Posted Octane Numbers

[Texasmark]

Just got back from a trip thru New Mexico and Arizona. On the northern leg of the trip I noticed the gas pumps dispensed 86 and 88 octane fuel whereas I am used to 87 and 89 in Texas, which is also what they dispensed in the lower parts of the two states.

Since my outboard and truck run on 87 min, I had to move up to the mid grade fuel.

Is the lower octane number for E10 Ethanol?

Thanks,

Mark

 

[JB]

Re: Posted Octane Numbers

I have noticed that in parts of the west, too, Mark. Premium is 91 there and 92/93 elsewhere. It was there before they started using ethanol to replace MTBF(or whatever that nasty stuff was).

I think it is because of the altitude and federally (EPA)mandated formulae.

 

[MikeV4]

Re: Posted Octane Numbers

Ethanol has a high octane rating, therefor a fuel with lower octane number will mostly not have ethanol.
There is no problem to go on a higher octane number than the minimum mentioned in your manual, on the otherhand going to a lower octane number can make the fuel/air mixture pre-detonate and will cause knocking, which can damage your engine.

 

[kenmyfam]

Re: Posted Octane Numbers

Higher octane no issue. lower octane can cause issues.

 

[Texasmark]

Re: Posted Octane Numbers

Now wait a minute Mike. I thought the BTU value of alcohol was lower than gasoline hence lower mpg with ethanol........or am I getting BTU's and octane mixed up again.

Let's see, BTU is heat content.....energy. Octane is the ability to withstand heat/pressure before self exploding (pre-ignition, knocking, pinging). Higher the Octane, the higher the compression tolerance...higher the engine compression can be.....?????? Right?????

Mark

 

[deejaycee_2000]

Re: Posted Octane Numbers

Here we have 95 octane (unleaded fuel) and 97 octane (leaded fuel) .... my racing motors run best on 97 leaded ......

 

[Silvertip]

Re: Posted Octane Numbers

I'm not a chemist but I don't believe octane rating and BTU are mutually inclusive. The faster burn rate of ethanol means the injectors need to have the pulse width widened by up to 30% to get the equivalent amount of fuel into the engine. The faster burn rate increases performance and decreases economy. The reading I've done indicates Ethanol (straight) has an octane rating of 105. Our E10 stickers on regular grade gas pumps indicates 87 Octane. My new Impala runs on E85 or E10. Although mileage goes down considerably on E85 the car does perform better on it. On newer cars and trucks, one octane point will not pose a problem as the knock sensor(s) will retard the timing to take care of any detonation. One must pay attention to this with outboards however. As an aside, E85 needs to be at least 40 cents/gallon cheaper than regular to make running it cost effective.

 

[Paul Moir]

Re: Posted Octane Numbers

Deejaycee, the octane numbering system used in North America is different from the one used in the rest of the world. Yours is Research Octane Number and ours is a combination of RON and Motor Octane Number. To make a long story short, our 87 is about 90, 91 is about 95 RON.
Naturally, TEL has been banished from the pumps here for a very long time.

 

[deejaycee_2000]

Re: Posted Octane Numbers

Yeah I figured that, you guys in the US always so complicated .. lol ... luckily here, we put it in and it runs .... (tipical 3rd world country) hahaha

 

[Texasmark]

Re: Posted Octane Numbers

Well Paul just answered another question I had about the newer gasolines.

Back in '88 I had a Ford p/u with a 302 ( or 305 forget which is Ford and which is Chev) and it would knock while clinbing a hill on the interstate going to work. So I had to go to a higher octane fuel than regular.

The new fuels came along and the truck was able to run on the cheapest with no knocking.

Now I know why.

Thanks.

Mark

 

[Paul Moir]

Re: Posted Octane Numbers

... you guys in the US...

You better watch out there, buddy.

 

[MikeV4]

Re: Posted Octane Numbers

Now wait a minute Mike. I thought the BTU value of alcohol was lower than gasoline hence lower mpg with ethanol........or am I getting BTU's and octane mixed up again.

Let's see, BTU is heat content.....energy. Octane is the ability to withstand heat/pressure before self exploding (pre-ignition, knocking, pinging). Higher the Octane, the higher the compression tolerance...higher the engine compression can be.....?????? Right?????

Mark

That depends on the compression ration of your engine, the higher the CR the higher octane you require to prevent knocking. 87 octane fuel consits of 87% octane and 13 %heptane, octane strings are better combustible too handle self ignition than the heptane (or other additives to give the same BTU) When high compression ratio engines were build (first in aviation,racing and than for commercial) the need for higher octane fuel was required to prevent the knocking. Lead (TEL)was found to be a good octane booster, but very environment unfriendly, after the lead was banned other octane boosters were required, methyl tertiary butyl ether (MTBE). MTBE unfortunally easily combines itself with water and can cause cancer. Ethanol is a very good octane booster,oxygenetic carbon and as hygroscpic as MTBE but but does not cause cancer..

