RE: Smog tests

["Ian Lomax" <sfalfisto@xxxxxxxxx>]

> > My '73 has the valve timing adjusted with a bit more 
> overlap than stock.
> > Under high loads and RPM, that can make it pull unburned 
> charge through
> > the exhaust right?  Could the opposite effect be obtained going the
> > other way?  
> 
> The answer is yes, but the reasons are backwards.  
> 
> The point of increased valve overlap is that the extra 
> air/fuel mixture is drawn into the cylinder at high rpm, and 
> then burnt in the cylinder, providing greater power.  The 
> downside of this is that unburnt charge is pumped out through 
> the exhaust at low rpm, giving high levels of unburnt hydrocarbons.  
> 
> To decrease unburnt hydrocarbons at lower rpms, the valve 
> overlap can be decreased, as you suggested.  This is 
> basically what the early Alfa VVT did, retarding the intake 
> cam, and so decreasing the valve overlap at  engine speeds 
> under 1800 (?) rpm.  
> 
> Regards,
> Anthony.  

And I'm pretty sure this is what they did on late 70's spiders, changing
the cam timing to improve emisssions.

Cheers,
Ian Lomax
71 Spider, 74 GTV, 75 Fetta GT
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