I don't mean to be ignorant here, but I have seen it in a couple of places now that the best economy is achieved with timing and minimal boost for the fuel injected. Why does minimizing the boost result in better economy (I assume there is more to it than just less fuel being injected for a specific mass of air going in)? Plse help clear this up for me thanks.

When you create boost on an LLY you also create more drive pressure (back pressure). Drive pressure robs HP. The more HP you need to make to do the same task the more fuel you use.

I hope that makes sense.

Makes sense, and such a simple answer too. Thanks.

So if you want great fuel economy, you need big ported heads, an air intake, a huge cam, and a huge exhaust.

It's not just less air....it's the right amount of air. It does cost energy (via backpressure) to make boost, but sometimes you're injecting enough fuel to need extra air (beyond what mother earth provides).

From what I've read, when there's enough fuel to burn 60% (equivalence ratio of 0.6) of the oxygen in the air charge, that's about optimal. More fuel, and you'll get incomplete combustion, soot, and hot EGT. Less fuel, and you might as well save some pumping costs.

In my econ tunes I recalculate boost based on fueling and expected airflow to get enough boost to maintian 0.6. Obviously, at low fueling there's still to much air, but boost can only go down to 1 atm absolute.

Above 70mm3 fueling, I leave boost stock for safety at high power, and because I also added extra fuel above 75 % throttle (about 75mm3 fueling).

I have monitored the boost table using the EFI scantool, and a calculated equivalence ratio PID based on measured airflow and known fueling. In the range where it moderates (30 to 70%) it almost always holds 0.6 nicely.

When running stock, I noticed how just driving empty at 65 MPH, the thing would be boosting like 6 psi. That was crazy. All that air just to go 65 empty? The stock tune is VERY generous with air, probably for emissions. Now I can run on the freeway with the vanes wide open, no boost, no back pressure and the eq ratio still about 0.4.

Decreasing air intake restriction won't do much for mileage, since even stock, there's too much air at cruise speed (which mostly determines mileage). Under boost, you might get a little less backpressure if it can draw and flow more, but it would be small. Most of the gain would be at peak power when airflow is highest, turbo is fastest, and a little more air might cool things off or burn a little better.

Exhaust system is helpful since backpressure not only causes pumping loss (loss of power and econ), but the backpressure itself increases "internal EGR", the exhaust gas still left inside the cylinder when the exhaust valve closes. More residual pressure means less air taken in during the intake stroke.

The turbine section of the turbo is also alot more efficient with low backpressure especially at high load.

OK, its starting to make sense now. I took my boost stick off yesterday based on what I read about your economy tunes. I was making 6-7 psi boost at 65 on the highway with the BS installed. Now I will see if my mileage increases any with the lower boost. I am very close to getting EFI as well, and that may be the best way to get better economy (with an economy tune). Thanks for the explanation.

Citi1991,

At one point, did you post your calculated equivalence ratio? I have a calc_pids.txt file that has an entry for Equivalence Ratio Calculated as: ({GM.FUELQ_MAIN_DMA}+{GM.PILOTRATE})*0.012236/{CALC.CYLAIR}

Is that the equivalence ratio you are referring to?

TIA,
kbk

citi1991, at one point, did you post your calculated equivalence ratio? i have a calc_pids.txt file that has an entry for equivalence ratio calculated as: ({gm.fuelq_main_dma}+{gm.pilotrate})*0.012236/{calc.cylair} is that the equivalence ratio you are referring to? tia,
kbk this is the one that i have. :1 = ({gm.mainbpw}+{gm.pilotbpw})*sqrt({gm.frpact})/({calc.cylair}*16115)
is valid

The one using fuel rate and airflow was mine. It assumes that when you request 50 mm3, you get 50 mm3.

The other one (with the square root, and using pulsetimes) was from fingers. He tries to estimate fuel flow using pulsetimes, since they are changed when you add hp, and the requested and actual fuel flows will be in error in the extra-hp-zone.

I still use the flow version since I think it's more accurate with stock pulsetimes, but the disadvantage is that it is wrong when you are in the cells with modified pulsetimes.

His estimate is better in the high-power zone.

Thanks for the clarification. I had not even thought about the implications for modified pulsetimes. Makes sense though.