No, not exactly. What you're probably hearing is your exhaust system allowing cold air to be sucked back into the pipes when you let off suddenly. Headers will make this much more pronounced as do performance mufflers.

Exhaust leaks can also make very annoying popping noises on deceleration.

Detonation is when the gas / air mix ignite before they are supposed to. This is sometimes referred to as pinging or spark knock. Essentially if everything works perfectly your car will be able to fire the spark plug so that the air / fuel combo ignites and actually starts to force the piston down right as the piston crosses over the TDC and starts it's power stroke. If detonation is occuring because of too advanced timing or some other reason, basically the fuel air mixture is being ignited early and is trying to push the piston down as it is still coming up on the compression stroke. This obviously robs horsepower tremendously and can be extremely detrimental to the health of the engine. Severe detonation will destroy an engine very quickly.

The car can vary timing for certain conditions and certain amounts, however in general the more advanced the timing (the sooner it fires) the more power you will make. You want to get as much out of the air / fuel charge that you can. Imagine if the timing is retarded such that the piston has already begun to travel down before it fires. The combustion process takes time, and if the piston has already moved past TDC and started down it's travel if it fires then it will be wasting some power. The explosion will expand but it will not be pushing against the piston because the piston has already moved down.

Advance the timing too far and you lose power due to detonation, retard the timing too far and you lose power as well. You see, a lower octane fuel burns faster, and mroe violently, because of this you can't advance the timing as far. You also can't compress it as much before it will explode on it's own without the influence of a spark plug.

Make sense?

Pinging can be heard as a tick tick tick sound under load conditions, such as heavy acceleration or when pulling a hill in a high gear, etc. However, this could also be an exhaust leak as well. If you're hearing a lot of popping when you let off the gas you might investigate the exhaust leak possibility.

Listen to this and tell me if it sounds familiar
http://www.wku.edu/~nathan.plemons/sounds/hot_cam1.mp3

That is the sound of my flowmaster mufflers and headers, coupled with about 10 small exhaust leaks. Now listen to this
http://www.wku.edu/~nathan.plemons/s...en_headers.mp3

This is running directly off the headers, no cats, no pipes, no mufflers. Essentially the stock exhaust serves to mask all of these noises, but it is not knock. However, a loud exhaust can sometimes trigger your knock sensor into reading "false knock" and retarding your timing anyway.

So if you think you are getting some knock timing retard you can test this with a scan tool, or some software such as Diacom. Under normal driving if you car getting knock retard, you might try a higher octane fuel until you don't get any knock retard for maximum performance. If you end up with 112 octane in the tank and you're still getting knock retard you might want to consider the possibility that it is false knock and look into your exhaust system and try and find any leaks or anything that would make excess noise on the car.

 

What I'm hearing IS backfire. And it happens only when it shifts. I never let off the gas and this all started after I changed the O2 sensors on my exhaust (corsas even then). It never happened before then. I never get it at any other time other than what I previously stated. :nonod:

 

I used to use the octane boost, but I found it wasn't really doing that much. I just used Sunoco Ultra 94 and the car runs great with that. Better 1/4 times by about 7 hundreths to one tenth, with Sunoco 94 rather than Premium+Octane boost. I also noticed w/ Sunoco 94 the fuel mileage average on the dash was reading higher. This is from my own research and experience.

 

Could just be the changes in ehxaust pressure when your engine is still spooled up but suddenly unloaded. For example if I wail on my car up to 6000 rpm's or so and just let off the gas and leave it in gear it will pop and crackle as it slows down with the tires.

If instead I put it in neutral or press the clutch it'll fall back to idle quietly. Exhaust does funny things depending on engine load. I don't think you've got too much to worry about. You might be running a little rich. I know my car was running real rich with my stock heads and headers, along with a few exhaust leaks I threw a 5 foot flame out of my tailpipes on the dyno!

I'm sure it's nothing serious.

 

So no matter what grade gas I put in, I would most likely get the same problem? I thought that maybe by running at 91 than the usual 93 I would be experiencing this problem more often. :confused:

 

Probably. Knocking and pinging isn't something you would typically hear out of the ehxaust. It certainly wouldn't hurt anything to try some better gas or an octane booster, but I wouldn't get your hopes up. If it's particularly annoying it's worth trying just about anything.

If you get under the car can you see any black carbon at any of the joints in the ehxaust system? If so that's a leak.

