Gas and octane longevity

[zrpilot]

I purchased some 93 octane gas early in our riding season and on my first long day of riding, I was getting some deto on heavy throttle - engine loading conditions.

How long does gas maintain its octane rating? I'm thinking that if it sat in the gas station tank for a couple of weeks, then in my quad for a couple of weeks, maybe the 93 octane I bought became 89?

I have a 416EX with an 11:1 Wisece and an HRC cam.


GPracer2500, got some wisdom?

[Dill]

gas does lose octane when sitting in plastic tanks. It will last much, much longer when stored in a metal container. That being said, I highly doubt 93 octane would lose 4 octane points in a month.

Most somewhat busy gas stations get deliveries of gas every few days. No way it would sit for a week or more.

[400exrider707]

Hmmm a well jetted 416 with 11:1 and a big overlap cam shouldn't have any problems with deto. You are sure your jetting is on correct? I dont want to say that it is supposed to be ok with 11:1, as some people have different results than others, but especially with that cam you SHOULD be ok, I agree, GPracer2500 you should shed some knowledge on us brotha!

[GPracer2500]

Get some fresh fuel in there and see what happens. If the deto disappears then it pretty much must be an octane rating issue.


If you're pushing the limits of what pump gas octane ratings will handle then I wouldn't use gas that's been sitting in the fuel tank for very long. If you do then you may find the gas isn't up to snuff anymore. Sounds like that could be what's going on in your case, zrpilot.

This is one reason why using a fuel that barely meets your engine's octane requirement is a fine line to walk. It's always good to use a fuel that exceeds the engine's octane requirement by at least a little bit so you've got some wiggle room. "Walking the line" is ok, I suppose, but you've got to be aware of it (and able to detect deto) so you don't unwittingly damage the engine.

How long can gasoline be safely stored inside a fuel tank? There's no single answer. Some of it depends on the gasoline supplier. At each gasoline companies' distribution terminal they may or may not add stuff (anti-oxidents) to aid in storage stability. And even beyond that, the RFG gasoline sold in Chicago might be more or less tolerant to storage then the conventional gasoline sold in Nebraska. The boutique gasoline sold in AZ might handle sitting in a tank differently than either one--and on and on. It's the more reactive molecules in gas that contribute to storage instability. Therefore, storage stability is directly linked to the fuel's particular composition.

The other big factor is the storage environment. High temperatures and venting to outside air are bad for fuel storage. Sunlight is bad too.

Stored fuel deteriorates in two main ways: evaporation and oxidation. When gasoline has access to outside air the light ends (the hydrocabons with comparitively low boiling points) are quick to evaporate out of the fuel. The loss of these light ends decreases octane rating (and hurt cold start performance). Plus, oxygen that comes into contact with the fuel can combine with certain components turning them into new molecules. This oxidation can also lower octane rating and it forms gums that are held in suspension within the gasoline. Heat speeds up both oxidation and evaporation.

As Dill suggested, when gasoline is stored in a sealed metal container it can be stored for months or even years without deteriorating. Storing fuel in a well-sealed plastic container is going to be better than letting it sit in your gas tank because it's not (as) exposed to air. A junky plastic container that lets fumes leak out isn't much better than just leaving it in the tank.

In the end, it's how your particular pump gas behaves in your particular engine that counts. It's not out of the question that even 1 week of sitting in your gas tank could cause deto that wasn't there before. I'd think that would be unusal but if the gas you're using isn't very tolerant of your storage methods and you're engine needs every last bit of octane rating the fuel can possibly supply....there ya' go. If months old pump gas never deto's in your engine then chances are you're taking advantage of the "wiggle room" I mentioned above.

[sc400ex_rider]

i had the same prob i built my motor over the winter and had about half a tank of old gas. 11.1 cam ect i filled it up with 93 and it would pop at WOT in 4th and 5th. i then put 1 gallon of 110 in it with no probs. i have since been runnin 93 for 2 or 3 tanks with no probs so it was last years stale gas. cant use old gas in 11.1

[zrpilot]

Thanks everyone, good information!!

My jetting is as follows:
39MM FCR
175 main
EMP Needle
52 pilot
No lid

I'm going to try a couple of things:
1) New 93 octane (non-oxygenated) from a different supplier
2) Up the main to a 180 (from a 175)
3) Return needle to the 4rd (middle) clip position form the top (currently 5th)

Then ride it again..

I'll give you a report.

[400exrider707]

Originally posted by zrpilot
Thanks everyone, good information!!

My jetting is as follows:
39MM FCR
175 main
EMP Needle
52 pilot
No lid

I'm going to try a couple of things:
1) New 93 octane (non-oxygenated) from a different supplier
2) Up the main to a 180 (from a 175)
3) Return needle to the 4rd (middle) clip position form the top (currently 5th)

Then ride it again..

I'll give you a report.


So do you think its a jetting issue or simply an octane issue, or both? I would try some fresh 93 and see if it still occurs, if so, then perhaps it is indeed jetting. What does your plug look like?