What gas octane rating is recommend in the USA?
10-13-2012, 08:13 AM
#1
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What gas octane rating is recommend in the USA?
The fuel door of my 997.1 (here in Switzerland) has a minimum octane rating of 98. What does say on cars in the states? is it the same? I don't ever recall seeing such high octane fuel there. Or is it a different measurement system?
10-13-2012, 10:56 AM
#2
There are a number of different standards for octane calculation. My manual recommends an RON octane of 98, (Research Octane Number), which is apparently equivalent to a 93 octane when the CLC (Cost of Living Council), or AKI (Anti Knock Index), are used. It gets even more complex in California, where the additives required for clean burning keep our premium octane down around 91.
But I digress. lol
Your question was about the recommended octane, which is 98/93.
But I digress. lol
Your question was about the recommended octane, which is 98/93.
10-13-2012, 01:15 PM
#3
Drifting
It simply no longer matters.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
10-13-2012, 01:35 PM
#4
It simply no longer matters.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
" 'Octane' is an anti-knock rating. The higher the rating the higher the anti-knock properties and the cooler the fuel will burn.
High compression engines require higher octane ratings to keep them from overheating in the combustion phase. That's why it's critical to use a premium fuel in a european automobile that requires it, otherwise you run the risk of knocking or 'detonation' as the proper term. Detonation comes from the fuel iginiting before the spark sets it off, causing an awful knocking sound in the engine. Excess heat stored in carbon build up and on the piston dome sets off the fuel charge. To excess it can burn through the piston dome.
The lower octane fuel runs hotter so it offers more power for a lower compression engine. In our old race bikes we would actually have to jet leaner with race gas because it burns cooler and thus combines a lower fuel-air ratio. Conversely, if they were setup for race gas and you used pump gas you needed to jet richer or they would tend to lean out.
For your car, if the manufacturer recommends 91 octane premium fuel, that's what you should burn in it. The newests models with all of their sensors and computerized control of combustion mixtures will sense a lower octane burn and retard the timing a bit to curb detonation. You will get better mileage and smoother running with the proper fuel."
... and this is straight out of the owner's manual:
"Porsche also recognizes that these fuels may not always be available. Be assured that your vehicle will operate properly on unleaded premium fuels with octane numbers of at least 95 RON (90 CLC or AKI), since the engine’s “Electronic OctaneTM knock control” will adapt the ignition timing, if necessary..." "Octane rating indicates a fuel’s ability to resist detonation. Therefore, buying the correct octane gas is important to prevent engine “damage”."
“The man who never alters his opinion is like standing water, and breeds reptiles of the mind.”
-William Blake
10-13-2012, 02:30 PM
#5
Race Director
It simply no longer matters.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
10-13-2012, 03:47 PM
#6
Yep
10-13-2012, 03:56 PM
#7
Thanks Macster. That's my new favorite explanation. For what it's worth... How much does one save in the long run? Even if a person uses a full tank every week, and supposing a price difference of .20 per gallon...
16 gallons x 52 weeks x .20 comes to a whopping $166.40 for the year. I spend more than that at Starbucks.
16 gallons x 52 weeks x .20 comes to a whopping $166.40 for the year. I spend more than that at Starbucks.
Well, I'm sorry but it does matter at least in Porsches and probably other car brands as well.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
Trending Topics
10-13-2012, 04:00 PM
#8
Rennlist Member
It simply no longer matters.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
These days if you fuel a high compression engine with low octane, below "spec" octane, the engine ECU will quickly detect that and slightly enrich the mixture when the throttle is wide open or nearly so. At least that's what I was told by the factory when we picked up my 2001 C4. I was told that during our subsequent european travels we might not always be able to fuel with preminum but not to worry...
These days most cars, for the above reason, with high compression engines, engines that in yesteryears REQUIRED premium fuels, now only "recommend" fueling with premium for best performance.
Yes, you CAN run any octane gas, and engine will be protected by ECU and won't be damaged. However, by doing so, you lose fuel economy and horsepower.
The loss of HP is actually very noticable. In Canada we have 94 Octane/0% ethanol gas, anytime I travel to US and use 91 octane/10% ethanol gas, the car feels like it lost at least 20-30HP, specially in the mid-range (as Macster explained).
10-15-2012, 11:59 PM
#9
Download this app and try and run ethanol free gas. My mileage is noticeably better running ethanol free gas.
Pure-gas.com
Pure-gas.com
10-16-2012, 11:25 AM
#10
Three Wheelin'
Well, I'm sorry but it does matter at least in Porsches and probably other car brands as well.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
10-17-2012, 01:02 PM
#11
Drifting
Well stated Wwest. I know that there have been many hotly debated conversations regarding fuel, not just here, but in many auto forums. I personally have a friend with a 2008 Corvette who often uses regular unleaded. At the risk of igniting another of these debates, I submit this response to the queston, which seems to be a very thorough answer to the octane riddle:
" 'Octane' is an anti-knock rating. The higher the rating the higher the anti-knock properties and the cooler the fuel will burn.
