[Carl Summers]

Rich Mauruschat wrote:
> 
> >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??

Anything with more than 1 degree of overlap @.050" will see different
scavenging levels depending on exhaust system...and on a speed density
system this will cause a different A/F ratio....
Also regarding the egr function with less backpressure,,,,the OBD 2
systems will turn on the check engine light if you remove the cat and
don't rework the EGR tables in the chip....you can however change it
mechanically by getting a "universal" EGR valve, check the size of
restrictor you are supposed to have in that year,make,model and simply
install a larger restrictor to allow more EGR compensating for the
backpressure loss in the exhaust system........
-Carl Summers




> 
> 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