Fuel to use on a 250

[kolle_hond]

So I have been running my truck on LRP (lead replacement petrol) ever since I got it, but the pump where I usually fill up no longer stocks it. Here in South Africa, I have two other choices: unleaded 93 or unleaded 95 (octane levels). I have heard that in some cases a supplement is needed for older engines. Most of my engine is stock as far as I know, so will there be any risk using the unleaded? And which one would you guys recommend? I am taking a long trip to my family in the countryside (about 400km) tomorrow, and I don't want to get stuck with a engine failure somewhere.

[Sixteen Tons]

I have the same engine. My experience has been this: when using 87 octane unleaded, I almost always experience some degree of dieseling (run on after shut off), though I am aware that there may be other factors contributing to that issue, with mid-grade, usually 89 octane, the problem is somewhat mitigated, but with premium 93 octane I have virtually no dieseling ever. If in SA you have access to a even higher octane, then I suspect from simply a fuel perspective, you should be fine. There is no recreating the fuels these truck originally ran on, though I have read on this site that a high zinc motor oil, if available, has beneficial properties for old engines.

[57taskforce]

What I would do for some 95 in New Mexico...

[BLT gmc]

If your cylinder head has the original valve seats, they are most likely the "soft type." These "soft seats" like to use the leaded gas/supplements as a impact cushion. If you use unleaded gas for prolonged peroids of time the hard valve will wear away the seat. I will have the seats replaced on my equipment with the hard seats(when it is time for a rebuild) to avoid having to up supplement in all the time. FYI, Bruce

[Daner]

Do they use ethanol or some other alcohol in SA to get those high (it's all relative) octane numbers? I'm just curious.

[CST10]

There is an additive called lucas upper cylinder lubricator that you add to your fuel. Does it work??? I don't know but I use it in case it does work. I buy it at Walmart by the gallon. 3 oz's per 10 gallons fuel.

[Steeveedee]

Use an additive if you wish, but running unleaded at this point shouldn't hurt it. Use as low an octane as you can get without spark knock, if there is a price difference.

Your octane number may be Research Octane Number rather than Motor Octane Number, which would explain the seemingly high value. In the USA, the octane published on the pump is the average of the two numbers. RON is higher than MON.

http://en.wikipedia.org/wiki/Octane_rating

[kolle_hond]

Thanks for all the replies, I used 93 and it worked fine, though I suspect there might be problems if using for prolonged periods of time. I did get 6.1 km/l (14.3 mpg), which is much better than my previous measurement of 5.8 km/l (13.6 mpg) (same trip, a year ago).

The reason I was afraid was because of the following table:



Source: http://www.naamsa.co.za/unleaded/databases/index.php

@Steeveedee
I believe you are correct, the number seems to be RON.
Source :http://www.naamsa.co.za/unleaded/faq.htm

[68 IRON]

Kolle, I take it that you're uneasy about the metallurgy suitablility? I'm going to say they are referring to the "softer" valve seats....yes they should have lead to cushion the impact of the valve. With that being said....I have the same engine in my 68 C-10 and it runs 100 miles a day on 87 octane. This engine has never been opened up either for repair let alone rebuild. The I-6 is one tough engine! I do get the the occasional dieseling effect which only requires me introducing 89 octane to cure. Have a good trip...that engine will get you there and back!

[chip46wis]

Just my 2 cents----when I drag raced we used aviation gas which I always heard was unleaded so I would fill up and use 1/4 to 1/3 small bottle of Marvel mystery oil right in the tank---it worked for me!