Calling in the pros.....A/F ratios and their effects on performance, longevity, etc..
01-07-2006, 10:15 PM
#1
Calling in the pros.....A/F ratios and their effects on performance, longevity, etc.. Ok, I went to the dyno today with some buddies. My car is down (rebuilding the tranny and installing a new clutch) so I just tagged along.
Anyhow....my first buddy got his car tuned for his current mods, headers, true duals, and lid/filter. He was setting at about 14.5ish a/f on the first pull. He layed down 322rwhp, and 330ish rwtq. After some tuning and tweaking the tuners settled the a/f at 13.5 and the car was at 333/347.
Next buddy had a car with a mailorder tune from a sponsor. His a/f was also at 13.5.
On my previous car, my tuner of yesteryears always shot for and acheived 13.0. Today I asked if they were gonna leave the car that they were tuning at 13.5 and they said yes, which puzzled me a bit. They said that if they set it at 13.0 it would be pig rich in the summer
The weather was about 45-50 deg today as far as that aspect is concerned.
The way I looked at it was this way, these cars don't have carbuerators with fixed jets, where a large change in weather would affect them. So were they right about the a/f changing when the warmer weather comes back? I really don't think it should.
Another thing. We looked at the run files of my car from the past times I was there. With only a lid I pulled 321/331, and my a/f was at 12.2ish. Then with the headers, ory, and catback I pulled 336/349, and a/f was at 12.5ish. The car has always been on the stock tune, still is. What I am curious about is what type of power is being left on the table with this rich condition, and isn't 13.5 a bit lean?
ANY and ALL input will be GREATLY appreciated. I am VERY interested in having a little more light shined on these subjects.
FWIW, the owner and tuner are extremely nice guys, no flames intended, I just have questions and greatly desire the "correct" answers
Anyhow....my first buddy got his car tuned for his current mods, headers, true duals, and lid/filter. He was setting at about 14.5ish a/f on the first pull. He layed down 322rwhp, and 330ish rwtq. After some tuning and tweaking the tuners settled the a/f at 13.5 and the car was at 333/347.
Next buddy had a car with a mailorder tune from a sponsor. His a/f was also at 13.5.
On my previous car, my tuner of yesteryears always shot for and acheived 13.0. Today I asked if they were gonna leave the car that they were tuning at 13.5 and they said yes, which puzzled me a bit. They said that if they set it at 13.0 it would be pig rich in the summer
The weather was about 45-50 deg today as far as that aspect is concerned.The way I looked at it was this way, these cars don't have carbuerators with fixed jets, where a large change in weather would affect them. So were they right about the a/f changing when the warmer weather comes back? I really don't think it should.
Another thing. We looked at the run files of my car from the past times I was there. With only a lid I pulled 321/331, and my a/f was at 12.2ish. Then with the headers, ory, and catback I pulled 336/349, and a/f was at 12.5ish. The car has always been on the stock tune, still is. What I am curious about is what type of power is being left on the table with this rich condition, and isn't 13.5 a bit lean?
ANY and ALL input will be GREATLY appreciated. I am VERY interested in having a little more light shined on these subjects.
FWIW, the owner and tuner are extremely nice guys, no flames intended, I just have questions and greatly desire the "correct" answers
01-07-2006, 10:33 PM
#3
Originally Posted by chrisj6
i have found that around 13.0 is best for my truck, some go leaner and some go richer, i think it pulls harder when its leaner.
Chris
Chris
Thanks for your input man
01-07-2006, 10:39 PM
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always here to throw in my two cents
I see what your saying about the safest, your right i could be running too lean, i guess the best way to check for that is your plugs. I checked mine last night and they are fine, although i havent had it leaned out for too long, so I will keep an eye on those.
Chris
I see what your saying about the safest, your right i could be running too lean, i guess the best way to check for that is your plugs. I checked mine last night and they are fine, although i havent had it leaned out for too long, so I will keep an eye on those.
Chris
01-07-2006, 10:50 PM
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I always shoot for 12.8 through the torque band and 13.0 in the power band on the cars I tune. That combined with the right timing will usually give you the most area under the curves. You can sometimes squeek out a little more peak leaner than 13.0, but can't run much timing without seeing knock. There is always some tradeoff. To me, it is better to stay safe and get more area under the curves than just shooting for peak. And the bottom line is that every single combination has to be tested to see where it makes the best power - that is why mail order tunes are ok, but possibly not the best you can do.
