I don't have to run PREMIUM fuel anymore !

[Kern Dog]

For over 23 years, I've been a slave to the high cost of Premium fuel when I buy gas for this car...

The first 440 was built and swapped in the summer of 2001. It was iron headed and around 9.2 to 1. I ran Premium 92 then...

When the octane dropped to 91, I don't know if I noticed a difference. I built another 440 in 2004, this one with 9.5 compression and aluminum heads but still ran Premium 91. That mill threw a rod at around 850 miles so I got another 440 block and got a 4.15" stroke assembly to get 493 cubes. I was now at 10.8 to 1 and still needed Premium fuel.
Over the years I've dealt with detonation. I've tried delaying spark advance, limiting the advance, making sure I wasn't running lean, bigger cam, thicker head gaskets, higher octane fuel like so:

In 2022, I pulled the engine to go through it after a camshaft went bad and ate several lifters. I took this opportunity to lower the compression a bit but add quench.
I'm now at 9.8 to 1 but I have a quench distance of .040. I've been careful and have kept using 91 octane fuel at every stop...until May of this year.
On the way home from a car show weekend in Van Nuys, the group I was with stopped for gas just off the freeway. Just for the heck of it, I topped off with 89 octane mid grade.
Now, I still run 35 degrees of total timing which is in around 2500 rpms. The engine did not knock at any time but just to make sure, the next fill up I tried 89 octane again.
Still no knock and no run on when I shut it down. It has me wondering if I could actually run it on 89 octane.
I've been careful to always run the highest grade gas for years so being able to get away with cheaper fuel in the lower octane is great! I don't mind paying for the better gas in this car since the car is not a daily driver.
Still, if I can save 20 cents a gallon, that is cool. Also, it is nice to feel okay if I stop at a gas station that only has 87 and 89 octane.
What is the highest compression ratio that you have ran on regular or mid grade gasoline?

 

[Geoff 2]

You could hear det with your engine using a particular fuel. So you know & can hear it [ hearing still good ]? If do not hear det now with a cheaper fuel, go for it.....
I have seen tests where higher octane fuel made less HP than a lower octane fuel, so no point spending extra $$ if it does nothing...

 

[Kern Dog]

That is another good point....that it may be possible to make more power with less octane.
I've read for years that you should run the lowest octane that allows you to avoid detonation and that using a higher than needed octane is just wasting money.
Again, it isn't a daily driver. It is a hobby car that usually gets less than 2000 miles a year put on it. Spending 20 cents more per gallon isn't that much when I'm only using 200 gallons in a year. It just feels like a little relief to be able to step it back a bit.

 

[Geoff 2]

They are so fickle these cars. My GTO had the usual induction system: high rise intake & 4bbl carb. 9.8:1 CR. I had to use 98 octane [ similar to your 95 ] to stop det. Had cold air induction, cool running engine. Could get det on very hot days.

Switched to a completely different induction system, still carbs though. 20 yrs now. Nothing else changed. Car runs on 91 [ about your 87 ] & I have never had it det under any conditions.

What this showed me is how important the distribution of the fuel & air is because when you hear det, might only be one cyl that is detonating because it is lean.

 

[Cheapsunglasses]

Congratulations! I’m sure it’s the same in California, but I know in Texas we have some pretty small towns that either don’t offer 91, or it’s just a few farmers that don’t need that so who knows how old that gas is, even if it can be considered gas anymore ! Lower octane is more common , and it is cheaper too, but if it’s not a main driver cost isn’t too much of a factor, it’s really also piece of mind that you don’t have to stress out about finding the Goldie locks gas station, you can go anywhere and fill up anywhere

 

[AR67GTX]

I’ve been using 91 no-ethanol in my old cars for a couple of years now and tuned them all with shortened advance curves to keep base timing up while cutting back max timing a degree or two. They do just fine, even my Vette with 12:1 compression and 256/266 cam, although it does have L88 heads. The Hemi is fine on it too.

 

[Hilljack68]

I’ve been using 91 no-ethanol in my old cars for a couple of years now

Agreed. The bigger factor for me is non-ethanol. I too have been using the 91 n-a, but have been adding an octane boost from VP Racing. The 340 may not actually need it, but the stroker I'm building probably will.

 

[j-c-c-62]

Have you tried DIY grade mixing at the pump, being its likely slightly cheaper and you can better dial in the octane your set-up needs?
An upgrade your combo is very ready for if not already reached, is almost any EFI set-up that includes timing control, a knock sensor and fuel sensor IMO.

 

[BSB67]

You should do a cylinder pressure check to see what it is. This is always good info to have when paired-up with your fuel quality needs.

 

[Don Frelier]

You still need to kinda monitor the situation.
I was able to run mid grade on my 383 for about 5 years after I rebuilt it.
Hypereutectic pistions, heavily milled 906 heads with 9.4 : 1.
But after some carbon built up over a few years it started pinging.
I backed off the timing but ended up going to "premium" at the time 93/94 octance.
That combo only had 145 - 150 psi but probably no the best quench setup with a cut down open port head.

 

[eldubb440]

and yet another reason to drive her across the country

 

[Unkle Krusty]

I have about 160 pounds of pressure in my 318, so I use 89 octane. Had to put 89 in my bike a week ago, and it ran quite sluggish, and sounded bad. Back with 91 and Bluzu is happy again. I think compression is about 10.5. Fuel injected.