I think the fuel sitting on the valve, can evaporate long before the intake valve opens, and some of of the fuel escapes, back into the manifold, or even out the throttle body sometimes, due to the reversion of the cam (bigger cams can spit fuel back into carbs etc...). The stock cam probably sips the intake runner effectively enough that the high column of vaporized fuel hanging in a well designed runner tube doesn't get much a chance to escape. I tried the 6.5~range settings with the OEM camshaft over a year ago and noticed very little if any difference so I put it back to stock 5.55 until recently.
So it would seem a general idea then emerges for those of us running shorter-modified runner manifolds and larger than stock camshafts, that is, to inject the fuel as late as possible in order to conserve it within the chamber or runner (direct injection is the next step) thus maximizing fuel savings.
couple risks
Since we cannot inject at very high pressure (1500psi or whatever) there is a risk of
fuel puddle if the fuel is injected cold, and the engine is cold. Furthermore there is no way to tell when the intake valves are
'very hot' or the engine is actually vaporizing fuel easily to avoid fuel puddling. Some of the fuel injected may be integral with
cooling the valve or have something to do with long term
stability of intake valve. I noticed they look quite different engines. You could say "but Civvie the exhaust valves do just fine" and to that I would reply "yes, but, we don't know if the intake valve manufacturer or engineers who designed the engine used different materials or science behind the design of the intake valves that depends on the cooling effect interactions of fuel vaporization"
I just
don't know is all I'm saying, from random search [
https://www.quora.com/What%E2%80%99s...exhaust-valve]
also random articles here on this site can give insight "valve design" if you had time, like this one
http://www.sbintl.com/tech_library/a...and_alloys.pdf
Fuel puddle/cling can remove oil film from surface of cylinder walls, it may facilitate engine cylinder bore wear. The rate of vaporization can vary and there maybe tipping point where so much fuel cools things down so far that sudden fuel begins to cling and puddle. The most annoying part of these risks is we have very little insight as to how well(how fast) and where/when the fuel is vaporizing from outside the machine.
technology
some computers can adjust injection timing per RPM, not just temperature.
Injectors will be starting to inject fuel much earlier at high RPMS anyways if it's anywhere near WOT at high duty cycle
In other words there is no way to avoid spraying fuel to the back of an intake valve at WOT if the injector gets to a high duty cycle. I think the risk at WOT is minimized for a variety of reason and this injector spray setting is mostly for cruise/idle conditions. You
want the late injection to keep the fuel where it belongs near idle and cruise when fuel mass is low and easy to vaporize.
experiment
First I tried having all the fuel in just a ways after overlap, 6.5 to 6.7 normal with the usual boundary 6.5.
Noticed immediate improvement in a/f ratios
(richer) in cruise and idle regions when fully warm with the mild cam TFS-30602001 @ 5.3L
Then I took my cam card and found advertised duration rough intake valve opening point 327* (open to .050" by 2*ATDC) and put my end of injection right near that,
so (
5.9+6.5)*90-786 =
330*
That way the fuel is injected later than factory, but still has a brief moment to touch and cool the intake valve just before it begins to crack open as the piston is moving up, which I think pushes the fuel away from the intake valve slightly for an instant. Then hopefully it can glide through overlap before any fuel is in the cylinder.
And I am still rich everywhere. I'm pulling fuel out of the VE map around 2% VE (a fuel savings of roughly 5%) some spots. I think this later spraying is conserving fuel within the intake runner and it's early enough that I keep most of what I spray now AND it still hits the back of the intake valves which I can tell by looking down the runners has a slight cleaning effect as well. I can't wait to test fuel economy difference next.