Quote:
Originally Posted by gmonsen
I don't know what types of injectors are used and what their flow rates are, but there could well be problems with this approach, because the secondary injectors should have more flow than the primaries and some injectors don't work well when the duty cycles are very low, which they would need to be down low in the rpm range. The whole premise of the motor was to make a lot of torque down low and yet have the air flow and fuel to make max power at the high end of the rev band. The approach you are thinking about will probably reduce torque at the bottom end and might add some to the mid-range. The top won't change at all. I would think trying to open up the secondaries a little sooner and retune would be a better approach.
Gordon
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Hello and welcome!
Moving from hypothetical to empirical LT5 (secondary facts and characteristics), I found these departures from the "textbook" as you may soon discover (hopefully):
- Both injectors are 21# ers.
- Short PW issues (apparently) become more prevalent with higher flowing injectors and or fuel pressures. I, and many others running sans secondaries can put to rest any concerns about these injectors able to operate at low PW under low throttle demands.
- In short, the secondaries open along an inverse, exponential curve (not a straight line) according to rpm and throttle position, e.g, just off idle (1500ish) at 70% throttle will trigger the secondaries, but as rpm increases, the amount of throttle demand resulting in the secondaries opening drops dramatically. Furthermore, once opened, they are programmed to remain open over a wide range of operating conditions until throttle and rpm levels drop below a much lower threshold curve than was required to open them.
- Practically speaking, unless the power switch is turned off, mid range power comparisons with or without the secondaries installed is somewhat moot due to the fact that under loads where mid range torque was needed, the secondaries are already open!
- Ah, the plenum...AND runner length, & the velocity vs. volume thing; bring Helmholtz into the picture, and things get really interesting, huh? To each his own, but build me a system that will give me a flat torque curve from just off idle all the way to the rpm limit of the motor...and the more (torque) the better, of course, and you have my attention. But, if peak torque or hp is of ultimate necessity, then usually one or the other has to suffer somewhere. I hope you are willing to share the "Holy Grail", should you find it.
I'm gonna guess and say that most that have eliminated the secondary throttle plates for one reason or another probably have other flow mods in place. Such is the case w/ me. So, the two graphs below are "apples to oranges", far as comparing goes, but one is with my stock engine, and the other is after being fully ported, optimized exhaust, and tune.
I don't know what significant information can be gleaned from the results of these drastically different motors, but in the context of the general discussion, I thought they'd be interesting in their respective right, no?
My stock LT5 (note the leaning was due to the secondary fuel pump failing)
This is the same motor after being fully ported according to "FBI" models that demonstrate proven characteristics I particularly like.
And, rules and theories and empirical data that apply to 2-valve motors struggle to apply absolutely to 4-valve, dual intake runner motors - or at least to the LT5's design. For comparison, look at the characteristics of a stock(?) LS7, the torque and power curves in particular, compared to the (350 ci) LT5.
For a comparison of an LT5 a bit closer to the LS7's cid, there is this: a 415 cid (stroked and sleeved) LT5:
I've rambled too much (as I tend to do..)
P.