As for the weather question - if tuned properly, the weather should not effect the tune.
As for the weather question - if tuned properly, the weather should not effect the tune.
01-07-2006, 10:53 PM
#6
Originally Posted by BLK02WS6
I always shoot for 12.8 through the torque band and 13.0 in the power band on the cars I tune. That combined with the right timing will usually give you the most area under the curves. You can sometimes squeek out a little more peak leaner than 13.0, but can't run much timing without seeing knock. There is always some tradeoff. To me, it is better to stay safe and get more area under the curves than just shooting for peak. And the bottom line is that every single combination has to be tested to see where it makes the best power - that is why mail order tunes are ok, but possibly not the best you can do.
As for the weather question - if tuned properly, the weather should not effect the tune.
As for the weather question - if tuned properly, the weather should not effect the tune.
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01-08-2006, 01:07 AM
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When I tune a N/A car I ask my customers what there primary intended use is for the car. I will get back to that in a moment and let me explain. Most street driven calibrations I shoot for a 12.8 to 13.2 Air/Fuel range. I do not personally leave a calibration at anything leaner than this.
When referring to Air/Fuels you must think of thermal management within the cylinder. The entire reason richer mixtures than stoichiometric are needed at Wide Open Throttle is simple to keep the motor from melting down. Richer A/F's help to pull some of the added heat during Wide Open Throttle away from the cylinders and in turn the cooling system. This is why you see so many factory calibrations much leaner than stoich while in cruising conditions. This is done for two primary reasons the first being fuel economy and the second is thermal management. At cruising conditions a given engine has a very low Tq and Hp output. This level of output requires very little additional cooling to keep the thermal temperatures in line.
Now on the other side of the spectrum you have a 320 HP peak LS1 at WOT that needs a lot of additional cooling to help keep the motor from melting a ring land for an example at WOT. The extra fuel does two things one helps quench knock and carries through the process of combustion excess heat out of the cylinder and into the exhaust and out into the atmosphere.
To answer your other question in personal testing I have done on the dyno I have seen very little power gains from running lets say a 13.0 A/F then when the car was at a 12.0 A/F. Think A/F are the thermal management and the ignition timing is the main thing that increases or decreases cylinder pressure and in turn power. The end does not justify the means. Here is why... The less exaggerated thermal load that is placed on the engine throughout the course of its life will increase in varying levels the longevity of the engine over a period of time.
Now... When I tune a customer’s car it is purpose driven how I set The WOT portion of the fuel map. If it is a road race car that will be under WOT and high load portions of the map for long periods of time. I will intentionally run richer mixtures than what I was speaking of at the beginning of this post. For the sake of keeping the engine together for a long life. Unless you are on the extreme ends of the spectrum such as to rich to induce a misfire or to lean causing a cylinder misfire. Changing your A/F's up or down .5 of a point will not make power it will greatly help determine the life span of the motor all other things being equal.
I hope this helps friend
Chris Macellaro
When referring to Air/Fuels you must think of thermal management within the cylinder. The entire reason richer mixtures than stoichiometric are needed at Wide Open Throttle is simple to keep the motor from melting down. Richer A/F's help to pull some of the added heat during Wide Open Throttle away from the cylinders and in turn the cooling system. This is why you see so many factory calibrations much leaner than stoich while in cruising conditions. This is done for two primary reasons the first being fuel economy and the second is thermal management. At cruising conditions a given engine has a very low Tq and Hp output. This level of output requires very little additional cooling to keep the thermal temperatures in line.
Now on the other side of the spectrum you have a 320 HP peak LS1 at WOT that needs a lot of additional cooling to help keep the motor from melting a ring land for an example at WOT. The extra fuel does two things one helps quench knock and carries through the process of combustion excess heat out of the cylinder and into the exhaust and out into the atmosphere.
To answer your other question in personal testing I have done on the dyno I have seen very little power gains from running lets say a 13.0 A/F then when the car was at a 12.0 A/F. Think A/F are the thermal management and the ignition timing is the main thing that increases or decreases cylinder pressure and in turn power. The end does not justify the means. Here is why... The less exaggerated thermal load that is placed on the engine throughout the course of its life will increase in varying levels the longevity of the engine over a period of time.
