what grade of gas to use?

[banko]

I have a OMG Cobra 3.0 I have been running lower grade gas and I am wondering if I will see better results if I switch to high grade gas and if the improvement would be worth it? Thanks in advance for everyones input

 

[matt167]

Re: what grade of gas to use?

Some of the marine 3.0L's actually called for premium as they had fairly high compression, but at least on the Mercruisers, the valve cover was marked as to needing premium. If it's marked to run 87 octane, there is no performance in higher octane

 

[HT32BSX115]

Re: what grade of gas to use?

I have a OMG Cobra 3.0 I have been running lower grade gas and I am wondering if I will see better results if I switch to high grade gas and if the improvement would be worth it? Thanks in advance for everyones input

Howdy,


In a word, NO. You will not see an improvement.

Now having said that, Some engines of the "past" had different (couple degrees advanced) ignition timing settings for higher octane fuels. When you advance timing on any engine, you can see incremental increases in HP (on a DYNO)

A LOT of 80's and 90's OMC (GM and Ford) engines had different fuel grade/timing settings. (8 degrees vs 10 BTDC etc)

You won't see a difference in operation though.

In general, different grade gasolines DO NOT increase performance in ANY engine. (that's an old wives tale promoted by the Petro advertising industry for many years)

Some very high HP engines with very high compression and ignition advance CANNOT run on low octane fuel because of detonation which WILL destroy them under load quickly.

It's not the fuel that increases the power, it's the high compression and (advanced) ignition timing (which REQUIRES the higher octane fuel)

High octane fuel has the same energy content (BTU/gallon, KCAL/Ltr) as low octane fuel.


Cheers,


Rick

 

[banko]

Re: what grade of gas to use?

Howdy,


In a word, NO. You will not see an improvement.

Now having said that, Some engines of the "past" had different (couple degrees advanced) ignition timing settings for higher octane fuels. When you advance timing on any engine, you can see incremental increases in HP (on a DYNO)

A LOT of 80's and 90's OMC (GM and Ford) engines had different fuel grade/timing settings. (8 degrees vs 10 BTDC etc)

You won't see a difference in operation though.

In general, different grade gasolines DO NOT increase performance in ANY engine. (that's an old wives tale promoted by the Petro advertising industry for many years)

Some very high HP engines with very high compression and ignition advance CANNOT run on low octane fuel because of detonation which WILL destroy them under load quickly.

It's not the fuel that increases the power, it's the high compression and (advanced) ignition timing (which REQUIRES the higher octane fuel)

High octane fuel has the same energy content (BTU/gallon, KCAL/Ltr) as low octane fuel.


Cheers,


Rick

that's why I ask this question I stumbled across the manual having two settings for tune up one states using RON 93 would be 4deg BTDC and running RON 90 it would be 1deg ATDC. I have been running 87 in it.

 

[HT32BSX115]

Re: what grade of gas to use?

that's why I ask this question I stumbled across the manual having two settings for tune up one states using RON 93 would be 4deg BTDC and running RON 90 it would be 1deg ATDC. I have been running 87 in it.

I would suggest you try it and make up your own mind. RON means "Research Octane Number"

From Wikipedia,

United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel[SUP][citation needed][/SUP]. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI (98 RON) is available.[SUP][26][/SUP] As of January, 2011, over 40 states and a total of over 2500 stations offer ethanol-based E-85 fuel with 105 AKI.[SUP][27][/SUP] Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI[SUP][citation needed][/SUP].

You may be able to actually hear detonation "pinging" when you "punch-it". If you do, back off immediately.

I also did a little searching around. 87 AKI evidently is equivalent to a higher RON, So midgrade may be approx equal to 93 RON.

In ANY CASE, if you induce detonation and you cannot hear it, the damage could be immediate and severe (read broken/"holed" pistons, broken rings, burnt valves, etc)



As you can see, ign timing/fuel octane is extremely important. This is why most (or all) marine gasoline engines today are designed to operate on 87 AKI gasoline, and MOST or all have "knock"-sensors that cause the ECM/electronic ignition systems to retard ign timing when they detect "knock"

 

[xdgt03]

Re: what grade of gas to use?

The important thing is to run the octane your timing is set for. You can change the timing to run the grade gas you want. You DO NOT want to run it with the "ping" or predetonation that people have been talking about. That will ruin your engine relatively quickly. I agree that there is no reason in these boats to run high octane for any performance reason, tho I would avoid anything lower than 87 and definitely not e85.

 

[Another retired guy]

Re: what grade of gas to use?

Assuming your engine is running fine on 87 without "pinging", one possible reason for using a higher octane fuel is "long term" storage. If you use lots of gas and are constantly topping up with fresh then no problem. But if a tank of gas is sitting around for an extended period of time, then the octane level will fall, so starting off with a higher octane will compensate for the loss. Use it, or lose it

 

[Oshkosh1]

Re: what grade of gas to use?

(that's an old wives tale promoted by the Petro advertising industry for many years)

C'mon...really? I just CAN'T believe "big oil" would lie to us!:laugh:

Say it isn't so...

 

[HT32BSX115]

Re: what grade of gas to use?

C'mon...really? I just CAN'T believe "big oil" would lie to us!:laugh:

Say it isn't so...

But OTOH, a some guy once said, "There's a sucker born every minute"!!

