Louie928

Hi Woody,
Under full power around 13.5:1 is going to be optimum. Richer mixtures are for thermal control where otherwise the combustion chamber will get too hot and either melt something or lower the detonation threshold. A very rich mixture can also be used to some extent like reducing spark advance. A too rich mixture will burn slower. At the low end where yours is around 11.5:1 I see no real reason for it to be that rich. Even though it's at WOT, the RPM is low enough so you aren't burning a lot of fuel per unit of time and combustion temp shouldn't be a problem. You'll see more torque if it was leaned to around 13.5:1 there. Above that where the mixture is very rich at less that 10:1 you've got danger of washing oil from cylinder walls and fuel diluting the oil in the crankcase. A lot of that fuel isn't even being burned. Above that area, the mixture is going leaner and that's the wrong way although it does end up being about right at the top for high RPM and WOT and boosted. What would be ideal is to have the mixture at about 13.5 - 14:1 at the bottom and progressively getting richer as the RPMs climb so it ends up at 11.5 - 12:1 for a SC car. Depends some on boost level too. Turbo needs richer. If the timing isn't correct and you have to add fuel to slow the burn, that's another thing.

A SuperMAF from John Speake, stock fuel pressure regulator, either 36 or 42 lb injectors, get rid of add-on fuel pressure controller. Do a sharktune, burn chips be happy. My opinion on it.

928SS

wouldn't you also need a WB02 sensor to dial it in at 13.5:1 or so cause the OEM stuff is tuned for 14.7:1 for max fuel efficiency and best emissions?? or can you just use the WB02 for tuning and re-use the OEM thingy...

Pizza

928SS,

I think most wide band O2 sensors can be permanently mounted and wired into the cars computer via an analog narrowband interface.

Louie928

When you are at more than about 70% throttle and also other conditions of high load and/or high RPM, the LH goes to open loop condition and does not control the mixture to strictly 14.7 anymore. With a WB O2 sensor and the Sharktuner, you then tune for the mixture you want. When at cruising, the LH uses closed loop and maintains the 14.7:1 using the stock O2 sensor.

Louie928

Yes, you can do that too. Most WB sensors also have a simulated narrow band output. That works fine and you can then use the same WB sensor to monitor your mixture through an external display also and supply the input to the LH.

PorKen

AFAIK, 14.7:1 is best for the cats.

I was wondering if it be possible to tune for a leaner mixture at cruise for increased mileage? Does the ST let you bias rich or lean compared to the oxygen sensor, or is the O2 loop post processed and/or hard coded?

At cruise is there a downside to a lean mix? Increased cylinder temps?

Louie928

The ST does not allow setting for biasing rich/lean from stoic in closed loop. Depending on the configurability of the narrow band simulated output of your wide band sensor, you may be able to bias that. Then when the LH gets a voltage that corresponds to stoic (14.7), or about 0.45 volts, it will adjust the mixture for that. If you had set up your wide band so the simulated NB output switchpoint was equal to 0.45 volts (14.7 AFR) when it was really 15.5, then the LH would keep the closed loop mixture to that value, or 15.5.

I tried that for a while and couldn't really see a measurable difference in fuel mileage. Maybe a couple tenths of a mpg higher, but that's all. Might have been wishful thinking too. Probably not really worth the effort. I didn't try anything leaner than 15.5 although the 928 seems to run well at up to about 17:1.

You have to remember that with a mixture leaner than stoic, you'll have oxygen left over in the hot combustion chamber. It's going to want to combine with whatever it can and the hotter it is the easier it can do that. At higher power levels the exhaust valve is probably the hottest thing around. Burned valves. At low cruising power levels there probably isn't any worry. A leaner than stoic mixture won't burn as hot as a stoic mixture. A stoic mixture will produce the hottest EGT. If you go leaner, the EGT falls off and so does the cyl head temp. These temps fall faster on the lean side of stoic than they do on the rich side.

