[mtdaniel]

A local gas station is now selling 91 octane straight petroleum gas. It has to be sold as a recreational gas and is 70 cents more a gallon than 93 premium.I was told by the station owner it is actually a 93 octane .
My last two tank fulls I have used the 91 octane. I have noticed the idle has picked up a few RPM's and the car seems to perform the same as with the ethanol blend. I do have some concerns though. The car is extremely loud ( Headers with full 3 inch exhaust and 10 series flow masters). If I were having detonation issues would I be able to hear it over the exhaust. Or would you fell it also. Like I said it runs fine , the temp is running the same as always, and the motor doesn't run on after the motor is shut down. I just don't want to cause any problems that I am unaware I'm doing.
The motor is a carburetored 488 with Alum. heads , 10.7:1 comp. with a non vacuum advanced MSD distributor and a total of 36 degrees of timing all in by 3200 rpm's. and I am running the autolite AR3924 plugs.
Just wondering what people's thoughts are on continuing to run the 91 or is it safest to run the 93 w/ ethanol in it.

[steve25]

And how are you confirming your not having full throttle detonation issues now my friend ?

[Formulajones]

Good spark plug and acceptable heat range, I use those plugs in a couple of aluminum headed engines here.

Best way to check detonation is to periodically remove the plugs and look for black spec on the porcelain, (looks like pepper)

Keep in mind 10% ethanol gas requires a .7 richer AFR to compensate. So the car will act like it's running a bit leaner on the ethanol blend if you don't make any other changes.

All we have here is 91 octane with 10% ethanol. We are running some engines here that push the envelope and working well enough on this cat pee. Fathers 571 with 10.84:1 and aluminum heads is working fine on it. Made best power on the dyno at 34 degrees timing and it's running perfect. I have an 11:1 iron headed engine here that is also running fine on it with 36 degrees.

Just have to keep an eye on things. Mild detonation may not even be audible.

[The Champ]

Originally Posted by mtdaniel

A local gas station is now selling 91 octane straight petroleum gas. It has to be sold as a recreational gas and is 70 cents more a gallon than 93 premium.I was told by the station owner it is actually a 93 octane .

The station owner is full of crap.

91 octane is 91 - regardless of whether it's E10 or 100% gas. If his pure gas was 93 octane, that is what it would say on the pump.

[cdrookie]

Citgo station right? I tried it when it came over here in the spring and wasn't impressed.

[1968GTO421]

All the stations here selling 10% ethanol gas are rated at 91 octane on the state sticker on the pump. The few local stations here selling non-ethanol gas have state stickers on the pump stating 93 octane. And, yes, it is about 50 cents again more than the 10% ethanol gas. I use the non ethanol mostly to prevent ethanol related problems in my fuel system. (And I like the 2 points more octane.)

[Hooter]

I run the ethanol free in my GTO and all my small engines(mowers, chain saws, etc). Works fine in all.

[Navy Horn 16]

If you don't have cats, 100LL from your local airport can be the ethanol free octane boost that you are looking for. It's under $4/gallon right now here in Texas.

[64speed]

I used to buy gas at the airport to the only gripe about it was they couldn't technically see you filling your car up with it so I had to carry two ten gallon gas cans in the trunk.

[PontGuy]

I ran 93 non-ethanol for years until very recently. I noticed some light pinging and discovered the station had reduced the octane of their non-ethanol gas to 91. So now my Lemans is getting some corn in its diet but at least the gas is 93 octane and no pinging.

If you get no detonation with 91 non-ethanol then that's what I would use. But if your car requires the octane then 93 ethanol is a better choice. Either that or pull back on the timing and lose power.

My Lemans exhaust is very quiet so its easy to hear when the engine is not happy with the gas. Its definitely harder to tell on a car with loud exhaust. Keeping an eye on the plugs is the best way then.