Having a chat about this last night, and going bigger myself prompted me to have a hunt round for some info on how to work out which sized injectors to go for.

Hope it helps - tells you all the calculations you need to work out which injectors to go for for you anticipated bhp

To convert cc's to lbs/hr divide the rated cc's by 10.5

450cc / 10.5 = 42.85 lbs/hr or rounding up a 43lb injector.

To convert lbs/hr to cc's multiply the rated lbs/hr by 10.5

26lb * 10.5 = 273 cc/min injector.

Brake Specific Fuel Consumption

Locating information about stock injector flow rates can sometime be quite difficult. If you can’t find your car’s injector size you can get a good idea of the injector flow rate using your car’s factory horsepower rating. To do this calculation you will need to determine your Brake Specific Fuel Consumption (B.S.F.C.). The B.S.F.C. is the number of pounds of fuel it takes to make 1 horsepower for 1 hour.

The B.S.F.C. values very from engine to engine depending on its efficiency. The more efficient the engine the lower the B.S.F.C. value, which translates into needing less fuel to make 1 horsepower for 1 hour. This changes for forced inducted applications as the extra fuel is used to cool the combustion chamber helping to prevent detonation. Here are some basic B.S.F.C. values.

Low to medium performance = .50

Performance engine with head work = .45

Performance engine with expert head work = .40 - .45

Supercharged and turbocharged engines = .55 - .60

Injector Duty Cycle

An injectors duty cycle defines how long the injector stays open. Injectors are always rated at 100% duty cycle so, a 26lb injector will put out 26lb's of fuel under 100% duty cycle. You never want to run your injectors above .85 or 85%. Anything higher puts you at risk of the injectors locking up. Choose an injector that puts out your desired lbs/hr at an 85% duty cycle rather than one at 100% duty cycle. If there isn’t an exact match you can raise your fuel pressure, which makes the injector act like a larger one but this trick is only good if you need a few extra lbs/hr.

Now that you have determined your B.S.F.C. and duty cycle it's time to calculate what your stock injector is rated at.

Here is the formula:

( H * B ) / ( C * D ) = estimated injector size.

H = horsepower

B = B.S.F.C.

C = number of cylinders

D = duty cycle

This is what it should look like for a Saturn.

( H * B ) / ( C * D ) = estimated injector size.

124 * .50 / 4 * .80

62 / 3.2 = 19.375 lbs per hour.

If you’ve installed too large of an injector and your car exhibits erratic idle during cold start or stumbling after it is warm you can attempt to remedy this by lowering the fuel pressure. But beware if you lower the fuel pressure to much it will effect your fuel injectors spray pattern so be careful of how much you lower it.

Injector Upgrade

If you want to upgrade your stock 19lb injectors by 10% but your not sure what the new flow rating will be simply multiply the injector’s lbs/hr rating by .10 then add the original lbs/hr rating of the injector. Example:

19 * .10 = 1.9 + 19 = 20.9 or a 21lb injector.

So this means we can upgrade to a 21 lb injector with no problem.

Injector Flow Rate based on Pressure Changes

To calculate an injector’s flow rate at a new desired fuel pressure you will need a calculator that has a Square Root function. Now this one can get a little tricky but pay close attention and this can help you out tremendously. Most of the injectors on the market today are rated at 43.5 psi. So, when you say you have a 19lb injector what you are saying is that your injector puts out 19lbs of fuel per hour at 43.5 psi of fuel pressure. So, how do we find out what our new desired fuel pressure going to be?

Here's an example:

Lets say we want to up the factory fuel pressure to 55psi. You will need to know the following information lbs/hr of the injectors you currently using (19lbs), original fuel pressure (43.5psi), and the new fuel pressure 55psi.

Here is the formula:

The square root of x multiplied by z divided by the square root of y = new injector flow rate

Where:

X = new fuel pressure

Y = original fuel pressure

Z = lbs per hour

First, find the square root of the new and original fuel pressure.

New fuel pressure: square root of 55 = 7.416

Original fuel pressure: square root of 43.5 = 6.595

Then divide the new fuel pressure square root sum by the original.

7.416 / 6.595 = 1.124

Then multiply this total by the size of the injector. In this case we are using the 19lb injectors.

1.124 * 19 = 21.35

At 55psi the injector now flows 21.35lbs/hr

Injector Flow Rate Chart

30psi

35psi 43.5psi* 45psi

50psi

55psi 60psi

16lb

17lb 19lb 19lb

20lb

21lb 22lb

20lb

22lb 24lb 24lb

26lb

27lb 28lb

22lb

23lb 26lb 26lb

28lb

29lb 30lb

29lb

31lb 35lb 36lb

38lb

39lb 41lb

32lb

35lb 39lb 40lb

42lb

44lb 46lb

*Stock injector size 19lb @ 43.5psi

Injector Size vs. Horsepower

Calculating how much horsepower the injectors can support is relatively straightforward using the following four variables:

Injector size: 26lb in this example

Number of cylinders: 4

Maximum duty cycle: .85

B.S.F.C. .50

Here is the equation:

Q * W * E / R = estimated horsepower for injectors.

26 * 4 * .85 / .50 =176.8 horsepower is what four 26 lb injectors will support.

Q = size of injector

W = cylinders

E = duty cycle

R = B.S.F.C.