AFR tuning theory
Jan 31, 2010, 05:27 PM
#1
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AFR tuning theory
Having too little timing can cause knock in certain situations.. I see this alot with K series acura motors. Also, enrichening the fuel actually increases burn speed. So if your knocking and its not due to cylinder temps/boundry layer, you can actually make the issue worse by richening the fuel.. (Also see this alot on LS1-7 "GM V8" motors)
Theres alot of missinformation out there on the dynamics of tuning. Theres no 1 formula and set of rules that apply to every engine. Usually when I see knock at lower IATs I lean it out a little. Most of the time the knock goes down and power goes up. If you see knock at higher IATs, then you may need to richen it up a little, but its not always the case. Don't just try one way, go both ways and see what works. I've tuned a couple EVO 9s that made best power at 12.8:1 on 26 PSI with no knock.. 95% of the people on the internet that think they know how to tune, only know the very very basics.. You should never have to go richer than 11.5s .. Anything past that, burn speed increases very rapidly.
Like I said, in most cases leaning it out will calm down knock unless its being caused by high cylinder temps due to IATs or debris in the cylinder that are staying hot .. obviously you don't want to go retarded lean. But if you go leaner and theres no knock and your not loosing power, then your doing fine. if you run rich for long periods of time you will build up carbon debris in the cylinders and this can create hot spots and embers that will cause preignition later down the line. It can become a vicious cycle if you don't truely understand whats going on..
Theres alot of missinformation out there on the dynamics of tuning. Theres no 1 formula and set of rules that apply to every engine. Usually when I see knock at lower IATs I lean it out a little. Most of the time the knock goes down and power goes up. If you see knock at higher IATs, then you may need to richen it up a little, but its not always the case. Don't just try one way, go both ways and see what works. I've tuned a couple EVO 9s that made best power at 12.8:1 on 26 PSI with no knock.. 95% of the people on the internet that think they know how to tune, only know the very very basics.. You should never have to go richer than 11.5s .. Anything past that, burn speed increases very rapidly.
Like I said, in most cases leaning it out will calm down knock unless its being caused by high cylinder temps due to IATs or debris in the cylinder that are staying hot .. obviously you don't want to go retarded lean. But if you go leaner and theres no knock and your not loosing power, then your doing fine. if you run rich for long periods of time you will build up carbon debris in the cylinders and this can create hot spots and embers that will cause preignition later down the line. It can become a vicious cycle if you don't truely understand whats going on..
Last edited by Bugermass; Jan 31, 2010 at 05:31 PM. Reason: Added Sig:
Jan 31, 2010, 05:40 PM
#2
Doesn't getting closer to stoich (14.7) actually increase burn temp?
ie, 11:1 is "cooler" than 12:1? I would have thought that burn speed would be fairly closley associated to burn temp, ie closer to stoich = faster flame front..?
Do you have any links to your burn speed information?
ie, 11:1 is "cooler" than 12:1? I would have thought that burn speed would be fairly closley associated to burn temp, ie closer to stoich = faster flame front..?
Do you have any links to your burn speed information?
Jan 31, 2010, 05:50 PM
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Burn speed is greatly influenced by mixture density. If you have more fuel in the mixture, the fuel molecules are closer together and the flamefront will progress more quickly across the chamber. (I have a graph that shows this relationship, I'll scan it later and post after I take my wife out to dinner 
) ...
Yes leaner mixtures will generally yield higher engine temps. BUT, not because it burns hotter nessisarily. First off, it will yield higher temps because the flame front is slowed down therefore the engine components are exposed to the heat longer (also one reason why leaner generally makes more power). The other reason it yields higher temps is because your reducing your boudry layer of fuel that acts as a barrier between the combustion and the piston crown and chamber. On lesser quality cast pistons, this boundry layer is more important because the materials can't cope with the heat as well. but on most engines that come turbo, or a fully built engine, this isn't as much of an issue and somtimes not an issue at all, depending on the fuel.
) ...Yes leaner mixtures will generally yield higher engine temps. BUT, not because it burns hotter nessisarily. First off, it will yield higher temps because the flame front is slowed down therefore the engine components are exposed to the heat longer (also one reason why leaner generally makes more power). The other reason it yields higher temps is because your reducing your boudry layer of fuel that acts as a barrier between the combustion and the piston crown and chamber. On lesser quality cast pistons, this boundry layer is more important because the materials can't cope with the heat as well. but on most engines that come turbo, or a fully built engine, this isn't as much of an issue and somtimes not an issue at all, depending on the fuel.
Jan 31, 2010, 06:39 PM
#5
Interesting rebuke from someone that sounds like they know what their talking about:
http://forums.nasioc.com/forums/show...2&postcount=11
I didn't really want to get into the whole AFR debate, since it seems to be one 
What does worry me is why all of a sudden I started getting lots of audible knock..
I guess I will need to load the old map back on, and do a pull in cooler whether, MAYBE I was tuned too close to the limit and the hot weather just pushed me over... Maybe thats why when I richened up the map the knock went away.
Thoughts?
http://forums.nasioc.com/forums/show...2&postcount=11
I have never heard anyone claim that anything as rich as 11:1 has the fastest flame speed, other than Hartman. I have heard a lot of slightly differing figures for it. EPA says, from their testing, that it occurs at 12.25:1 with standard pump gas. Considering that they're actual scientists, vs car guys, I'd tend to believe them.
It'll also vary slightly with other factors such as dynamic compression ratio, exact fuel composition, etc.
It's pretty easy to find, though. The AFR with the fastest flame speed will be the one that allows you to run the LEAST timing. When you change the AFR and then change timing, you changed timing because the flame speed changed. This means that richer AFRs don't allow more timing, they require more timing.
Look, the whole point of timing is to get the peak cylinder pressure (PCP) to occur when the piston is in a specific place (15-20* ATDC). Timing is how far before that point you have to light the fuel to get PCP 15-20* ATDC. Some things make the flame burn faster (such as increasing boost, increasing the compression ratio, etc) and require less timing to keep PCP at 15-20* ATDC. Some things (reducing boost, running way rich, etc) slow down the flame and require more timing to get PCP 15-20* ATDC.
It'll also vary slightly with other factors such as dynamic compression ratio, exact fuel composition, etc.
It's pretty easy to find, though. The AFR with the fastest flame speed will be the one that allows you to run the LEAST timing. When you change the AFR and then change timing, you changed timing because the flame speed changed. This means that richer AFRs don't allow more timing, they require more timing.
Look, the whole point of timing is to get the peak cylinder pressure (PCP) to occur when the piston is in a specific place (15-20* ATDC). Timing is how far before that point you have to light the fuel to get PCP 15-20* ATDC. Some things make the flame burn faster (such as increasing boost, increasing the compression ratio, etc) and require less timing to keep PCP at 15-20* ATDC. Some things (reducing boost, running way rich, etc) slow down the flame and require more timing to get PCP 15-20* ATDC.
What does worry me is why all of a sudden I started getting lots of audible knock..
I guess I will need to load the old map back on, and do a pull in cooler whether, MAYBE I was tuned too close to the limit and the hot weather just pushed me over... Maybe thats why when I richened up the map the knock went away.
Thoughts?
Jan 31, 2010, 07:43 PM
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Interesting rebuke from someone that sounds like they know what their talking about:
http://forums.nasioc.com/forums/show...2&postcount=11
I didn't really want to get into the whole AFR debate, since it seems to be one
What does worry me is why all of a sudden I started getting lots of audible knock..
I guess I will need to load the old map back on, and do a pull in cooler whether, MAYBE I was tuned too close to the limit and the hot weather just pushed me over... Maybe thats why when I richened up the map the knock went away.
Thoughts?
http://forums.nasioc.com/forums/show...2&postcount=11
I didn't really want to get into the whole AFR debate, since it seems to be one
What does worry me is why all of a sudden I started getting lots of audible knock..
I guess I will need to load the old map back on, and do a pull in cooler whether, MAYBE I was tuned too close to the limit and the hot weather just pushed me over... Maybe thats why when I richened up the map the knock went away.
Thoughts?
Basically if your knock limited (meaning your not at MBT yet but you can't add timing cause theres knock), and the cause of the knock is boundry layer heat issues, then yes you can go richer and it may reduce the knock and allow you to run more timing to get closer to MBT. But running richer definatly does not REQUIRE more timing due to slower flame speed..
My point from before is that most people pull timing and go richer when they come across knock. So what you are doing here is loosing power both from going richer AND from pulling timing. What I am saying is that alot of times, not always, but often, you can leanout the mixture, this will slow down flame speed (reduce uncontrolled or runaway burn "knock") and more often than not increase power at the same time. Again this is not always the anser, but I've found it to work in many situations.
While yes a slower burn can be more prone to knock, thats not always the case and is very dependent on other factors.. It comes down to why is there knock in the first place. There are so many factors that can create knock..
Theres no one formula that works every time for every engine/setup like many people believe. I see it all the time.. "11.5:1 AFRs in boost, add timing till it knocks then back off 2 deg.. " This is the biggest load of incomplete information there is lol..
The truth is there are MANY formulas that can be apllied in different situations. Its not that the standard methods of thinking are incorrect, its that the whole picture and all the possible variables that can effect those methods are not being explained or understood..
Jan 31, 2010, 07:53 PM
#7
thanks Chris, your input is welcome.
From your graph - 0.8 lambda is peak flame speed? ie about 11.8AFR.
When you say you can reduce knock by leaning out the mixture, presumably thats lean out AWAY from 11.8?
ie if you are tuned for 11.0 and run into knock, the worst thing would be lean out to 11.5, since that would be compounding the problem...
Also, surely a slower flame speed REQUIRES more timing to make the same power?
From your graph - 0.8 lambda is peak flame speed? ie about 11.8AFR.
When you say you can reduce knock by leaning out the mixture, presumably thats lean out AWAY from 11.8?
ie if you are tuned for 11.0 and run into knock, the worst thing would be lean out to 11.5, since that would be compounding the problem...
Also, surely a slower flame speed REQUIRES more timing to make the same power?
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Jan 31, 2010, 08:01 PM
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A slower flame speed will produce more power as long as peak cylinder pressure still occurs before 20ATDC.. So the anser to your question depends on how far from MBT you are before so start slowing down the flame speed. And yeah I'd go from 11.0 to 12.0-12.5 and see if it helps. It may, it may not.. But in alot of cases leaner mixtures can reduce knock and usually will increase power at the same time.
Jan 31, 2010, 08:05 PM
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don't forget 12.5 . . On race gas "Q16" I usually find best power between 13.0 and 13.4 and thats at boost levels from 30-55PSI... But for pump gas, to be safe I wouldn't go any leaner then 12.5 without being on a dyno and reading your plugs. i've been leaner on pump but it depends on how the motor is liking it. every motors different.
Jan 31, 2010, 08:08 PM
#11
it seems to me from that above graph - that the flame speed on either side of 0.8lambda is about the same.
ie 0.7 lambda (10.3 AFR) = 30cm/s and 0.9 lambda (13.2 AFR) = 30cm/s
let's make it more useful to us..
ie 0.75 lambda (11 AFR) = 30.5cm/s and 0.85 lambda (12.5 AFR) = 30.5cm/s
So what is the difference between running those two AFR's? Do they result in different cylinder pressures?
ie 0.7 lambda (10.3 AFR) = 30cm/s and 0.9 lambda (13.2 AFR) = 30cm/s
let's make it more useful to us..
ie 0.75 lambda (11 AFR) = 30.5cm/s and 0.85 lambda (12.5 AFR) = 30.5cm/s
So what is the difference between running those two AFR's? Do they result in different cylinder pressures?
Jan 31, 2010, 08:15 PM
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With the leaner mixtures you'll have a cleaner burn and i can almost 100% garuntee you'll make more power and the car will feel better.. There are the occasional engines that make more power richer, but they are the exception to the rule..
And remember this stuff specifically applies to gasolines, Alcohols (ethanol/methanol) are different..
And remember this stuff specifically applies to gasolines, Alcohols (ethanol/methanol) are different..
Jan 31, 2010, 08:19 PM
#13
Got any links to info on cylinder pressure vs lambda/AFR?
What about the theory of non-uniform mixtures? ie you can't guarantee a 100% uniform mix of fuel to air, so in order to stop hotspots from forming and causing pre-ignition you run richer?
Also when you say they make more power, is it like 1% or 5%? To me the danger of running lean vs rich would not be worth 1% or even 3% more power.
Unless you are getting a guaranteed fuel then its not worthwhile.. Pump can be pretty random in terms of quality!
What about the theory of non-uniform mixtures? ie you can't guarantee a 100% uniform mix of fuel to air, so in order to stop hotspots from forming and causing pre-ignition you run richer?
Unless you are getting a guaranteed fuel then its not worthwhile.. Pump can be pretty random in terms of quality!
Last edited by tephra; Jan 31, 2010 at 08:30 PM.
Jan 31, 2010, 08:49 PM
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The increase in power is definatly engine/setup dependent. But generally its significant enough to be worth while. The worry of poor mixture uniformity applies more to older engines with poor port and chamber design. Due to emmisions requirements most modern day engines have much better mixture uniformity. And now with direct injection and piezoelectric injection its getting even better. For example, a stock cobalt SS or HHR runs in the 13s at 20+ psi of boost from the factory. If your unconfortable with your particular engine then obviously I'd be concerned and run it rich out of pure safety. But I've tuned a few of them already and havn't had any issues running them between 12.0-12.5 @ 22-26psi.. Granted this is on stock turbos and things vary between setups, but just keep an eye on your knock sensor and check your plugs for signs of problems and you'll be ok. When you start running leaner you'll notice your plugs start looking alot better, less deposits and will typicaly last longer before fouling out.
Like i said it depends on the motor/setup. I'd never tune a stock subaru that lean on pump gas cause their motors are horrible lol.. But the EVO motors seem to be pretty good in general.. Keep an eye on your knock sensor and check your plugs and use your judgement and you'll be fine.
Like i said it depends on the motor/setup. I'd never tune a stock subaru that lean on pump gas cause their motors are horrible lol.. But the EVO motors seem to be pretty good in general.. Keep an eye on your knock sensor and check your plugs and use your judgement and you'll be fine.