300XS octane: DANG IT! Put in a half tank of 89 Octane! Thanks, Marina!!

[Philster]

I fueled up (topped off) at a fuel dock next to the premium pump. I just need fuel, I tell them, and the dock hand hands me the pump, and I pump away, knowing it ain't diesel since the handle wasn't colored (always check that).

I pull away and somewhat later I happen to be putting my receipt and credit card in my little Ziploc baggie, and notice the receipt says, "pump 1" and the total gallons/price, and I pulled out my smart phone and used the calc funtion and realize that ain't the premium price.

I had a half tank of 91 in there already with a little 92 octane, so going in I was at a tick above 91 octane, which I just diluted with 89. Based on what I had in the tank in gallons and the # of 89 octane in gallons, I weighted out the average and came up with 90 octane or slightly less. Guess what? The Smartcraft system reduced engine power and it was by a lot. This is no coincidence. I am probably at 90 or just below in total tank octane.

I can probably squeeze in about 10-12 gallons of 93 that I can get at BP but this raises my octane to about 90.

Should I have someone drain about half (30+ gallons) and top off with 93, or can I just toss in 10 gallons of 93 and some octane booster?

Bare minimum is 91 for the 300XS.

[native2]

I have run 89 in mine when in a pinch. The rating from merc says 90. So I am sure you could get away with 89. I wouldn't run WFO or put a big wheel on her... If it helps you sleep, go get 5 gals of 100 oct racing gas and add it to the tank.

[Guillaume]

If it helps you sleep, go get 5 gals of 100 oct racing gas and add it to the tank.

This.

Buy 5 gallon of something like C16 and fill it up.

[patchesII]

Around here you can buy 5 gal of 112 for around 45-50. That's what I'd do unless there is some kinda reason you can't run leaded fuel thru an XS

[Guillaume]

Around here you can buy 5 gal of 112 for around 45-50. That's what I'd do unless there is some kinda reason you can't run leaded fuel thru an XS

Oxygen sensors can be sensitive to leaded fuel but I really wouldn't worry about it since it's blended.
Guys blend race fuel with pump fuel at the race track all the time and it doesn't do any harm to the sensors.

Does the XS have oxygen sensors??

[skatr186]

In my experience even cars that are to run high test are fin with a slight mix up. Just don't beat on it running wide open or get on it to heavy on the start. Smooth and easy take off, no reason you couldn't burn off what you need and not damage the motor. If you have heavy mods and need more then 93 octane I would be more concerned (machined heads, turbo, supercharger).

[Euroski]

Oxygen sensors can be sensitive to leaded fuel but I really wouldn't worry about it since it's blended.
Guys blend race fuel with pump fuel at the race track all the time and it doesn't do any harm to the sensors.

Does the XS have oxygen sensors??

300xs doesn't have O2 sensors

[Rayzor]

I agree with earlier posts. Go get yourself some 110 leaded race fuel and get as much in there as you can, you'll be fine. Being out here in California, we barely get 91 octane, so I would always cut my 2.5/280 fuel 20% with leaded 110 race gas. 7 years running 8,200 rpm and never had a problem!