I still don't have a Lightning yet (can't wait till my lease runs out...six LONG weeks), but I've been reading the forum pretty regularly and found a number of post regarding "Detonation".

In a stock 02 lightning, I know premium fuel is recomended, but can you get away with running midgrade (or even 87)? Not all the time, but occasionally. This will be my daily driver

Also, I've heard the term quit a bit, don't fully undersand exactly what detonation actually is. are detonation and spark knock one the same?

 

Don't even be tempted to run anything less than premium gas, even in a stock truck. You might void your warranty as well, with anything less than 91 octane.

 

Detonation is the 10 letter version of the word BOOM. For example, if you replaced a few words in the sentence "I lit the fuse and the dynamite detonated," you'd have "I hit the gas, and the engine detenoted.

Either way, it's destructive.

Spike

 

My understanding of detonation is this (as it referes to octane rating) The higher the octane the more precise the fuel will ignite. At the point of stroke where the preasure is the most is (or should be) when the cylinder is at it's peak. That is when the spark fires. Higher octane will wait till that point to ignite. Lower octane may fire x many degree's before or after that point. Some engines can handle it by changing the timing (the phrase 'so many degree's of timing' comes to mind). The more the timing is off the worse millage and more engine wear occurs. Think about it; if the cyl is not all the way to the top and it ignites, you're actually getting retardation (I think I used that right). The explosion actually pushes against the flow. So run the recommended level.

one more thing. The sounds that denote detnoation, ie. pinging, knocking, valve rattleing all have to do with detonation. If you've heard someone's valves knocking, try putting higher octane in.

It's amazing how in step these things (any engine really) have to be. Each rev is a stroke, in a 4-stroke eng it fires 1000 times per min @ 4000 RPM. And that's per cyl. If not they'll shake, rattle, and blow themselves up. Hence DENTNOTATION = BOOM

Ok now somebody come in and fix all the misunderstandings I have.

 

Not exactly. The spark ignites the fuel and you don't want it to explode. Rather, you want a controlled burn. A higher octane will slow down the flame front given the same cylinder pressure and assuming no hot edges which could ignite create another flame front which would collide with the one created by the spark plug.

For the experts, how close am I?

Spike

 

I don't think either of you have it dead on. The unleaded pump gas you buy all have very similar properties (Lower Heating Value, Stoichiometric A/F ratio, Heat of Vaporization, ...). The only difference is the octane number. There's plenty of chemists and chemical engineers that could explain the chemistry behond detonation, but its not really important. Just understand that the higher the octane number, the more resistant the fuel is to auto ignition (detonation).

The power, fuel economy, etc. will be essentially identical between all of the fuels. The spark plug sparks, you have an ignition delay (the same for all of the fuels), the when it does burn, you have the burning velocity (a function of the lamilar burning velocty and the in cylinder turbulence) and that is still the same for all of the fuels.

The only way you make more power with a higher octane fuel is yuo can be more agressive on fuel and spark without the risk of detonation.

Most likely you could put 87 octane in and be fine, so long as you don't run under high load. But then why are you wasting 30K on a L to save $2.50 on your next top off?????

 

-----------------------------
How Much Octane?
Street-legal racers are always looking
for better performance; 76's 100-octane
gasoline in one answer
By Tim Wusz
Reprinted with permission from National Dragster
February 7, 1997

(a portion of the article)

WHAT IS DETONATION?
The octane number of a gasoline is a measure of its resistance to detonation. Detonation occurs when the octane number is too low for the engine and its operating conditions. When the spark plug fires, the flame moves through the air/fuel mixture, burning it very rapidly. Detonation occurs if a portion of the unburned air/fuel mixture gets raised to a temperature and pressure it cannot tolerate and ignites before the flame front gets to it. Detonation causes the maximum pressure in the combustion chamber to be reached before the piston reaches top dead center and pushes down the piston before it has reaches the top of its travel. Much of the gasoline's energy is wasted in trying to move the piston up while the high-pressure gasses are trying to push it down. The extreme temperature and pressure developed can cause broken rings, rod-bearing damage, piston overheating, and erosion of the aluminum. Pistons sometimes end up with holes in their tops from the high temperatures and high pressures.
-----------------------------

 

My thought's exactly. Premium is what, $0.20 more per gallon than 87 octane? (depending where u go) So basically even on a full tank up, you're only spending $5.00 more to get the better gas. So with that, go with a water instead of that last beer at the bar and the rest is the change in your pocket at the end of the week

 

A lot of that excerpt is a little bit of a stretch. The peak pressure changes just aren't all that much different when the engine is knocking. Maybe very hard knock, but a noise that bad usually makes drivers back out of the throttle.

The reason the damage occurs is because of the increased in-cylinder air motion and heat trasfer. Combustion temperatures can be north of 2500 Kelvin. That is way above the melting point of metals. The metals survive because they don't have to to equilibrate to the combustion temps before expansion lowers the temperature and pressure. Heat trasfer is a product of the temperature difference and the heat transfer coefficient. The coefficient is a function of materials, air velocity, and air turbulence. When knocking, the increased turbulence (due to pressure fluctuation) increases the coefficent, and therefore heat transfer. This added heat is what destroys parts within an engine.

The peak pressures just aren't enough different to cause damage. You probably get a much larger change is peak pressure by putting on a 4# pulley than you do if you are detonating a stock engine.

 

So it has nothing to do with peak pressure occuring at the wrong time?

Spike

 

Just don't do it. Premium only! If I remember correctly, 91 octane (M+R)/2 is the absolute minimum. I never use less than 92. If you need to lower your fuel bills you are looking at the wrong truck. I agree with previous comments - the one thread common to all the blown engines seems to be lean detonation.