Now... When I tune a customer’s car it is purpose driven how I set The WOT portion of the fuel map. If it is a road race car that will be under WOT and high load portions of the map for long periods of time. I will intentionally run richer mixtures than what I was speaking of at the beginning of this post. For the sake of keeping the engine together for a long life. Unless you are on the extreme ends of the spectrum such as to rich to induce a misfire or to lean causing a cylinder misfire. Changing your A/F's up or down .5 of a point will not make power it will greatly help determine the life span of the motor all other things being equal.
I hope this helps friend
Chris Macellaro
01-08-2006, 01:29 AM
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the main factors are heat and knock. Your car runs at 14.7:1 AFR most of its life and never complains, this is becuase it is at light/moderate load.
Heat is the #1 factor, leaner mixtures mean hotter cylinder temps which in turn heats valves, seats, and exhaust components. There is a temperature at which these components will fail. examples of this are melting the cats, tops of pistons, cracking cylinder heads or mushrooming exhaust valves. You need some good quality EGT's to measure the compbustion temps and then try to estimate component temps.
Knock is related to timing but also cylinder temps. Leaner mixtures will tend to take less timing and knock earlier. Knock is far more damaging to engine components in a very short time, the basic rule being ANY knock is bad. Knock WILL break stuff very quickly if not detected. eg. pistons, head gaskets, damage ring lands, bearings etc.
In an engine dyno situation the operator will have a limit to where he likes the exhaust temp to be based on the components that are being used. ie. are they steel, titanium, aluminum, ceramic coated, etc. They will then work AFR and timing to maximize torque within the allowed exhaust temp maximum.
The intake temp, cylinder head design and compression ratio also play important roles as generally LS1's will make peak torque a few degrees back from the knock point. Also, depending on the intake temps a richer mixture may allow more timing and higher torque than a leaner mixture and less timing.
Most tuners will tell you that some engines seem to like richer AFRs than others. One i hear quite a lot is the LS2 likes richer AFRs to make power and you just can't get as much timing into them as the LS1's, probably due to the almost 11:1 comp they run
So in the end the answer really is "it depends" (like so many other things) but most dyno operators will shoot somewhere between 12.0 and 13.0:1 on the AFR for NA applications and then tweak the timing until they see max torque on the dyno and no knock.
Hope that helps.
Chris...
Heat is the #1 factor, leaner mixtures mean hotter cylinder temps which in turn heats valves, seats, and exhaust components. There is a temperature at which these components will fail. examples of this are melting the cats, tops of pistons, cracking cylinder heads or mushrooming exhaust valves. You need some good quality EGT's to measure the compbustion temps and then try to estimate component temps.
Knock is related to timing but also cylinder temps. Leaner mixtures will tend to take less timing and knock earlier. Knock is far more damaging to engine components in a very short time, the basic rule being ANY knock is bad. Knock WILL break stuff very quickly if not detected. eg. pistons, head gaskets, damage ring lands, bearings etc.
In an engine dyno situation the operator will have a limit to where he likes the exhaust temp to be based on the components that are being used. ie. are they steel, titanium, aluminum, ceramic coated, etc. They will then work AFR and timing to maximize torque within the allowed exhaust temp maximum.
The intake temp, cylinder head design and compression ratio also play important roles as generally LS1's will make peak torque a few degrees back from the knock point. Also, depending on the intake temps a richer mixture may allow more timing and higher torque than a leaner mixture and less timing.
Most tuners will tell you that some engines seem to like richer AFRs than others. One i hear quite a lot is the LS2 likes richer AFRs to make power and you just can't get as much timing into them as the LS1's, probably due to the almost 11:1 comp they run
So in the end the answer really is "it depends" (like so many other things) but most dyno operators will shoot somewhere between 12.0 and 13.0:1 on the AFR for NA applications and then tweak the timing until they see max torque on the dyno and no knock.
Hope that helps.
Chris...
01-08-2006, 04:49 AM
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In one of my reference books Im sure I read that you run richer than stoich also to ensure that all of the precious air in the cylinder is burnt, in fact Id go so far as to say that is the primary reason other than thermal management, thermals are really the job of the cooling system. I read somewhere that the heat disipated by richer fuel is statistically insignificant hence why water injection and intercooling is used.
Additionally it ensures you dont run into a over lean condition which definately does heat things up significantly.
So I think you run rich 1) to get more air burnt and 2) to give you a margin to stay away from lean. Not specifically to "cool" the engine. JMO
Additionally it ensures you dont run into a over lean condition which definately does heat things up significantly.
