gas & engine knock
10-24-2004, 10:50 PM
#1
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Join Date: Jul 2004
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Posts: 118
gas & engine knock
ok teachers, school me.... whenever i run 89 or 87 gas have engine rattle when i stomp on it. so, i run 93 all the time. i've heard the engine knock is pre- detination, carbon build up on the valves. rebuilding the heads is not an option right now. anything else to do? guess i should keep running 93. it's like running a turbo ya know!! just kidding, i know better. have read some of the other posts, but didn't answer my questions. the car has MANY miles on it. that could be the problem? thanks.....
10-24-2004, 10:52 PM
#2
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Join Date: Apr 2004
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Posts: 9,172
RE: gas & engine knock
I would say take it to Jiffy Lube and have them run cleaner straight through a vacume line and try to clean it out. It will smoke like hell as its doing it but it helps. I dont know what else to tell you if you cant do a rebuild.
10-25-2004, 02:28 AM
#6
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Join Date: Oct 2004
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Posts: 76
RE: gas & engine knock
One of the most frequently misunderstood issues among all folks concerned with performance and appropriate care and feeding of their motors is the subject of gasoline, and more specifically the mysterious octane number. Oil companies have gone a long way to foster this mystery, by marketing gasoline with higher octane numbers as “premiumâ€, and inferring that the golden road to more performance, cleaner widgets, and hot chicks, is to spend the extra dough on the higher-octane stuff.
Well, sorry to be the one to break this to you, but you’ve been had. So lets take a quick look at octane, and then get to the big point.
Now the truth is, octane is actually a chemical, which comprises an important part of the chemical soup that is gasoline. But here’s the thing – octane the chemical has absolutely nothing to do, with “octane†the pump measurement. So what is it?
In a nutshell, the octane rating of a fuel is a measure of its ability to resist detonation, ping, pre-ignition, or knock.
The number we most often associate with octane is the "Anti-Knock Index", or the "Pump Octane" number. This rating is an average of two different measurements - the Motor Octane Number, and the Research Octane Number. Both of these measurements are taken using a special single cylinder test engine that has a variable compression ratio. The RON measures the knock resistance of a fuel during low RPM, light load conditions, while the MON is representative of high-speed, high load operation. As a result the MON will always be lower than the RON, but for our purposes the Research Octane Number is more significant because it more closely represents the way a low rpm motor is operated (normal driving).
So “Octaneâ€, has absolutely nothing to do with the energy content or quality of gasoline. It’s just a measurement of when the gas will make a motor of a specific configuration begin to knock.
Lets take a moment here to define a couple of things important to understanding this discussion.
Knocking (also called pinking or pinging) in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug but burns too quickly, combusting completely before the optimum moment during the compression phase of the four-stroke cycle. The resulting shockwave collides with the rising piston, creating a characteristic metallic "pinging" sound. The fuel is normally ignited slightly before the point of maximum compression (the spark advance) to allow a small time for the flame front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the point of maximum compression. It is only when this flame front arrives too early, for whatever reason, that the knocking effect occurs. If allowed to persist, knocking can cause vibration and damage to engine parts.
Knocking is a different phenomenon from pre-ignition, which occurs when the air-fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by heat buildup in engine components or overheating of the air-fuel mixture during compression, and cannot be prevented by delaying spark plug firing. As such, if pre-ignition is allowed to continue for any length of time, severe engine damage can result. Pre-ignition is bad bad stuff, and changing the octane of the gas won’t affect it (just thought I'd through that in there).
Generally speaking, a higher-octane gasoline prevents knocking by either slowing the burn rate of the gas, or by increasing the difficulty of lighting it up. The idea here is to prevent the gas from burning too fast, and causing knocking though.
So the octane number has no bearing on the quality of the gas. But it does have a bearing, on how well your motor can utilize the gas and extract the maximum energy from it. However, getting too far into details of this would require a pretty damn big article, so I’m going to generalize a bit, for the sake of brevity here.
First, consider that gas burns faster under pressure. The more pressure, the faster it burns. Therefore, a high compression motor will burn gas faster than a low compression motor. most of the Mustangs are a relatively low compression motor. So, we don’t really want a slow burning gas (unless it has been modified).
