No Subject

[Rich Mauruschat]

>From: martin at mgass.demon.co.uk
>Date: Mon, 15 Dec 1997 17:34:38 +0000
>Subject: Re: oxygen sensor readings

>On a vehicle with a closed loop system and cat what is the likely 
>effect of removing the cat and replacing it with a straight pipe ?
>I reasoned as follows :
>a) removing cat will alter back-pressure
>b) back pressure alteration will affect air-fuel ratio.
>c) in a closed loop system the lambda will detect the alteration of 
>air-fuel ratio and instruct the intake to correct ?
>d) this may cause a gain or lowering of power and torque depent on 
>the engines characteristics.
>e) What effect will the EGR valve now have if deployed by the ECU ?

Martin,
My opinions as follows;
a)      Yes possibly to some extent, may not be very significant compared to
whole exhaust system, modern catalysts don't appear to be as restrictive as
they used to be provided they are designed and sized correctly. ??is this
still true with turbo applications??

b)      I'm not sure this is strictly true, the back pressure variation
which may occur will affect the ability of the engine to breath, it need not
affect the afr. 
??anyone any comments or experience on this??

c)      Yes, if the control system remains unaltered with the lambda sensor
retained, the system will still operate in closed-loop and isn't aware that
the catalyst is missing; with due consideration of any small benefits in
airflow at (a) the fuelling and ignition timing will not be altered.

d)      Because of (c) the management sytem will still maintain fuelling
around lambda=1 and consequently no change in engine characteristic would be
neccessarily apparent.

e)      Same effects as before conversion I guess.

The benefits in the de-cat process are to be had by running the engine in
open-loop so the fuelling and ignition timing can be optimised without any
consideration of the stringent requirements of the cat. to operate with AFR
lambda=1. To do this the lambda sensor and closed-loop control function has
to be disabled in the ECU. The benefits are unlikely to be great at WOT, as
the system would normally run open loop anyway under these conditions even
with the cat. 
Any comments on the above welcome - I am always willing to learn from
others' experience; mine is limited to small capacity (2L) 4cyl. multipoint
port injection engines, where I've seen very little difference in
performance without the cat. fitted when the control system is not modified
to suit.
Hope this helps.
Rich