TBI vs Port injection

[Gaz64]

Originally Posted by eschaider

The low speed idle is not that difficult to manage with large injectors if the tuner is willing to do some work in terms of getting the injectors properly characterized.

I use dual 2200cc injectors on a 'little' 281 inch SVT Cobra race engine. We needed the dual injectors because at high boost, on methanol,l the engine's appetite for fuel substantially outstripped a single 2200cc injector's capabilities. After a day racing and before the car is put up each evening we drain the methanol, put in gas and start the car on its gas tune to flush out any remaining methanol to protect the fuel system.

Even with dual 2200cc injectors the car idles around 1000 rpm and is very drivable on gas with the big injectors. The drivability issue revolves around having properly characterized the injectors using SAE's J-1832 low pressure gas fuel injector standard and then properly using that data in the tune.

This J-1832 characterization gives the tuner the ability to know exactly what the injectors do, rather than assuming generic published data for the injectors. The differences especially in terms of the low slope or idle range of the injector can be substantial. Knowing the injector flow characteristics in that range goes hand in hand with good idle and off idle throttle response.

For a n/a engine I usually recommend sizing the injector to the owner's target power level and adding another 15% of head room for 'good air'. For a supercharged engine you want to determine the maximum useable boost the supercharger is capable of and size for that boost even if the owner is claiming he is not going to go that high — he eventually will.

If you don't build in headroom for increased boost, the owner will finding himself spending lots of money to take an incremental but necessary step up in injector size to accommodate his new found boost levels. Of course even at the high boost levels you want to leave another 15% of head room for those 'good air' days.

The biggest challenge with idling the engine on big injectors is the low slope flow characteristics of the injector. It is tedious and uses up a lot of dyno time to map out the fuel and ignition tables at engine idle and just off idle speeds for varying load and voltage situations. The tuners ought to be doing this as SOP no matter what injector is being used. Sadly some tuners will skip over this because of the drudgery and the 'lack of time' to get the injector characterized.

Most injector flow tests are done as a static max flow exercise. Dynamic flow tests are run across a wide range of pulsewidths and voltages to properly characterize an injectors’ behavior from its initial point of opening, through its low slope nonlinear range, up into linear and finally static flow.

The availability of dynamic flow test data in the hands of a tuner who knows how to put it to use brings with it a much better drivability with significantly improved low speed and mid range throttle response.


Ed

I like reading your replies Ed. I hope others are absorbing your writings as well.

Gary