Second, the burn timing in your motor is hugely important when considering how well the motor can develop power from a given amount of gas, and this timing is determined by the ECU, which can manipulate the spark advance of your engine. The ECU can and does retard or advance the spark timing to respond to certain running conditions of the motor, like coolant temperature, or any of a number of other conditions. But none of these parameters are accessible (yet) by you, the itinerant tuner. So, you have no way to manipulate the spark to match the burn speed of your gas unless you change somethig, or something fails. Since the Mustang is designed by the factory to use 87 octane gas (except for higher compression engines like the mach1), if you run something different, the only possible way you have to tune your motor to use a different octane, is to manipulate the compression ratio of your pistons. or timing, So here’s the thing:
Code:
Unless you have changed the pistons, or timing in your Mustang, a higher octane gas will have the effect of de-tuning the motor, and therefore reduce its performance.
Now, the amount will probably be minimal, but it’s there.
That’s the meat and potatoes guys, but there are a few more things we can infer.
First, at higher altitude your compression will be slightly lower, and therefore you can get away with running a slightly lower octane gas. In fact, in areas like Denver, where the altitude is up around 5,000 feet, you’ll find gas stations selling 85 octane gas. But you’ll never find a gas station in Texas selling 85 octane. Since the altitude is much lower, the octane rating needs to be a little higher, so regular gas is typically 87 octane.
Second, gasoline quality is dependant on a lot of things, but octane isn’t one of them. In fact, there is almost no real difference in “premium gas†other than the octane number. Federal law dictates the amount and type of most of the additives in gas. So by definition, they’re all pretty similar in nature. Except. Gasoline is formulated according to climate in the US, and gasoline refiners use 6 basic formulations based on the expected temperature in the area they are expected to be sold in. Gas companies vary this by season, and by location (it also tends to be colder in Denver than in Dallas). So that gas you bought in Dallas might not give you great performance in Denver. These days, unleaded oxygenated gas is far superior in performance and energy characteristics as compared to the old leaded gas of yesteryear. We generally have very good gasoline available to us at the pump.
Lastly, just remember this simple rule of thumb:
You should always use the octane rating recommended by the manufacturer for your motor. But, if you are riding in a climate significantly different than sea-level at 80 degrees Fahrenheit, then use the lowest octane gas you can find that will not cause your motor to knock.
One more thing – many cars these days have anti-knock sensors built into them, and those sensors are used by the ECU to manipulate the spark timing. So, if you run an octane significantly different from the manufacturers recommended octane number, the ECU will detune the motor in real time, and you won’t get any knocking – but you absolutely will lose performance. Use the recommended gas.
If you’ve been running a higher octane gas, and spending the dough to do it, then I hope this little article will save you some bucks, and in the process gain you some performance. Good luck!
Well, sorry to be the one to break this to you, but you’ve been had. So lets take a quick look at octane, and then get to the big point.
Now the truth is, octane is actually a chemical, which comprises an important part of the chemical soup that is gasoline. But here’s the thing – octane the chemical has absolutely nothing to do, with “octane†the pump measurement. So what is it?
In a nutshell, the octane rating of a fuel is a measure of its ability to resist detonation, ping, pre-ignition, or knock.
The number we most often associate with octane is the "Anti-Knock Index", or the "Pump Octane" number. This rating is an average of two different measurements - the Motor Octane Number, and the Research Octane Number. Both of these measurements are taken using a special single cylinder test engine that has a variable compression ratio. The RON measures the knock resistance of a fuel during low RPM, light load conditions, while the MON is representative of high-speed, high load operation. As a result the MON will always be lower than the RON, but for our purposes the Research Octane Number is more significant because it more closely represents the way a low rpm motor is operated (normal driving).
So “Octaneâ€, has absolutely nothing to do with the energy content or quality of gasoline. It’s just a measurement of when the gas will make a motor of a specific configuration begin to knock.
Lets take a moment here to define a couple of things important to understanding this discussion.
Knocking (also called pinking or pinging) in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug but burns too quickly, combusting completely before the optimum moment during the compression phase of the four-stroke cycle. The resulting shockwave collides with the rising piston, creating a characteristic metallic "pinging" sound. The fuel is normally ignited slightly before the point of maximum compression (the spark advance) to allow a small time for the flame front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the point of maximum compression. It is only when this flame front arrives too early, for whatever reason, that the knocking effect occurs. If allowed to persist, knocking can cause vibration and damage to engine parts.
