WOT AFR tuning Why?

[kanusky1990]

hey guys, I know there are recommended AFR's for WOT, but my question is why those numbers? I don't want opinions, I am looking for facts. If that's what works because its been tried and true, then okay, but I'm really looking for some scientific explanation to this. Not just for my personal knowledge but for the community as well. Instead of tuning to a number someone told you, I figured if people understood why you're supposed to tune to those numbers it would benefit the understanding of the tuning process. Thanks in advance

 

[Calan]

Target AFRs are ideally chosen for best torque (given a certain amount of timing advance)...but additional fuel is needed in boosted engines to provide cooling and prevent detonation. So we normally end up with target AFRs between 10.5 and 11.5 for pump gas, and slightly leaner AFRs for fuels that are more tolerant to knock.

The higher the engine load, the more rich a mixture is required; so target AFRs tend to drop as load and/or RPM increase. At low loads below atmospheric pressure, the stoichiometric ratio of gasoline (14.7:1) is usually ideal for best power and fuel mileage.

 

[bastarddsm]

There's quite a bit to it. The AFR need's to be richer than stoich for many reasons. One of which is that the flame will have an excess of oxygen, and looking for more fuel to burn. Run it hard enough like this and it turns into a cutting torch, burning your piston/head/block...

So running it richer keeps the flame fuel rich, and of course cooler. It also sort of slows down the flame so it has more of a push, instead of a hammer. As you start enriching it up, more of the fuel isn't completely burnt, it burns to CO instead of CO2, and when this happens less energy is released, causing the engine to make less power.

I don't know of any "formula" to tell you what afr is good. There is basically a range of AFR's that work. Pretty much it boils down to 11-12:1 for a turbo motor on gas, 12-13:1 on n/a gas motor, 9.5-8.0:1 for a ethanol motor.

Generally the leaner the more power, until you get it leaned to the point it wants to detonate.

 

[Calan]

As you start enriching it up, more of the fuel isn't completely burnt, it burns to CO instead of CO2, and when this happens less energy is released, causing the engine to make more power..

Ehhh..... huh?

 

[jtmcinder]

Pretty much it boils down to 11-12:1 for a turbo motor on gas, 12-13:1 on n/a gas motor, 9.5-8.5:1 for a ethanol motor.

Are you really saying that people run around stoich - or even above - on ethanol?

 

[bastarddsm]

Ehhh..... huh?

, Fixed it.

Are you really saying that people run around stoich - or even above - on ethanol?

No, should have specified E85. Stoich for E85 is like 9.7, so 9.5 would be pretty lean, but I've done it and the car seemed to like it. My dyno guy says some N/A guys are in that area.

 

[Calan]

Fixed it.

Thanks. For a minute there I thought one of us was loosing their marbles, and I wasn't sure which one of us it was.

***********

These conversations always confuse the hell out of new tuners, but it's actually easier to think of the AFR in terms of lambda rather than a true AFR for any given type of fuel. Besides, that is what widebands are actually measuring; the output just happens to be calibrated to show the equivalent AFR for pure gasoline (iso-octane).

A lambda value of 1.00 is the same for any fuel, since it is measuring the ratio of excess air in the mix. A lambda value of .90 would be 10% rich, 1.10 would be 10% lean. What this means in a nutshell is that you can still think in terms of gasoline AFR values when tuning ethanol and other fuels, so you don't need to worry about doing a lot of conversion. If you get used to looking at lambda instead of AFR (which most widebands will show), it's even easier. If the gauge shows .91 for example, you are 9% richer than stoich...no matter what fuel you are running.

With all that said, best power on the rich side is somewhere between lambda .84 and .92...which is right at 12.6:1 for gasoline, or 8.7:1 for E85 (IIRC). The reason you can run leaner on E85 is because of it's burn characteristics, excellent cooling properties, and higher octane (resistance to knock).

Of course this doesn't take into account any extra fuel needed for cooling, incoming charge temperature and pressure, spark strength, timing advance, fuel blend....well you get the idea. As bastarddsm said... there is a lot that goes into it, and it's different for every setup.

 

[kanusky1990]

Thanks guys, that's a lot of good info. So with I've read to tune for 10.5 for a basic tune:

http://www.dsmtuners.com/forums/articles-tuning-ecu/407715-how-get-basic-tune-ecuflash.html

That's just to set it up. So I assume you slowly lean the AFR out to make the best tune, so do you slowly lean out this value until you see knock? Or will damage occur (high temps) before the knock occurs if you run at a leaner value continually? (if you get what I am asking here)

So i guess my question is, if I'm not seeing any knock, is there anyway that I could be damaging anything with the AFR and timing being used.

Basically, I'm looking for a "step by step" general guideline to tuning, for both AFR and timing. I'm pretty sure I read somewhere (not sure where) that once the AFR's are tuned, you can start advancing timing until you see knock, then retard the timing as necessary.

I just want to make sure I understand what I'm doing before I go messing with everything . Again, I know its easier to just listen to what people say to do, but I like having the confidence in understanding what I'm doing and why.

Thanks again, the info so far is great, lets keep it coming

 

[Calan]

I just want to make sure I understand what I'm doing before I go messing with everything . Again, I know its easier to just listen to what people say to do, but I like having the confidence in understanding what I'm doing and why.

You sound like someone who could appreciate this:

Amazon.com: Engine Management: Advanced Tuning (9781932494426): Greg Banish: Books

Amazon.com: Designing and Tuning High-Performance Fuel Injection Systems (9781932494907): Greg Banish: Books

By far the best $30 you'll ever invest.