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scott0999

E85 AFR's

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Hello, my first post here

 

im interested in tuning my car for E85. I'm confused on how your supposed to tune it for such low AFR's though. for instance stoich is around 9.8 AFR, rich max power 7 AFR ect..

 

all the widebands I've seen arent accurate that rich so what do you do?

 

thanks

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You are confusing true AFR with the gasoline calibrated AFR that is reported on most wide bands.

When working with E85 or E85 blends you are much better dealing with the lamba value reported by the wideband. It is accurate regardless of the fuel in the tank. The AFR reported by most widebands assumes the fuel being used is gasoline. The wideband O2 sensor, actually reads the fuel air mixture in terms of Lambda and then internally computes (converts it to) a gasoline calibrated AFR where stoich mixture is assumed to be 14.7:1.

 

Stoich for any fuel is Lambda 1.0 Most users find that in closed loop, the wideband will report Lambda 0.98 -1.02 regardless of fuel (if the ECU has enough fuel trim authority to get back to stoich).

 

Most of us shoot for a Lambda of about 0.78 - 0.85 on E85. To determine what gasoline calibrated AFR the wide band will display with those values simply multiply the Lambda value by the gasoline stoich AFR of 14.7.

 

That means if you are running a lamba of 0.85 on E85 the wideband will tell you your AFR is (0.85 x 14.7 = 12.495) gasoline calibrated AFR, even though your true AFR (on pure E85 with a stoch of 9.8 AFR) would really be AFR(e85) of 8.33:1.

 

Most folks adopt one of two strategies, either think only in terms of Lambda values or only use gasoline calibrated AFR's. Since most tuners are familiar with what is rich and what is lean in gasoline calibrated AFR's it is easy to think in those terms. On E85 the cars run quite well on gasoline calibrated AFR's near 11.5 - 12.5 in most cases. Heavily turbocharged or supercharged will be toward the rich end of that scale (or a bit richer) and NA cars toward the lean end.

 

Larry

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I completely forgot you can set the fuel type in the wideband logging software lol

 

so stoich is the same for a narrowband o2 sensor between regular gas and E85?

 

just add approx 30% fuel until the fuel trims are the same as prior to E85?

 

with spark advance adjustment and all..

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narrow band sensors are a different issue, they only accurately measure the oxygen content of the exhaust gases near stoich mixture. Yes you are correct the ECU will try to get to stoich according to the O2 sensor regardless of fuel as long as the car is in closed loop operation (ie paying attention to the O2 sensor). All that goes out the window when you go WOT and switch to open loop fueling. At that point you are at the mercy of your fuel tables in the ECU, and in most cases the engine will go lean in open loop on E85 unless you have done something to prevent it like raising the fuel pressure, installing larger injectors, remaping the ECU fueling tables or fiddling with injector duty cycles.

 

The quickest and simplest way is as you mention just increase total fuel flow by about 30% if you will be running straight E85. The ECU will see the very rich mixture under closed loop and try to trim the fuel air mix back to lambda 1 in closed loop operation. Under open loop operation the extra fuel will keep you from going lean.

 

For all practical purposes E85 uses the same ignition timing as gasoline. It can "tolerate" more advance but it does not necessarily need it.

 

If you can just set the software to report Lambda numbers and don't worry about it. With lambda values you don't even need to know the true fuel mix in the tank to do a basic tune.

 

Larry

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sweeet!!

 

my ECU is pretty good about target AFR's for WOT if the part throttle fuel trims are dead on. but thats on a gas scale.

 

I understand tuning with a wideband is the best way, but im without one right now.

 

if you were reading E85 AFR on a gas scale, what would be a good AFR for WOT?

 

reason im asking is I have a wideband o2 datalog of my car from when I last had a WB.

 

so I could estimate WOT fueling for E85 based on how much more/less it should be then

 

and if I set it a bit extra on the rich side, check plugs, ect I should be ok

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A quick and dirty correction for Gasoline AFR's to true E85 AFR's is to divide the gasoline AFR by 1.5.

 

9.03:1 AFR is the stoich mixture for pure ethanol, typical E85 has a stoich AFR of 9.7 - 10:1 AFR depending on if it is summer blend (85% ethanol or winter blend of 70% ethanol).

 

Fuel                    AFRst     FARst     Equivalence   Lambda
----                    -----     -----       Ratio       -----
=======================--=========================================

Gasoline   stoich          14.7    0.068       1           1
Gasoline Max power rich    12.5    0.08        1.176       0.8503
Gasoline Max power lean    13.23   0.0755      1.111       0.900

=======================--=========================================

E85 stoich                 9.765    0.01235    1          1
E85 Max power rich         6.975    0.1434     1.40       0.7143
E85 Max power lean         8.4687   0.118      1.153      0.8673


=======================--=========================================

E100 stoich                9.0      0.111      1          1
E100 Max power rich        6.429    0.155      1.4        0.714
E100 Max power lean        7.8      0.128      1.15       0.870


=======================--========================================= 

 

Larry

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