You’re going to see a gain if your new injectors mix the correct amount of fuel into the air better than your old injectors did.
An engine needs about 1 part of gasoline for every 14.7 parts of air for the mixture to burn properly. If you’ve floored the accelerator and the engine is spinning at 8000 rpm, you’re pumping as much air through the combustion chambers as you can. Your injectors need to flow enough fuel to keep up with that. If you inject even more fuel than your engine needs, you’re going to see a drop in horsepower after a certain point.
The higher the injector’s flow rate, the briefer the period of time it’ll need to be open to inject the proper amount of fuel. At full throttle, injectors with too high of a flow rate will be yawning with boredom and at idle, they may be open for so few milliseconds that they don’t atomize the fuel well anymore. If your engine isn’t inhaling enough air to need 440cc, 550cc, or 725cc injectors, installing bigger injectors isn’t going to get you a horsepower gain but it may mess up your idle.
Modern injectors can atomize fuel better and can have a better spray pattern than the OEM injectors in our engines, which are based on 1980’s technology. Better injectors can get you more horsepower even if they have the same flow rate as stock.
If you want to upgrade your injectors, you could try to figure out what flow rate you need and which injectors give you the best spray pattern and atomization. Honda installed 240cc injectors into NA1s with 270 hp. So Honda’s Formula 1 engineers felt that about 0.89cc’s per horsepower was enough for an NSX engine. Multiply your crank horsepower figure by 0.89 to see what minimum flow rate you should shoot for.
The next issue is spray pattern. Ideally, it would be good to have an injector that sprays fuel down the backs of the intake valves without wetting the walls of the intake ports. Given the geometry of an NSX’s cylinder head and the positioning of the injectors, each injector should ideally spray two cones of fuel (because there are two intake valves per cylinder) with a total spray width of about 30° to achieve that.
Then we come to atomization. Modern injectors with multiple holes in the spray nozzle can atomize fuel better than older single hole injectors do.
So what injectors give you the best flow rate, spray pattern, and atomization – irrespective of cost? Six 410cc RDX injectors flow enough fuel to support 460 crank hp in an NSX using the OEM NSX injector duty cycle. That’s more than any naturally-aspirated NSX using the stock intake manifold is likely to need. Their spray pattern is just about ideal given an NSX’s cylinder head and intake manifold geometry, they atomize the fuel well, and despite their size, seem to give a stable idle.
There may be an even better injector out there for naturally-aspirated NSXs with better atomization, a lower lag time, perhaps a lower flow rate, etc. If you hear about a better injector, whatever the price, please post about it.