Ford Focus ST Forum banner

1 - 13 of 13 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter #1
I recently started using the COBB OTS stage 1 93 oct map, and the AFR has been around 15.7 at idle and 44.53 at WOT. Is that AFR too high? I've seen usual AFR numbers around 12 idle and 15-16 WOT. Is it dangerous/unsafe to run the car with this high AFR number? I'm going on a trip soon and don't want to run the car if it has potential of hurting it.
 

·
Administrator
Joined
·
10,656 Posts
I recently started using the COBB OTS stage 1 93 oct map, and the AFR has been around 15.7 at idle and 44.53 at WOT. Is that AFR too high? I've seen usual AFR numbers around 12 idle and 15-16 WOT. Is it dangerous/unsafe to run the car with this high AFR number? I'm going on a trip soon and don't want to run the car if it has potential of hurting it.
I don't have to much to lend to this, but the 44.53 is decel in gear without throttle input, and should be disregarded. 15.7 seems too lean, someone more in tune with whats up should chime in shortly.
 

·
Registered
Joined
·
116 Posts
I recently started using the COBB OTS stage 1 93 oct map, and the AFR has been around 15.7 at idle and 44.53 at WOT. Is that AFR too high? I've seen usual AFR numbers around 12 idle and 15-16 WOT. Is it dangerous/unsafe to run the car with this high AFR number? I'm going on a trip soon and don't want to run the car if it has potential of hurting it.
I'm not sure how different car might run on stage 1 93 oct but I'm running stage 1 91 oct and my idle AFR is at about 14 and WOT around 12.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
9,130 Posts
Be cautious about running too lean when its not hot outside. Running lean can also cause wear on OEM pistons it seems, this could cause problems in OEM ringlands like bust a hole in them.
 

·
Registered
Joined
·
9,130 Posts
A simple thermodynamic reason why more power is made at rich mixtures is that hydrocarbons release most of their energy on combustion to CO; so for any given displacement/compression ratio/engine speed, etc. complete combustion doesn't lead to highest power output.

Consider the combustion of methane as an example:

CH4 + 2O2 --> CO2 + 2H20

If you took introductory chemistry, recall that standard enthalpies of formation (Hf) allow calculation of the enthalpy (or heat output) of reaction:

Hrxn = Hf,products – Hf,reactants

Tables of Hf can be found in most general Chemistry textbooks. So for methane combustion we have:

Hrxn = [-395 + 2(-242)] – [-75 + 2(0)] = -804 KJ/mol

Now for incomplete combustion the reaction is:

CH4 + 3/2O2 --> CO + 2H2O
Hrxn = [-110 + 2(-242)] – [-75 + 3/2(0)] = -519 KJ/mol

So essentially 2/3 of the energy available from hydrocarbon combustion is generated in the first step (CH4 --> CO), leaving only 1/3 in the second step (CO --> CO2). Conceptually at least, for a given displacement/compression ratio/engine speed the most power would be produced IF we could stop the reaction after the first half of combustion, dump the CO and pull in a fresh charge of CH4. This is hardly practical, but does occur to a certain extent when the A/F ratio is slightly stoichiometrically rich. There are plots of emissions and power vs. A/F ratio in the literature showing that often maximum engine output and maximum CO emission (untreated) coincide.

Clearly in a fired engine both kinetic and thermodynamic effects are operative. But the size of the heat of combustion difference suggests to me that the chemical effect may be the dominant factor.

Founds some nerdy stuff.
 

·
Registered
Joined
·
9,130 Posts
One more point to consider regarding running "lean" - if you have more free oxygen available, you are more likely to literally "burn" the iron in the pistons. With a rich or near-stoiciometric mixture, there is little free oxygen to oxidize the iron.
 

·
Administrator
Joined
·
10,656 Posts
A simple thermodynamic reason why more power is made at rich mixtures is that hydrocarbons release most of their energy on combustion to CO; so for any given displacement/compression ratio/engine speed, etc. complete combustion doesn't lead to highest power output.

Consider the combustion of methane as an example:

CH4 + 2O2 --> CO2 + 2H20

If you took introductory chemistry, recall that standard enthalpies of formation (Hf) allow calculation of the enthalpy (or heat output) of reaction:

Hrxn = Hf,products – Hf,reactants

Tables of Hf can be found in most general Chemistry textbooks. So for methane combustion we have:

Hrxn = [-395 + 2(-242)] – [-75 + 2(0)] = -804 KJ/mol

Now for incomplete combustion the reaction is:

CH4 + 3/2O2 --> CO + 2H2O
Hrxn = [-110 + 2(-242)] – [-75 + 3/2(0)] = -519 KJ/mol

So essentially 2/3 of the energy available from hydrocarbon combustion is generated in the first step (CH4 --> CO), leaving only 1/3 in the second step (CO --> CO2). Conceptually at least, for a given displacement/compression ratio/engine speed the most power would be produced IF we could stop the reaction after the first half of combustion, dump the CO and pull in a fresh charge of CH4. This is hardly practical, but does occur to a certain extent when the A/F ratio is slightly stoichiometrically rich. There are plots of emissions and power vs. A/F ratio in the literature showing that often maximum engine output and maximum CO emission (untreated) coincide.

