85 octane?
Wow, and I thought we had it bad in Phoenix where the lowest octane at the pump is 87 octane. Premium fuel is 91 octane.

 

I'm fairly sure. Std grade is 85, midgrade is 87 and premium, 90. Not only do we have cheap quality gas, its about 25 cents higher than all the other towns in the state...even the true "one horse" towns! But our town has this little cartel going...but, I'll stop here. I'm getting political.

 

Bummer about the octane ratings...
Anyway, I could definitely see a 9.5 motor not running well on 85 octane! Let us know if octane booster or the next tank of gas runs better.

 

I doubt your engine needs to run on premium gas, and if it did, it would just ping under load. It wouldn't have much effect at idle, especially on a 9.5:1 compression engine.

I run 87 octane (lowest and cheapest we can get in California) in the 289 in my '66, and the car runs just fine. I can't tell any difference at all between the 87 and 91, excpet my wallet is a little lighter with the 91. This engine has about 9.3:1 compression with 351w heads.

I only run 91 octane in the '69, but its compression is 10.18:1. I haven't tried it, but I bet it would run fine on 89 octane.

You may have just gotten bad gas, not just gasoline too low in octane.

BTW, putting octane boost in the tank will not be a good indication of anything. It will boost a full tank from 85 (assumming that's what you really have) to probably 85.05 *LOL*.

 

Thanks for the info. I may need to tune the carb a little too, which is probably the underlying problem with rough running and idle quitting. The timing is set up right and the engine idle is about 700 RPM. May need to kick the idle up a little bit with that cam. Now that the 351W valvetrain is getting broke in with a new cam, it may need a little tune-up tweaking. Just seems odd I noticed an immediate change in the engine with one fill up. I may dump some booster in there anyway just to see...couldn't hurt, though I agree with what you say.

Ya know, now that I'm grabbing at straws; I may have some slight condensation in the tank as its been humid the last couple weeks here and the stang has sat for awhile with a low gas level....hmmmm.

 

It could actually be the gas.
I generally run any sort of cheap gas (except in my motorcycles) and never have any problems other than some minor pinging from time to time.
Some years ago my mom wanted to borrow my truck for a couple of days so we swapped cars. She told me only to use premium from Exxon or Texaco. Of course I filled it with cheapo before returning it. I figured why the heck does a Civic need with premium. And I caught hell. She knew immediately what I had done and I was allowed to keep the car again until I ran all the crappy gas out and I had refilled it properly. She wasn't kidding. The car was low mileage and had always run faultlessly. On cheap gas it stumbled upon acceleration and idled rough to the point of even sometimes dying at stoplights. Oddly enough it never did ping though. Once refilled with her choice of gas it ran just fine again.

 

could have been moisture in the gas too, that will cause a rough idle, and bad acceleration.

 

I agree with johnpro 100%, I just don't see how the lower octane would cause your problem. With that said, there is more to premium gas than just higher octane. Most name brand gas includes more or better detergents to go with the higher octane. The vast majority of cars on the road don't really need the higher octane, so to get the public to spend the extra money for premium it's formulated with more additives like detergents and lubricants so your car actually runs better on it. So when you by cheap gas, you really are getting cheap gas!

 

Octane is simply the resistance to pre-ignition. If octane rating were the sole measure of a quality fuel, we would all run deisel as it has a very high octane rating.

It was likely some gas with too much moisture. Try a bottle of dry gas in addition to your carb tuning. If it is bad enough, you may have to drain your tank.

BTW, gas stations generally have two fuel tanks, one for regular and one for premium. Mid grade gas is simply a mixture of the two that occurs within the pump. In Wisconsin regular is 87, midgrade is 89 and premium is 93. You can save a few cents by mixing 1/3 premium with 2/3 regular. However, at <50 cents a tank savings, it is hardly worth the effort.

 

Octane is simply the resistance to pre-ignition. If octane rating were the sole measure of a quality fuel, we would all run diesel as it has a very high octane rating.

Diesel is Cetane rated:
Cetane vs. Octane The cetane number of diesel fuel is an indication of how easily diesel fuel will ignite under compression whereas the octane rating of gasoline indicates it's resistance to auto-ignition. It's preferable, in both instances, to have a high number but, whereas a high cetane number indicates an ease of ignition, a high octane rating indicates a resistance to ignition. (When low octane gasoline is used in a high-compression engine it has a tendency to ignite, in much the same way as a diesel engine, before the spark from the plug. This produces 'engine knock'.)

So the numbers work opposite of each other
Average cetane is 40 for good diesel fuel
that is about equal to 40 octane.
100 Octane is approx 18 cetane
Where a Mustang will run great on 100 octane a Powerstroke diesel won't even start on 18 cetane.

 

Thanks Greg. I learned something today so now I can go home. :lol: (I go home early a lot)

 

Thanks Greg. I learned something today so now I can go home. :lol: (I go home early a lot)

LOL! I do too!

 

I use to run premium, but changed to "regular" when prices went way up a few years back. Did notice any difference. Our octane is 87 for low grade also. But I use Amoco. Don't know if the brands make a difference.

Oh, welcome back stranger!