Technical Question: A/F Ratio
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2008 C63
Technical Question: A/F Ratio
Hi everyone,
This is the first naturally aspirated car I have owned, and after seeing the dyno graphs from almost everyone's tune, I must have something wrong. From my experience with turbo cars, the ideal A/F ratio is somewhere mid to high 11's, and anything above that tends to be unreliable.
However, every tune I see for the C63 seems to have AFR's in the high 12s low 13s area. I must have something wrong. Is it the fact that the car is naturally aspirated that allows it to have a higher afr that is safe, or is it compression ratio? Perhaps something else?
A little clarification would be appreciated!
This is the first naturally aspirated car I have owned, and after seeing the dyno graphs from almost everyone's tune, I must have something wrong. From my experience with turbo cars, the ideal A/F ratio is somewhere mid to high 11's, and anything above that tends to be unreliable.
However, every tune I see for the C63 seems to have AFR's in the high 12s low 13s area. I must have something wrong. Is it the fact that the car is naturally aspirated that allows it to have a higher afr that is safe, or is it compression ratio? Perhaps something else?
A little clarification would be appreciated!
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2012 C63;1971 280SE 3.5(Sold);2023 EQS 450 SUV 4 Matic (Wife's)
Max power at WOT should be in the 13:1 AFR area with modern engine and head construction methods. The higher the compression ratio the higher the fuel octane rating that will be required to avoid detonation. This will also be affected by spark timing at WOT. For safety sake the AFR will be set in the 12.2:1 to 12.8:1 area depending on spark timing and fuel quality octane speaking. There will be a balance of spark timing and AFR to achieve max power depending on piston & head design, compression ratio and exhaust tuning. Heat is the killer.
In FI the air is compressed before it enters the engine and this creates huge amounts of heat that must be dissipated as much as possible because it is an engine killer. For this reason it is usual to run 11.5:1 AFR or even lower depending on type of FI, quality of fuel, spark timing, engine design, etc. The richer mixture adds a safety margin to avoid detonation and adds extra fuel to cool the intake charge and the combustion cylinders. Max power is then set using boost pressure and spark timing and AFR is set to allow a safety margin and to help cool the cylinders.
The combination of these factors to achieve max power is determined by experience and tuning and testing while maintaining a reasonable safety factor depending on a persons risk aversion. After a while acceptable ranges are developed for a particular package and what is perceived as acceptable risk. These ranges must be tightened down by tuning and testing to achieve max power within a persons goals and risk aversion.
Disclaimer: I am not a tuner and the above is intended to be a general discussion only.
In FI the air is compressed before it enters the engine and this creates huge amounts of heat that must be dissipated as much as possible because it is an engine killer. For this reason it is usual to run 11.5:1 AFR or even lower depending on type of FI, quality of fuel, spark timing, engine design, etc. The richer mixture adds a safety margin to avoid detonation and adds extra fuel to cool the intake charge and the combustion cylinders. Max power is then set using boost pressure and spark timing and AFR is set to allow a safety margin and to help cool the cylinders.
The combination of these factors to achieve max power is determined by experience and tuning and testing while maintaining a reasonable safety factor depending on a persons risk aversion. After a while acceptable ranges are developed for a particular package and what is perceived as acceptable risk. These ranges must be tightened down by tuning and testing to achieve max power within a persons goals and risk aversion.
Disclaimer: I am not a tuner and the above is intended to be a general discussion only.
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From: Canada
M156 and LS7
good explanation
Max power at WOT should be in the 13:1 AFR area with modern engine and head construction methods. The higher the compression ratio the higher the fuel octane rating that will be required to avoid detonation. This will also be affected by spark timing at WOT. For safety sake the AFR will be set in the 12.2:1 to 12.8:1 area depending on spark timing and fuel quality octane speaking. There will be a balance of spark timing and AFR to achieve max power depending on piston & head design, compression ratio and exhaust tuning. Heat is the killer.
In FI the air is compressed before it enters the engine and this creates huge amounts of heat that must be dissipated as much as possible because it is an engine killer. For this reason it is usual to run 11.5:1 AFR or even lower depending on type of FI, quality of fuel, spark timing, engine design, etc. The richer mixture adds a safety margin to avoid detonation and adds extra fuel to cool the intake charge and the combustion cylinders. Max power is then set using boost pressure and spark timing and AFR is set to allow a safety margin and to help cool the cylinders.
