This question has been on my mind again and I can't really seem to get a straight answer through google.

My Barth is a 1977 22 foot with a 350 4bbl. The air cleaner says the engine wants 91 octane. But so did my VW from that year.

I'll be heading back to Burning man this year. She'll be fully loaded and towing a trailer.

Can she run lower Octane? Bottom line, I'd like to save some cash but I do not want to hurt the engine!

Thanks.

I do not claim to be an expert. but in my experience (53 years and a million or so miles in various vehicles).

You can go with less octane, until it starts knocking. Your engine is more than 35 years old. technology for the plugs, wires, the gas itself have all changed.

I would suggest that you carry a bottle of octane booster with you and then try lowering it by trying midgrade then regular. if it doesn't knock great. if it does add the booster and fill up with the higher octane next time.

Been running my lexus 430 on midgrade for years. I never noticed a difference.

Of course, I drive like a little old lady, so it could be me.

johnny

Knocking is the real problem, to rephrase an old line, if the engine isn't knocking don't go rocking. I had a '69 Plymouth with a 318 that knocked under any load. I decided to opt to about 8 ounces of diesel to each tankful and found this took care of the problem. My reasoning was the diesel fuel was intended for much higher compression and should reduce detonation and if not at too high a level it should not foul the plugs or cause other problems. Your results may vary, worth what you paid.
Johnny, your Lexus has a computer to adjust the timing to reduce knock so it will probably put up with whatever it gets fed and may not even change mileage.
Tim

Try running the tank low, put in a 1/4 tank of lower octane and try it. When it's good and hot, run it up some hills with high throttle setting. If it's gonna knock, you'll know then.

If it doesn't knock, fill it up and run it. If you get knocking, try retarding the timing a couple degrees and retest. Run the timing as advanced as you can without the knocking.

I'm not sure what octane was desired for my 454 Regal, I've always run the low octane stuff (contaminated with alcohol here in the states...) GM called for 4 degrees BTDC in 1986, Thorley suggests 12 with their headers. Sure enough, I advanced to over 12 before I got any knock, I ran it at 12. (Now I have the computer and fuel injection, so base timing is 6 BTDC and the computer does the rest)

If you can't get it to quit knocking by playing with timing, you could try the additives. If still no luck, fill it up with the higher octane and carry on as you did before.

Mileage was better when timing was advanced, too. Gasoline without alcohol (hard to find here) gives me consistently 10% better mileage also.

My new 454HO has higher compression, I suspect it will be marginal on regular gas but hopefully it will be OK. I'll know soon when I finish the install.

Haven't seen you around much, I still watch your Burner video now and then, awesome. Gotta love the "bar car" trailer! Maybe someday I'll make it.. probably kill me but I'd die with a smile on my face! Have a good time, again.

Octane rating is determined by two different tests, done with identical one-cylinder engines, but the parameters are very different. The less stringent test provides "Research Octane" and the more severe test yields "Motor Octane". For years, the number at the pump was research octane, but the feds changed that so current numbers are the average of motor and research. 87 octane now is the equivalent of the old 91.

BTW, the number is derived by determining the percentage of a mixture of normal heptane (octane rating of zero) and iso-octane (100).

The Lexus LS series' ECU retards the spark to adjust for lower octane, but I use only premium in mine.

Three things cause "pinging":

Knock: Timing too far advanced for the fuel's octane rating and the engine's power setting
Detonation: Too lean mixture for the power setting
Preignition: Cylinder deposits ignite the mixture prematurely