Guys, you might find this interesting. I had my truck tuned on Friday. It is bone stock except for the headers. When we first checked it, the fuel curve ranged from 10.2 to 10.7. I was very surprised to find the OE fuel mixture so rich. Needless to say, it runs much better now and I am hoping for a bump in fuel economy too. I have to wonder if GM sets all the OE fuel mixtures that fat on trucks?

 

coulda been from the headers throwing it off since it wasnt tuned?

any noticeable increase?

 

^ Wouldn't that cause a lean condition?

 

I would think it would cause more of a rich condition?

Depending if you still had your cats on or not right?

it was hard to tell for my header swap because when I did it I chopped the cats off and ran with the engine light for cat deficiency.

Edit: I see he has cats. So then it shouldnt have changed really. Other than the fact of more scavenging and better flow. I would still think it would burn more rich

 

GM does a lot of things I don't understand. MAybe they were thinking they could drown out any potential spark knock if they poured enough coals on the fire. Maybe they thought it would be more emissions friendly. They are all about burning twice the amount of fuel to make lower emissions.

 

They typically run them richer under WOT so susie soccer mom doesn't destroy the engine running WOT late to practice with the AC blaring and in 120 degree temps. Running that rich keeps the truck from overheating and keeps the thing from spark knocking.

 

True, it's for the soccermom doing WOT romps, a.w.a. for towing & hard work trucks are put thru that cars are not. Trucks are notorious for being rich from factory.

 

I live in northern IL who did your tune if you dont mind me asking? I just bought a 07 sierra denali awd im wanting to get more power and more MPG. Do you feel a big difference with the headers and tune?

 

Thanks for the comments! I agree that trucks are notoriously rich from the factory. I also agree GM does this to reduce spark knock and to cool the engine under heavy load. Now to answer a few questions. Did I feel a big difference with the headers and tune? Yes and yes. May have lost a bit on the bottom end, but barely noticeable. Very nice mid-range bump with headers. After the tune, at around 1500 ~ 2000 there is a noticeable increase throughout the powerband. It literally feels like you lost 1000 lbs. Something I did not expect. After headers with stock exhaust, it was loud as hell in the mid range with a Corvette style bark. After the tune, that is mostly gone. Still has some bark, but sounds a lot like it did stock. Who tuned it? I took it to FC Customs in Springfield, IL. There is a gentleman there named Ross and he is pretty sharp and has a great attitude. There are several tuners in Chicago with a good name, but I wanted someone closer to home. I did not have much luck finding a tuner on this site. Found a lot of information on the Corvette forum, www.corvetteforum.com.

 

The 99-07 trucks were about the same from the factory. I figured it was so rich since GM assumed the truck might be run WOT for extended periods while towing, hence the super-rich, super-safe AFR. WOT timing is only about 15 degrees as well.