RB25 AFRs (Cruising)

[S14_BOSS]

Just curious to know what a safe good AFR reading would be while NOT under load just cruising around the city/freeway.

I got my RB25 tuned today (street tune) and would like to know if I'm able to run anymore lean than what it is as I'm going for maximum gas mileage with still being SAFE. Mind you I barely use about 10% throttle when driving anywhere and hardly never step on it.

SETUP: S1 RB25, Greddy IM, FMIC, Boost @ 10psi, Aeromotive FPR, Z32 Fuel filter, 3" From turbo back, walbro fuel pump, AFC-neo

My AFR's while cruising are about 14.7-15.2. I was just curious if I could go leaner.

Looking forward to seeing what others are running.

[fliprayzin240sx]

I'd leave it alone, thats good enough. I'd just be worried about the stock ECU pulling timing on your setup.

[S14_BOSS]

Well I mainly went so I can get a low throttle tune as I mostly do city driving and barely use like 10-15% throttle all the time. For some reason a 0% low throttle to me looks odd as of we weren't tuning for normal cruising. Most of the time I was just flooring it (WOT).

AGAIN, I went in for a tune STRICTLY for better gas mileage. Someone with SAFC knowledge please chime in.


My settings

LOW THR - 0%
HI THR - 45%

NePoints (HI / LOW)
500 (-2 / +15)
950 (-2 / +15)
1500 (-2 / +15)
2000 (-2 / +5)
2500 (-2 / +2)
3000 (-2 / +0)
3500 (-2 / -2)
4000 (+2 / -2)
4500 (+2 / -2)
5000 (+2 / +0)
5500 (+1 / +0)
6000 (-3 / +0)
6500 (-3 / +0)
7000 (-3 / +0)
7500 (+0 / +0)
8000 (+0 / +0)

[nextproject]

Ive street tuned my 3sgte MR2 as lean as 16:1 in very low load cruising. up to 60kpa of load i used 16:1, EGTs are usually highest around stoichoimetric for pump gas. This differs slightly for E85. So once my loads increased past 65kPa and headed towards 100kPa, or zero vacuum, i would have the tune set to get the AFRs around 13.5:1. This is all in efforts to keep the EGTs down and get good gas mileage. I was able to get a cammed 3sgte to get 30mpgs on the highway. Original EPA figures from 1992 stated that the mr2 turbo only got 26mpg on the highway.

This was tuned on an MSPNP.

[2.5T_/<ouki]

I'd like to know this also. Probably going the same route soon.

[S14_BOSS]

Ive street tuned my 3sgte MR2 as lean as 16:1 in very low load cruising. up to 60kpa of load i used 16:1, EGTs are usually highest around stoichoimetric for pump gas. This differs slightly for E85. So once my loads increased past 65kPa and headed towards 100kPa, or zero vacuum, i would have the tune set to get the AFRs around 13.5:1. This is all in efforts to keep the EGTs down and get good gas mileage. I was able to get a cammed 3sgte to get 30mpgs on the highway. Original EPA figures from 1992 stated that the mr2 turbo only got 26mpg on the highway.

This was tuned on an MSPNP.

Thanks for your input. I did see that another member on this forum running an RB25, stock ECU, and SAFC2 managed to have his AFRs pulled to 16-16.4 while cruising. I'll have to figure out how to not let the o2 change the back the AFRs after I make adjustments in the Afc-neo.

Anyone have any idea about my throttle percentages? Why would my tuner set my low throttle to 0%? He was saying so anything after 0% will be using the low map but I've read that low map is utilized with anything that is below what the low throttle % is set at, hi map is used with anything above the hi throttle setting, and everything in between is a mixture from both.

[Huntsman2007]

That rb25 mpg thread got everyone thinking about their fuel economy hahaha

[Kingtal0n]

The problem is the narrowband O2 sensor will correct for your SAFC adjustment during cruise. The solution is use a wideband O2 sensor to generate a narrowband signal and set the signal such that 15.5:1 (any desired A/F) is 0.500 volts off the wideband analog output. For instance 15.7 can be 0.02volts and 15.3 can be .997 volts.

