what do these emission numbers mean?
Rennlist Member
Joined: Feb 2004
Posts: 12,312
Likes: 10
From: Roseville, CA
Contrast that with an early 911's MFI or carb engine where we strove for 4-4.5% CO. You could literally gas yourself into unconsciousness tuning them if you didn't do it outside.
Thread Starter
Race Car
Joined: May 2005
Posts: 4,902
Likes: 13
From: The Bluegrass State
My last 2 CA inspections (which are pretty tough) were zero HC and zero CO so I don't think your result is a fluke. The fact that the SAI has been removed shouldn't matter as IIRC it only runs at startup so with a hot engine it's not a factor in emissions, only to the OBD II in the later cars which periodically checks to see if it's working.
To answer the OP's question, in a perfect world the only combustion products your engine would produce is H2O and CO2. That never happens as you get some incomplete combustion which produces CO and HC (partially burned and unburned fuel) as well as a few other byproducts (like NO2). The FI system should keep the CO and HC numbers pretty low if it's working right as it meters in the correct air fuel mix quite efficently. The cat is just a catalyst (duh!) that completes the combustion of whatever HC and CO comes out of the combustion chambers. The main function of the SAI is to provide extra O2 to help complete combustion in the exhaust port and help get the cats hot enough to start working. If all is working right, and up to operating temp, the cats should clean up most of what's left. However, add a little oil blowby from bad valve guides or a bad fuel air mix due to a faulty MAF or the like and the numbers go to hell. Cats can also wear out but generally last a very long time unless they are stressed from too rich a mixture (which can make them overheat) or fuel additives that kill the catalyst. When all is right your emission numbers should be at or close to zero.
To answer the OP's question, in a perfect world the only combustion products your engine would produce is H2O and CO2. That never happens as you get some incomplete combustion which produces CO and HC (partially burned and unburned fuel) as well as a few other byproducts (like NO2). The FI system should keep the CO and HC numbers pretty low if it's working right as it meters in the correct air fuel mix quite efficently. The cat is just a catalyst (duh!) that completes the combustion of whatever HC and CO comes out of the combustion chambers. The main function of the SAI is to provide extra O2 to help complete combustion in the exhaust port and help get the cats hot enough to start working. If all is working right, and up to operating temp, the cats should clean up most of what's left. However, add a little oil blowby from bad valve guides or a bad fuel air mix due to a faulty MAF or the like and the numbers go to hell. Cats can also wear out but generally last a very long time unless they are stressed from too rich a mixture (which can make them overheat) or fuel additives that kill the catalyst. When all is right your emission numbers should be at or close to zero.
Mike
