Leaning out the 7mgte. How far can I go?

[3.0 GT]

Im running 14.7:1 afr under normal conditions and 16:1 afr for inerstate driving under no boost whatsoever. Im using a zetronics wideband to control the afr. Iv got 550's and lexas afm. would this be safe to run 16:1? Without being to hot or burning my valves? Its only under light crusing and I want to make sure its ok befor I take a 8 hour trip for spring break.

 

[supra90turbo]

What are you looking to gain? I'd say 16:1 is about as lean as you should go with a lex/550/safc(?) combo. I don't think a few MPG's are worth the rebuild.

What is the MPG difference between 14.7:1 and 16:1 cruising on the highway?

 

[mkiiichip]

Cruse settings are not to be adjusted. You are only to adjust high throttle settings, as thats where the adjustments are beneficial and will make more power.

The whole point of pulling fuel at WOT is because the stock settings are slightly rich (safe) at these areas. The factory settings are king at low throttle, this is where toyota spent most of their time correcting and maximizing the drivability (and safeness) of their vehicles. There is no room to go leaner at cruse.

Leave the low throttle settings alone.

 

[3.0 GT]

Yes i have the safc2, with lexas and 550s. I am hoping to gain mpg from leaning this due to how much interstate drive in do commuting for school. Its seeming to work but I dont know how safe or what problems it could potentialy create in future like burnt valves and such.

Whats your oppinion running this lean or should i lower it to 15.5:1 to be safe?

What i did was put a toggle switch from a narrow band O2 sensor and the wideband so i can swich from 14.7 to what ever i set for the wideband one which is 16

 

[jetjock]

In steady state cruise you can run as lean as you like without harm to the engine as long as it's making less than 75% power. In fact it'll run cleaner and cooler slightly lean of stoich. Just make sure it's rich when it needs to be.

 

[destrux]

So you are setting the wideband to output a richer narrowband output to fool the ECU into going leaner than normal at cruise?

 

[shaeff]

Lean burn cruise, eh? I plan on the same thing on a 7M in my pickup using the MaftPro. Keep us updated and post your mileage!

 

[IJ.]

Did a LOT of testing when I had my 7M, anything leaner than 15.5>15.8 would make it unresponsive and create a slight lean miss, in turn to drive at that you needed more throttle % so ended up using as much and sometimes more fuel then a low 15:1 cruise.

In the end I was able to have a 500+rwhp 7M that returned 29US mpg on the highway.

 

[supra90turbo]

3.0 GT: Whats your MPG? I like that this has JJ's approval :bigthumb:

Very cool, Ian! I will be doing the same with my maftpro, and was aiming for 30mpg. Glad to see it's doable!

 

[jetjock]

What he's asking about and what Ian is alluding to is exactly how high performance aircraft engines are operated for hours on end at power settings far greater than the appx 25% a typical car needs going down the highway.

The result is cooler, fewer deposits, and improved fuel economy. Burned valves not. Too lean and you'll get misfire, lose power, and run even cooler (after all, the fire is going out) but no engine damage will occur. Why would it?

The best instrumentation for running this way is EGT, preferably for each cylinder. Assuming the injectors are balanced how lean one can go before the onset of misfire in any particular cylinder depends on how tuned the intake is.

A well balanced system will allow somewhere around 200 degrees lean of peak EGT (peak is stoich) before misfire sets in. There's no risk of engine damage even if you romp on the throttle under such conditions because all that'll happen is lean misfire.

Now doing the opposite, going lean *while* making lots of power, is an entirely different story...

 

[3p141592654]

Albeit, with high NOx emissions.

 

[supra90turbo]

Many thanks jetjock, I'm quite enjoying the direction this thread has gone!

 

[jetjock]

The primary differences between doing this in aircraft and cars is that with aircraft the cruising "road" is always flat, there's full manual control of the mixture, and the throttle is basically locked at one position.

That's tough to do in a car. Changing road conditions and having to dick with the throttle means once everything is set up you'll either have to tolerate occasional entry into lean misfire or run less than optimally lean to avoid it.

Long stretches of flat highway using cruise control. Probably more trouble than it's worth but the point was there's no risk of burning valves. And 3P is right about NOx. I'd overlooked that

 

[supra90turbo]

But I'm sure it can be done successfully, so long as you're not tuning for too lean an A/F ratio, no?

 

[3.0 GT]

Cool! very good information! I just wanted to make sure no damage would be made while cruising, Im running afr of 16:1 and have a little miss but not much so might go to 15.8 or so like someone said. But milage is a lot better im getting over 300 miles per tank now when i would only get 250 before. So thats sweet with gas prices going up and all. More money for parts now. Thanks for everyones thoughts

 

[arknotts]

Love the discussions here so far! I've been reading my way through a couple turbocharging/EFI tuning books and everything you guys have said has matched what is in the text. Light load (cruise) lean mixtures are fine, but will produce more NOx emissions. Rich mixtures will produce more hydrocarbon emissions. The closed loop 14.7 AFR that most factory cars target is simply to minimize the sum of both and make the catalytic converter's job that much easier.

I'm looking forward to experimenting when I get my MAFT Pro up and running (hopefully within the next couple weeks!). Without a standalone, how are you guys tuning closed loop to something other than 14.7? Do you disconnect the stock O2 sensor to force open loop?

 

[ValgeKotkas]

Could it be done using the VE table as the ECU can only correct the settings a given % ?( Vf would be at 5?)