... engines are engineered to dump extra fuel (~12:1) in when a large demand is placed on them (such as at WOT) in order to keep temps down to prevent detonation, overheating etc. …
Also, at idle, AFRs are approaching 20:1 in stock engines now-a-days, to keep pollution levels down. …
Yes, you’re right – an air/gasoline mixture doesn’t burn only when the ratio is exactly 14.7:1. That is the “ideal” stoichiometric ratio but gasoline will burn when the air/fuel ratio is within a certain range.
As you said, when a large demand is placed on the engine such as at Wide Open Throttle, engine management computers adjust the air/fuel ratio to be richer than 14.7:1. And in light load situations, many modern cars adjust the air/fuel ratio to be leaner than 14.7:1.
In our NSXs, I believe the stock EMS doesn’t adjust the air/fuel ratio to be leaner than 14.7:1. In light load situations, I think it monitors the output from the oxygen sensors and adjusts the short and long term fuel trims based on that to maintain an air/fuel ratio of 14.7:1. Adjusting the fuel trims based on the output of the oxygen sensors is called “closed loop mode”.
In high load situations, the EMS no longer targets 14.7:1. It wants to go richer. Since NSXs come from the factory with narrow-band O2 sensors that can only tell you whether you’re above or below 14.7:1, they’re pretty much useless if your target is, say, 12.6:1. So when the EMS targets air/fuel ratios richer than 14.7:1, it goes into “open loop mode”. It no longer uses the signals from the O2 sensors to determine how much fuel to inject but simply looks at the base maps programmed into it and at the fuel trims it learned in closed loop mode (which compensate for the fuel pump perhaps slowly getting tired, the fuel filter getting a bit stopped up, etc.).
In any case, when you’re driving your car, the EMS (stock or aftermarket) targets a certain air/fuel ratio, whether it’s in closed loop mode or open loop mode. Your injectors have to be able to flow enough to meet that target.
Installing larger injectors can help if the engineers who designed the car really screwed up and installed injectors that were too small. However, what are the chances that Honda’s F1 engineers specified injectors that weren’t able to meet the fuelling requirements of a stock NSX engine, didn’t catch that mistake over the car’s 15 year production run, and that the engine turned out to be pretty much bullet-proof anyhow? I think larger injectors in and of themselves aren’t going to help in a stock NSX engine. Better atomization, better spray pattern, etc. – I can see that helping, though.
Will there be anything to gain by only installing the injectors and NOT going through any tuning. In other words will there be any gains with installing the RDX injectors and running stock ECU maps?
Installing larger injectors without a retune isn’t going to work well. The engine management computer doesn’t tell the injectors how much fuel to inject. It tells them how long they should be open – what their “duration” should be. The injector’s nozzle size, the fuel pressure, and the duration taken together determine how much fuel is injected. The stock injectors in an NA1 inject 240 cc’s of fuel in some time period at some fuel pressure. In the same time period at the same fuel pressure, RDX injectors inject 440 cc’s, I believe. So when installing RDX injectors into an NSX, those injectors should be open only 55% as long as the stock injectors to maintain the targeted air/fuel ratios.
If you don’t change the duration, your air/fuel ratio will be something like 6.9:1 or 8.0:1 instead of the 12.6:1 or 14.7:1 you want to see. At an air/fuel ratio of 8:1, you’ll lose power, foul your spark plugs, and get black smoke out of your tailpipes. If you can get the car to start, that is. The stock engine management computer can adjust the fuel trims in closed loop mode and then keep applying those trims in open loop mode but I expect 440 cc injectors are going to be beyond the limits of its adjustment capability. And I think most aftermarket engine management systems don’t apply any fuel trims at all in open loop mode, so then you’re really screwed [edit: if you don’t do any tuning for the new flow rate and if you’re still using the stock narrow-band O2 sensors].
