Just wondering what you guys run at.

I have GT35R and my idle A/F is around 12.8 and cruise is around 13.5, I believe it's a little rich. I always thought idle should be in the 13's and cruise should be in the mid 14's. From my understanding that's with stock twins and as far a logic allows me it should be the same for a single right?

Feedback greatly appreciated.

M-

 

I have mine to idle right at 13afr but at cruise its alot leaner. Mine sits at about 15 afr if its at a constant cruise speed. Works perfect, It seems to be able to cruise as lean as high 15's without any problems but 16's and it stutters.

 

My 1/2 bridge idles much smoother in the mid 12s than it does in the 13s. I had it in the 13s for a while, but the idle would become strange at times. It would lose the brap and idle funny. it wasn't missing, it was just odd. I moved it back down in the 12s and it will maintain a very healthy brap that shakes the car.

 

I idle around 13- 13.5 and my cruise on the highway is 15.5-16 at low speeds in town its right around 14.5. Off boost, I usually try to lean it out as much as I can without having it stutter or shake.

 

thanks guys for confirming that my car is too rich at idle/cruise.

M-

 

At idle and cruise, your engine is in a vacuum state (which basically mean it's NA). So why would you think either turbo would have an effect on your A/F ratio? If your engine is ported for more overlap, that will have the biggest effect on the accuracy of your A/F ratio reading. More overlap will allow more unburned fuel to exit the exhaust port. Also some don't realize that with more overlap, it's possible to get the impression the engine is running too rich when it really isn't. Think about it!

 

What kind of porting do you have?

I have found on a couple street ported motors that idle stability really decreases when you go much leaner than 13:1 on the wideband. If you carefully datalog hot starts you will see that the motor will always pull a little less vacuum (putting you in a different cell on the fuel map) and the IAT sensor will heatsoak to a degree (leaning out the mixture).

If you still have your ISC installed you may then experience surging as a result of this. If you don't have it installed, it will just start to idle low and maybe have unpleasant vibration. I aim for a range between 12.5 - 13:1 on these street ported cars (street ported FD and big streetported FC). That way when you hot start it doesn't get too lean (assuming your air temp correction is set up properly to account for heatsoak). You MUST account for hot start situations. It will drift always drift a little lean , unless you just overcompensate for the IAT change and then live with running rich under boost when it's hot out. It becomes a very tricky balancing act if you want a lean, stable idle under all weather conditions. There is a reason why they use O2 sensors on factory ECU's.

My cruising AFR's get leaner as vehicle speed increases. Under about 45-50 it's in the high 14's, and on the highway it's in the high 15's. That's on a streetported FD and a big streetported s4 T2.

 

^ Ditto

Mine gets 24 mpg Highway and no boosting.

 

12's at idle and mid 15's cruise 22 mpg at 75/80 mph.

The exhaust looks kool after a trip very clean!

 

I'm glad you guys posted this, when I told a few that I was getting 22-24 MPH they thought I was crazy. With no boost and cruising in the high 14s low 15s I get the exact same mileage.

Anthony

 

the big difference I noticed is the exhaust tone at idle. When leaned out it sounds like tapping on an empty can. But it sounds "padded" when overly rich.

 

When i went to see Steve Kan tune late last year we put the car on the dyno and it got 500 h/p but the big deal was he tuned the driveability and thats when it went from about 14/16mpg to 22 mpg and that was at 75/80 with the j-spec 5th gear.
Now its 4:30 dif and j-spec trannie oh well 21mpg!

 

idle in the 12s isnt so bad.

Your cuirse afrs are rich..

 

every engine and aftermarket ECU is different on how to set the proper cruise and idle a/f. For example, apexi PFC and Haltech and go pretty close to stoich without and issues. Mircotech and AEM for Rx7 works best at around 13.5-14.0 for cruising. This is on a slightly ported or stock port engine. more aggressive street port and bridge engine require more fuel. I think alot have to do with how the ECU runs the injector phasing and how it stage the injectors. The rpm at which you cruise could vary a/f as well. anything below 2500rpm seemed to run best in the 13.5 a/f range and around 14-15 range at steady state. 13.5 a/f at crusing isn't too bad. You might gain another 3-4mpg by leaning out more but what I find is that the response time may suffer a little on certain injectors no matter how you adjust the throttle pump on the ECU when tuning cruising closer to stoich. The best way to tune cruise a/f is to dyno it on a steady state dyno and find the max hp at the leanest a/f and advance timing. It takes alot of time to do it this way and alot of the facility don't offer a steady state dyno.

 

Stock intake ports with ported port throats, and mild exhaust porting.

Idle with no load: 12.5
Idle with A/C load: 13.5
Cruise is tuned from 14.5 at about 35mph to 16.5 at 70mph.

20.9mpg at 70 mph with JDM short 5th gear.

The only tuning issue is around 58mph when the secondary TB ports just start to crack open.
It can cause a momentary leaness as primary port air velocity drops. That is tuned 16.

Bad Mazda design between air flow through the ports and when secondary injectors start to work which is about 100mph cruise.

Check it out sometime with a DL if you have a PFC.

 

thanks for all of your responses guys, a lot of useful info in here. So I guess the idea behind the richness is to keep the EGT's low due to the cooling properties of the fuel, thereefore so I won't blow up using this crappy 91 octane that we have here. So the thinking is to start with a lower EGT's before I hit boost.

This is for the time being as once I get my AI there won't any worry about high EGT's at 15 psi with 91.

 

I wouldn't worry about detonation or EGT's when you aren't under load, and a rich mixture won't help cooling anywhere near as much as a good intake and intercooler setup.

I thought this thread was about low load AFR's but now you are talking about boost...?

 

You are correct. But after this thread I questioned my tuner on why so rich, and his reply was "I try to keep the EGT's lower at cruise so when you WOT (whenever that is) the starting temp is lower from the get go". Anyway, thats the logic behind the madness.

 

Just for people who crap on about how every engine is different blah blah blah.

If your building engines with same specs and they are wanting different tunes. Id be looking at retiring as an engine builder.