My Fuel Injector Research

My Fuel Injector Research:
Tonight I set out to learn everything I needed to know about fuel injectors. As most of us know stock LT1 injectors are 25lb/hr injectors and will reach about 80% duty cycle on a stock vehicle which is already the safe limit, so before any serious modifications can be done you need to upgrade injectors. I know a popular injector choice is the Ford SVO injector, however these are expensive, and I am on a budget. I know from my research into the 3800 Series II S/C engines that they come with 36lb/hr injectors from the factory, and using stock injectors can be easily found at a reasonable price. Hence my plan was to acquire two sets of stock 3800 Series II S/C injectors and use the best matched 8 of them. I have an old fuel injector testing bench that I got from my neighbor, who in turn got it from an auction from a Grand National shop that went out of business. So tonight I removed the injectors from my LT1 and started testing them to see what they really flowed at. I set the control to 3000rpm at 16ms for one minute at 43psi, and got the fallowing results:
Injector………Flow
1……………..190ml/min
2……………..195ml/min
3……………..195ml/min
4……………..195ml/min
5……………..195ml/min
6……………..190ml/min
7……………..190ml/min
8……………..190ml/min
Okay, now to figure out what lb/hr and gal/hour that translates to so I know how to check the new injectors to make sure I set the tune properly. First step was to determine what duty cycle the test was conducted at. To find out the duty cycle I took the rpm and multiplied it by the pulse width.
3000rpm X 16ms = 48000ms or 48 second
48 of 60 seconds is an 80% duty cycle
So, now that I know our flow at the duty cycle I can find out what the flow is at full duty.
80 / 100 = 195 / X
80X = 19500
X = 243.75
Now I determined that my stock injectors would flow 243.75ml in one minute at full duty. To determine flow in an hour I simply multiply by 60.
243.75 X 60 = 14625
14625ml/hr = 3.86gal/hr
In the factory tune GM used 6.538lbs/gal, so I did the same.
3.86 gallons = 25.24 pounds
So my injectors ranged from 25.24lbs/hr to 24.61lbs/hr. That is only a 2.5% variance.
When the new injectors get in I’ll update this with the new numbers I get.

Cool.

My injector research part II

Okay, today the first set of 3800 series II S/C injectors showed up. The good news is they are the same length and have the same o-rings so they will hook right up. I have not checked to see if the clips will work properly with them yet. My flow testing, again at 16ms, 3000rpm, 43PSI, resulted in the fallowing:
Injector……..Flow in ml
1…………….265
2…………….260
3…………….260
4…………….265
5…………….255
6…………….255
Using the math in my previous post you’ll get the fallowing flow rates for them:
265 ml = 5.25 Gal / hr = 34.327 lbs / hr
260 ml = 5.15 Gal / hr = 33.680 lbs / hr
255 ml = 5.05 Gal / hr = 33.032 lbs / hr
As you can see these did vary slightly more from injector to injector, about 4% compared to the 2.5% observed with the LT1 injectors. However, seeing as these come in sets of six, I’ll be able to choose the best eight matching injectors to run.

Now to figure out how much horsepower these injectors can support. To do that we need to know about B.S.F.C. (Brake Specific Fuel Consumption). B.S.F.C. is the amount of fuel, in pounds, that the engine will use per hour, per horsepower it produces. The recommended levels of fuel consumption are; .45-.50 lbs per hr per hp for naturally aspirated engines, .55-.60 lbs per hr per hp for supercharged engines, and .60-.65 lbs per hr per hp for turbo charged engines. To determine what horsepower levels an injector will support use the fallowing formula:
injector size (in lbs / hr) X % duty cycle / B.S.F.C = Horse Power supported per injector
For this example:
34.327 X .80 / .50
27.4616 / .50
54.9232 hp per injector, or a 439 hp N/A V8

To determine what size fuel injectors your engine will need you would use the fallowing formula:
Horsepower X B.S.F.C / Number of injectors X Duty cycle ( 80% )
For example:
400 X .50 / 8 X .80
200 / 6.4
31.25 lbs / hr injectors

To determine what size fuel pump an engine will need you would use the fallowing formula:
Horsepower X B.S.F.C. + 15% (for recirculation and flow loss in piping)
For example:
400 X .50 = 200
200 + 15% = 230 lbs / hr, or 35.18 Gal / hr

To determine the increased flow rate of n injector at a greater fuel pressure use the fallowing formula:
√ ( new pressure / old pressure ) X old flow rate = new flow rate
For example:
√ ( 50 / 43 ) X 34.327
√ 1.1628 X 34.327
1.0783 X 34.327
37.015 lbs / hr @ 50psi (It would now support a 470 hp N/A V8 engine)

Conversions: (number used by GM in stock calibration)
1 Gallon = 6.538 pounds
1 Gallon = 3.785 liters
1 Gallon = 3785.411 ml
1 pound = .153 Gallons
1 pound = .5791 liters
1 pound = 579.1 ml

You're missing the square root function in your flow vs. revised pressure calculation. You did take the square root of the pressure ratio, but you didn't show it in the formula or your calculations....

(new pressure / old pressure) ^ 0.50 X old flow rate = new flow rate

(50/43)^0.50 = 1.0783

did the "√ " not show up properly?
I did have a square root sign, but i'm not sure if it posted correct. (works from my computer)

1 pound = .5791 liters
1 pound = 5.791 ml
I'm pretty sure that should be 579.1 milliliters. :)

did the "√ " not show up properly?
I did have a square root sign, but i'm not sure if it posted correct. (works from my computer)
It shows up fine on my computer.