So I think you run rich 1) to get more air burnt and 2) to give you a margin to stay away from lean. Not specifically to "cool" the engine. JMO
01-08-2006, 05:14 PM
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no doubt intercooling and water injection cools the intake charge more than richer mixture does. The fuel mixture plays a very important role in the combustion temperature, ie. the temperature of the burning mixture not the intake charge. Leaner mixtures definitely create hotter combustion temps. Think of an oxy torch, how the flame temp gets hotter as you add more oxygen (leaning it out so to speak).
Also, think about how all car manufacturers implement catalytic converter overtemperature protection (and other exhaust component protection) or piston protection routines. They richen the mixtures up to cool things down.
In the end, the name of the game is to completely burn as much fuel as possible. If you have more fuel than needed, the fuel is taking up space that could have been filled by air, if you have less fuel than needed your wasting opportunity.
This all comes back to the stoichiometric ratio of the fuel you are using. For gasoline its around 14.7:1. This means that if you mix the air and fuel at 14.7:1 then all of the fuel will be burned and nothing will be left over. In theory (ideal world) this is the hottest temperature the combustion can achieve (although in practice its not exactly that way). So why have a power enrichment mode and run richer than 14.7:1 at all? The answer is because combustion is not perfect and the cylinder is not the ideal place to light a fire.
The cylinder is not a nice bubble with a lot of space or time for every oxygen molecule to find every fuel molecule and burn in a nice controlled way. It is a volatile place that gets increasingly unfriendly to controlled burning as temperatures increase. Also as the engine speed increases you have a decreasing amount of time to burn everything in a controlled way. The extra fuel helps in a number of ways, it gives the air molecules more of a chance to find the fuel and hence produce the most energy and it helps stabilise the flame front in the cylinder allowing for a controlled burn. This in turn reduces the chance of detonation (uncontrolled burning). The richer fuel mixture aids in a number of ways which all lead to the tradeoff between maximum power and stoichiometry.
The opposite is true for good fuel economy, as you go leaner than stoich the temperatures also start to drop off. The engine doesn't need much power at light loads. This is the basis of lean burn / lean cruise. You burn less fuel and also the power drop allows you to open the throttle more reducing pumping losses (remember the engine is most effecient at WOT). It becomes a balancing act between how much can you lean things out, open the throttle more, mess with the timing and still keep the combustion process under control to maximise economy. Experiments show you can go quite lean in various levels of emmisions friendliness (you need a degree in chemical engineering to understand this stuff).
There are a lot of good books on combustion processes and the influencing factors. They tend to have a lot of chemical equations and use chemical process lingo but give you a great insight into how complex the process is and how many factors come together when each cylinder ignites!
Chris...
Also, think about how all car manufacturers implement catalytic converter overtemperature protection (and other exhaust component protection) or piston protection routines. They richen the mixtures up to cool things down.
In the end, the name of the game is to completely burn as much fuel as possible. If you have more fuel than needed, the fuel is taking up space that could have been filled by air, if you have less fuel than needed your wasting opportunity.
This all comes back to the stoichiometric ratio of the fuel you are using. For gasoline its around 14.7:1. This means that if you mix the air and fuel at 14.7:1 then all of the fuel will be burned and nothing will be left over. In theory (ideal world) this is the hottest temperature the combustion can achieve (although in practice its not exactly that way). So why have a power enrichment mode and run richer than 14.7:1 at all? The answer is because combustion is not perfect and the cylinder is not the ideal place to light a fire.
The cylinder is not a nice bubble with a lot of space or time for every oxygen molecule to find every fuel molecule and burn in a nice controlled way. It is a volatile place that gets increasingly unfriendly to controlled burning as temperatures increase. Also as the engine speed increases you have a decreasing amount of time to burn everything in a controlled way. The extra fuel helps in a number of ways, it gives the air molecules more of a chance to find the fuel and hence produce the most energy and it helps stabilise the flame front in the cylinder allowing for a controlled burn. This in turn reduces the chance of detonation (uncontrolled burning). The richer fuel mixture aids in a number of ways which all lead to the tradeoff between maximum power and stoichiometry.
