I work at our local airport and have access to 100LL, is it okay to pour some of this into my fuel mix? My 67 has a slightly modified 289; cam, Holley 4 brl, intake, MSD ignition, MSD Coil, exhaust... Will this destroy my engine, and how much should I put in?

 

Directly relates to your compression ratio (mostly) and timing.

So what is your compression ratio? And are you willing to recurve your distributor? Do you have blower, that you want to up the boost on?

Anything less than 10:1 and your car will run best on Pump gas, w/o blower. Blower changes everything, but i figure you wouldn't be asking this question if you had installed a blower anyways.

And yes you can actually damage an engine running too much octane, Mostly due to heating issues, but also can cause valve burning, in extreme cases.

Remember Octane is not power. It is the rating given to gas to denote the amount of energy required to get it to ignite. The higher the Octane number the more resistant it is to ignition.

So if you run a high compression engine you need a fuel that takes more energy (in this case compression pressure) to ignite, so that you do not get pre-ignition.

Another way to say it is, with a stock 9.5:1 compression ratio, you will loose power running above 91 Octane as the air/fuel mixture will ignite too late/incomplete in the cycle.

HTH,

 

So what should a stock 289 take?

 

My owners manual says 94 octane, but I'd be damned to see a pump here that sells anything above 91. Midgrade or premium depending on my economic condition is what I put in there.

And yes, I do have the original owners manual still in the car.

 

Well, two days ago was the first day i put premium it in. The engine didn't sound any different or run any different. I might just stick with the regular, since it's 20 cents cheaper.[:@]

 

The owners manual was written when the rating was done on "Research Octane Number" that is a much higher number then what we use today which is the average of R.O.N (Research Octane Number) and M.O.N. (Motor Octane Number). On the pump you should see somewhere in small print P.O.N. (Pump Octane Number).

The old 94 RON Octane is fairly close to 89 Octane PON.

A couple of rule of seat of the pants tests. Does your vehicle lose MPG on lower Octane fuel? and does you vehicle ping on a grade under load (allowing that you are not running higher than 34* max advance)?

I run my 6 on 91 Octane because I get the best fuel efficiency and adjusted the advance curve acordingly. If I get a bad tank of gas,(unusual) or put the wrong grade of fuel in (most likely) I can tell as soon as I put my foot into it. I hear a slight ping, and have to back out the advance.

There is some great information about Octane ratings on the web. Much better than I have described here.

HTH,

 

could i mix 10LL and cheap stuff?

 

But what would the point be?

Im not sure how octane chemistry works, but wouldnt mixing half jet fuel/half regular be like putting premium in it?

 

I dunno... it was just an idea... we get a lot of guys at the airport getting 100LL for their drag racers or circle track cars. Do you think premium grade gas is worth the difference in price?

 

Well, drag cars are just a bit different. You cant really put 89 octane in a drag car.