Is this true?
 

I recently heard that a stock engine will run worse with 93 octane fuel if it doesn't require it.
Is that true and if so what's the explanation behind it?

Re: Is this true?
 

A engine will run harder on the lowest octain that you can get away with. High octain is to make an engine live. It is like a cam, most people go way overboard with it. :)

Re: Is this true?
 

Octane rating tells you how long the fuel will burn.

In a high compression engine, you need a slow burning fuel that won't detonate. In that instance you sacrifice a little power by going to a higher octane fuel, but that power loss is insignificant to the power loss you'd experience from the mis-firing of the cylinder.

For an engine that doesn't need the higher octane fuel, all you're doing is slowing the combustion process, and weakening the cylinder pressure because it's slowing the expansion from the burn. Since cylinder pressure is one of the main contributors to torque, it also directly effects the horsepower production from the engine.

Re: Is this true?
 

I've done some testing, on different fuels, vp 103 motorsport, 93 pump, midgrade ect....I found my car ran best on midgrade, i do belive my motor is low compression because of the heads.
however My buddies 93 gt loves vp fuel.....so it depends on timing, mods, maybe even weather?

Re: Is this true?
 

Like 5302 mentioned, octane is added to fuel to resist burning, the higher the rating the higher the resistance to burning, it basically provides a more controlled burn. In diesel fuel they use a "cetane" rating which works the exact opposite of an octane rating. Just some useless knowledge I had to release. :)

Re: Is this true?
 

High compression is just one example (most common) for why vehicles require higher octane fuel from the factory. The further advanced your timing, again, the slower the burn needs to be. Carbon build ups, lean fuel mixtures, and hot running engines can also increase the need for greater octane rating.

Re: Is this true?
 

My 01 cobra ran way better with regular unleaded fuel. I think the high altitude has something to do with it.

I also run regular unleaded in my 90 coupe and it traps 107

Re: Is this true?
 

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, compression has been increased, etc. 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: detonation, 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.

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 a contributor to 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.

Re: Is this true?
 

ok i got 1988 gt which fuel then i should use regualer mid or super please let me know because i'm really confuse i got stock engine ,

Re: Is this true?
 

In a nutshell:
If your running regular and are not getting any knock/ping: Keep using regular.

If you play with the timing, etc and the car starts to knock, then try the next grade up, repeat as needed. :D

If the car is stock, more than likely regular is all you need.

J

*Edit* WOOT WOOT, 400 posts, and it only took me about 5 years! :rolleyes: