Optimum AFR to Avoid Detonation?
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Optimum AFR to Avoid Detonation?
I know there are a lot of guys on this forum that do a LOT of tuning, so this should be a good place to get some real-world, experience based input.
Accepting that we use fuel (gasoline in this case) to reduce in-cylinder temps, and that is why we use mixtures richer than max power mixtures for non-turbocharged applications, what is the optimum point? It seems like I read in some technical paper a long time ago that you could go too rich and actually lose some of the detonation inhibiting properties of the extra fuel.
I've been shooting for 11.5 as my higher boost AFR, but then I got to thinking that this might not be quite rich enough, especially if my injectors aren't matched quite perfectly, and thus I might have a cylinder or two that are running a little on the lean side.
Any thoughts?
Thanks in advance as always! ! !
Accepting that we use fuel (gasoline in this case) to reduce in-cylinder temps, and that is why we use mixtures richer than max power mixtures for non-turbocharged applications, what is the optimum point? It seems like I read in some technical paper a long time ago that you could go too rich and actually lose some of the detonation inhibiting properties of the extra fuel.
I've been shooting for 11.5 as my higher boost AFR, but then I got to thinking that this might not be quite rich enough, especially if my injectors aren't matched quite perfectly, and thus I might have a cylinder or two that are running a little on the lean side.
Any thoughts?
Thanks in advance as always! ! !
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I'm in for more knowledge on this too. What I've read from the better tuners on here, people who are known for that, is that 11.8:1 is ideal.
I do know some oem setups just dump fuel through the motor as a precaution, like 10.8:1. Esp when the motor is hot. BMWs supposedly do that horribly. I took a class in aircraft engines years ago, we learned that there is a point where knock propensity actually increases with richness, although I can't remember what it was exactly. I'm guessing somewhere under 10:1? Probably depends a lot on the setup too.
I do know some oem setups just dump fuel through the motor as a precaution, like 10.8:1. Esp when the motor is hot. BMWs supposedly do that horribly. I took a class in aircraft engines years ago, we learned that there is a point where knock propensity actually increases with richness, although I can't remember what it was exactly. I'm guessing somewhere under 10:1? Probably depends a lot on the setup too.
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Re: (rorik)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rorik »</TD></TR><TR><TD CLASS="quote">I took a class in aircraft engines years ago, we learned that there is a point where knock propensity actually increases with richness, although I can't remember what it was exactly.</TD></TR></TABLE>
Think you might be able to dig up a textbook or anything?
I have seen that with the Chrysler PCMs (powertrain control modules), they go to 10:1, it's just a question of at what RPM does it happen. Even the Mopar Performance ones do it. I figure it's kind of like another layer of rev limiter. . . .
Think you might be able to dig up a textbook or anything?
I have seen that with the Chrysler PCMs (powertrain control modules), they go to 10:1, it's just a question of at what RPM does it happen. Even the Mopar Performance ones do it. I figure it's kind of like another layer of rev limiter. . . .
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Re: (Tjabo)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tjabo »</TD></TR><TR><TD CLASS="quote">
Think you might be able to dig up a textbook or anything?
</TD></TR></TABLE>
Corky Bell's Maximum Boost.
Timing has more to do with detonation than small AFR changes.
Think you might be able to dig up a textbook or anything?
</TD></TR></TABLE>
Corky Bell's Maximum Boost.
Timing has more to do with detonation than small AFR changes.
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Re: (Tjabo)
There isnt going to be a general optimum AFR for every engine, there all different.
For instance, my h22a likes 11.5-11.8 from what i have seen on the dyno..
My former setup (same turbo) different engine (b20a5) lost power when AFR was leaned from 10.9ish to mid 11's i wasnt picking up any detonation at either AFR either
Ignition timing has a much bigger effect on detonation VS AFR. Depending on how aggresive or conservative timing is set at will also effect how "easy" the engine will ping per AFR settings. So i guess my point is, there are far to many varibles to have a general optimum AFR setting IMO..
Im sure someone else will chime in with a more elaborate explanation.
For instance, my h22a likes 11.5-11.8 from what i have seen on the dyno..
My former setup (same turbo) different engine (b20a5) lost power when AFR was leaned from 10.9ish to mid 11's i wasnt picking up any detonation at either AFR either
Ignition timing has a much bigger effect on detonation VS AFR. Depending on how aggresive or conservative timing is set at will also effect how "easy" the engine will ping per AFR settings. So i guess my point is, there are far to many varibles to have a general optimum AFR setting IMO..
Im sure someone else will chime in with a more elaborate explanation.
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Re: (thewrai6th)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by thewrai6th »</TD></TR><TR><TD CLASS="quote">
Corky Bell's Maximum Boost. </TD></TR></TABLE>
I have that book, it doesn't cover anything about tuning..
Anyway, I should edit my first post. 11.8:1 isn't the best mixture for reducing the propensity for knock, it's just the mixture most tuners had mentioned as ideal for safe power with boost.
However, the 02 may not 100% accurately reflect what is going on either, as any air (oxygen) in the exhaust on the overlap cycle will cause it to read leaner than it really is. Just thought I'd throw that out there too. As far as how much leaner that could be, idk.
Corky Bell's Maximum Boost.
</TD></TR></TABLE> I have that book, it doesn't cover anything about tuning..Anyway, I should edit my first post. 11.8:1 isn't the best mixture for reducing the propensity for knock, it's just the mixture most tuners had mentioned as ideal for safe power with boost.
However, the 02 may not 100% accurately reflect what is going on either, as any air (oxygen) in the exhaust on the overlap cycle will cause it to read leaner than it really is. Just thought I'd throw that out there too. As far as how much leaner that could be, idk.
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Re: (rorik)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by rorik »</TD></TR><TR><TD CLASS="quote">
I have that book, it doesn't cover anything about tuning..
</TD></TR></TABLE>
Are you serious? Shoot for 11.8 AFR. Tune the timing on a dyno. Some people just throw fuel at the the knock instead of correcting the timing. You can have perfect AFR across the board and detonatate from too much timing. That's my only point.
I have that book, it doesn't cover anything about tuning..</TD></TR></TABLE>
Are you serious? Shoot for 11.8 AFR. Tune the timing on a dyno. Some people just throw fuel at the the knock instead of correcting the timing. You can have perfect AFR across the board and detonatate from too much timing. That's my only point.
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Re: (thewrai6th)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by thewrai6th »</TD></TR><TR><TD CLASS="quote">
Shoot for 11.8 AFR. Tune the timing on a dyno. Some people just throw fuel at the the knock instead of correcting the timing. You can have perfect AFR across the board and detonatate from too much timing. That's my only point.</TD></TR></TABLE>
So how little timing do you think is okay to run?
The point here is to assume good timing, and just talk about the effect of AFR. Do you think we could please at least address that issue, even if you also want to address other issues you see as relevant?
At any rate, it appears that either this isn't a very interesting topic, most people don't have opinions other than the one that posted an answer on point already (thanks B20luda!), or most of the experienced tuners aren't interested in sharing. Which I can understand I guess. . .
Shoot for 11.8 AFR. Tune the timing on a dyno. Some people just throw fuel at the the knock instead of correcting the timing. You can have perfect AFR across the board and detonatate from too much timing. That's my only point.</TD></TR></TABLE>
So how little timing do you think is okay to run?
The point here is to assume good timing, and just talk about the effect of AFR. Do you think we could please at least address that issue, even if you also want to address other issues you see as relevant?
At any rate, it appears that either this isn't a very interesting topic, most people don't have opinions other than the one that posted an answer on point already (thanks B20luda!), or most of the experienced tuners aren't interested in sharing. Which I can understand I guess. . .
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Re: (98vtec)
Well that's really helpful. . . Was it supposed to be humorously annoying, or do you just not grasp the topic for discussion? 
If it was meant to actually BE helpful, then I apologize for being a jerk.
