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Larger Injectors

Posted: Wed Jul 15, 2015 12:07 am
by Renovelo
Another popular modification is adding larger injectors. This is a necessity when doing some N/A modifications such as a combination of 3.5" intake, cams, and headers. For F/I applications, larger injectors are definitely required.

When tuning for larger injectors, the fuel injector constant must be scaled appropriately. You'll notice the injector constant in Domino Tuning Suite is the raw number (not a value which represents something in cc/min or lb/hr). The stock value for a M3 is 43478 which corresponds to 16lb injectors (rated at 3.0b). Assuming you're swapping to 24lb injectors (also rated at 3.0b), you'll need to scale the injector constant by the same ratio.
  • 16 / 24 = 0.667
  • 0.667 * 43478 = 28985
The new value for your injector constant would be approximately 28985. Unless you've had your injectors tested on a flow bench to know exactly what their flow rate is, this final value will be an estimation. You'll have to tweak it slightly to get everything perfect.

If your injectors are rated at a different pressure, you'll have to adjust accordingly. There are many calculators available online to help determine this.

Additionally, every value that references engine load must be scaled by the same factor. This includes any value within the body of a table, axis, or single constants. There are many references to engine load in the OBD1 DME, so these files are a good starting point!

This methodology of scaling works well for injectors up to 42lb. When tuning for injectors larger than this, additional techniques must be used which are described in later posts below.

Attached below are some base tunes for 24lb injectors (rated at 3.0b fuel pressure). This should cover any N/A modifications and low boost (less than 6psi) F/I applications. There is also a 42lb tune attached below which could be the starting point for a turbo tune.

Warning: The only thing that has been changed with these tunes is scaling for generic 24lb injectors. Some slight tuning may still be necessary to the injector constant to dial in your specific injectors, even if they're 24lb. Other tweaks will also need to be made to the fuel tables to run perfectly. These tunes should only be used as a starting point!

Note that these tunes have been scaled assuming that larger injectors are the only modification done to the vehicle. If your vehicle has forced induction, then modifications will have to be made to the load axes on the fuel and ignition tables where additional room for extra load is needed. Also note that the maximum MAF controls have not been touched in these base tunes. Until these are scaled appropriately for forced induction, fuel cuts could be applied at higher engine speeds.

Re: Larger Injectors

Posted: Mon Dec 18, 2017 3:05 am
by Triple-R
I'm curious of the different techniques to scale injectors of larger sizes.

Re: Larger Injectors

Posted: Mon Dec 18, 2017 10:27 pm
by Renovelo
I'm curious of the different techniques to scale injectors of larger sizes.
As you'll notice, the fuel maps in the stock tune as well as these 24lb tunes are relatively flat. This is because the fuel maps for these DMEs are actually for deviations in expected opening time. If the engine were a perfect air pump, then these tables would be perfectly flat. We've managed to tune for injectors up to 42lb by maintaining relatively flat fuel tables.

What you'll notice is that when scaling all of the engine load axes and tables for 42lb injectors, the values on the low end approach zero. Unless you start modifying the axis breakpoints, it might be difficult to tune for the low end of all tables. You might find that the engine doesn't run as smoothly either since the low end of most tables is where the engine idles (and where you would probably notice injectors which weren't tuned perfectly).

Tuning for even larger injectors can be done by making the fuel tables resemble traditional fuel maps where higher engine loads have higher injector opening times. The fuel maps change from being relatively flat to being almost linearly sloped along the engine load axis. The engine speed axis remains relatively flat. Attached is a screenshot from the 60lb tune on our shop car.


Re: Larger Injectors

Posted: Thu Apr 02, 2020 5:23 am
by harry_de_best
Any chance you could post this as .bin files?

Re: Larger Injectors

Posted: Thu Apr 02, 2020 5:29 pm
by Renovelo
Unfortunately not...our software uses *.dme files.

Re: Larger Injectors

Posted: Sat Jul 18, 2020 1:58 pm
by Lwinder
Can you post this 60lb injector tune?

Re: Larger Injectors

Posted: Sun Jul 19, 2020 9:48 am
by Renovelo
We don't have a base tune which is just tuned for 60lb injectors.