Clearly in a fired engine both kinetic and thermodynamic effects are operative. But the size of the heat of combustion difference suggests to me that the chemical effect may be the dominant factor.

Founds some nerdy stuff.
Wow. You need a drink bro.

Blind siding a man with adjacent formulas at this time of night/morning, your eagerness may have gone too far. Lmao!!!
 

·
Registered
Joined
·
816 Posts
A simple thermodynamic reason why more power is made at rich mixtures is that hydrocarbons release most of their energy on combustion to CO; so for any given displacement/compression ratio/engine speed, etc. complete combustion doesn't lead to highest power output.

Consider the combustion of methane as an example:

CH4 + 2O2 --> CO2 + 2H20

If you took introductory chemistry, recall that standard enthalpies of formation (Hf) allow calculation of the enthalpy (or heat output) of reaction:

Hrxn = Hf,products – Hf,reactants

Tables of Hf can be found in most general Chemistry textbooks. So for methane combustion we have:

Hrxn = [-395 + 2(-242)] – [-75 + 2(0)] = -804 KJ/mol

Now for incomplete combustion the reaction is:

CH4 + 3/2O2 --> CO + 2H2O
Hrxn = [-110 + 2(-242)] – [-75 + 3/2(0)] = -519 KJ/mol

So essentially 2/3 of the energy available from hydrocarbon combustion is generated in the first step (CH4 --> CO), leaving only 1/3 in the second step (CO --> CO2). Conceptually at least, for a given displacement/compression ratio/engine speed the most power would be produced IF we could stop the reaction after the first half of combustion, dump the CO and pull in a fresh charge of CH4. This is hardly practical, but does occur to a certain extent when the A/F ratio is slightly stoichiometrically rich. There are plots of emissions and power vs. A/F ratio in the literature showing that often maximum engine output and maximum CO emission (untreated) coincide.

Clearly in a fired engine both kinetic and thermodynamic effects are operative. But the size of the heat of combustion difference suggests to me that the chemical effect may be the dominant factor.

Founds some nerdy stuff.
I appreciated this response since I just took 4 semesters of chemistry. Physics and chemistry for the win! Wooooo!

OP you should see around 11.5-12 at WOT and about 14-15 at idle for AFR give or take


Sent from my iPhone using Tapatalk
 
  • Like
Reactions: Darkdayz23

·
Registered
Joined
·
2,252 Posts
No throttle decel in the 40s is expected, 15.7 seems a bit lean for idle. You may want to have your tune looked at. It would be worth paying for tune and log review from Stratified or something like that so you can optimize that tune. The car will feel a lot different with that than any of the Cobb OTS tunes.
 

·
Registered
Joined
·
816 Posts
No throttle decel in the 40s is expected, 15.7 seems a bit lean for idle. You may want to have your tune looked at. It would be worth paying for tune and log review from Stratified or something like that so you can optimize that tune. The car will feel a lot different with that than any of the Cobb OTS tunes.
Or Freektune! Justin just checked one of my logs because I thought I was running rich, but upon further inspection I was hitting target AFR. Put your mind at ease with one of the tuners here


Sent from my iPhone using Tapatalk
 
  • Like
Reactions: Bryman13

·
Registered
Joined
·
2,252 Posts
Or Freektune! Justin just checked one of my logs because I thought I was running rich, but upon further inspection I was hitting target AFR. Put your mind at ease with one of the tuners here


Sent from my iPhone using Tapatalk
Aboslutely. Any of the tuners on here will be a big upgrade from the cobb ots stuff.
 

·
Registered
Joined
·
7 Posts
Discussion Starter #12
Thanks for all the help guys, greatly appreciated. I actually just ordered a flash tune from Stratified, so hopefully it's everything that i'm hoping for. :Rally:
 

·
Registered
Joined
·
816 Posts
Thanks for all the help guys, greatly appreciated. I actually just ordered a flash tune from Stratified, so hopefully it's everything that i'm hoping for. :Rally:
Even an OTS based tune from stratified feels much better than the Cobb OTS. And then you get into the realm of custom tunes and you unlock your cars full potential. I will be going custom tune soon from Freektune and I can't wait


Sent from my iPhone using Tapatalk
 
1 - 13 of 13 Posts
Top