Injector sizing

[zigzzagz]

On another forum I attend someone brought up an interesting point about injectors and sizing, and the biggest not necessarily being best which of course is sort of self evident as the 'right size' is always best, but he brought up a few good points I hadn't thought of. I'm using the 6.19gm/sec injectors in my 124" twin cam, they were the ones to use according to those who were building the succesful big inch motors when I was putting together my parts list a few years back. I've learned recently from talking to T-man that he no longer uses these but uses a somewhat smaller injector, the V-Rod injector as he says it helps in tuning at idle and low rpm. Well this person on my other site who also tunes bikes was having some difficulties in tuning a 124" due to the large amount of fuel these injectors provide. This bike had the zippers thundermax setup and he was using the 120" map and he had to take out anywhere from 15 - 25% fuel to get it to run properly. I'm assuming that you can set injector size with thundermax and this was not his problem. Anyways he suggested that with a smaller injector under more pressure I could get better atomisation and less blow through for a better burn. Are these injectors maybe too large, and perhaps leaving fuel unburned because of it, or is it unlikely they could hurt peak numbers. I do know that at one point HPInc was at least expirimenting with higher fuel pressures with some success. Anyway, it's just an idea that I thought might have some merit and was worth pursuing.

[Mike]

Actually T-Man's use of the V-Rod injector came from converations with us. This is what we use in our t-bodies from 130-160hp "turquiose band", above 160 the "yellow band" HP Inc/Ducati injectors or "blue band" Destroyers may be more appropriate. I do not know the specified flow rates for these and there can be up to a 20% variance in flow from specification. Because of this variance, many racers send their injectors to people like RC Engineering to have them calibrated to flow and spray the same (by the way, the O2 sensors in 06 dyna's are there mainly to account for this variance, among other manufacturing variables). Ideally, you want the smallest size that will satisfy the hp needs at sea level. This is an effort to get the best low speed performance and drivability. Basically, controling flow in a garden hose vs a fire hose is easier and more accurate. Trying to control a big injector at idle can be weird which is why some import bikes and cars (and NOS/Turbo systems) use primary and secondary injectors (there are some aftermarket dual injector systems for H-D also, dual meaning two per port or runner). Big injectors tend to have only one orifice making contor even more difficult at low pulse widths. Fuel pressure increases are one way to increase flow but only if the max capable flow is not acheived with the stock pressure (we use an overpressure regulator in our AMA XB). If max. flow is achieved then increasing pressure will increase the force of the spray but not the volume. Atomization is also a design function based on the number of orifics and spray angle/pattern but can be improved on some injectors with increased pressure. The correct thing to do is use the design specified pressure and a correctly sized injector for the application.
Regards, Mike

[zigzzagz]

So as long as I have good low speed drivability there would be no specific benefit in changing the injectors in regard to a possible gain in power.?

[Mike]

Correct. If you are not experiancing idle or drivability problems (esp. around 1800-2200 rpm) then no need to change. Control at high rpms is not normally an issue so no power gains available (only when they are too small).
Thanks, mike