The combination of these factors to achieve max power is determined by experience and tuning and testing while maintaining a reasonable safety factor depending on a persons risk aversion. After a while acceptable ranges are developed for a particular package and what is perceived as acceptable risk. These ranges must be tightened down by tuning and testing to achieve max power within a persons goals and risk aversion.
Disclaimer: I am not a tuner and the above is intended to be a general discussion only.
In FI the air is compressed before it enters the engine and this creates huge amounts of heat that must be dissipated as much as possible because it is an engine killer. For this reason it is usual to run 11.5:1 AFR or even lower depending on type of FI, quality of fuel, spark timing, engine design, etc. The richer mixture adds a safety margin to avoid detonation and adds extra fuel to cool the intake charge and the combustion cylinders. Max power is then set using boost pressure and spark timing and AFR is set to allow a safety margin and to help cool the cylinders.
The combination of these factors to achieve max power is determined by experience and tuning and testing while maintaining a reasonable safety factor depending on a persons risk aversion. After a while acceptable ranges are developed for a particular package and what is perceived as acceptable risk. These ranges must be tightened down by tuning and testing to achieve max power within a persons goals and risk aversion.
Disclaimer: I am not a tuner and the above is intended to be a general discussion only.
Thread Starter
Senior Member
Joined: Sep 2010
Posts: 378
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2008 C63
Thanks for the explanation!
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
Member
Joined: Jun 2009
Posts: 209
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From: Canada
M156 and LS7
generally speaking low load, low rpm, hot ambient temperautre
Thanks for the explanation!
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
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2008 C63
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Thanks for the great lesson
Thread Starter
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2008 C63
Fuel on it's own, won't ignite. Fuel with a limited quantity of air will catch fire. Fuel with unlimited quantity of air, will be even more intense.
A real world explanation is when you have a fire drill at work, the first thing they tell you is to close all your doors behind you. This is to restrict air flow to any potential fire, which slows the spread of fire.
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From: Canada
M156 and LS7
SI engine, gasoline fuel for stoichiometric combustion, A/F ratio is 14.7, more air(A/F>14.7, lamda> 1) is lean burn, slightly lean burn will give you better fuel efficiency (think diesel engine case), but if the charge(air-fuel mixture) is too lean, you need higher ignition energy(ie. plasma), slightly rich, will give you better power, too rich, you just dump fuel into the exhaust, rise TWC temperature, if too much rich, the charge may not get ignited (ie. A/F ~8-9)
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2012 C63;1971 280SE 3.5(Sold);2023 EQS 450 SUV 4 Matic (Wife's)
Thanks for the explanation!
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
Does anyone have an idea when detonation starts to occur on a C63? I realize there are many different factors here, but assuming average temperature, 93 octane pump gas, etc.
The reason for all of these questions, is I am considering buying a tune in the spring, but I want to make sure that I'm getting something safe.
MBWorld Fanatic!
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2012 C63;1971 280SE 3.5(Sold);2023 EQS 450 SUV 4 Matic (Wife's)
While this is certainly true and the ping we can hear at times, it is not the detonation that will kill an engine quickly. This detonation may damage an engine over time if not corrected by reducing timing, improving cooling or using a higher octane or better quality fuel, which is the usual cure. This detonation can be contained by reducing throttle angle or shifting down a cog as our 7 speed autos will do. Also why they say not to LUG an engine because it builds cylinder pressure which can damage piston crowns, connecting rods and rod bearing and crankshaft bearing surfaces.
Unrestrained detonation at WOT on the other hand can and will damage an engine in a very short period of time. This type of detonation may not be as audible due to ambient nose from the intake, combustion, exhaust and drive train. Heat is generated very quickly and damage to pistons, rods and heads can happen in a matter of a few seconds. Thus the need to avoid detonation at WOT by building in reasonable safety margins in a tune.
Unrestrained detonation at WOT on the other hand can and will damage an engine in a very short period of time. This type of detonation may not be as audible due to ambient nose from the intake, combustion, exhaust and drive train. Heat is generated very quickly and damage to pistons, rods and heads can happen in a matter of a few seconds. Thus the need to avoid detonation at WOT by building in reasonable safety margins in a tune.
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2012 C63;1971 280SE 3.5(Sold);2023 EQS 450 SUV 4 Matic (Wife's)
No, too much fuel will cause an engine to bog and reduce power drastically. The cylinder will fill with fuel that will not be burnt and will resist atomization and compression, it will cool the cylinder below optimum operating temperature and will be lazy to move out of the cylinder to allow the new mixture to enter. This is probably oversimplified but overly rich fuel mixtures do not make power but quite the opposite. The same is true for mixtures that are too lean on the other end of the scale.
