There are a lot of injector calculators on the internet, but none seem to calculate very well and two I used suggested a stock M50B25 should be running no less than 24 lb/hr injectors (stock is 17.5, and those handle an extra 30 HP easily). So let’s learn a quick an easy way to choose injectors with just a little math.

(“wait, math? I don’t think so!” No worries! Scroll to the bottom of the page for a calculator!)

This is a very simple method and does not account for many factors, so just use it as a way to estimate what you’ll need. That being said, it does tend to be quite accurate!

Simply take your target horsepower at the wheel*, divide by number of cylinders, and then divide by two.

i = ( h / c ) / 2

###### * to find WHP: multiply BHP (brake horsepower; “crank horsepower”) by 0.92-0.88 for FWD, 0.9 to 0.85 for RWD, or 0.88 to 0.82 for AWD. What is WHP/BHP? (external link)

As proof of concept, we will look at a few examples.

First the RWD M50B25TU (with VANOS):

Horsepower (h): 172 WHP (192 BHP)
Cylinders (c): 6
Stock Injector Size: 17.5 lb/hr @ 50.75 PSI

i = ( 172 / 6 ) / 2
i = 28.66 / 2
i = 14.33

Since 14.33 is less than 17.5, we know it can handle the specified power level. However, it’s a good idea to add an operating and error margin of 15% to this number for safety. To do this: multiply 14.33 by 1.15. This brings us to 16.5, quite close to the stock number but still leaving us with room to grow.

Speaking of growing: the S50B30, which BMW claimed output 243 BHP, actually outputs around 190-200 WHP (210-220 BHP). It also used the same 17.5 lb/hr injectors as the M50. Generally the stock injectors don’t allow much room for growth over stock, but can take the vehicle to about 210 WHP before upgrading is necessary. (There are a few dyno examples you can see for yourself (external link))

Let’s use the equation to see if it matches our observed maximum supported power.

i = ( h / c ) / 2
i = ( 210 / 6 ) / 2
i = 35 / 2
i = 17.5

Excellent, it does! However this is absolutely maxing out the stock injectors and there is no safety margin.

One more you say? Okay! Let’s look at the M54B30, which comes with 23.5 lb/hr injectors stock and is rated at 231 BHP (or 207 WHP with our brake-to-wheel conversion of 0.9, which is makes exactly according to a dynojet).

i = ( 207 / 6 ) / 2
i = ( 34.5 ) / 2
i = 17.25 ; 19.84 with safety margin

Fantastic! So we know we have room to grow here before upgrading injectors. Sure enough, we’re usually able to see up to a maximum of about 250 WHP with this injector size.

Let’s work backwards with this next example. We’ll be looking at how much horsepower our 42 lb/hr injectors support on a 6-cylinder engine (too many people running 200 WHP engines seem to think they need 42 lb/hr injectors for some reason).

We take flow in lb/hr, multiply by two, and then multiply again by number of cylinders.

h = i * 2 * c

h = wheel horsepower
i = injector flow in lb/hr
c = cylinder count

h = 42 * 2 * 6
h = 42 * 12
h = 504

504! That’s quite a lot! But let’s be safe and apply our 15% safety margin by multiplying this number by 0.85. This gives us 428 WHP safely supported.

That’s right, your 42lb/hr injectors support a whopping 428 WHP with a safety margin, or 504 completely maxed under ideal conditions.

Now, we can get more complicated with all of this based on airflow values and AFR, but this method works quite well for the basics. If you’d like to be more specific, keep reading!

But wait! Why can’t I just get massive injectors even if they’re more than I need? Who cares?!

As your injector’s flow rate increases, the time at which the injector opens becomes increasingly small. At idle and low engine speeds this can lead to issues. It also decreases the total length of piston stroke over which fuel is added to the cylinder. Increasing the spread of fuel over the length of the stroke allows for better mixing of fuel and air, which leads to a decreased possibility of knock and pre-detonation, as well as better usage of fuel.