The opposite is true for good fuel economy, as you go leaner than stoich the temperatures also start to drop off. The engine doesn't need much power at light loads. This is the basis of lean burn / lean cruise. You burn less fuel and also the power drop allows you to open the throttle more reducing pumping losses (remember the engine is most effecient at WOT). It becomes a balancing act between how much can you lean things out, open the throttle more, mess with the timing and still keep the combustion process under control to maximise economy. Experiments show you can go quite lean in various levels of emmisions friendliness (you need a degree in chemical engineering to understand this stuff).
There are a lot of good books on combustion processes and the influencing factors. They tend to have a lot of chemical equations and use chemical process lingo but give you a great insight into how complex the process is and how many factors come together when each cylinder ignites!
Chris...
01-08-2006, 06:51 PM
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Originally Posted by AaronR
What makes more power?A lean A/F with less timing or rich with more timing?
The trick is to get as much fuel burnt as possible in a controlled way such that the downward force (pressure) on the piston after TDC is maximized over as longer time as possible. Timing starts the burning and the AFR (and many other things, including when the burn started) affect the burn and the final pressure exerted on the piston. What's worse is that the previous cycle (unburnt gases and residual heat) also effects the next burn and also everything takes a certain amount of time to happen.
It's all about tradeoffs. A common one in racing engines is to open the exhaust valve early, sacrificing a bit of available work on the piston to more completely evacuate the exhaust gases before the next cycle begins. Forced induction has another set of conditions and they are different for turbo's since they are driven by exhaust pressure (heat) and superchargers that don't care as much for what the exhaust is doing.
the only real way to find the answer for a particular engine is by experiment.
01-08-2006, 07:15 PM
#15
Anyone else agree that the a/f shouldn't change when the weather does? Like having to have it set at 13.5 so it won't be too rich in the summer?
Myself along with BLK02WS6 agree that it should remain the same regardless of the weather conditions.
Myself along with BLK02WS6 agree that it should remain the same regardless of the weather conditions.
01-09-2006, 07:49 AM
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Originally Posted by BriancWS6
Anyone else agree that the a/f shouldn't change when the weather does? Like having to have it set at 13.5 so it won't be too rich in the summer?
Myself along with BLK02WS6 agree that it should remain the same regardless of the weather conditions.
Myself along with BLK02WS6 agree that it should remain the same regardless of the weather conditions.
Hello Brian
You must take into account for the air density changes from different air temperatures and the small effect it will play on A/F's.
True speaking of the MAF cars this is the reason the OEM's use sensors such as this. The Air density is directly and physically measured. As you know the MAF directly measures the density of the incoming air (colder air gives us a denser volume of oxygen molecules and hotter air gives us less oxygen molecules per given volume) So when you have a 35 degree day out with xx amount of air flowing into the engine the sensor returns the signal then the ecu references this value against the look up chart and the proper fueling is applied. So on a day that it is 90 degrees outside you have yy flow through the motor the ecu will use the proper fueling for this amount of air being introduced into the engine.
On speed density cars the ecu uses a component of mechanical physics to calculate air flow it is called PV=NrT.
P = pressure (MAP sensor input)
V = Volume (total swept cylinder volume)
N = Mass (*this is what we are trying to find from the mathematical equation with the other numbers in the formula)
r = Real Gas Content (fixed number within the ECU it does not change)
T = Temperature (Air Temp Sensor input)
This is the equation a speed density ECU uses for air density entering the motor and as you can see Air temperature is a required input for the equation to be complete.
Yes I have noticed very small changes in A/F's from car to car with dyno testing in a controlled environment. But to answer your question If a car is PROPERLY tuned it should not have to be RETUNED if the calibration was done correctly the first time from season to season.
I hope that helps gentlemen
Chris Macellaro
01-10-2006, 02:06 PM
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I agree with just about everything in this post. I did most of the tuning in Gen VI DFI on my old car with a 355 LT1.... It made 422RWHP and 395 RWTQ with a stock crank, 6" rods, SRP flattops and some heads that only flowed 258cfm max on the intake at .600 lift.... The motor didn't have a ton of cash in it just off the shelf stuff. Still put down a lot of power though considering it was only a 355... Once the 150 shot was on power went up to 525 and 597 respectively.... I can't give you a good estimation of the n2o a/f ratios since they were up in the very lean 14.5:1 area you spoke of due to a swap of a stock intank and NOS inline pump that I had to a 255lph intank only that severely hurt fuel flow for some reason.
