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What Fuel Injectors Do I Need?

What Fuel Injectors should you use? It's a very common and important question.

This Tech Article is a work in progress. This chart was started using information from Trevor James of Arizona from a thread he made on 3SI. His thread had a good basic shell of a chart to start with. I have added the currently available larger injectors he left off his chart, I have also added E85 Data information to aid in choosing the right fuel injector for engines running E85.

I will be adding more in depth information soon, including but not limited to variables, like fuel type being used, that will change the Injector size you need to use.

Check back often to keep informed with the updates.

Please Contact us, using the Contact Us link at the far right bottom of the site if you have any suggestions for adding technical information or personal experiences using a specific setup if you have LOG files to show what works in the real world. Because as we all know, theory is great, Real Life is frequently different

I was asked on a FB thread about this chart and how it could be used to help choose the right fuel injectors for a 4G63, a DSM 4 cylinder.

Since this chart is for reference to help 3S 6G72 6 cylinder owners, the numbers are all rated for 6 fuel injectors. So we need to convert the numbers to 4 injectors.

Pretty simple. Power is a constant. It doesn't matter what engine, xxxcc of fuel makes xxx power because xxxcc of fuel has xxx BTU. Run at a correct Air/Fuel that injector can supply a static constant hp no matter what engine it is supplying fuel to.

There is an efficiency issue, Normally Aspirated versus Forced Induction and even between Turbo and Supercharged.

B.S.F.C is brake specific fuel consumption - How much fuel you are using per horsepower per hour.
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher.

Normally Aspirated engines run 12.5:1 A/F wide open throttle while Turbocharged engines run 11.5 A/F wide open throttle. It takes more fuel to run the same HP on a turbocharged engine than on a normally aspirated engine.

The information listed here is for a turbocharged engine application. If running normally aspirated you will need to correct the numbers for .50 B.S.F.C

To convert to 4 injectors you will neec to knock 2 injectors off the listed HP ratings.
Divide the rated HP for a given fuel injector by 6. That gives you the HP for a single Injector of that size. Then multiply by 4 to get the HP rating for using 4 of that cc injector.

(6)360cc injectors are good for 320hp-399hp (at 43.5psi)
320hp ?6= 53hp per inj
399hp ?6= 66hp per inj
53hp x 4 = 212hp
66hp x 4 = 264hp
So (4) 360cc injectors are good for 212hp to 264hp

Here is an example using a 4 cylinder, a DSM with (4) stock 450cc
DSM uses 450cc inj stock. As listed below:
(6)450cc injectors are good for: 400hp - 499hp
400?6=66hp per inj 499?6=83hp per inj

So a DSM on stock 450cc inj should be able to make, on pump or race gas which both have pretty equal volume/hp numbers, crank hp:
264hp to 332hp

A real easy way to look at it:
A 4 cyl only uses 4 injectors. To make the same power a 6 cylinder makes you need to run larger fuel injectors than the 6 cylinder. In short, for the same power on a 6 cylinder and 4 cylinder you will need the same total cc worth of fuel injectors on both setups.

So if a 6 cylinder is making xxxhp using 6 1000cc injectors, 6000cc total fuel volume, you will need 4 1500cc injectors, 6000cc total fuel volume, to make the same power on a 4 cylinder.

Choosing the fuel injector suited best for your setup is very important. Especially for setups using a piggy back fuel controller that does not have the ability to adjust timing.
When using a piggy back fuel controller you are intercepting the MAF (Mass Air Flow Meter) air count wire before it connects to the ECU. The piggy back fuel controller is then used to trick the ECU into thinking less air is coming into the MAF. Doubling the size of the fuel injectors, 360cc to 720cc requires a 50% decrease adjustment in the MAF Air Signal to correct fueling.
So with this setup when using a piggy back fuel controller without timing control is that you are telling the ECU that there is 50% less air flow coming into the engine, so that the ECU reduces fueling by 50% in order to hit the same Air Fuel Ratio while using injectors twice as large.
The problem with this scenario is that at 18psi and 360cc injectors on stock parts or even with a piggy back fuel controller set basically at 0 the MAF is in the upper range of Air Flow Signal output, letting the ECU know the engine is running in high boost, also known as a High Load situation. As Load increases (boost increases) the ECU is mapped to reduce ignition timing advance. Without this the engine would run way too much ignition advance for the high load situation, causing catastrophic engine damage.
Now install 720cc injectors which will require a 50% air flow signal correction in order to stay within correct air fuel ratios. BUT what this tells the ECU, actually tricks the ECU, is that at 18psi, high load, the engine is running half that load, maybe 8psi. So the ECU uses the low load ignition timing advance map which is very advanced versus the high load map.
So with 720CC injectors and a piggy back fuel controller with no timing control when you end up running 18psi, the ECU gets half that air flow signal so the ECU runs 8psi ignition timing advance which is WAY to advanced for 18spi. Catastrophic engine damage.

If you are using a piggy back fuel controller without timing control it is critical to choose the correct fuel injector size. In order to get correct ignition timing advance to your engine you need to ensure that when the engine is running high load the ECU is getting a high load MAF air signal. To do this you need to choose the fuel injector that you will run in the 85% to 90% IDC area when wide open throttle and at the boost levels you will run. Doing this will ensure the ECU is using the high load ignition timing advance map and save your engine from catastrophic detonation.

If using a piggy back fuel controller that has timing control or a standalone ECU that has timing control this becomes less of an issue. But there are other issues to consider. Many of the larger fuel injectors available, over 1000CC, do not like, can not mechanically work, with the VERY short injector pulses required to idle an engine on large fuel injectors so your idle may suffer. Sometimes this can be fixed by raising the idle RPM.
It also makes no sense to spend the extra money that larger fuel injectors cost when all you really need is a smaller injector. Do Not be that person that buys 2000CC injectors just so you can say I have 2000CC injectors when all your setup requires is 800CC worth of fuel.

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