Sizing fuel injectors to engine

[Anonymous]

Here is some food for thought: Many people with Multi-port injection seem to think that the 'in' thing is to replace a stock size injector (say 19 lbs. /hour) to a larger injector in the name of "performance"; e.g. going to an injector that is 24 lbs. /hour.

I'm aware that the engine will only get the fuel needed for stoich combustion - swapping injectors will simply decrease the "On Time" of the injector - provided the engine's PCM can compensate.

It seems to me, that when the injector shoots just before the valve opens, that the smaller injector will shoot a finer mist for a longer time, and the larger injector would shoot for a shorter time - more like a big burst.

My Question: Is there any advantage either way?

I know that a lot of systems "gang fire" the injectors - so it may be a bit before the injected fuel gets taken in, waiting for the valve to open.

Any thoughts?

 

[Anonymous]

The 300 is a batch fire system. Ford put in small injectors and overpressurized them to get the flow needed. So for a 300, the injector swap might help out even if you're stock because as injectors clog up or fuel pump gets older, it can't flow as well as it did so you end up leaning out. In any case, most injector swaps won't net you much if any power. And don't forget that you're usually swapping in new injectors, so they will probably spray cleaner than your old injectors anyway.


-=Whittey=-

 

[StrangeRanger]

the stock injectors are rated for 14lbs./hr. @ 39 PSI. On a 300 @ 58 PSI they put out 14 * (58/39)^0.5 = 17.1 lbs./hr.

If you assume a BSFC of .5 lbs/HP-hr. and a duty cycle of 80% then they provide enough fuel for:

17.1 * 6/0.5 * 0.80 = 164 HP.

Actually BSFCs can run a bit lower and duty cycle can run a bit higher so this is a conservative figure, but it is the "standard" for sizing injectors. If you're going to do any mods at all to the 300, you have to add fuel capacity. On an otherwise stock engine they give you nothing. Some people who have made the swap report better throttle response with the larger injectors but that is pretty subjective and may represent wishful thinking. The later 300s (mine's a 96 ans SEFI not batch-fire) have a knock sensor that pulls out timing at the first sign of detonation. If, and this is a VERY big if, the knock sensor comes into play when you go to WOT then it would make sense that more available fuel would prevent knock and keep the timing advanced. In that case there'd be some genuine performance gains not just seat-of-pants reinforcing wishes. I'd still call the injector swap something you do to support other mods, not something you do expecting it to add performance in itself. I've got all the parts accumulated but didn't do the swap till I figure out what I'm going to do about the cam and head.

 

[Anonymous]

That's kind of where I'm at - trying to sort out the facts from the hype, before I just start throwing time and money around.

The only thing I see that would make a real difference, so far, is if one scheme is better at atomizing the fuel better than the other.

In the real world, the difference in injector times is so short, any gain would be small, I'd think.

Just wondering. Thanks.

 

[xctasy]

The info that StrangeRanger uses is also found here. The link was posted earlier this year.

Enjoy it!

:arrow: http://www.ultrastang.com/Tech02.asp