ak_guru

I have read over and over about 14.7 stoichiometric for cars equiped with cats. What about if you don't have cats? I don't have cats and do not have emmision testing. In my experience my car seems to run fine at 14.7afr but runs better at say 15.2afr but I was wondering what should be ideal/right or to much?... I'm running vette alum. heads on 350, 11.25:1 compression forged, 95 octane, mild cam .438/.452, 3" cat back exhaust, built tranny, gutted maf, accel ignition and diy proms at 14.7/12.5 ran 14.00 with stock ltr and 3.27 rear, one wheeling half the time(street tires), now have stealth ram, 1 3/4" headers and 4:11 Moser rear, and ET streets. This is in an 87 iroc best tuning on 32B but dont have any problems running 6E. A long post to ask about AFR's...
Also not just the 14.7 but what about wot 12.5

Using Moates flash-n-burn and TunerPro, A+ to Craig and Mark!

roarin_mouse

The best ratio of air to fuel will depend upon the current operating conditions of the engine. While cruising, leaning the mixture somewhat is ok and helps with fuel economy. Surely there will be contention about this but here's my take: The safest way to determine how lean is safe is by measuring the EGT. In airplanes, we find the mixture that provides the highest EGT and then richen it until the temperature drops by 25 dgrees C. This gives the best economy and is safe for the engine. Under heavy loads (higher power conditions), we find the peak EGT and then enrich for a 100 degree C drop.

As RPM and load increases, so does the residual heat from the previous power stroke. This residual heats causes the charge to burn faster (sometimes too fast). As fuel evaporates, it removes heat so that any extra fuel will keep your pistons, rings and heads from going into meltdown. It is for this reason, we want a richer mixture under heavy load and WOT. Just as you don't want to be too lean (light load) as temperatures can soar, an overly rich mixture will decrease the life of your engine because the additional fuel washes the oil from your cylinder walls.

Grumpy

Yep, 14.7 is just for the cats..
Once you get away from them, then you can run whatever makes the engine happy. Leaner in cruise usually (or can) make for better mileage. It all depends on your combo., and what it wants. The trick is figuring out what makes your combo the happiest. It's all too easy to be too rich, and with too much timing, it takes true testing to figure out what's really working. The human Buttometer is a poor test guage....

RednGold86Z

iTrader: (1)

But, for a mild street car, I always recommend the set-it-and-forget-it closed loop with a narrowband. Open loop just can't quite keep the consistency, especially when you throw humidity and temperature changes and altitude changes at it, unless you want to spend months and months tuning it. And then you get a bit different gas, and it all goes to heck, especially when you're balancing the fine line of the lean economy and lean misfire, and knock.

Sure, run highway mode if you want. But at least let the ECU check for stoich to give it a much better chance.

Otherwise, find a way to run WB closed loop on the street.

ak_guru

Thanks for the replies guys.

RednGold that brings up another question. I'm using the Zetronics WB, so would using the simulated narrow band output be equivalent to WB closed loop?

RednGold86Z

iTrader: (1)

I'm not familiar with the Zeitronix WB's outputs (I use LM1's), but, I'd rather suggest finding a way to use an A/F "map" to get the desired results. Then, target an A/F based on load and RPM and "mode" such as PE, and have the Open loop code be close to the desired, and the WB be the fine tuning.

-Now if the WB companies could alter the target vs inputted MAP and RPM via an AUXBOX (LM1), then it would be easy (except the open/closed loop code would not match the desired output). Or perhaps a continual closed loop ECU, with a switched WB output (LM1 has dual outs, could be switched via a throttle switch), but again, it would require the ECU to see the changed output at switching and adapt to it in some time period, which may not be acceptable.

Altered code would be the best way. Super duper $8D or $6E would be in order.

ak_guru

Thanks for the info. I have a copy of the 6E code with wb, I haven't gotten into it that deep yet, but I'll give it a try.

Grumpy

The only down side to all of that, is that your assuming the engine is happiest at 14.7:1. But, 14.7 is just about the cats, and has nothing to do with best mileage, engine life, etc..
While the summer to winter gas change might make for some tweaking, it's no big deal.

PLANT PROTECTION

iTrader: (6)

I agree. I won't bother with an open loop tune on a MAP car again, only to rough out the upper RPM/MAP VE table with a WB