S4-on-your-back-door

This leads me to a post I posted a while back. Cant one just unhook the full throttle wire and some how feed that signal to the WB 02 and adjust WB's narrow band output to call for more fuel from the LH that way the LH will try to maintain a 13.1 fuel ratio at wide open ( it will stay in close loop condition)? No more needing to tune or burning chips or FMUs the car will do all the work. What do you think Porken can you make that work?

S4-on-your-back-door

well?

Louie928

Short answer is, No.
You could run your O2 sensor into an Op Amp or a Schmitt trigger circuit. Have the trip point adjustable so you set it where you want the mixture to be. Set it a to 0.4 volts and the engine will run a little leaner. Set it to 0.55 volts and it will run a bit richer all in closed loop. The S4 LH will switch to open loop under several circumstances other than when the WOT switch is activated. You can also use a Perfect Power SMT to change the O2 sensor output to be different than the input. You could also configure your Lh to run as with no O2 sensor (no cat mode). Set your idle mixture with the required pot and set your overall mixture with fuel pressure. S4's usually run too rich at the top end anyway when perfectly stock so no real need to do anything to get more fuel to them. Work to get more air in, then fix the fuel if you need it.

S4-on-your-back-door

Im srry Im talking for a SC 928

Louie928

Ok. You have plenty of air then. Sometimes, I feel like a preacher standing on a street corner in Las Vegas.
1. You'll never have good mixture control using the band aid approach with the fuel pressure adjustment devices whether rising rate, BFGI, whatever they are. They work better on the centrifugal type SC because the boost rises with RPM so it sorta works. Doesn't work on the TS. It also sorta works on the Eaton SC because the boost is usually around 5 psi so it's close anyway. By doesn't work I mean you cannot get a AFR of 14:1 at the low RPM end and transitioning to 12:1 at the top end while retaining good closed loop idle and part throttle mixture. Tony has come the closest after what I'd consider extraordinary effort. For timing, you depend on the built in safeties of the EZ-K to keep detonation disaster at bay.

2. The SMT-6 or similar piggy back controllers can help to a certain degree to take care of the over richness at idle and part throttle that usually occurs when using the fuel controllers. At the top end you are still left with fuel pressure alone to control mixture. There is no automatic adjustment for different air pressure. Set it up at sea level, then go to the mountains and you'll be too rich. Set it up at 4000' and go to sea level, you'll be too lean.

3. John Speake's Sharktuner when used with his SuperMAF and injectors of 36 or 42 lb along with stock fuel pressure regulator works for both fuel mixture and timing control.

You are sure free to experiment with anything else that seems like it would work, and if you find something it would be great.

928SS

preach away mr ott!! as always - your "sermans" are great

Pizza

Amen!!!!

Louie928

Ok, back on the street corner....
Here is some mixture info from the textbook from the EFI 101 course I took.

For starting, whatever it takes to get the engine to fire, but not richer than necessary.

Idle: Lambda 0.9 to 1.0, AFR 13.25 - 14.7 although 928s will idle smoothly at up to AFR of 16 - 16.5.

Cruise: Lambda of 1 - 1.05, AFR 14.7 - 15.5

Transition to high load, and high load NA: Lambda 0.88 - 0.90, AFR of 13 - 13.5.

Low to medium boost ( up to about 12 - 14 psi boost): Lambda 0.82 - 0.87, AFR 12 - 12.8

High boost (14 to 30 psi boost): Lambda 0.80 - 0.85, AFR 11.75 - 12.5. Turbo and air cooled could go as rich as Lambda 0.7 - 0.75, AFR 10.3 - 11:1.

From this you can see that if your 928 will operate closed loop at idle and cruise (that's around AFR 14.7), then it's perfect as is. Don't mess with it. If it won't go into closed loop at idle and part throttle then fix that problem until it will. It could be a bad MAF or some (deleted descriptor for a common item that screws up the AFR curve), bad O2 sensor.

At WOT, NA, the LH with a good MAF usually runs plenty rich as it is. Don't try to add more fuel. With the max boost level that's reasonable with stock compression ratio, probably in the 8 - 10 psi range, there is no need for excessive richness at AFR less than 12 - 12.5:1.