Ordered a new 347 shortblock, need bigger injectors?

dderek · Mar 23, 2007

well, since my stock bottom end 302 blew some headgaskets.
i ordered a fully forged shortblock from CHP, with flattop pistons.
so ill have a 347, 10.5:1~ish compression.. cnc ported/polished twisted wedge heads.. ill order a custom cam soon, intake, full ignition. etc etc.

anyways, on the old motor, i was running 24lb injectors.

i wonder if that will be enough for the new 347 thats coming in a few weeks?

what do you guys think?
it will be N/A, no turbo or s/c.. and no spray. at least no spray for now

poneypower89 · Mar 23, 2007

I believe you could possibly get away with the 24lbers...

Im sure there will be a lot of people telling you to upgrade... which you could also probably use. I guess the question is, how much do you want to stress your fuel system?

Aryan15 · Mar 23, 2007

Yeah, I think 24s will work. I'd buy one of those wideband a/f gauges just to be sure. The motor deserves it.

Grn92LX · Mar 23, 2007

No WAY are you going to get by with 24's!!! I have a similar set up but with unported TW heads and my 30's are near maxed out up top! 36# MINIMUM for your combo!If it were me, i'd go 42's

BlownFiveLiter · Mar 23, 2007

I have to agree with Grn92LX on this one. Those CNC ported heads should be moving some pretty good air. I'd vote for the 36lb injectors, but you could probably make due with some 30s and a slight bump in fuel pressure.

just4bob50 · Mar 23, 2007

Im having a Dart based 347 built right now that will have TW heads on it also that Im hoping to shift between 6500-7000 rpms. Im staying with my 42lb inj. so that way there is room to grow. Where did you get your head work done.

5spd GT · Mar 23, 2007

30's N/A.

What is your powerband, or where do you think you will be shifting?

Fuel Injector Selection:

Injector HP Ratings: divide flow rating by .5 and multiply the result by the number of injectors. The following examples use a 100% duty cycle. These ratings are for N/A engines.

Example:

19 lb injectors/.5 = 38 - 38 x 8 = 304 HP
24 lb injectors/.5 = 48 - 48 x 8 = 384 HP
30 lb injectors/.5 = 60 - 60 x 8 = 480 HP
36 lb injectors/.5 = 72 - 72 x 8 = 576 HP
42 lb injectors/.5 = 84 - 84 x 8 = 672 HP

The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

304 HP x .85 = 258 HP for 19lb injectors
385 HP x .85 = 326 HP for 24lb injectors
480 HP x .85 = 408 HP for 30lb injectors
576 HP x .85 = 490 HP for 36lb injectors
672 HP x .85 = 571 HP for 42lb injectors

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

See http://users.erols.com/srweiss/ to get the calculators used in these examples.

The above information is courtesy of jrichker!

Grn92LX · Mar 23, 2007

Yep listen to those with first hand experience 36's minimum!! Your combo will flow more air than mine and my 30's are near maxed out up top!!

5spd GT · Mar 23, 2007

The most accurate formula I have found by experience is this one:

Grn92LX's suggestion x .80 seems to be accurate.

24's would be great on my h/c/i car like on many. 30's are too big for it currently.

30lb injectors are 20% bigger than 24lb injectors.

347 cubic inches is 12% 'bigger' than a 306. Do the math

Grn92lx - A suggestion would be to increase your fuel pressure. Very simple fix.

To the user wanting to know, a big injector like 42's or 36's will give you a richer fuel curve from idle to desired rpm. This effect's power. If you have access to a good dyno tuner or software program like twEECer and you would have no problems.

My personal belief is do not stick anything on the car that causes some type of compensation to be made.

I like to have parts on my car that work, not work with some work.

If you just stuck 42lbers on your car with matching MAF and then 30lbers with matching MAF, and that is all, I guarantee the 30's will run better.

Good Luck! Sounds like a great combo!

dderek · Mar 23, 2007

looks like ill be getting 36lb injectors
i went to a shop north of me, and he shipped them out somewhere to be cnc ported.
i do have a tweecer, and a laptop, and wideband i will be welding in the exhaust somewhere.

so tuning should be nice, i hope.

5spd GT · Mar 24, 2007

Tuning will definately help you, good luck and post up dyno numbers when you get them

PM's are always nice.

Grn92LX · Mar 24, 2007

dderek said:
looks like ill be getting 36lb injectors
i went to a shop north of me, and he shipped them out somewhere to be cnc ported.
i do have a tweecer, and a laptop, and wideband i will be welding in the exhaust somewhere.

so tuning should be nice, i hope.

Find out where he sent them. If he sent them to like total engine airflow or fox lake you're going to end up with a KILLER sbf head!

Thats good that you have the tweecer stuff, it'll work out nice!

A little tid bit of info: Back when I had my 30's and pro m maf and NO tuning device the car would cold start like crap. Why? Because it was so VERY VERY VERY lean!!!! I had to add in +30 - +40% in my cold comp menu in the PMS to fix it. This is with both engines i've had them on

5spd GT · Mar 25, 2007

Idle vs. wide open throttle is much much different. The computer/pcm had totally different signals to send.

I'm quite sure the original poster is wondering what injectors his car can handle under stressed conditions, not 'how do they start up'.

My 30's however are extremely rich at idle. Makes my eyes water like I am cutting onions, but I love the smell

My car starts fine in the cold, when my IAC isn't clogged up.

dderek · Mar 25, 2007

just want to know what injector would be comfortable for the car.
not overkill, and not running lean.
i asked my friend to price out 42lb injectors for me.

5spd GT · Mar 25, 2007

As long as it has a 3x in the number your golden

To many people get caught up in 'more is better' or 'bigger is better'. I have found out that is not the case. Either by my car or my stang buddies in their quest to be quick and efficient.

There are many bias answers out there, just watch for them

An N/A engine of your size/flow will not need 42lb injectors, that is a guarantee. It will run with them though.

Are you running a stock block? Nitrous won't be much use to you if you are...

5spd GT · Mar 25, 2007

Here is some info from the 'new' Pro-M:

Why do you recommend some form of tuning for injector sizes larger than 42lb/hr?

-For two reasons:

1) The further you rescale the meter from the stock injector size, the lower the MAF voltage will be at idle. Once you go below a certain voltage, the computer will think something is wrong, and either go into limp-home mode or throw a check engine light. This will cause the computer to stop reading the MAF, and therefore assume the car still has stock injectors.

2) The further you rescale the meter from the stock injector size, the further the load calculations inside the ECU get skewed.

So with the TwEECer you will have, you can take care of that.

Basically what I am saying and what they are saying, is that when someone post to get a 'big injector', they need to specify to have a tuning device or get a tune. Otherwise it can be a headache later, and none of us want that.

Ordered a new 347 shortblock, need bigger injectors?

dderek

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