But back to my point. When the car was on the dyno NA and made the 422 I was at about 12.8:1 on the torque peak and leaned it out to about 13:1 at the power peak (about 6200RPMS with the cam I had).... I leaned it out in the midrange which picked up a bunch of torque under the curve and tried some different things here and there but peak hp and torque stayed about the same mostly... I did not attempt to venture much higher than 13.2:1 for safety's sake....
With my new combo I think I'm going to attempt to reach roughly the same medium. Maybe a tad less since my new motor has three times the money in it as that thing but I'll still shoot for like 12.5:1 at the torque peak and lean it out to like 12.8:1 at the power peak for safety and longevity of parts.... Lean might be mean but it can be expensive as well....
But back to my point. When the car was on the dyno NA and made the 422 I was at about 12.8:1 on the torque peak and leaned it out to about 13:1 at the power peak (about 6200RPMS with the cam I had).... I leaned it out in the midrange which picked up a bunch of torque under the curve and tried some different things here and there but peak hp and torque stayed about the same mostly... I did not attempt to venture much higher than 13.2:1 for safety's sake....
With my new combo I think I'm going to attempt to reach roughly the same medium. Maybe a tad less since my new motor has three times the money in it as that thing but I'll still shoot for like 12.5:1 at the torque peak and lean it out to like 12.8:1 at the power peak for safety and longevity of parts.... Lean might be mean but it can be expensive as well....
01-11-2006, 11:31 AM
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Just my .02 on if the afr sholud change with wether. First i would like to say that cars didnt always have mafs and that one of the holden cars out still dosent have a maf and these cars are not taken to the dealer as the weather changes. The way I understand it iat shold be able to compensate for weather changes at least so you wont have to retune it.
01-11-2006, 05:19 PM
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Personally, I would rather a car run richer than leaner. The difference between 12.2 and 13.0 is marginal for peak power but significantly more so for peak torque.
Case in point, my car is tuned for 12.2 at WOT around peak cylinder pressure and 12.4 to redline. I gain substantially more torque up till 5000RPM richer than running lean, say at 12.7AFR. Peak power difference whether running 12.7 or 12.2 is a marginal 5rwhp. I'll give that up for the increased torque and longevity/safety any day.
Now each car is different, depending on its setup. For a stock cam car running closer to 13 is more ideal for linear torque and power production. Cams like a little more timing but more so fuel. This achieves 2 things - it keeps exhaust port temps down and cools the cats. Heat causes restrictions as much as a smaller diameter pipe. Heat also increases fatigue and wears components down more quickly. Another thing, runnign the engine rich will also help keep cylinder temps down and as a by-product lower the operating temps of the valvetrain/springs. Hot springs will valve bounce easily. So IMO its all a balancing act - matching the performance limitations of all your running gear, from valvetrain to exhaust. A more restrictive exhaust will like to run richer because it cant expel the gases as quickly as a less restrictive one and so relies on cooling effects of unburnt fuel to manage temps.
In essence, I guess, it comes down to managing energy throughout the engine and where it is dissipated. You want to preserve as much energy as possible in the combustioin chamber and anything left over you want to expel as quickly and efficiently as possible.
Case in point, my car is tuned for 12.2 at WOT around peak cylinder pressure and 12.4 to redline. I gain substantially more torque up till 5000RPM richer than running lean, say at 12.7AFR. Peak power difference whether running 12.7 or 12.2 is a marginal 5rwhp. I'll give that up for the increased torque and longevity/safety any day.
Now each car is different, depending on its setup. For a stock cam car running closer to 13 is more ideal for linear torque and power production. Cams like a little more timing but more so fuel. This achieves 2 things - it keeps exhaust port temps down and cools the cats. Heat causes restrictions as much as a smaller diameter pipe. Heat also increases fatigue and wears components down more quickly. Another thing, runnign the engine rich will also help keep cylinder temps down and as a by-product lower the operating temps of the valvetrain/springs. Hot springs will valve bounce easily. So IMO its all a balancing act - matching the performance limitations of all your running gear, from valvetrain to exhaust. A more restrictive exhaust will like to run richer because it cant expel the gases as quickly as a less restrictive one and so relies on cooling effects of unburnt fuel to manage temps.
In essence, I guess, it comes down to managing energy throughout the engine and where it is dissipated. You want to preserve as much energy as possible in the combustioin chamber and anything left over you want to expel as quickly and efficiently as possible.