Fueling & Injection - Injectors?

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01-21-2007, 10:47 PM
Ok, I am sure this question has been asked before but here it is again. I am building my motor and trying to decide what injectors to go with. If I were to put in a 42lb injector and the motor only need say a 36lb is that going to affect me in any way or can the tuner just tune down the 42lb. I know shit about injectors so if anyone can clear this up for me I would really appreciate it.

01-21-2007, 11:47 PM
You would have no problem. The tune would compensate for it. It would also allow you room to grow instead of having to upgrade the injectors down the road (no pun intended).

01-24-2007, 01:15 AM
as long as pulse width required at idle is above minimal pulse width for the given injectors, you're good to go

01-24-2007, 01:28 AM
as long as pulse width required at idle is above minimal pulse width for the given injectors, you're good to go

What the fuck does that mean?

01-24-2007, 06:58 AM
Since you stated in the original post that you have no knowledge of injectors, that post was probably too technical for you.
What he was saying is that there is a minimum and a maximum time that an injector can spray. This is called the pulse width. It is how long the injector is open and spraying fuel into the cylinder. The longest pulse width recommended is 80%. If it exceeds that, you need a larger injector.
The computer will tell the injector how long to spray. If you put in too large of an injector, say 96 in an engine that only needs 28, the injector would not be able to go down to that small of a requirement. Think of it as putting a 1080 cfm carb on an engine that only needs a 250 cfm. It would just be underwhelmed and would not run.
In short, as long as the injector you put in there is close enough, even though it is too large, the computer can compensate.
Hope this helps.

01-24-2007, 07:23 AM
What the fuck does that mean?

It means that you have a lot of reading to do :)

Good luck with the car.

01-24-2007, 08:49 AM
It means that you have a lot of reading to do :)

Good luck with the car.

01-24-2007, 09:33 AM
I especially like the use of the f word when Supra was only trying to help. :eyes:

01-24-2007, 10:26 AM
This is the formula to help you decide:

(projected BHP x .5) / 6.4 = min injector to meet 80% duty cycle

01-24-2007, 11:45 AM
ok, i'll try to go slow, i know a guy who's screenname is similar to yours, so i assumed you know more.

injector sizing happens on two ends: idle, and wot.
at idle, you are limited by the physical ability of a given injector to open, spray minimal amount of fuel, and close. different injectors have different characteristics, but generally speaking, the bigger the injector, the bigger the minimal amount of fuel it wants to spray. if this minimal amount of fuel injected is bigger than the need for the fuel at the given airflow consumption, you will dump more fuel than it needs, cause carbon buildup, mess up spark plugs, eventually kill O2 sensors, and it generally going to run like poo since it's basically overfueling, which usual symptom is surging. with big cams, or any other 'race' modifications, the efficiency of the motor at idle range is significantly deminished, which needs a lot less fuel than stock setups. that combined with need for bigger injectors (and longer minimal pulse width) often clashes, as the bigger injector is not capable of injecting smaller amount of fuel, so it goes with the minimal amount it can do, and you're back to overfueling and surging. that's why you have to raise idle rpm on such setups--bigger cams get up to normal range efficiency very quickly with more rpm. so you basically have to figure out at what rpm the airflow requires more fuel than the minimal pulse width. there's a lot of variables involved, and few of them are linear, so this is not an easy task.

at WOT, the situation is different: you just have to make sure that at any given airflow, the amount of fuel needed isn't bigger than ~85% duty cycle of the injectors. on NA this is simple, as the airflow usually just goes up with RPM. on boosted setups, you can boost a lot more in midrange than up top with fairly small injectors--that's what the 'chipped' 1.8t audi/vw's do, 17psi at 4000rpm, and back to like 7psi by redline, so the dinky stock injectors have enough time to spray in required amount of fuel.

duty cycle is an often misunderstood. it's a ratio between pulse width required for proper AFR and the length of time you have to inject that amount of fuel, dictated by the rpm and the otto cycle. so 20ms pulse width at 3000rpm is just fine, because the 'window of opportunity' is 40ms. but the same 20ms pulse width at 6000rpm is 100% duty cycle becuase the 'window of opportunity' is also 20ms.
very often on these forums you get people that say dumb shit like 'my car runs just fine at 130% duty cycle, so the stock injectors must be underrated' what they really mean is their injectors dont have the capacity to provide enough fuel in the time alloted. so not only the ignition will happen without all the fuel necessary for given rpm, but the fuel is still spraying during the power stroke, and then turn off and recharge for another spray cycle half way through a cycle where they should be spraying fuel. so while one cycle might be fueled fine, another is gonna be lacking. it causes severely unpredictable fueling and AFR, fluctuating EGT's, and generally making it unpredictable and difficult to tune. in addition to that, you're stressing the injectors beyond their operational range, overheating them, and possibly causing a pre-ignition.

without proper injector sizing your car is going to be running like shit (at best), and possibly cause a meltdown (at worse). it's just not worth it. injectors are cheap, i will never understand people who are willing to drop 3k in heads that flow few cfm better than others, but won't even think of spending 300bux to make sure that the airflow can be matched with fuel. dont be a moron, be fast and reliable instead.

01-24-2007, 01:45 PM
I especially like the use of the f word when Supra was only trying to help. :eyes:

Obviously wasn't meant in a negative way...some people just talk like that

01-25-2007, 09:45 AM
Obviously wasn't meant in a negative way...some people just talk like thatGood point. I worked for the navy for 9 years, and it is true about the way sailors talk. I just try to be more careful when I don't really know the background of the people I am conversing with.

01-26-2007, 03:51 PM
I especially like the use of the f word when Supra was only trying to help. :eyes:

I was in no way trying to offend anyone. I just meant that was way over my head. I am not one of those dicks that ask for help and then turn around and insult you. Anyway I appreciate the help and the info.


01-29-2007, 07:09 PM
based on my your equation I would look at running 32 lbs injectors. I'm just cam only for right now, so its safe to say I can use 42 lbs injectors.

Is that equation correct/useful/accurate?

01-29-2007, 07:39 PM
for LSx applications it's really simple:
boltons, tiny cams: redtops
cam, heads, intake: greentops
up to ~10 psi FI or big cubes: 60s
beyond that it really depends as you gotta go to low-Z injectors

01-30-2007, 05:39 PM
RedHardSupra, thats good info. Thanks for posting that.

01-30-2007, 06:17 PM
Nice work RedHardSupra.