Knocking is a different phenomenon from pre-ignition, which occurs when the air-fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by heat buildup in engine components or overheating of the air-fuel mixture during compression, and cannot be prevented by delaying spark plug firing. As such, if pre-ignition is allowed to continue for any length of time, severe engine damage can result. Pre-ignition is bad bad stuff, and changing the octane of the gas won’t affect it (just thought I'd through that in there).
Generally speaking, a higher-octane gasoline prevents knocking by either slowing the burn rate of the gas, or by increasing the difficulty of lighting it up. The idea here is to prevent the gas from burning too fast, and causing knocking though.
So the octane number has no bearing on the quality of the gas. But it does have a bearing, on how well your motor can utilize the gas and extract the maximum energy from it. However, getting too far into details of this would require a pretty damn big article, so I’m going to generalize a bit, for the sake of brevity here.
First, consider that gas burns faster under pressure. The more pressure, the faster it burns. Therefore, a high compression motor will burn gas faster than a low compression motor. most of the Mustangs are a relatively low compression motor. So, we don’t really want a slow burning gas (unless it has been modified).
Second, the burn timing in your motor is hugely important when considering how well the motor can develop power from a given amount of gas, and this timing is determined by the ECU, which can manipulate the spark advance of your engine. The ECU can and does retard or advance the spark timing to respond to certain running conditions of the motor, like coolant temperature, or any of a number of other conditions. But none of these parameters are accessible (yet) by you, the itinerant tuner. So, you have no way to manipulate the spark to match the burn speed of your gas unless you change somethig, or something fails. Since the Mustang is designed by the factory to use 87 octane gas (except for higher compression engines like the mach1), if you run something different, the only possible way you have to tune your motor to use a different octane, is to manipulate the compression ratio of your pistons. or timing, So here’s the thing:
Code:
Unless you have changed the pistons, or timing in your Mustang, a higher octane gas will have the effect of de-tuning the motor, and therefore reduce its performance.
Now, the amount will probably be minimal, but it’s there.
That’s the meat and potatoes guys, but there are a few more things we can infer.
First, at higher altitude your compression will be slightly lower, and therefore you can get away with running a slightly lower octane gas. In fact, in areas like Denver, where the altitude is up around 5,000 feet, you’ll find gas stations selling 85 octane gas. But you’ll never find a gas station in Texas selling 85 octane. Since the altitude is much lower, the octane rating needs to be a little higher, so regular gas is typically 87 octane.
Second, gasoline quality is dependant on a lot of things, but octane isn’t one of them. In fact, there is almost no real difference in “premium gas†other than the octane number. Federal law dictates the amount and type of most of the additives in gas. So by definition, they’re all pretty similar in nature. Except. Gasoline is formulated according to climate in the US, and gasoline refiners use 6 basic formulations based on the expected temperature in the area they are expected to be sold in. Gas companies vary this by season, and by location (it also tends to be colder in Denver than in Dallas). So that gas you bought in Dallas might not give you great performance in Denver. These days, unleaded oxygenated gas is far superior in performance and energy characteristics as compared to the old leaded gas of yesteryear. We generally have very good gasoline available to us at the pump.
Lastly, just remember this simple rule of thumb:
You should always use the octane rating recommended by the manufacturer for your motor. But, if you are riding in a climate significantly different than sea-level at 80 degrees Fahrenheit, then use the lowest octane gas you can find that will not cause your motor to knock.
One more thing – many cars these days have anti-knock sensors built into them, and those sensors are used by the ECU to manipulate the spark timing. So, if you run an octane significantly different from the manufacturers recommended octane number, the ECU will detune the motor in real time, and you won’t get any knocking – but you absolutely will lose performance. Use the recommended gas.
If you’ve been running a higher octane gas, and spending the dough to do it, then I hope this little article will save you some bucks, and in the process gain you some performance. Good luck!
10-25-2004, 02:52 AM
#8
1st Gear Member
Join Date: Oct 2004
Location:
Posts: 76
RE: gas & engine knock
It does, because he wanted to be schooled on engine knock/pre-ignition/fuel octane. another thing is he still hasn't given enough info for a diagnosis, for example, did this just start happening all of a sudden/ did he go to some sh$tbag gas station that sells a gallon a week? has he made any changes to the way the motor fires (retard or advanced timing), added any mods where he could have advertantly changed something? I could go on forever, but I think it does relate to his question, and a 98 stock is set up to run on 87 octane (correct me if I'm wrong) we're all on this board to learn and help.
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