From Wikipedia, the free encyclopedia
Jump to: navigation, search
"There's a sucker born every minute" is a phrase often credited to P. T. Barnum (1810–1891), an American showman. Though this phrase is often credited to Barnum, it was actually spoken by a man by the name of David Hannum, who was criticizing both Barnum and his customers.

I would say that the people that bought off on Slick-50 would fall into that category.....


Caveat Emptor



Rick

 

[southkogs]

Re: what grade of gas to use?

I have a OMG Cobra 3.0...

I don't have much to add to the discussion, but just got a laugh outta' this. I now have a new descriptor to my driveline, "I have an OMG Stringer!"

... that's what most people say when they see it anyway.

 

[Maclin]

Re: what grade of gas to use?

I must protest the use of "high grade" and "low grade" terminology. The fuel quality is the same, only the octane rating makes them different. Most refining methods "set" the octane last.

Octane rating is used to compare the burn rates. Lower octane is faster burn and will detonate more easily so engines designed for lower compression and lower ignition advance call for lower octane fuel. A slower burn rate will not detonate as easily at higher ignition advance and higher compression and thus is suitable for the engines tuned for higher performance (higher compression, etc.).

The method of rating Octane must be taken into consideration as HT and others have mentioned. The RON method is "Research Only Number" used worldwide, except of course in the US. Most ratings in the US are (R+M)/2 which is an average of RON and "MOTOR" method (where an actual engine of known configuration is used to test in a lab).

As mentiond earlier, RON of 91 is about 87 in the (R+M)/2 world. RON 93 is about 89-91 R+M.

Most marine engine callout stickers and manuals will state both. Not everyone who reads that understands the difference, and the RON method seems to be first in line in most cases and that has propogated many untruths about what any given marine engine needs for octane.

Using higher octane when not called for can leave byproducts in the sparkplug and combustion chamber and the engine can actually lose performance. Advancing timing may help but other factors such as compression ratio and duty cycle will keep the higher octane from being a performance booster in most engines that only require the lower octane fuel.

 

[boaterinsd]

Re: what grade of gas to use?

In my area the higher octane fuel is 100% gas unlike the ethanol blend lower ones. I do know that the carbs / fuel systems will not like the alcohol content. Bad on seals gaskets Etc.

 

[Another retired guy]

Re: what grade of gas to use?

In my area the higher octane fuel is 100% gas unlike the ethanol blend lower ones. I do know that the carbs / fuel systems will not like the alcohol content. Bad on seals gaskets Etc.

Now you've opened up a whole new can of worms
Many threads on the ethanol vs non-ethanol topic, and everyone has an opinion. Just to prove that I'll say that I use regular ethanol gas in my '88 4.3 without any problems at all. Even with a 7 month winter storage season!

 

[Maclin]

Re: what grade of gas to use?

I will just throw this out there...Ethanol lowers the BTU content but raises the Octane "reading". Engines designed for Ethanol (above E10) have higher compression and advance the timing much more when they sense they are on E85 to take advantage of the detonation resistance and make up for the loss in BTU's.

 

[HT32BSX115]

Re: what grade of gas to use?

The method of rating Octane must be taken into consideration as HT and others have mentioned. The RON method is "Research Only Number" used worldwide, except of course in the US. Most ratings in the US are (R+M)/2 which is an average of RON and "MOTOR" method (where an actual engine of known configuration is used to test in a lab).

Also Known as "AKI" (Anti Knock Index)

Most marine engine callout stickers and manuals will state both. Not everyone who reads that understands the difference, and the RON method seems to be first in line in most cases and that has propogated many untruths about what any given marine engine needs for octane....................................

Using higher octane when not called for can leave byproducts in the sparkplug and combustion chamber and the engine can actually lose performance. Advancing timing may help but other factors such as compression ratio and duty cycle will keep the higher octane from being a performance booster in most engines that only require the lower octane fuel.

This is largely true in low compression aircraft engines that specify 80 octane AVGAS but are running either 100LL or the older and generally no longer available 100-130 Octane AVGAS.

Because the engine doesn't produce high enough exhaust temps, the (mostly lead based) combustion products end up fouling plugs, tops of pistons and valves etc. (poor lead scavenging)

ELIMINATE SPARK PLUG LEAD FOULING!
Since the introduction of 100LL as the main fuel for low-compression engines, Alcor has been a leader in providing pilots a fuel treatment that eliminates lead before it can cause fouling. TCP is the only FAA approved product of its kind. A simple one-shot treatment with every fill-up prevents lead build-up on spark plugs and valves (recommended dosage is 1 oz. of TCP for every 10 gallons of leaded fuel).

This is NOT a problem in low compression engines running unleaded (REG/MID/PREM) fuel however.

Although you could run "premium" fuel in all of our boat engines, (fuel cost aside) I doubt it would cause any other noticeable problems.

I must protest the use of "high grade" and "low grade" terminology. The fuel quality is the same, only the octane rating makes them different. Most refining methods "set" the octane last.

I totally agree..... There is no "quality difference between the grades and if you hang around a refinery for a while, you'll find that just about ALL brands can come from the same refinery in a particular area!

I will just throw this out there...Ethanol lowers the BTU content

Yes, 10% Ethanol lowers the BTU content approx 3% or so.....it's almost insignificant.

 

[Maclin]

Re: what grade of gas to use?

^^^^

What he (HT) said... :thumb:

 

[banko]

Re: what grade of gas to use?

I was wondering if someone was going to catch that lol