[Gmecm] Dare we revisit fuel economy ?

[Russ Harms]

I'm hoping for some answers to a few
> questions re: Highway Mode. From those who've tried
> it,
>
> 1. Does it actually make any difference in economy?
> If so, what sort of percentage was found?
>
> 2.  How dangerous is it, in terms of engine damage?
> Can you really go so lean you have misfires (even
> though that's counterproductive) without melting
> things?
>
> 3.  Do people enable Hwy mode forever, or are you guys
> finding it causes problems of some kind?
>
> 4.  Is Hwy mode usually a "stand alone" part of the
> PROM, with distinct fuel and spark maps, or does
> messing with it change other things as well?
>
> Thanks for helping me out with the truth regarding
> something that seems almost too good to be true when
> you first hear about it!


I have highway cruise mode enabled on my 403 Olds '730 ecm powered 
motorhome.

1) I think the fuel economy is a little better but I haven't made any long 
trips to get any good data, at ~12 mpg I don't expect a big improvement.

2) This a really big but universally held misconception that lean mixtures, 
in and of themselves are damaging to engines, the real villain is detonation 
not lean mixture, in fact according to Lycoming, once a lean enough mixture 
is  reached ~16 to 1, detonation is no longer possible let alone likely. It 
is the transition around 14.7 to 1 that is the most likely cause detonation 
damage under heavy load.
GM designed highway cruise into there ECM's  but never, AFAIK, enabled it on 
any car because of emissions, specifically the catalytic converter needs to 
be ~ 14.7 to do its job.

3) No problems so far, this is a 100,000 mile engine pulling a 10,000 lb 
vehicle.


Russ Harms