High compression engines require higher octane ratings to keep them from overheating in the combustion phase. That's why it's critical to use a premium fuel in a european automobile that requires it, otherwise you run the risk of knocking or 'detonation' as the proper term. Detonation comes from the fuel iginiting before the spark sets it off, causing an awful knocking sound in the engine. Excess heat stored in carbon build up and on the piston dome sets off the fuel charge. To excess it can burn through the piston dome.
The lower octane fuel runs hotter so it offers more power for a lower compression engine. In our old race bikes we would actually have to jet leaner with race gas because it burns cooler and thus combines a lower fuel-air ratio. Conversely, if they were setup for race gas and you used pump gas you needed to jet richer or they would tend to lean out.
For your car, if the manufacturer recommends 91 octane premium fuel, that's what you should burn in it. The newests models with all of their sensors and computerized control of combustion mixtures will sense a lower octane burn and retard the timing a bit to curb detonation. You will get better mileage and smoother running with the proper fuel."
... and this is straight out of the owner's manual:
"Porsche also recognizes that these fuels may not always be available. Be assured that your vehicle will operate properly on unleaded premium fuels with octane numbers of at least 95 RON (90 CLC or AKI), since the engine’s “Electronic OctaneTM knock control” will adapt the ignition timing, if necessary..." "Octane rating indicates a fuel’s ability to resist detonation. Therefore, buying the correct octane gas is important to prevent engine “damage”."
“The man who never alters his opinion is like standing water, and breeds reptiles of the mind.”
-William Blake
" 'Octane' is an anti-knock rating. The higher the rating the higher the anti-knock properties and the cooler the fuel will burn.
High compression engines require higher octane ratings to keep them from overheating in the combustion phase. That's why it's critical to use a premium fuel in a european automobile that requires it, otherwise you run the risk of knocking or 'detonation' as the proper term. Detonation comes from the fuel iginiting before the spark sets it off, causing an awful knocking sound in the engine. Excess heat stored in carbon build up and on the piston dome sets off the fuel charge. To excess it can burn through the piston dome.
The lower octane fuel runs hotter so it offers more power for a lower compression engine. In our old race bikes we would actually have to jet leaner with race gas because it burns cooler and thus combines a lower fuel-air ratio. Conversely, if they were setup for race gas and you used pump gas you needed to jet richer or they would tend to lean out.
For your car, if the manufacturer recommends 91 octane premium fuel, that's what you should burn in it. The newests models with all of their sensors and computerized control of combustion mixtures will sense a lower octane burn and retard the timing a bit to curb detonation. You will get better mileage and smoother running with the proper fuel."
... and this is straight out of the owner's manual:
"Porsche also recognizes that these fuels may not always be available. Be assured that your vehicle will operate properly on unleaded premium fuels with octane numbers of at least 95 RON (90 CLC or AKI), since the engine’s “Electronic OctaneTM knock control” will adapt the ignition timing, if necessary..." "Octane rating indicates a fuel’s ability to resist detonation. Therefore, buying the correct octane gas is important to prevent engine “damage”."
“The man who never alters his opinion is like standing water, and breeds reptiles of the mind.”
-William Blake
NO.
Before the advent of EFI, the days of carburetion, this may have been the procedure, but even then that only applied to "spark knock", "detonation" resulting from the ignition spark itself.
But with EFI, or nowadays DFI, enriching the mixture slightly to alleviate detonation, even spark knock, is a much better solution, except for knocking arising from lugging the engine.
10-17-2012, 01:07 PM
#12
"..and retard the timing a bit to curb detonation.."
NO.
Before the advent of EFI, the days of carburetion, this may have been the procedure, but even then that only applied to "spark knock", "detonation" resulting from the ignition spark itself.
But with EFI, or nowadays DFI, enriching the mixture slightly to alleviate detonation, even spark knock, is a much better solution, except for knocking arising from lugging the engine.
NO.
Before the advent of EFI, the days of carburetion, this may have been the procedure, but even then that only applied to "spark knock", "detonation" resulting from the ignition spark itself.
But with EFI, or nowadays DFI, enriching the mixture slightly to alleviate detonation, even spark knock, is a much better solution, except for knocking arising from lugging the engine.
10-17-2012, 01:31 PM
#13
Drifting
Well, I'm sorry but it does matter at least in Porsches and probably other car brands as well.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
...it retards ignition timing...
No.
Besides which, just what good would that do? If the detonation is occuring prior to the spark then obviously NONE...!! If it's "spark knock" then it would only delay detonation, and possibly result in "self ignition" as the piston continues to rise, increasing compression.