How about coming at it from a different angle then (for everyone):
Is there an AFR on the low end, where you have found that you started to incur detonation because your mixture was too rich?
How about coming at it from a different angle then (for everyone):
Is there an AFR on the low end, where you have found that you started to incur detonation because your mixture was too rich?
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Re: (Tjabo)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tjabo »</TD></TR><TR><TD CLASS="quote">Well that's really helpful. . . Was it supposed to be humorously annoying, or do you just not grasp the topic for discussion? If it was meant to actually BE helpful, then I apologize for being a jerk.
How about coming at it from a different angle then (for everyone):
Is there an AFR on the low end, where you have found that you started to incur detonation because your mixture was too rich?</TD></TR></TABLE>
being that understanding what a spark plug should look like under optimum conditions is greater than what some gauge is going to display, i would say its pretty helpful advice coming from a "tuner"
the spark plug is a tattle tail. You may find something useful in the link in my sig if you scroll down the page.
How about coming at it from a different angle then (for everyone):
Is there an AFR on the low end, where you have found that you started to incur detonation because your mixture was too rich?</TD></TR></TABLE>
being that understanding what a spark plug should look like under optimum conditions is greater than what some gauge is going to display, i would say its pretty helpful advice coming from a "tuner"
the spark plug is a tattle tail. You may find something useful in the link in my sig if you scroll down the page.
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Re: (Tjabo)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tjabo »</TD></TR><TR><TD CLASS="quote">
So how little timing do you think is okay to run?
The point here is to assume good timing, and just talk about the effect of AFR. Do you think we could please at least address that issue, even if you also want to address other issues you see as relevant?
At any rate, it appears that either this isn't a very interesting topic, most people don't have opinions other than the one that posted an answer on point already (thanks B20luda!), or most of the experienced tuners aren't interested in sharing. Which I can understand I guess. . .</TD></TR></TABLE>
98vtec was trying to help. There is no one on this board or in the world that can tell you how little timing to run? Reason being you havent even specified a motor. Some motors like more timing than others (b16 vs a d16y8). And then ontop of that you have individual motors wanting different amounts of timing. i.e. my d16y8 might like 10* at 10lbs/7500prm where your d16y8 might like 8* at 10lbs/7500rpm.
You read the plugs and get on a dyno to find at what point your motor likes timing. I have yet to really learn how to read plugs, but that is where you will get lots of information about what the motor is doing.
Modified by Civicman86 at 12:27 PM 9/24/2008
So how little timing do you think is okay to run?
The point here is to assume good timing, and just talk about the effect of AFR. Do you think we could please at least address that issue, even if you also want to address other issues you see as relevant?
At any rate, it appears that either this isn't a very interesting topic, most people don't have opinions other than the one that posted an answer on point already (thanks B20luda!), or most of the experienced tuners aren't interested in sharing. Which I can understand I guess. . .</TD></TR></TABLE>
98vtec was trying to help. There is no one on this board or in the world that can tell you how little timing to run? Reason being you havent even specified a motor. Some motors like more timing than others (b16 vs a d16y8). And then ontop of that you have individual motors wanting different amounts of timing. i.e. my d16y8 might like 10* at 10lbs/7500prm where your d16y8 might like 8* at 10lbs/7500rpm.
You read the plugs and get on a dyno to find at what point your motor likes timing. I have yet to really learn how to read plugs, but that is where you will get lots of information about what the motor is doing.
Modified by Civicman86 at 12:27 PM 9/24/2008
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Re: (Tjabo)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tjabo »</TD></TR><TR><TD CLASS="quote">
So how little timing do you think is okay to run?
</TD></TR></TABLE>
Only a dyno with your car on it can tell you that, the AFR question has been addressed. 11.8 or richer is fine.
I'm just trying to help you prevent detonation, not change the subject.
So how little timing do you think is okay to run?
</TD></TR></TABLE>
Only a dyno with your car on it can tell you that, the AFR question has been addressed. 11.8 or richer is fine.
I'm just trying to help you prevent detonation, not change the subject.
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