[Kingtal0n]

In your attempt to get best fuel economy leaner isnt always better. I advice you to shoot for best vacuum first, then adjust road speed until injector duty cycle is lowest at a most reasonable pace. As you get leaner than 15:1 you will experience a drop in engine vacuum. To compensate for the loss, the throttle body will open more, and this can be demonstrated by using cruise control and monitoring TPS voltage/position while removing fuel injector duty. Engine load will increase and you will be in another portion of the engine's timing map- a portion not designed for cruise, and there will be less timing, and fuel economy will suffer.


if you had access to the map you could increase timing in those portions and re-gain some efficiency and the EGT would theoretically drop, but I've never done this or measured the difference, usually I keep the engine around 15.2 for various safety/longevity precautions.

[S14_BOSS]

In your attempt to get best fuel economy leaner isnt always better. I advice you to shoot for best vacuum first, then adjust road speed until injector duty cycle is lowest at a most reasonable pace. As you get leaner than 15:1 you will experience a drop in engine vacuum. To compensate for the loss, the throttle body will open more, and this can be demonstrated by using cruise control and monitoring TPS voltage/position while removing fuel injector duty. Engine load will increase and you will be in another portion of the engine's timing map- a portion not designed for cruise, and there will be less timing, and fuel economy will suffer.


if you had access to the map you could increase timing in those portions and re-gain some efficiency and the EGT would theoretically drop, but I've never done this or measured the difference, usually I keep the engine around 15.2 for various safety/longevity precautions.

Thanks for your input!

Using the wideband o2 to output desired narrowband voltages does seem like a good idea. This seems like it would take other devices or tuning instruments to set the voltages that you wanted at certain AFRs.

Now, for best fuel economy would NIStune be something to invest in? I truly could care less about the numbers at WOT and truly only am seeking a tune for partial throttle/closed loop to improve gas mileage

[Kingtal0n]

Thanks for your input!

Using the wideband o2 to output desired narrowband voltages does seem like a good idea. This seems like it would take other devices or tuning instruments to set the voltages that you wanted at certain AFRs.

Now, for best fuel economy would NIStune be something to invest in? I truly could care less about the numbers at WOT and truly only am seeking a tune for partial throttle/closed loop to improve gas mileage

With my innovative there is a laptop program and a serial cable used to adjust the narrowband output voltage. I believe the AEM unit can also be adjusted.

best fuel economy takes more than just a computer and time. You also need to pay attention to the vehicle itself, adjust rolling resistance and vehicle weight for instance. Lightweight components, such as flywheels, driveshafts, wheels make a difference. Loosen those belts, remove the back seats, carbon fiber hood, new wheel bearings, fresh tie rods and alignment, proper tire pressure, proper cruise RPM/gearing, etc..

The computer side is important of course, but you need to do several hours of logging and comparisons to determine if one particular timing/egt/injector PW/cruise rpm is better than the next. And even then, variables are in every corner, for instance injector duty can give you raw lbs/hour of fuel, but different fuels have different specific weights, and injector duty must be factored in with fuel pressure, so unless you log injector duty AND fuel pressure you cannot accurately estimate actual fuel usage unless you use a "nice round fuel pressure" based on engine vacuum at the time, and a "rough idea" for fuel estimated specific weight. In other words your best fuel economy numbers may come more from random guess work combinations than actual recorded data because nobody records fuel pressure and compares it to injector duty then posts the results at different speeds online for you to compare with. Its a very personal vehicle specific job, one map does not fit all. As you lighten the vehicle you should find that more ignition timing can be used without knock and less fuel can be used at lower load values and that will kick up fuel economy significantly. We are not interested in adjusting the air/fuel ratio so much as we are the raw fuel input (duty cycle)

I do not feel nistune will be capable enough in this respect. something more commandable such as power FC with data-logit is capable of logging this important data including aux input for fuel pressure sending units if you wanted to.

[S14_BOSS]

I understand what you're saying and it all makes perfect sense, however I don't understand why running less fuel to go the same distances wouldn't increase my fuel consumption/economy. Ex. Drive 5 miles w/ AFRs @ 14.8 and it uses 3 gallons of gas. Now if I were to change the AFRs to 16.2 and drive the same miles, wouldn't I now only be using 1.5-2 gallons of gas to go the same distance?

A power FC seems so overkill and god damn expensive just for a pretty much stock RB25. It's like $800+ for the unit and another $600-800 for a tune.

