I was just thinking about possible reasons for my hard-to-idle 428 and I was wondering. High-compression engines need a high octane fuel to run properly right? Well the 428's compression is 10.5, and there's 87 in it right now. Do you guys think if I filled it with 92 it would help?

 

It might. Dieseling is basically auto-ignition of the fuel in the chamber, i.e., detonation. Higher octane fuels resist auto-ignition better. Higher compression ratios (or use of forced induction) result in higher temperatures of the charge, and are more likely to cause auto-ignition. That's why higher compression engines and booster engines require higher octane fuels.

The rule of thumb I've always heard is that 9.5:1 is the maximum safe compression ratio for 87 octane gas, and 10.5:1 is the maximum safe ration for high octane (92-94) pump gas. Of course, "your results may vary." Your 428 was designed to run on fuel loaded with tetraethyl lead (TEL), i.e., "leaded" gasoline. TEL both lubricated that valves and significantly increased the octane rating of the gas. Modern 87 octane unleaded is probably 10 points lower than the has your engine was designed for.

That's not to say that switching to hi-test will fix your problem, but it's worth a shot. Run your tank down as low as possible, and refill with 92 (94 if you have a Sunoco nearby). Run a couple of tanks of the premium stuff through and see if it helps. Then report back here with the results.

 

Well, being that I've got to replace the starter before I can do anything, it may be awile. But I will definetly report what happens when I find out! So, I actually need to put some octane booster in then? So what I may do is drain the tank right where it sits and then put a couple of buckets worth of 92 along with some octane booster in there to get the octane rating up to what it needs to be.

 

So, uh just for the sake of argument, theoretically, if a person wanted to run leaded gas, where might a person find it?

 

Leaded gas has been illegal for use in on-road vehicles since the mid 90s (phase-out was started in the early 70s). Racing and aviation fuels do still typically use TEL as an octane booster, but they are expensive and difficult to purchase. They're also illegal to use in your on-road truck, and will quickly destroy any emissions equipment you have.

You shouldn't need leaded gas, anyway. 92-94 octane unleaded should be fine. If your engine still knocks on that stuff, then something is probably wrong with your engine. You should run a tank or two of the highest octane unleaded you can get locally and see if it still knocks. If so, back off your ignition timing a couple of degrees.

If you really really want leaded fuel for some reason, your best bet is to buy unleaded and use a TEL additive. Lots of companies make "octane boosters", but few contain TEL, and there's not much data to show whether they actually work or not. At least one company does sell a 100% TEL additive, but it's hard to find and expensive. Like I said, though, you should be fine without it.

 

Toluene is a cheap and apparently safe octane booster. See:

http://home.kscable.com/ssutton/miata/engine/toluene.html

http://www.vtr.org/maintain/gasoline-octane.html

 

Every tank, I run 93 with a bottle of Outlaw Octane Booster. Once I did mess up and top the tank off (filled it up the week before, and only drove it once) fuel guage didn't work so I decided to top it off. Dumped another bottle of Outlaw in the tank, and proceeded to put in 4 gallons, filled it up....Truck never ran better.

Two bottles of octane booster wre now in the tank, filled with 93 octane. Man it couldn't be beat!