E10 is unleaded fuel with 10% of ethanol, which is approved for all unleaded engines, E85 is unleaded fuel with 85% ethanol and can only be used for engines which are specially designed for this (flexible fuel vehicles) It is high octane and for high compression ratio engines. Check with your engine manufacturer if you can use it!
Ethanol has a heating value of 76,000 BTU per gallon, which is approximately 30 percent less than gasoline's heating value (which is approximately 109,000 to 119,000 BTU/gal)

Conclusion over all:
If you want to increase the power (HP) of your engine by increasing the octane--you will have to increase your compression ratio otherwise it will not help! and it's loss of $$$ by using a higher octane fuel than the minimum recommended by manufacturer.

 

[MikeV4]

Re: Posted Octane Numbers

Now wait a minute Mike. I thought the BTU value of alcohol was lower than gasoline hence lower mpg with ethanol........or am I getting BTU's and octane mixed up again.

Let's see, BTU is heat content.....energy. Octane is the ability to withstand heat/pressure before self exploding (pre-ignition, knocking, pinging). Higher the Octane, the higher the compression tolerance...higher the engine compression can be.....?????? Right?????

Mark

That depends on the compression ration of your engine, the higher the CR the higher octane you require to prevent knocking. 87 octane fuel consits of 87% octane and 13 %heptane, octane strings are better combustible too handle self ignition than the heptane (or other additives to give the same BTU) When high compression ratio engines were build (first in aviation,racing and than for commercial) the need for higher octane fuel was required to prevent the knocking. Lead (TEL)was found to be a good octane booster, but very environment unfriendly, after the lead was banned other octane boosters were required, methyl tertiary butyl ether (MTBE). MTBE unfortunally easily combines itself with water and can cause cancer. Ethanol is a very good octane booster,oxygenetic carbon and as hygroscpic as MTBE but but does not cause cancer..

E10 is unleaded fuel with 10% of ethanol, which is approved for all unleaded engines, E85 is unleaded fuel with 85% ethanol and can only be used for engines which are specially designed for this (flexible fuel vehicles) It is high octane and for high compression ratio engines. Check with your engine manufacturer if you can use it!
Ethanol has a heating value of 76,000 BTU per gallon, which is approximately 30 percent less than gasoline's heating value (which is approximately 109,000 to 119,000 BTU/gal)

Conclusion over all:
If you want to increase the power (HP) of your engine by increasing the octane--you will have to increase your compression ratio otherwise it will not help! and it's loss of $$$ by using a higher octane fuel than the minimum recommended by manufacturer. Because they have the engine designed with a certain compression ratio (common is 8 to 1)

 

[Texasmark]

Re: Posted Octane Numbers

Thanks Mike. Nice reply. You answered my questions about the need for octane and the fact that ethanol does have a lower BTU content per gallon than gas. Buttttt it's hard to visualize that 10% alcohol would result in a 30% drop in BTU. If you took the alcohol completely out and only had 90% of a gallon of pure gasoline, you would have only lost 11,000 BTU's of the 110,000. :/

Mark

 

[MikeV4]

Re: Posted Octane Numbers

Mark, you are totally wright about that: 100% Ethanol has 79,000 BTU/Gl (this is approx 30% less than gasoline), and 100% 87 gasoline has 109,000-119,000 BTU/Gl, mixed together with a 10/90 ratio would only make a total loss of 3% of the original 109,000 BTU/Gl.This is not a great loss, worn gearbox or badly inflated tyres can create the same loss on miles/gallon.

 

[Texasmark]

Re: Posted Octane Numbers

Agree Mike. Thanks for the followup.

Mark

 

[bruceb58]

Re: Posted Octane Numbers

If you were driving through a high altitude area that would be the reason you had a lower octane number on the pumps. The higher the altitude, the lower octane your vehicle needs.

 

[waterinthefuel]

Re: Posted Octane Numbers

Here we have 95 octane (unleaded fuel) and 97 octane (leaded fuel) .... my racing motors run best on 97 leaded ......

Take a spin by your local airport. Avgas is 100 octane low lead, I just sold some to a guy for his racing car just this past week. The racers love the stuff.

 

[Texasmark]

Re: Posted Octane Numbers

Hey water, what's the compression ratio of a Continental Opposed 6?

Mark

 

[bruceb58]

Re: Posted Octane Numbers

Aircraft piston engines have lower compression than some auto engines but require higher octane because of load requirements and that they are air cooled.

Foe example, a Lycoming 360 has a compression ratio of 8.5:1. A Chevy 350 is 9.4:1