What octane fuel is best for my car?
 

I have a BONE STOCK 91 lx 5.0 vert and I went to fill up today. I decided to put in 91 octane rather than 87 and I noticed an immediate difference. The car started easier, idled smoother, and the throttle was more responsive. What do you guys run in your stangs?
Also a gas related question. When I fill my tank up, my gas gauge needle goes above full. Being nervous about running out of gas, I have been filling up at a quarter of a tank. Is this a normal thing for the gas gauge to do, and is it still accurate?

I'M NOT SURE ABOUT THIS! but i believe that 93 octane would be the best for your car. I know some engines don't like the higher octane but i know that I try to use premium as often as possible and love the results. You should probably get PKRWUD's opinion since he know just about everything. Something i am sure about is that it is normal for the gauge to go above full. When you "top off" the tank you can actually fill the fill tube so the gauge will read above full until you use the gas in the fill tube, then it will read "normal full" when only the tank is full. I'm sure that you could go below 1/4 tank without filling up but why would you want to? The last quarter of tank has all the settlement from 11 years worth of (sometimes) crap gas. Filling up before with a 1/4 tank will help prevent some of that from getting stirred up and clogging your fuel filter or in the worst case getting past the fuel filter.

Sounds like the 91 is better for your car. I have to run 92/93 in mine because of the blower but with the price of gas right now, I'd be using 87 if I could. If you bump the timing then the higher octane is needed.

Its not a bad idea to fill the tank at the 1/4 mark. The gas is actually what keeps the fuel pump cool and keeping it cool will help it live longer.

91-93 gas if what the car likes :) But you dont have to get it But I like to baby May car HEHEHE

Cool, thanks guys. Im going to run premium from now on. Where can I find out about bumping my timing? Are there risks? What are the benifits?

benefits of timing increase are a few hp but can't say how muc every car is diffeerent, only risks are if you go too far or don't have enough octane and engine will detonate. to "bumb" your timing firsr find the balancer and there should be lines there with a 10 and a 20 with smaller lines between them, get some chalk or a white marker and highlight between there, then next to the distrbutor is a short set of wires with a grey plug in there pull it out and put it in your pocket so you don't loose it! loosen distributor hold down you'll need a long extension and a swivel to get to the holddown bolt loosen just enough so you can move it with a little effort but not so looose that it turns too easy. hookup timing light and start car and aim at balancer and see what your timing is. try setting it around 13-15 degrees which would be about half way between the 10 and 20 marks. tight dist hold down and check again to make sure it didn't move and then shut off car, put back in the plug and take for a ride and see how it feels.

Unfortunately, many people assume that higher octane fuel is better for all engines, no matter what. This, as I'm sure you know, is absolutely false. Ultimately, the best octane for any vehicle is the lowest one you can run without detonation (pinging) occuring.

Running too high of an octane will ultimately make it necessary to continue using that octane.

Just to clarify, it's not the fuel that increases the performance, it's the fact that the timing has been advanced. The higher octane permits this to happen. Basically, the way it works is something like this:

Octane represents a fuel's stability. When you compress an air fuel mixture, it becomes quite unstable, and explosive. In fact, this is why diesel engines don't require spark plugs. They commonly have compression ratios of 22:1 (as opposed to a stock Mustang with 9:1), which makes the air/fuel mixture so explosively unstable that it ignites itself when the piston reaches TDC.

Octane, for all intents and purposes, is used to slow down, or control the burn rate. Modifications that increase performance, such as increased compression, or advanced ignition timing, will cause the mixture to become too unstable, and pre-ignition (aka: pinging) will occur. This is when the mixture fires on it's own, at the wrong time. Severe damage will occur if left untreated. Anyway, the only way to perform the mods that increase the power is to further slow down and control the burn rate of the air/fuel mixture, and the method used to do this is increased octane.

People often think of high octane fuel as being more flammable, and easier to explode, whereas that is actually the opposite of the truth. 110 octane race gas is much tougher to light than low grade 87 octane. BUT, because of that fact, it is the use of 110 octane fuel that permits the compression to be increased to 12:1 without melting the pistons.

On a side note, nitrous oxide works on a very similar principal. With nitrous, the power comes from the added oxygen. The problem has always been trying to find a way to add oxygen safely. As I'm sure you know, oxygen is EXTREMELY flammable, and if you were to try and add it straight to your engine, you would likely die, and not even be able to explain to people why it was you died. It was discovered that the nitrogen in the nitrous oxide was just enough to keep the oxygen from burning uncontrollably, thus permitting it to be used in a gasoline engine. In other words, the nitrogen is to nitrous oxide what the octane is to gasoline, except for the fact that octane is a calculation, and nitrogen is an actual element, but you get the idea.

Okay, back to the story...

Use of a fuel that has a higher octane than is required by the engine and the way it has been tuned will result in unburned deposits being created and left behind in the combustion chamber, and on top of the piston. Often referred to as carbon deposits, these cause the combustion chamber to become very unstable, for several reasons, all of which will ultimately require you to use a higher octane fuel to restore stability. Among the reasons, are increased compression (the deposits take up space, and actually increase the compression enough to make it unstable), retained heat (deposits retain heat at times when those surfaces shouldn't, increasing the likelyhood of pre-ignition), and the development of sharp edges (like sand under the waves, carbon deposits can be "shaped" by their environment, and can develop sharp edges, which act to the compressing mixture like a hat pin would to a balloon.

Take care,
~Chris

I loaned my '89 to my room-mate a couple of times, and in return, they filled the tank with 91 or 94 octane, thinking they were doing me a favor with the expensive stuff.


I noticed no increase in performance or fuel milage. Your "immediate difference" in performance statement just doesn't reflect the experience I have had.

PKRWUD, so I am actually doing damage to my engine? It is jsut wierd it seems to run so much better.:confused:

I'm not saying that without being able to see it for myself, but if it didn't ping with 87, 91 is a waste of money, and will not help you at all.

If you have your heart set on using 91, try bumping your timing up a couple more degrees. The higher octane should allow that.

:)

Take care,
~Chris

Cool, thanks. I'm going to look into bumping my timing up, and run 91.

In street tune my car runs better with 87 octane because of the low compression ,boost,and btm . i actually slowed down when i put 104 in and didnt crank up the timing and boost.