"dan martin" <nospamdwmartin@idirect.com> wrote in
news:FK6dnQy8k86uA47fRVn-pA@look.ca:

> I beleive it's due to less back pressure. I have seen my fuel consution
> skyrocket from a open straight pipe.
>
> Dan
>
> <KnifeKnut@gmail.com> wrote in message
> news:1108522596.679172.14270@f14g2000cwb.googlegro ups.com...
>>
>> vtecracing wrote:
>> > Yes, a cracked exhaust manifold will result in excessive gas
>> > consumption.
>>
>> Why?
>>
>
>

On a o2 sensor equipped car, it will make a difference. Exhaust "pressure"
is not constant, a good part of the time when not at high throttle
settings, a negative pressure exists in the manifold . Air enters through
the crack, and the oxygen in the added air is read by the o2 sensor, and it
compensates by adding more fuel to try to correct the "excess" oxygen. I
have not had direct experience with it but a friend had a Fiero, his
stainless headers cracked constantly, and he said he could tell by
monitoring his performance and gas mileage.
Scott