Cam changes and fuel maps (theory)

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If you where to change the volumetric efficiency of your engine by a cam change but everything else remained constant would the car run rich? I changed my cam to a performance cam, the motor now runs very rich. Is this because there is less vacuum and the computer thinks the engine is under (more) load?

My original theory was that the computer would run lean with a more efficient cam profile.

Any thoughts?

 

I can change my fuel map but how much? (%)

 

In the old days I would just go up a few jet sizes so this seems backward.

[Owen]

It should run leaner as the bigger cam is allowing the car to breathe better. But because of the vacuum, maybe the computer is dumping in more fuel like you say. Is the computer programmable and does the system use a wideband O2?

 

How much fuel you add or subtract would depend on what air/fuel ratio you wanted at a certain rpm and vaccum. A good WBO2 is helpful to measure this, an EGT also helps. From what I hear, the "cheaper" ($150 or so) air fuel gauges are not very accurate, especially with a narrow band O2.

 

FYI...what I've learned from the people at FAST (Fuel Air Spark Technology) is "give the car (ECU) what it wants to make it drivable, powerful, etc. don't just rely on what textbooks say the A/F ratio should theoretically be".

 

Owen

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The computer is an old Accel DFI and uses a narrow band O2 sensor. The car can be run with the lap top on and shows all the sensors’ readings. I was interested in the theory to program some leaner maps to save time. I have to get in the narrow ball park to make my O2 sensor work. Right now things are too far off. The fumes are outrageous.

I suspect I will have to lean out the low MAP/ RPM map and enrich the WOT part.

[TimZ]

The computer is an old Accel DFI and uses a narrow band O2 sensor. The car can be run with the lap top on and shows all the sensors’ readings. I was interested in the theory to program some leaner maps to save time. I have to get in the narrow ball park to make my O2 sensor work. Right now things are too far off. The fumes are outrageous.

I suspect I will have to lean out the low MAP/ RPM map and enrich the WOT part.

 

Your suspicions sound basically correct. If you have a speed-density system (MAP sensor), then ANY time you change the ve curve (it's really a curved SURFACE, not a line, and definitely not a single number) you will need to recalibrate the fuel delivery table(s).

 

I'd be careful about trying to shortcut the tuning process by wholesale leaning out your fuel maps. I'm not exactly sure what you have available to you with the DFI, but if you can datalog, that would be how I'd go about figuring out how to change the fuel delivery tables. Don't just guess - that's for people with carburetors.

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Today I started out by bench testing the O2 sensor. Propane touch and a volt meter. It turned out fine. When I was done I found “ash†from the burnt off carbon and blew it off. This solved most of my problems right there. From there I was able to recalibrate my low end throttle map. It took a few hours and some bloopers by it’s done.

I will have to wait until the motor is broken in before taking on the WOT end of the map.

I think the main issue was the fouled O2 sensor. On start up there were some leaky injectors to deal with. Most of the fuel got dumped into the manifold and probably plugged the sensor with carbon…but I’m just guessing.