fanbelted

Hi all,

My apologies because I am sure this is a noob question, but I couldn't find any posts on the topic. I am about to get an AFR gauge + wideband for my car. After shopping around, I noticed that all AFR gauges had a lower limit of 10:1 or 11:1, and an upper limit as high as 18:1. Now, I have always heard that you don't want your ratio to ever go over 12 or 13, or you could blow up your engine. In agreement with this idea, a couple of years ago I had my car dyno'ed, and part of the dyno display was my Air/Fuel ratio. When the throttle was jammed down, the ratio plunged below 7! So it would seem to me that you really need an AFR gauge to have a range from 5-14. Why are the gauges on the market all calibrated with such a high range? If my knowledge is accurate, then they would have a useful dial sweep of only 15 degrees or so, with the rest of the dial just wasted space. And a 10-18 calibrated gauge would spend a lot of time with the needle stuck on the lower peg. Can anyone explain this?

Thanks a lot!

P.S. My car is an Evo IX MR

fanbelted

Hmmm... I see 18 people viewed this thread, but nobody responded Could somebody who currently has an AFR gauge write a reply? I am really curious about how this is all handled!

thanks a lot!

Paul

baylorar

iTrader: (8)

Here is a picture of my stock fuel map. You'll see at low load, low rpm (i.e. cruising), the AFR is 14.7. Now at cruise, this is the value the wideband will read 14.7.



The lowest value, after my tune, that the wideband sees is 9.8 (this was a 1st thru 4th pull and it hits 9.8 when I get into 4th at wot). Correspondingly, the lower afr value in my current map is 9.6.

Hope this helps. I'll subscribe and answer any questions I can.

Adam@MMS

iTrader: (7)

A properly tuned Evo should never dip below 10 to 1 on the a/f. Most people target around 11 to 1 on pump gas at WOT and 14.7 to 1 at cruise and idle. 7 to one is really rich, I would think the car would start to sputter at that point. Hope that helps.

fanbelted

wow, thanks a bunch guys! I really appreciate the info. I guess I remembered wrong about the ratio not being allowed to go over about 13:1. My memory must be failing lol. About my ratio plunging below 7, that's what my dyno sheet shows (there's a separate graph below the main torque & HP curves). The dyno could be inaccurate, but I always heard that Mitsu's stock map is super rich so I didn't question it. One thing still bugs me though. Isn't stoichiometry at 14.1:1 ? Or maybe it's 14.7:1 ? but 18:1 sounds like it's way beyond dangerous, and that's what most AFR gauges have at the max end of their dial, of the ones that I've seen online. If your AFR is never permitted to even get close to that value, it seems like wasted dial area to me.

markymark1082

iTrader: (4)

14.7 is stoich.

fanbelted

cool, thanks for the confirmation! So how lean is too dangerous? Is 18 on the gauge dial a ludicrous number?

Adam@MMS

iTrader: (7)

At full throttle, yes, engine go bye bye. At idle no, but the car will probably start to sputter.

Mr. Evo IX

iTrader: (5)

When you get way to lean the engine will probably not make enough power to do damage. The most dangerous AFR for WOT is probably right around 14.7:1 or maybe some ridiculously rich, ring washing AFR. However 14.7:1 is the most efficient AFR for cruise and idle. For closed loop tuning the target AFR is 14.7 (idle and cruise), for open loop (WOT) you should be somewhere between about 10.5:1 and 12.5:1, depending on intended use and your specific engine. This additional fuel during high load conditions keeps the internal engine components from overheating during the combustion process.. Also, during decel injectors turn completely off, and your gauge will just read lean as there is no fuel being injected.

Essentially there are different correct AFR's for different areas of the map "landscape", different run conditions require different fueling to produce adequate thermal management for the engine.

baylorar

iTrader: (8)

You're kidding right? When you run too lean, there is premature detonation (knocking). Sure, the ecu pulls timing, but it won't save you if you go way lean for some reason (i.e. fuel pump going out... I know from experience. AFR went to 17:1, maxed the knock counter, and the ecu pulled 15 deg of timing. I got lucky, no white smoke, no burning of oil, no damaged spark plugs, etc.).

Mr. Evo IX

iTrader: (5)

No, I'm not kidding. What I was saying is that 14.7 is more dangerous than 17:1. At 17:1 and leaner your engine was probably misfiring and not making much power as very lean mixtures are difficult to ignite under boost. Also, we're not talking about fuel system failures here, as that's another story all together. Had your engine actually pre-ignited due to a lean condition no amount of ignition timing retard would have saved it as it would ignite the fuel on it's own (without spark) for glowing hot internal components.

baylorar

iTrader: (8)

I mistook your post. I think I see what you're saying. Since it's so lean (17:1 and up), it's hard to actually ignite due to the mass amounts of air vs fuel? Whereas, 14:7 is richer, thus easily ignited? On that note, going super lean and maxing the knock counter out made a terrible sound, but it did lose considerable power, so seems to make sense.

Mr. Evo IX

iTrader: (5)

Yes, as cylinder pressures increase it becomes increasingly difficult to ignite a lean mixture and get a complete burn in the alloted time. I was not advocating running an engine lean. What I was really trying to empasize is that there is no magic AFR for all running engine conditions. A typical engine will run at different AFR's for different conditions.

typical example for gasoline engines - just shooting some numbers off the top of my head, engines should be tuned for the application / intended use. In other words these numbers are not absolutes.. just generalities for discussion purposes.

15.5-16.0 or fuel cut for decel (overrun)
14.6-14.7 idle and Low Load - ideal burn or stoich (emissions 100% water and co2)
13.5-14.5 Medium Load
12.5-13.5 High Load (Max load for NA cars eg 100kpa or 0psi relative)
12.0-12.5 Low Boost
11.5-12.0 High Boost

Richer mixtures may be tuned in for idle quality with cams, or additional cooling of combustion chambers under boost conditions. Obviously Mitsubishi wanted to be on the safe side of things if you look at the above fuel map.

fanbelted

ok, here's a related question. I live in Oregon, and in the winter months (sometime around Halloween) all the gas stations are required to add something to help with emissions. I don't know what they add, but it makes my gas mileage drop from 20mpg to 17mpg. There is obviously less energy contained in each drop of gas. Should this affect the tuning map?