After seeing between 126-132 BLMs on the highway running 165, I enabled highway fuel mode and set my A/F ratio to 15.7:1 and was rewarded with 25MPG. I then swapped to 730, tuned my highway BLM to 127-129 and set my A/F ratio to 15.7 but can barely muster 18MPG. The enable temp, speed, and delays were all the same with both ECMs. Why such a hugh change?

 

With that manifold, gears, and cam, 18 ain't bad in my book. Was it the same setup with the 165?.

Speculation mode.
If the same setup then it would seem to justify the statement of MAFs being very filtered in steady state conditions. The MAF sensor is actually faster sensing then the MAP, but the down side is that it takes more filtering to get it to work. So you wind up with a more filtered signal. In this filtering, is where you get the *forgiveness* in how easily tunable they are.

Is the tubing from the manifold to the sesnor very rigid?. If your really to the stage of doing accurate tuning then you might even go with copper tubing to rid the system of any dampening of the signal to the MAP.

Might try doing an open loop chip, and leaning it down some that way.

Is TCC working the same?.

I could see a couple MPG difference but your's does seem excessive. One of those times, it really would be nice to fire up both ecms on an ecm bench and see what the differences really are.

Have you tried rolling back the timing a little?.

 

If you're crusing on the highway couldn't you just look at the "desired AFR" compared with engine speed (and vehicle speed) VS pulse width? The has to be a reason other than just the swap. Are you sure your injector constant is correct in the 730?

 

It is the same setup as 165. My vacuum line to my MAP is actually just a rubber vacuum line, is this affecting readings? My TCC is working the same with both setups also. As for timing, I was using stock ARAP timing on 165 and stock AUJP timing on 730, so there are some differences there. Unfortunately, I am unable to compare any scans from 165 because my laptop HDD just crashed, leaving me with only my most current bin which is in my car.

 

Working from memory here, but I'd bet the ARAP timing has a bit more advance then the AUJP.

Regular vac line can bellows as the engine runs, effectively dampening the MAP signal. GM always, well on every car I've seen either uses a hard plastic line, or mounts the sensor actually into the manifold to prevent that.

 

Just a thought but I had the same problem, alot of it had to do with me disabling the CCP causing the ecm to stay in cell 4 and never realizing it.

 

Highway fuel mode will not enable unless CCP is enabled anyway. This seems to be a different problem. I am going to track it with highway fuel disabled to see if there is any change.

 

My car is originally Sd equipped and ive run a lot of timing to roughly 34-36 degrees at the present for crruising area...

My mileage in town is roughly figure in the 16ish or so range. When i am on the highway i only get 18-19 mpg or so...I've tried everything in vain to raise it some more, including highway mode and its just like its stuck there(been out of the loop due to wiring issues with the formula, but still)

I do believe i should be able to muster mileage in the 20's on the highway without too much of an issue, trouble is i havent figured out how to yet(grumpy, you've seen my calibration)

I've talked to u before about this Plant P , wonder how other people with the hotcam and SD are doing? It really smooths out once u hit the upper teens and 2k range specially on the highway with the TC locked, dont even have to make the tach move to make the speedo climb in a hurry

The mileage mystery is just that for me as well hmmmmmmmmmmmm

my 2 cents if any helped
Jeremy
PS i dont have anymore bright ideas to try , seems like you've tried everything i have