 

"pinging" sounds similar to ice cubes rattling around in the engine. It's not a uniform timed sound like a drum beat, it's more like alot of ice cubes rattling around loose in the engine. If you find a steep hill, or put the car under extreme load, like towing a heavy trailer, you can usually hear it occur. It happens while you're accelerating only, not after you let up on the throttle. Sometimes, it's not real loud, so you need to turn off all of your accessories (i.e. radio, a/c blower, ect.) and roll up the windows, so that all you're hearing is the engine. If you know someone with good hi-tech tune up equipment or someone who works at a smog inspection station, they can find the "pinging" and tell you what is causing it. If you are experiencing "pinging", be sure to get it checked out. Uncorrected, it can cause a burnt/damaged valve or even a burnt /damaged piston (usually a hole in the head of the piston). Then you've definitely got an unwanted project on your hands.

 

I agree that buying higher octane than needed is a waste of money.

The parts I've never heard are:

The definition I read most often is that octane is a measure of a fuel to resist detonation. This is primarily a function of how fast the flame front progresses through the combustion space. In a well designed, well maintained engine this can be most distructive.

There are other causes of "pinging": i.e. predetonation - combustion initiated before spark usually caused by hot spots in the combustion space, not normally cured by higher octane; dieseling - combustion initiated before spark usually caused by poor fuel, sometimes cured by higher octane; timing too far advanced for rpm, might result in backfire through the intake system, most often cured by tune-up.

While this might be true depending on how the octane is achieved, in most modern blends, there is so little difference as to be unmeasureable in engine performance testing.

Here in San Diego (elevation 0-6500', most of it around 10'-200'), unless you really hunt for racing fuel, the best octane you find is 91. Fortunately, stock (or mostly stock) Vettes don't seem to mind.

I'm probably use spliting hairs on the definitions. As I said before, I had just never seen these explainations before (in about 35 years of reading :D ).

JR

 

Ask yourself exactly how you define detonation? Perhaps we should say it's ability to resist PREMATURE detonation, because really your engine runs on detonation. Higher octane resists this because it is less apt to detonate under higher compression situations, and as you mentioned doesn't burn quite so quickly. In this case you can actually introduce your spark sooner and it won't detonate, explode, burn, whatever you want to call it until after TDC. If you ignore the possibility of the fuel igniting without spark due only to compression than you can only look at the speed / rate / violence /energy of the combustion process.

Assume for a minute that you have an engine tuned with a proper sprak advance so that the expansive force of the burning happens right after TDC and it forces the piston down with all of it's force. If this engine is running premium you're good. If you put a lower octane grade in there and don't retard the timing, what happens? It explodes early and actually uses some of that power to try and push the piston the other way because it bruns faster, more violent, however you want to state it.

Now you do the same thing in a low compression motor, that is running perfectly with maximum spark advance on low octane. If you put high octane in there nothing bad will happen, but it will be less efficient. While running on regular the mixture explodes right as the piston passes TDC and forces the piston down with it's peak force because it is at it's highest compresison point. Now if you fire at the same time with high octane you will be giving up some power because the piston will have already started it's downward travel. Although the burn was started at the same time, the premium burns slower, and allows the piston to travel away from it more as it burns. This means that some of the expansion by the explosion will be used just to fill the void in the chamber, not necessarily push the pison downward.

You're right it might well be a negligable amount, but for the reasons above it does still happen. You might not be able to measure the performance decrease and there will never be a increase, so is it worth paying $.20 more per gallon on something that isn't helping you out any?

Remember, California is the home of the smog ****'s. They would probably rather you're car run in such a way as to make the least amount of power possible. I'm gonna have to chalk that one up to the state alone. There are regional differences in gasoline formulation as well. But it is true that in gerneral the higher the elevation the lower the octane rating will be because it isn't necessary. California excluded! There are people on the forum here who say they run 94 or 95 octane, which is absolutely not available here. We go from 93 to 100+ race fuels in one step.

I don't think anything wrong with it. I think you understand what I'm saying and you already know what I'm saying, you've just never heard it explained exactly this same way before. There are usually many different ways to explain the same situation.

:cheers:

 

California!!!!!!!!!! Don't remind me about the smog laws. THEY SUCK!!!!!!! You are required to have catastrophic converters, can't run anything but the stock type mufflers, NO HEADERS, and I know this being a retired cop. The vehicle code section covering this is "modified exhaust" which is a catch all for headers, mufflers, cut-outs (I know you young guys are wondering "cut-outs?"). On top of that, some entrepenuers decided to get their state smog station inspection licenses, invest in a testing machine and nobody came. It was alot more convenient to pull into a gas station and have the car inspected. If it didn't pass, they could repair it and make it passable. Well these entrepenuers weren't making any money, so they went to the State and now the State requires that you go to these "test only" stations every other time, to make sure that these guys get some business. The problem is, they can't fix the it, so you have to go somewhere else to get it fixed. Good old politically correct California. BTW, most of the cops here don't enforce the "modified exhaust" law, unless it's really loud or the driver doesn't pass the attitude test.