

one of the greatest advances to fight knock is the use of direct injection. unlike port injection where the fuel is mixed with the air during the intake stroke and compressed, direct inject sprays fuel directly into the combustion chamber at ungodly high pressures at the very last instant before combustion is to begin. the fuel simply isnt there most of the compression stroke to create a knock event.

this advance has allowed OEM's to not only build cleaner burning engines (emissions compliance) but they are able to raise engine compression ratios to make more power at the same time as the tolareance for knock is higher with DI. its a win-win really. add knock sensors and modern computers......its why knock is a thing of the past on modern OEM engines.

 

[Sleek Chode]

You should run what the OEM says... when you smoke the motor you can blame the OEM and not the forum.

11.5:1 427 BBC 100 octane and 35 degrees all in advanced 1600-5600 RPM. All depends... different builds have different requirements.

 

[Shlbyntro]

ethanol, which is a fancy name for alcohol.......does offer additional knock resistance. mainly because alcohol has a greater "cooling effect" when mixing with air (think of pouring alcohol on your palm and its cold) and temperature is one of the main components to bring on pre-ignition. however, in our 87 pump gas it is usually about 10% of the fuel blend so the adnatages arent huge.

one of the greatest advances to fight knock is the use of direct injection. unlike port injection where the fuel is mixed with the air during the intake stroke and compressed, direct inject sprays fuel directly into the combustion chamber at ungodly high pressures at the very last instant before combustion is to begin. the fuel simply isnt there most of the compression stroke to create a knock event.

this advance has allowed OEM's to not only build cleaner burning engines (emissions compliance) but they are able to raise engine compression ratios to make more power at the same time as the tolareance for knock is higher with DI. its a win-win really. add knock sensors and modern computers......its why knock is a thing of the past on modern OEM engines.

Except that manufacturers have been finding that theyve been having problems with carboning the intake valves on direct injected motors with the recycled pcv because there's no gas to help clean the valves as it blows by into the cylinders.

As for soot on the transom. You should just live it and wipe it off at the end of the day. So long as it's not creating a performance issue, soot means the engine is burning rich and safe (for the internals of the engine.) regular replacement of the spark plugs can help a little with the soot.

When I send ECMs out, I always direct my guy to tune them on the rich end of the spectrum for the application.

Use the manufacturer suggested fuel. All pump gas in US is rated in R+M/2. I only recommend using a higher octane if you plan to let the boat sit for a hot minute or if you have done any modifications to the engine. Otherwise, you are just throwing money away for gas you are not burning.

 

[DaveH]

Except that manufacturers have been finding that theyve been having problems with carboning the intake valves on direct injected motors with the recycled pcv because there's no gas to help clean the valves as it blows by into the cylinders.

As for soot on the transom. You should just live it and wipe it off at the end of the day. So long as it's not creating a performance issue, soot means the engine is burning rich and safe (for the internals of the engine.) regular replacement of the spark plugs can help a little with the soot.

When I send ECMs out, I always direct my guy to tune them on the rich end of the spectrum for the application.

Use the manufacturer suggested fuel. All pump gas in US is rated in R+M/2. I only recommend using a higher octane if you plan to let the boat sit for a hot minute or if you have done any modifications to the engine. Otherwise, you are just throwing money away for gas you are not burning.

i

i agree that DI has its down sides. the well known issue with the intake valves is just the beginning. there have been issues with the injectors themselves stemming from exposure to the combustion event. DI is very expensive to implement due to the high fuel pressures, high voltage the injectors run at and expensive to repair.

DI isnt new and been around for a while, and like anything else will only get more refined as the technology progresses.