First I should say I am not 100% positive that the 5.4 has the knock sensor. It is my understanding that it does. My 4.2 had them.

And I completely agree about “nature of the beast” and was just passing on information I had learned for a few different web sites that I don’t have links to. I found it interesting about the “knock sensor and supercharger vibration” and like I believe you are referring to it most likely wouldn’t matter if these trucks had knock sensors because they would probably be useless…

 

hmmm, ok i'll have to check that out. The 4.2 i could see having it since it's a 3.8 which is stroked or something, so maybe they kept em. I was under the impression that the 4.6 and 5.4 didn't have them because, they by nature knocked a little at idle. I'll have to check into it.

And knock sensors would actually be great if they could work and work with the pcm. I can't remember who it was but i think it was easter that was really interested in getting some put on his truck.

 

I posted a couple a while back...

http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

http://www.misterfixit.com/deton.htm

NOTE: The following link has 8 pages...

http://www.streetrodstuff.com/Articl...ne/Detonation/

 

This splains it well
http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

 

I stand corrected. Octane ratings appearantly relate directly to a fuel's resistence to detonation. However, that doesn't necessarily mean that such a propensity is unrelated to the concentration of long-chain hydrocarbons.

I have a friend who is now a chemical engineer for an oil company. I'll give him a call and see what he says about differences in fuel.

 

Correct. The guys with higher horsepower engines have a greater chance for damage to be caused by detonation.

See page 2 about higher horsepower...

http://www.streetrodstuff.com/Articl...ion/Page_2.php

 

SuperFords....to sound in and answer "HOW" you get detonation on the downstroke is simple...YOU DON'T. The two are similar but different at the same time.

Pre-ignition is the ignition of the fuel/air mixture in the cylinder, prior to the firing of the plug, most often by a "hot spot" in the cylinder, the head, on the piston assembly or on the plug itself. It can be a carbon buildup, an overheated spark plug electrode, or a sharp bit of metal, like you might get in the head if the engine "swallowed" something and dinged up the top of the piston and the head.

Detonation, is an entirely different animal. Detonation occurs when the fuel/air mixture ignites from the combination of heat and pressure within the cylinder during compression. Like pre-ignition, this ignition occurs independent of the spark plug firing, but, unlike pre-ignition, it can and does occur after the plug has fired. The existing heat in the cylinder and head, combined with the rapidly rising pressure as the A/F charge expands from the spark ignition, exceeds the fuel's ability to resist spontaneous combustion, and it explodes. The result is the collision of two independent flame fronts and the results are violent!

To answer the question as to A/F ratio's play on detonation. Simple....your "Octane" rating in the fuel is just that...IN THE FUEL. Not in the air. Second, the fuel "quenches" the combution chamber, valves and surrounding area, thus adding a cooling affect.

Just remember, octane does nothing to improve performance in and of itself. All it does is measure the ability of the fuel to resist detonation. And it inhibits flame propagation (flame speed) across the combustion chamber and so, fuel with too high an octane rating may actually reduce engine performance.

 

Sure a lot of myths flying here...

 

i didn't see any, care to elaborate?

 

YOU DO get detonation on the downstroke...

Quote:

"I want to reinforce the fact that the detonation pressure spike is very brief and that it occurs after the spark plug normally fires. In most cases that will be well after ATDC, when the piston is moving down. You have high pressure in the chamber anyway with the burn. The pressure is pushing the piston like it's supposed to, and superimposed on that you get a brief spike that rings the engine. "

See this link for the quote...

http://www.streetrodstuff.com/Articl...ion/Page_3.php

 

Detonation usually happens first at the pressure wave's points of amplification, such as at the edges of the piston crown where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine - this is called constructive interference to yield a very high local pressure. If the speed at which this pressure build-up to detonation occurs is greater than the speed at which the mixture burns, the pressure waves from both the initial ignition at the plug and the pressure waves coming from the problem spots (e.g. the edges of the piston crown, etc.) will set off immediate explosions, rather than combustion, of the mixture across the combustion chamber, leading to further pressure waves and even more havoc. Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top. Piston tops, ring lands and rod bearings are especially exposed to damage from detonation. In addition, these pressure fronts (or shock waves) can sweep away the unburned boundary layer (see figure 2 above) of air-fuel mix near the metal surfaces in the combustion chamber

 

I know but it sure is a barrel of laughs.

There are some fairly good answers and some very good questions in this thread though...



Actually, detonation is cause by a little green imp's with little ball pein hammers that like to live in engine cylinders...

They bang on the cylinders if it gets to hot to let you know to turn the damned heat down...

 

Hmmmm. I will have to research the "Detonation on a downstroke". Again, Fuel is my job and in my training, we learn that Detonation's sound is shockwaves colliding of pressure waves (which gives it the knock and ping sound) and be you have mulitiple pressure waves because there are mulitple flame fronts. This can only occur during the compression part of the stroke.

I will look this up to be sure. But I can say this....some of you guys sure know alot about fuel. Anyone want a job? Hahaha

 

The GTP has a knock sensor and is supercharged. I think the knock sensor is actually in the coolant. Not sure.

Supercharged/Turbo engines are more prone to knock because the intake charges are usually well over 140 degrees. How often does a NA car see 140 degree intake temps?

Roots engines are hard to add knock sensors to because the intake happens in pulses. Its another thing the computer listening to the knock sensor has to know how to ignore. I think thats why GM embedded theirs in the coolant passage that way its listening more to the cylinder walls then anything else.

 

Just a couple, knock sensors causing detonation?...interesting. Oxygen in the atmophere at 13 or 14%(forget which was suggested), OSHA safety regs put it at <19% at deficient, >22% is O2 rich. There's more and I'm not trying to pick at anyone in particular.