Gas and Oil Gurus needed

[RTBandit]

I drive a car with a 5.7 Hemi engine - The recommended fuel is midgrade 89 octane. I have noticed the car runs better if I use premium 91 octane fuel. My question - Is there a greater risk of fuel dillution to my 5w-20 oil running 91 vs 89 octane gas. I usually use Castrol Syntec 5w-20.

 

[HTSS_TR]

Fuel dilution is at the same rate regardless of Octane of the fuel you are using.

 

[RTBandit]

I have heard that higher octane gas burns longer and slower than lower octane fuel. My concern is there may be a greater amount of gas residue remaining in the cylinders.

 

[dtt004]

Hi.

The information you heard about higher octane burning slower and longer is misleading. It is all about the compression ratio of the engine. If you have a high compression engine, you need high octane gas. We are talking usually, within the limits of today's operating environments, 11.0:1 and up. You will also need to use a high octane fuel if you are using a forced induction engine, as they compression ratio is automatically kicked up when larger amounts of air enter the chamber.

HTSS_TR is correct. Fuel dilution is the same regardless of octane. What affects that rate is the condition of your engine, not the gas or the oil. If your engine has badly worn cylinders, or improperly installed and tensioned rings, you will see an increase in fuel dilution because of the lack of optimum sealing and a large amount of blow by entering the oil. Although it is becoming a rare occurrence today because of the qualities of fuel available, your injectors and combustion chamber are as clean as the combustion process itself. The oil plays a part in keeping you engine lubricated, and also helps to keep it clean.

If you are worried about fuel dilution, it is only a common occurrence in poorly made and maintained engines. Yours doesn't seem that way, so don't worry about it. Using higher octane will not decrease fuel dilution and vice versa.

 

[CATERHAM]

Originally Posted By: dtt004

If you are worried about fuel dilution, it is only a common occurrence in poorly made and maintained engines.

Fuel dilution is anything put uncommon even in engines in perfect condition.
Small amounts of fuel does inevitably get past the rings particularly when intially started and until the engine is up to temperature. Once up to temp' it is less of a problem as the high internal temp's rapidly vaporize it. But if the engine is run frequently without being brought up to temp' fuel will build up in the oil. It's not unusual to have up to 5% of fuel present in the crankcase oil of short commuting vehicles during the cold winter months.

 

[ARB1977]

If your worried get a uoa. What ever gives you better numbers to with that. My guess you'll see no difference.

 

[NateDN10]

Originally Posted By: CATERHAM

Fuel dilution is anything put uncommon even in engines in perfect condition.
Small amounts of fuel does inevitably get past the rings particularly when intially started and until the engine is up to temperature. Once up to temp' it is less of a problem as the high internal temp's rapidly vaporize it. But if the engine is run frequently without being brought up to temp' fuel will build up in the oil. It's not unusual to have up to 5% of fuel present in the crankcase oil of short commuting vehicles during the cold winter months.

Yes, but a higher octane fuel still won't make a difference.

 

[CATERHAM]

Of course.