So I have a discussion going on hptuners forum about safe AFR on the DI motor from GM. Some of the peeps there are saying min 12.0's which makes me a bit uneasy. I get that we are not spraying fuel on the backside of the valve and have more precise fueling. When my truck was on the dyno it made the most power at 11.0-11.5, but still seems like its falling on its face up top.

Thoughts?

 

Yes, you can run leaner than normal in these DI engines.

 

That's pretty vague, how lean are we talking with boost?

 

I would have to default to a tuner. These DI engines can safely run leaner than previous lsx's. As far as ideal, I don't know. Not lots of FI or even modded DI setups yet.

 

Thats what I was thinking, i know the ecotec cobalts run up to 12.x-13.x.
I will make small changes and see how it does, but probably leave the heavy load cells a bit rich for now.

 

Not surprised to hear this, keep in mind on a DI engine the fuel is sprayed into an already closed and in compression cylinder.
As a result your adding 11.5 parts of additional fuel to an already full charge of air. The net result is in a way, your supercharging with the pressurized fuel. Even though it's richer you have more volume stuffed into the cylinder.

Compare that to multi port. Let's say for easy math a cylinder is 100 cc's your going to draw in at best 100 cc's of air and fuel.
With DI you get 100 cc's of air and your adding additional fuel on top of that already full cylinder. The richer you make it to a point the more total volume you have to ignite.

This is the same reason flex fuel engines that use DI make more power on e85 because your stuffing yet 33% more fuel into the same cylinder. It's nearly the same as adding 33% more air and fuel into the same space as a multi port and a small supercharger

Not to mention the additional cooling effect of spraying the fuel directly into the cylinder.

 

I know the dynamics of the injection type will change this discussion to a point but your stoich AFR is determined by what fuel type you are burning, not injection type.

 

and i learned something today

 

i run the cobalts at 12.6-13.0 afr on 25+psi with zero issue. start with 12.5 and watch KR, fuel pressure {big one} and monitor injection timing as this leads to major misfire issues and loss of power if the injection window is missed.

 

With port injection, a good portion of the fuel doesn't even have any positive contribution to the actual combustion. When you richen it up to 11.5 AFR on gasoline, basically all of the fuel added between 12.5 and 11.5 is for knock prevention. During the compression stroke, that additional fuel is vaporizing, absorbing heat from the compressing air and metal surfaces in the cylinder. It also serves to slow the combustion down to make sure your peak cylinder pressure occurs at a certain crank angle to maximize torque on the power stroke.

When the fuel is direct injected, it doesn't need to be injected in surplus to absorb heat. All it needs to do is do its job in combustion and that's it. So a lot less fuel is needed. Like Area57 said, the optimal range to shoot for is high 12s AFR. 12.8, plus or minus 0.3.