Go Back   986 Forum - for Porsche Boxster & Cayman Owners > Porsche Boxster & Cayman Forums > Performance and Technical chat

Reply
 
LinkBack Thread Tools Display Modes
Old 06-11-2015, 09:19 PM   #1
Registered User
 
Join Date: May 2013
Location: Orange County, California
Posts: 266
Understanding Catalytic Converters

I'm trying learn more about the catalytic converters on our cars, and have a few questions.

Why are there 2 on each bank? Does the integrated cat function the same as the downstream cat?

Why is the integrated cat called the precat, even though it's the one monitored by the O2 sensors? Why have two but only monitor one?

Since they don't do the sniff test anymore, the downstream cat shouldn't make a much of a difference in terms of getting tested, right?
hancock1701 is offline   Reply With Quote
Old 06-12-2015, 09:36 AM   #2
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 5,100
Quote:
Originally Posted by hancock1701 View Post
I'm trying learn more about the catalytic converters on our cars, and have a few questions.

Why are there 2 on each bank? Does the integrated cat function the same as the downstream cat?

Why is the integrated cat called the precat, even though it's the one monitored by the O2 sensors? Why have two but only monitor one?

Since they don't do the sniff test anymore, the downstream cat shouldn't make a much of a difference in terms of getting tested, right?
the pre cat is closer to the engine and smaller, so it gets hot really quick on a cold start, lowering starting emissions.

Once the main or three way cat is hot, it has much greater conversion capacity than the pre cat, so it is the only one monitored.

Removing the main cat will lead to problems in two areas: The car will code as the O2 sensors have nothing to go on. With your CARB inspections, the cats have to physically be there, they actually look.
__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein
JFP in PA is offline   Reply With Quote
Old 06-12-2015, 10:36 AM   #3
Registered User
 
Join Date: May 2013
Location: Orange County, California
Posts: 266
Quote:
Originally Posted by JFP in PA View Post
the pre cat is closer to the engine and smaller, so it gets hot really quick on a cold start, lowering starting emissions.

Once the main or three way cat is hot, it has much greater conversion capacity than the pre cat, so it is the only one monitored.

Removing the main cat will lead to problems in two areas: The car will code as the O2 sensors have nothing to go on. With your CARB inspections, the cats have to physically be there, they actually look.
Just redid my homework, and I think I made a mistake. Haven't been down under the car for a while, but looking at the diagram from ********************************************, there are actually three O2 sensors for both the starter cat and the main cat on each bank, correct?
hancock1701 is offline   Reply With Quote
Old 06-12-2015, 10:58 AM   #4
Artist, 986S tinkerer
 
NewArt's Avatar
 
Join Date: Aug 2013
Location: Montreal, Canada
Posts: 1,529
The location of the sensors depends on the year. On my '01 S, there are 2 sensors for each bank, one before and one after the pre-cat, nothing attached to the 2nd cat. Often diagrams are drawn/labelled to cover more than one model and may show various possibilities.
__________________
James
2004 986 S
987 air intake TB and plenum, Mantissport 2 litre sump, UDP, Bilstein B6 with H&R lowering spings, re71-rs for track, other stuff.
https://www.youtube.com/user/12shoes2
NewArt is offline   Reply With Quote
Old 06-12-2015, 11:12 AM   #5
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 5,100
Quote:
Originally Posted by hancock1701 View Post
Just redid my homework, and I think I made a mistake. Haven't been down under the car for a while, but looking at the diagram from ********************************************, there are actually three O2 sensors for both the starter cat and the main cat on each bank, correct?
No, there are only two sensors on each bank, one ahead of the three way cat, and one behind it.
__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein
JFP in PA is offline   Reply With Quote
Old 06-12-2015, 11:49 AM   #6
Registered User
 
Join Date: May 2013
Location: Orange County, California
Posts: 266
Just want to make sure we're talking about the same things. You were saying the starter/pre cat gets hot quicker because it's closer to the engine, and once the main cat (closer to muffler) gets hot, it'll convert more than the starter cat. But isn't the pre cat monitored by the computer with two sensors? There isn't another O2 sensor after the main (2nd) cat to monitor its performance, so how does the computer know it's there?
hancock1701 is offline   Reply With Quote
Old 06-12-2015, 12:00 PM   #7
Registered User
 
The Radium King's Avatar
 
Join Date: Apr 2010
Location: Canada
Posts: 2,888
the sensor before the pre-cat is monitoring combustion so as to control engine fueling. the sensor after the pre-cat is monitoring whether the pre-cat is working. there is no sensor for the main cat.

the pre-cat, and secondary air injection system on the engine, are north America specific systems due to higher start-up emissions standards. you typically don't find these systems on 'rest of world' cars. that's why one of the tricks if putting catless headers on your car is to get a 'row' flash, because the computer no longer monitors whether the pre-cat is working or not.
The Radium King is offline   Reply With Quote
Old 06-12-2015, 12:03 PM   #8
Need For Speed
 
KRAM36's Avatar
 
Join Date: Feb 2013
Location: Funville
Posts: 2,084
Garage
It also depends on what year your car is.

What year is your car?
KRAM36 is offline   Reply With Quote
Old 06-12-2015, 01:03 PM   #9
Registered User
 
Join Date: May 2013
Location: Orange County, California
Posts: 266
Quote:
Originally Posted by kram36 View Post
it also depends on what year your car is.

What year is your car?
2001, c.a.r.b.
hancock1701 is offline   Reply With Quote
Old 06-12-2015, 03:28 PM   #10
Need For Speed
 
KRAM36's Avatar
 
Join Date: Feb 2013
Location: Funville
Posts: 2,084
Garage
Yours should be like what NewArt posted. Nothing is monitoring the back cats.
KRAM36 is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -8. The time now is 09:12 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page