Fuel Injectors: Why doesn't the math add up??

[Chris-Nelson]

Ok, so I bought the book "High Performance Fieros" by Robert "Greg" Wagoner. It's great, and it covers tons of info like turbocharging, the 3.4L PR swap, and a LSI swap.

Anyways, in it is the basic Fuel Injector formula used in calculating what size fuel injector to use based on your power output speculation or goal:



So, using this formula, I discovered that I should be using about about a 39 - 40 lb/hr set of injectors to reach my horsepower goal of 350crank horsepower.

Why is it that people on .nl talk about not needing to upgrade the injectors more than a 19lb injector when doing upgrades like turbocharging etc...

From what I've been reading of other peoples' experiences, they've been getting good power out of their engine, but on very small (relatively) injectors.

Do I really need to get a set of 40lb injectors if I eventually want to hit 300whp? Or could I swing it with 30lb/hr injectors?

help a brotha' out... :3some:

[Series8217]

From what I've been reading of other peoples' experiences, they've been getting good power out of their engine, but on very small (relatively) injectors.

Who over there (I assume you're talking about stuff you've been reading on NL..) has dynoed over 200 hp on a turbo OHV 60 degree? I haven't even seen a 3.4 turbo over there.. 2.8 turbo setups are making a little over 200 hp..

[Pyrthian]

From what I've been reading of other peoples' experiences, they've been getting good power out of their engine, but on very small (relatively) injectors.

Who over there (I assume you're talking about stuff you've been reading on NL..) has dynoed over 200 hp on a turbo OHV 60 degree? I haven't even seen a 3.4 turbo over there.. 2.8 turbo setups are making a little over 200 hp..

exactly. 4-6# boost. just over 200 HP's. 17-19# injectors....

[Shaun41178(2)]

The people on .nl aren't dynoing their cars so they have no idea what they are making. They also have no idea what their air fuel ratios either because they refuse to run a wideband.

So they aern't making good power. But I guess it depends on what your definition of good power is.

7 lbs of boost on a T3 turbo might get your 210 crank hp on a 2.8 3.4 mihgt get you to like 230 crank hp. 19 lb injectors can support that, but at nearly 100% duty cycle.

You don't want your injectors running like that.

If your goal is 350 crank hp, then you need the injectors to support that hp goal. Fuel in = hp out. If you dont' have the fuel, you can't make the power.

I would look for 36# injectors for 350 crank hp.

[p8ntman442]

redo your calcs with 100% duty cycle, and see if they are making the power by maxing their injectors. Thats what I assume.

[donk_316]

a safe assumption with a 6cylinder turbo engine is to say 350cc injectors support 350hp and so on.

[Series8217]

a safe assumption with a 6cylinder turbo engine is to say 350cc injectors support 350hp and so on.

considering that 6 cylinder engines commonly range from 2.5L to 4.3L i would say thats not an accurate assumption.

[The Dark Side of Will]

Horsepower = fuel flow. Displacement is irrelevant in that equation. You know this.

[Series8217]

Horsepower = fuel flow. Displacement is irrelevant in that equation. You know this.

Yeah I don't know why I said that. Brain fart I guess. Only # of cylinders should matter. My bad!

[Kohburn]

looking at the formula that you posted, those are the calcs for a 4cyl.

200hp on 6 injectors is 20.83 at 80% duty cycle.. 19lb injectors at 200 hp would be at 88% duty cycle

[The Dark Side of Will]

looking at the formula that you posted, those are the calcs for a 4cyl.

200hp on 6 injectors is 20.83 at 80% duty cycle.. 19lb injectors at 200 hp would be at 88% duty cycle

Haha! Here's to double checking the numbers! Good catch.

[BigRedDeckSpoiler]

Using that Formula, I would need 21# injectores to make 200 HP with a V6. As it is, even 19s are too big for most 3.4s without pulling out a lot of fuel.
Earlier, using the 2p8 w/15# injectors as a base for the calculations, I figured that 17s would be just a bit too small, but 19s would be a bit too large for a 3.4. That's assuming a linear increase in VE, relative to displacement. (I know better than that, but you've gotta start somewhere.)
If I was going to turbo a 3.4, I'd probably go with 24s, and would be very careful.

Look for old Dennis LaGrua threads. He's running a 3.4 with a turbo.

Edit - Ooops. Everyone in the damned world beat me to it.

[Dirty Sanchez]

Here is a link to the .pdf article written by Holley on selecting the correct injector size: http://www.holley.com/data/TechService/ ... jector.pdf

[Sinister Fiero]

HP / Number of Injectors x BSFC / Duty Cycle = Injector size required

Injector Size X Duty Cycle / BSFC X Number of Injectors = HP supported

(BSFC=Brake Specific Fuel Consumption. For N/A engines, this factor is typically 0.50. For SC/Turbo apps, this factor is typically 0.65. Duty Cycle is the maximum amount of on time the typical electronic fuel injector can sustain without going static, and this factor is typically 0.80 or 80%.)


lbs/hour = cc per minute / 10.2

New Flow = Old Flow X square root of (new pressure / old pressure) <-- If you want to find out how pressure changes impact the flow rate of your injectors. Remember, to double the flow thru a given oriface, you must have 4 times the pressure.

-ryan