Anyone have any ideas why my fuel mileage went bad after I installed the Vortec heads? This is on a 95 truck, stock Vortec heads, GM Vortec intake manifold bored out for a big block throttle body.Stock BB injectors, 12 psi fuel pressure, base timing set @6deg. Mileage dropped from 16-20 to 10-14mpg.BLM's are right around 128.
Thanks

 

It has to be in your tune because you should have seen a 2-3 mpg increase does it feel like its pulling stronger?

 

Are you running a larger TBI unit (with your bored intake)? Are you running the stock cam? Vortecs will require different timing than the swirl ports that were on there. Your motor is out of tune and needs chip work.

 

Yes to both, stock cam and bored TBI. I'm still in the process of learning chip tuning, the guy that did mine said everything is correct because the BLM's are in the right range.

 

The BLMs are only a small part of the tune. The spark tables are much more difficult and objective. Tuning the part throttle VE is rather easy, because you have good feedback in the BLMs INTs, and o2 values. Its simple math from there.

Getting your spark table spot on, and your WOT fueling is a more difficult chalange.

FWIW, I usualy start with a base spark table that I made, and then adjust from there based on G-tech, and track data to develope the WOT timing curve. I then make sure sure I have a smooth timing table. With no knock, and a smooth idle. Then it is a matter of smoothing the timing curve.

AFTER the timing is ALL worked out, then I look at my part throttle VE tables, and recorrect them to the 126-128 range, and make sure I am in a pretty good ball park to what the car wants.

Then I visit WOT fueling, using a WB02 sensor, I nail everything down at about 12.8 AFR across the entire RPM range. I make sure I do this testing in at least 3rd gear, so that I know the engine is fully loaded.

Then I look at pump shot, pump shot tuning on the stock computers, is sort of a dark artform, a little black magic. Really, it is trying to use the WBO2, along with the feel of things, to make sure it is nice and drivable, and that I don't lean spike when I slam the throttle.

Once this is done, I hit the dyno with that, and make more timing adjustment, and more WOT fuel adjustments to make the MOST hp and tq. After this, I usualy go back and do part throttle VE changes again, since my timing may have changed.

Hope that all made sense. Once you have done that, you should be all tuned. With Hi-way mode active, I would expect 25mpg+ on the freeway, and a good running, good performing reliable tune.

 

I’d bet that you have too much timing, are getting some knock and the timing retard is killing your gas mileage.