[Kingtal0n]

I understand what you're saying and it all makes perfect sense, however I don't understand why running less fuel to go the same distances wouldn't increase my fuel consumption/economy. Ex. Drive 5 miles w/ AFRs @ 14.8 and it uses 3 gallons of gas. Now if I were to change the AFRs to 16.2 and drive the same miles, wouldn't I now only be using 1.5-2 gallons of gas to go the same distance?

A power FC seems so overkill and god damn expensive just for a pretty much stock RB25. It's like $800+ for the unit and another $600-800 for a tune.

You are talking about tuning it yourself which would be free. I do PFC tunes all day for $250 if you bring the vehicle to me.

And you need a stand alone with data-log capability and real-time adjustments if you want to squeek out every last drop of gasoline.


As to your first question, consider this scenario.
You are driving 75MPH on the OEM ecu. fuel injector duty is 10% and fuel pressure is 35psi. ECU is in open loop (no narrowband sensor to correct) Air fuel ratio is 15.2:1 on the wideband. timing is 39* btdc in this cell.

You remove fuel. Now you are around 16.2:1 air fuel ratio. Engine vacuum drops and fuel pressure increases to 38psi as a result. The load map also changes and the timing is now 36* btdc. You also lose power due to being leaner and having less timing, so now your car is decelerating instead of cruising at the same throttle position. The result is you need more air to give the same cruising speed with the new air fuel ratio. Fuel pressure rises even more, timing decreases even more, and fuel injector duty goes up.

If you had a power FC at that point, you could add timing in the new load cells, bring it back to 39* and watch the knock sensor carefully if the internals are OEM for spikes. I would advance as far as 45* to verify it does not ping at that load with that A/F ratio and watch the EGT for any odd behavior such as a sudden rise to 1600*F+ that might indicate the ecu (if capable) is pulling timing because it detects knock. It is not uncommon to see 1500*F while cruising on the highway with the wastegate connected on small displacement turbocharged engines (which is one reason I prefer a V8-V10-V12)

[S14_BOSS]

You are talking about tuning it yourself which would be free. I do PFC tunes all day for $250 if you bring the vehicle to me.

And you need a stand alone with data-log capability and real-time adjustments if you want to squeek out every last drop of gasoline.


As to your first question, consider this scenario.
You are driving 75MPH on the OEM ecu. fuel injector duty is 10% and fuel pressure is 35psi. ECU is in open loop (no narrowband sensor to correct) Air fuel ratio is 15.2:1 on the wideband. timing is 39* btdc in this cell.

You remove fuel. Now you are around 16.2:1 air fuel ratio. Engine vacuum drops and fuel pressure increases to 38psi as a result. The load map also changes and the timing is now 36* btdc. You also lose power due to being leaner and having less timing, so now your car is decelerating instead of cruising at the same throttle position. The result is you need more air to give the same cruising speed with the new air fuel ratio. Fuel pressure rises even more, timing decreases even more, and fuel injector duty goes up.

If you had a power FC at that point, you could add timing in the new load cells, bring it back to 39* and watch the knock sensor carefully if the internals are OEM for spikes. I would advance as far as 45* to verify it does not ping at that load with that A/F ratio and watch the EGT for any odd behavior such as a sudden rise to 1600*F+ that might indicate the ecu (if capable) is pulling timing because it detects knock. It is not uncommon to see 1500*F while cruising on the highway with the wastegate connected on small displacement turbocharged engines (which is one reason I prefer a V8-V10-V12)

Ahhh! I completely understand now. I was missing a lot more variables in my equation. I'm planning on going NIStune and i have read you are able to do all of what you described with adding the timing on the loads. I appreciate your response to my questions.

[nextproject]

This is true, in my specific case, with the 3SGTE/SW20 platform, i had to add half a degree of timing in certain cells to keep it from misfiring due to lack of fuel (running too lean). I dont/didnt have an EGT gauge hooked up, i was going based off of information i had received from Matt at DIYAutotune (the people that originally dynotuned the setup). I did the street tuning on my own as it wasnt possible for them to do it when they had the car due to expired tags.

So yes, ideally you want a way to add timing in the areas you are also leaning out, as you approach 16:1 you will probably kill your converter (if you are running one, they arent meant to run steady-state that lean) and run into slight misfire issues, which can be tuned out by adding timing (NOT ALOT!!!).