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racer1989

cold start afrs and max lean afr?

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Hello  maybe someone maybe able to help me out. I am looking for cold start AFRs for different air and engine coolent temps.... is there a chart somewhere or a standard % to add verses the base afr of 9.76?

 

Also what would be the max lean afr you can run for basic everyday driving for max mpg? would a afr of say 9.9-10 be too lean?

 

Thank you in advance for your help

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99 Black GTP HPT tune, fender-well intake, 180 stat, AL 103 plugs, 42.5 lb inj. 3" DP w/Hiflo conv., E85 fuel conv, 3.4 sc pulley, custom rebuilt trans. AMSOIL 0W-30 in engine and ATF in trans AeroShell 500 turbine oil for SC oil

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Racer- can you share more about what you are doing? I saw your post over on E85forum also- and as Hotrod pointed out there is nothing in the public domain I am aware of --HOWEVER- some of the Evo and Turbo Buick guys seem to have licked the cold start issues. Have you looked at them or need a link?

 

Perhaps Corey also has some thoughts on this.

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1Outlaw, I am just trying to do some fine tuning at this point... yes if you could send me a link to the Turbo Buick site that would be great... as far as the max lean... I am just trying to max out my mpg under normal driving around town and hwy, I am not worried about the emission issues as we don't have testing here, I just don't want to cause any engine damage either as in too lean to burn a piston or valve...

 

Thanks for your help

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Racer- here is one such thread:

http://www.turbobuick.com/forums/e85-technical-forum/335043-e85-cranking-start-values-xfi-she-doesnt-like-cold.html

 

Here is the overall alternative fuel section for Turbo Buick;

http://www.turbobuick.com/forums/e85-technical-forum/

 

I know there is a whole lot more in the threads from last fall but short on time to look for them- the site is full of good info for at least the turbo v6 models.

 

If you are doing a 4cyl then the Evo forum ( http://forums.evolutionm.net/alternative-fuel-225/ ) has a lot of good info as also does the NASOIC forum for Subies (but Hotrod is a key poster in that and did not know of such cold start charts--but here is Larry's 5 year excellent thread on Subies; http://forums.nasioc.com/forums/showthread.php?t=803341 ).

 

 

What scale are you quoting AFR from - gas or alcohol? Do you have Lambda available?

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"some of the Evo and Turbo Buick guys seem to have licked the cold start issues"

 

What's that all about?

 

TD- many models where guys simply put in larger injectors without finetuning the cold start enrichment will still experience cold start issues unless they are able or willing to add a bit more gas. It MAY also have to do with a need to adjust timing to allow for longer ignition periods in the first 2-3 seconds. Remember in extreme cold we are actually trying to have enough ignitable gas present to fire off if the alcohol is unheated- if you look at it that way the first couple of seconds we may need a ton of E85 to E70 to bring the 15-30%  gas vapors to correct concentration. At -20 the ethanol just does not have enough vapor pressure and flash point to do the first flame in a cold cyl and we are relying on the gas side of the mix for the first couple of seconds.

 

I would love to see the cold enrichment map for an FFV vs the same FFV's gas cold enrichment map.

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Well, lets see...The following is what I have developed for my car, it could probably use some refinement, but I only get 2 'true' cold starts per day - where there is no heat in the engine at all. Plus I started in the middle of winter and needed something which worked right then. I think it will be much easier to tune as the weather gets progressively colder this fall. Also note, I am running 12.5:1 CR, so that helps the cold start some.

 

I guess the simple answer when the engine first starts (~32F, cold water, cold air) I'm running about 0.8 lambda at idle and may dip down a bit more under load. This ramps up to ~1.0 at normal operating temp. If you want to talk about actual 'fuel correction curves', I don't know what options you have, but Kpro allows for both air and water temperature correction at both low and high engine loads, so that is 4 correction curves with air and water running simultaneously. I have the following:

 

fueltrim-1.gif

 

width=500 height=470http://i178.photobucket.com/albums/w269/coreyonline/fuelcomp-2.gif[/img]

 

If you have the capability, I've changed the following to allow:

 

As mentioned, water and air temp correction. It seems as though E85 wants a bit more enrichment for the cold water temps. I think this is not so much a chemical equilibrium as it is the fact you only have a certain percent of the fuel vaporizing. So to get a total amount of vapor, you have to inject more fuel. Though this extra fuel can be taken out pretty rapidly as even a 50-60 degree engine is getting 'warm' with respect to fuel vaporization.  Plus a running engine generates more heat of compression than a cranking one. The air temp correction is more along the standard lines you would expect for simple air density changes.

 

I also have a cold idle thermo valve which directs a jet of air through the injector spray - this creates turbulence and helps with atomization.  The thermo valve is un-metered air which is also compensated with the cold enrichment.  So if you don't have a thermo valve, you could probably get away with a bit less enrichment.

 

Ignition dwell angle - The stock program is a smooth curve starting with ~15 degrees of dwell at idle and getting larger as RPM's increase. More dwell should put more power into the spark plug making for a bigger/hotter spark - at the expense of more stress on the coils and plugs. I've tweaked mine to be only two steps - a fairly moderate value down low and a separate for up high, though both values are larger than the respective values they replace.

 

My reasoning is- if the ignition can run a certain dwell continuously (ie freeway cruising) then it should be able to run that same dwell all the way down to idle.  I kick the dwell way up at low RPM to aid in starting. On the high rpm end, my engine only sees brief times in the 4500-9000rpm range, so I run the same dwell across the entire range - if I can run an angle at 9,000 it should be good at 4500 as well.

 

Cold start base cranking pulse - This defines how the injectors fire while the engine is cranking. And again another location where an excess of fuel (in addition to what would be 'chemically needed) is required with very cold temps.

 

Ignition advance - I have about 10 degrees extra advance dialed in at 32F...I don't know if this is optimal, but it seems to work.

 

 

The way I've gone about this:

 

If the engine cranks and cranks, but won't start = add more cranking fuel

 

Engine cranks, starts and immediately dies = crank fuel OK, more cold water / idle-light load fuel

 

Engine cranks, starts, idles fine, but sputters when trying to accelerate = add more fuel to medium load fuel

 

Water temp fully warm, cold air idle = water temp compensation should be 0 and tweak air temp compensation at idle-light load to get STFT near 0

 

Water temp fully warm, cold air cruising = water temp compensation to 0 and tweak air temp compensation at medium-high load to get STFT near 0

 

Water temp fully warm, ~86F air temp - this is 'standard conditions' all corrections should be at zero and the fuel tables are adjusted for stoic / 1.0 lambda at cruise.

 

Water temp fully warm, hot air cruising = water temp comp to 0 and tweak air temp comp for higher temps.

 

If you are looking at max lean ratio, that seems to run ~1.06 - 1.1 lambda at least for this engine.  Beyond that, you start to get lean misfires and incomplete combustion.

 

OK - this got pretty wordy, but hopefully there is some info you can use.

 

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Just a note here- on another forum Racer told me he found a heat cycle for injectors in an '09 Tahoe ECU :o  So does anyone know if indeed some of the new units are already using heated injectors or just that the ECU on the Tahoe was programmed/ purchased as such but the function is unused and there for the future or on another vehicle? :confused: I have not done a search for injector features in the newer Tahoes.

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Dunno - I have not heard anything and GM is not known for being on the cutting edge of technology...especially in light trucks.  Though I believe it was Delphi who invented / did work with the heated injector, so it might tend to show up in GM vehicles.

 

I don't see anything special listed for an '09 injector

 

http://www.drivewire.com/vehicle/2009-chevrolet-tahoe/fuel-injector/

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