Speed density uses a MAP sensor - manifold absolute pressure. It sits on the firewall, and a vacuum line connects it to the manifold so it can sense pressure in the intake. The same sensor in a mass air car is called the BAP sensor - barometric absolute pressure. It's NOT hooked to the manifold in a mass air car, and simply senses barometric pressure.
Simplistically, the speed density system looks at temps, throttle position, rpm and manifold pressure, and calculates (estimates) how much air is entering the engine, and from that it determines how much fuel to inject. It's fairly sensitive to changes that impact manifold pressure/vacuum. Many cams (including the E303) will cause a big change in idle and low speed vacuum levels. It's not uncommon that with common mods, and E303 cam will reduce idle vacuum from a stock reading of 18-20" down to 11-13". Less vacuum at idle means there is MORE absolute pressure in the intake. More pressure there (less vacuum) and the engine thinks more air is flowing than actually is, and it therefore injects more fuel than it should. Once the car goes into closed loop and the O2 sensors start to provide signals, the system can trim back the excess fuel, but there's a limit to how much adjustment it will make that way.
So, when you start changing cams, heads, manifolds on speed density cars you just need to know that the more you alter the pressure in the intake from the what the designers designed it for, the more difficult you make it for the system to provide accurate amounts of fuel. There are people running the E cam, but generally it's considered to be right on the edge of what you can get away with if you want decent low rpm/street manners. It depends a lot on the other components as well, and the way you install the cam. Advancing it on the install usually helps to increase idle vacuum levels a bit, so that can help too. See if some of the speed density gurus can chime in and give you some more counsel.
Generally, if you stick with improved flow in the manifolds and the heads, and leave the cam alone, the speed density system should manage just fine. Just be careful with the cam choice. Why not leave the cam stock and throw a set of 1.7's on it if you want a bit more 'cam'? In any event, the E is recommended to used with different heads, intake, exhaust and gears -- so unless you really know what you're doing with your porting, it doesn't sound like a good match for your other changes anyway, not to mention how the SD is gonna react.
Having said all that, hook up with the SD gurus out there. The system can support certain mods quite well - and there are plenty of really quick SD cars on the prowl. You just have to know what the limits are.