## Forced Induction Help

There’s one small change we make to the equation for forced induction that tends to require some calculator usages. We change the final denominator from 2.0 to 1.8. So the new equation looks like this:

i = ( h / c ) / 1.8

Simple enough? That’s because it’s too simple. Realistically we need to adjust for a number of factors, including target AFR, but hey, this equation is all about simplicity and will get you a very quick and easy estimate. However, it does tend to be quite accurate.

The stock 565 cc/min (53.8lb/hr) injectors on the 2005-2014 STi tend to max out at 360-380whp.

Horsepower (h): 380
Cylinders (c): 4
Stock Injector Size: 54/hr

i = ( 380 / 4 ) / 1.8
i = 95 / 1.8
i = 52.77

Well darn if that isn’t close. But maybe you want to get a little more in-depth with it? Check for target AFR? Let’s change the final denominator to a function.

### The Long-Way Around

i = ( h / c ) / d
d = 1 + sqrt( t / 14.7 ) – [ r / 3600 / ( c * 10) ]
i = injector flow
h = WHP
c = cylinder count
t = target peak power AFR
r = peak power RPM (or redline if you are unsure)
sqrt means square root.

Let’s use this new equation to check the EJ255/257 with 565cc/min (53.8 lb/hr) injectors again.

i = ( 380 / 4 ) / d
i = 95 / d
d = 1 + sqrt( 10.5 / 14.7 ) – [ 6000 / 3600 / ( 4 * 10 ) ]
d = 1 + sqrt( 0.714 ) – [ 1.944 / ( 40 ) ]
d = 1.714 – 0.486
d = 1.7993
i = 95 / 1.7993
i = 52.80

Okay, that’s just about spot on, right? You can also adjust the variables to make sure the injectors are still sufficient at redline (you’ll use WHP at redline for this). This full equation should also be used for N/A vehicles, however generally the standard d = 2 works just fine for that application.

As proof, we can size up the stock N/A M20B25TU again. Plugging in 172 WHP, 13 AFR, and 6000 RPM we get:

d = 1.9126
i = 28.667 / 1.9126
i = 15.00 ; 17.25 with 15% safety margin

There we go, with safety margin it’s almost exactly what the stock injectors flow.

## Auxiliary: How much HP will I make by going forced induction?

One of the most common questions we’re asked is about what size injectors to choose for a turbocharger or supercharger build. In order to understand this, I feel the need to explain how much power you’ll actually be gaining from going forced induction.

A general rule of thumb here is that your kit will add 8-12 WHP per pound of pressure added. So if you’re aiming for 8 PSI, you’ll be adding 64-96 WHP onto your naturally aspirated numbers. The actual number varies upon so many factors, and this is not the article for that, so for sanity’s sake let’s stick with the standard and average 8-12 — 8 being closer to smaller TD04L style, and 12 being closer to HX35 style. (the actual number varies with your turbo flow dynamics and engine efficiency.)

An M50B25 can run up to 16 PSI on TD03/04 with simply ARP head studs and an MLS gasket. At 170 WHP stock, this will put you near 330 WHP. (A completely stock M50B25 with stock head bolts and gasket should not be running more than 10psi for safety.)

Let’s quickly size injectors for this build!

i = (h / c ) / 1.8
i = ( 330 / 6 ) / 1.8
i = 55 / 1.8
i = 30.55 ; 35.14 with safety margin applied

Taking it a bit further with our “probably more than you need to worry about” equation detailed in the section above, we plug in 330 WHP, 11.2 AFR, and 6 cylinders.

i = ( 330 / 6 ) / d
i = 55 / d
d = 1 + sqrt( 11.2 / 14.7 ) – [ 6600 / 3600 / ( 6 * 10 ) ]
d = 1.8174
i = 55 / 1.8174
i = 30.26 ; 34.80 with 15% safety margin

There ya go! If you have any questions or concerns, please feel free to send us a message and we’ll reply as quickly as possible!