Whereas enriching the mixture slightly....
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
Our cars NEVER run "open loop". In idle or cruise mode the lead oxygen sensor is used to control the A/F mixture. During acceleration or with the engine under a heavy load the A/F mixture is controlled via the MAF/IAT sensors. Since detonation is highly unlikely to result in idle or cruise that is not an issue here.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
Again, if detonation is occurring prior to the ignition spark just what good would it do to delay the spark? And if spark knock?
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
If detonation is occuring co-incident with ignition then what good would delaying the (BTDC) ignition accomplish..?
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
With all your downsides to delaying ignition why wouldn't ANY design engineer choose slightly enriching the mixture as a more reasonable solution?Or maybe even pre-cooling the fuel via the A/C
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Yes, agreed, absolutely.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Sorry, BTU's are not a part of "this" equation. Diesel fuel has the highest BTU content and then BTU's get lower and lower a you climb the octane scale.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
First the DME does not add extra fuel when it detects the effects of running a below octane grade of gasoline, it retards igntion timing.
...it retards ignition timing...
No.
Besides which, just what good would that do? If the detonation is occuring prior to the spark then obviously NONE...!! If it's "spark knock" then it would only delay detonation, and possibly result in "self ignition" as the piston continues to rise, increasing compression.
Whereas enriching the mixture slightly....
The only time I can see 'extra fuel" added is under hard acceleration when the DME in order to satisfy the torque demand of the driver enrichens the fuel mixture to richer level which is condusive to more power generation.
In doing so the DME switches from closed loop mode to open loop mode which lasts as long as this hard acceleration is in effect.
Our cars NEVER run "open loop". In idle or cruise mode the lead oxygen sensor is used to control the A/F mixture. During acceleration or with the engine under a heavy load the A/F mixture is controlled via the MAF/IAT sensors. Since detonation is highly unlikely to result in idle or cruise that is not an issue here.
The DME will retard spark but it does so upon detecting detonation arising from the wrong (too low) octane.
Again, if detonation is occurring prior to the ignition spark just what good would it do to delay the spark? And if spark knock?
This retarded spark timing lowers the pressure in the combustion which eliminates the detonation but this retarded spark timing has its downsides.
If detonation is occuring co-incident with ignition then what good would delaying the (BTDC) ignition accomplish..?
Sure, it is preferred to letting the detonation continue but the trade of is like deciding to smash a little toe with a hammer vs. a big toe because at least the little toe being smaller will hurt less.
One downside is retarded timing raises exhaust gas temperature which subjects the combustion chamber, valves and all downstream from the chamber to higher operating temperatures.
Another downside is the efficiency of the engine is reduced. That exhaust heat represents wasted energy.
Which brings up another downside: reduced fuel economy.
And another downside: driveability is affected.
With all your downsides to delaying ignition why wouldn't ANY design engineer choose slightly enriching the mixture as a more reasonable solution?Or maybe even pre-cooling the fuel via the A/C
High compression is a very nice way to help an engine be responsive where it most needs to be responsive, at part throttle operation where passenger vehicles spend the bulk of their time.
To derive maximum benefit from this high compression requires the correct grade of octane gasoline.
Yes, agreed, absolutely.
Both regular and premium gasolines have nearly the same BTUs. In fact IIRC premium gasoline's BTU's range is slightly above that of regular gasoline, but not by much and lest someone get the wrong idea certainly not enough to justify anyone switching to premium gasoline when his car's engine is designed to run on regular gasoline.
Sorry, BTU's are not a part of "this" equation. Diesel fuel has the highest BTU content and then BTU's get lower and lower a you climb the octane scale.
Premium gasoline doesn't burn slower or colder than regular it simply has additives/is blended to resist forming pre-ignition compounds ahead of the advacing flame front of combustion and to thus resist pre-igniting.
The flame front advances smoothly from the triggering spark to the end of combustion.
The DME can trigger ignition early enough (sometimes on the order of 35/36 degrees BTDC!) to get combustion going so that piston is best positioned to convert the pressure of combustion into mechanical power.
Since Porsches are sold worldwide and not every region has a good selection of high octane gasoline Porsche and other makers allow some leeway. IIRC though in my owners manual no octane below 90 should be used so there is a guess a limit to how low one can go.
Run whatever grade of gasoline you want in your car's engine but if you run anything but the proper octane of gasoline or as close to it as you can get you are shortchanging yourself regarding the experience of a properly fueled high compression engine and subjecting your car's engine to extra wear/tear and burning more gasoline in the process.
Sincerely,
Macster.
10-17-2012, 01:37 PM
#14
Drifting
10-17-2012, 04:49 PM
#15
Correct me if I am wrong, please, but is not detonation simply caused by the ignition of the fuel before TDC? The retardation of spark would then seem to be the perfect solution for the ECU to perform.