If I remove the SAI system...
03-13-2009 | 12:38 PM
#17
Addicted Specialist
Rennlist Member
Rennlist Member
Joined: Jun 2001
Posts: 6,185
Likes: 386
From: So.CA
Ooooh, great pics ...drooool! 
Dang, I wish this thread had been up when I did my clutch!!! I just replaced the clutch a couple of months ago and did a bunch of engine-out maintenance items, one of which was addressing some of that air-pump crap (which literally disintegrated as y you touched it)! Ugh!! Oh, well, the next time the enigne comes out ...
Great post. Any pics of the SAI-ectomy??
Edward
Dang, I wish this thread had been up when I did my clutch!!! I just replaced the clutch a couple of months ago and did a bunch of engine-out maintenance items, one of which was addressing some of that air-pump crap (which literally disintegrated as y you touched it)! Ugh!! Oh, well, the next time the enigne comes out ...
Great post. Any pics of the SAI-ectomy??
Edward
03-13-2009 | 01:08 PM
#18
Seared
Rennlist Member
Rennlist Member
Joined: Jul 2005
Posts: 16,787
Likes: 419
From: Ellicott City, MD
Andreas
03-13-2009 | 03:07 PM
#19
Thread Starter
Race Car
Joined: Sep 2003
Posts: 3,928
Likes: 3
I am just obsessed with yellow zinc
, so I figure while I'm ripping everything apart, I might as well protect the nuts and bolts...
03-14-2009 | 02:27 PM
#23
Drifting
Joined: Apr 2008
Posts: 2,650
Likes: 3
From: Calgary, Alberta
Jon:
It stands for secondary air injection, and is a system that keeps emissions down for a few seconds after engine startup. Just search SAI here, and you'll get the whole sad story.
If you do an advanced search, you can restrict it to 993's.
It stands for secondary air injection, and is a system that keeps emissions down for a few seconds after engine startup. Just search SAI here, and you'll get the whole sad story.
If you do an advanced search, you can restrict it to 993's.
03-14-2009 | 03:10 PM
#24
Rennlist Member
Joined: Jan 2005
Posts: 1,045
Likes: 19
From: SLC, Utah
My personal opinion is that people who disable a functioning SAI are the same as industries who knowingly cheat emmision laws.
03-14-2009 | 03:40 PM
#27
Rennlist Member
Joined: Jul 2007
Posts: 5,942
Likes: 154
My Dad used to say, "Steal $5, they throw you in jail. Steal $5 billion, they build statues of you". I used to think he was crazy. Now, when I read the papers, I realize he was right.
03-14-2009 | 06:32 PM
#28
Rennlist Member
Joined: Jan 2005
Posts: 1,045
Likes: 19
From: SLC, Utah
One that isn't functioning is already disabled. I would repair it. If it cost a head job to repair it I might wait until the heads had to come off for some other reason.
But I don't think it's right to take the functioning system off of the car and make your emmission system less effective. Depending on how you drive your car you may prematurely plug up the catalytic converters too.
It's obvious that few people understand what the SAI does. If you understand how the system works and you still choose to take it off then we just disagree. Here is a post I made in an earlier thread
But I don't think it's right to take the functioning system off of the car and make your emmission system less effective. Depending on how you drive your car you may prematurely plug up the catalytic converters too.
It's obvious that few people understand what the SAI does. If you understand how the system works and you still choose to take it off then we just disagree. Here is a post I made in an earlier thread
Originally Posted by hoggel
A catalytic converter uses a catalyst to convert nitrogen oxides, hydrocarbons, and carbon monoxide into nitrogen, oxygen, carbon dioxide, and water. For the catalyst to be effective it must be at a relatively high temperature. If the catalyst is not hot enough it doesn’t operate efficiently and the catalytic converter can get clogged with hydrocarbons. One of the ways used to get the catalytic converter up to operating temperature is the SAI system.
On cold start the secondary air pump is turned on by the ECU. The ECU also sends a signal to open a vacuum control valve that operates a vacuum valve allowing the secondary air into the plumbing. The secondary air is injected into the exhaust before the pre cat O2 sensor.
The ECU monitors the pre cat O2 sensor. It is a closed loop control system that adjusts the air fuel mixture to make the engine operate as close to the stoichiometric point as possible. On cold start the fresh air from the secondary air pump creates a very lean condition at the pre cat O2 sensor. The ECU compensates by injecting more fuel resulting in a fuel rich condition in the exhaust from the head. That unburned fuel in the exhaust combines with the oxygen from the SAI system and burns thereby heating the catalyst up to operating temperature.
So what happens if there is no SAI system? The catalyst takes longer to come to operating temperature, harmful pollutants are released into the atmosphere, and the catalytic converter can clog over time.
In an OBDII car the ECU checks for the presence of the pump. On cold start the ECU checks that the O2 sensor signal goes through the stoichiometric, lean, rich, stoichiometric cycle with the SAI.
It is difficult to kluge the system so that it will pass the SAI test because the O2 sensor signal is a variable duty cycle square wave. In order to fool the system a square wave signal must be generated that is timed with what would have been the SAI cycle. But the pre cat O2 signal is also used for normal operation of the car so the kluge system must be smart enough to fool the SAI check without impacting the normal feedback loop.
Some tuners of other marques have modified ECU software to delete the SAI check but that is a Federal offense. If you install a kluge in your car to bypass the system and present it for inspection it could be a Federal offense.
It is my opinion that 993 implementation of the SAI system is definitely a poor design. But, we should make every REASONABLE effort to keep the system on our cars operating properly.
On cold start the secondary air pump is turned on by the ECU. The ECU also sends a signal to open a vacuum control valve that operates a vacuum valve allowing the secondary air into the plumbing. The secondary air is injected into the exhaust before the pre cat O2 sensor.
The ECU monitors the pre cat O2 sensor. It is a closed loop control system that adjusts the air fuel mixture to make the engine operate as close to the stoichiometric point as possible. On cold start the fresh air from the secondary air pump creates a very lean condition at the pre cat O2 sensor. The ECU compensates by injecting more fuel resulting in a fuel rich condition in the exhaust from the head. That unburned fuel in the exhaust combines with the oxygen from the SAI system and burns thereby heating the catalyst up to operating temperature.
So what happens if there is no SAI system? The catalyst takes longer to come to operating temperature, harmful pollutants are released into the atmosphere, and the catalytic converter can clog over time.
In an OBDII car the ECU checks for the presence of the pump. On cold start the ECU checks that the O2 sensor signal goes through the stoichiometric, lean, rich, stoichiometric cycle with the SAI.
It is difficult to kluge the system so that it will pass the SAI test because the O2 sensor signal is a variable duty cycle square wave. In order to fool the system a square wave signal must be generated that is timed with what would have been the SAI cycle. But the pre cat O2 signal is also used for normal operation of the car so the kluge system must be smart enough to fool the SAI check without impacting the normal feedback loop.
Some tuners of other marques have modified ECU software to delete the SAI check but that is a Federal offense. If you install a kluge in your car to bypass the system and present it for inspection it could be a Federal offense.
It is my opinion that 993 implementation of the SAI system is definitely a poor design. But, we should make every REASONABLE effort to keep the system on our cars operating properly.
