Injector size vs power expectation rb26dett

Hey Yall!

hope this year was great and the one coming up will be even better with no knock and just raw power!

I had a question about the relation between injector size and power. I read up on alot of posts out there and sites that have a injector to power ratio but it just seems bit weird to me how this can be determined.

Im currently running the following setup on my rb26dett

- 2x t517z turbos with modified cores ( still disco potato) with billet impellors : 52mm ( like 18g mitsubishi) td05 HL housing

- 800 cc deatchwerk injectors low impedence on stock fuel rail

- wiseco 10:1 ( approx) 87mm with 1.1 mm mls HG pistons...getting 180 psi on all 6

- sard FPR running 45 psi approx on idle

- some aftermarket fuel pump ( dont know brand but came with the car)

-stock intake mani with greddy intercooler piping + apexi FMIC

- vipec ecu

I havent taken it on the dyno yet as i am planning to do some more street tuning next spring after diagnosing a misfiring issue all summer 2024 long ( thanks to those who helped me on here for heping me diagnose)

My qualm that im having is that im running very high injector duty cycle already ( 70%) at around 12 psi ( wastegate duty cycles set to 0%) .

before i changed turbo cores, i remember being at like 99% duty cycle at like 18 psi of boost...

Anyway, like i said, i dont have teh dyno figures but i heard that 800cc injectors on a rb26 are enough to push 550 awhp... i dont think im getting that based on my butt dyno and the logic of pushing 550 awhp on an rb26 at 15 psi of boost on 18g turbos.

So my question is, can my injectors be faulty? or am i just skipping steps and i should get ont he dyno and figure out what im really making before making these assumption

Cheers guys and happy new years!

Well, the easiest thing to do is to check each injector on the flow bench to ensure there are no bad ones. If they are ok the next step is to ensure the fuel pump is producing fuel pressure required. Also checking the intake part of the system in terms of air leaks would be good.

As far as relationship between injectors size and power- it's quite simple. The more power you make the more fuel you need to cool down combustion chamber. That makes engine less effective in terms of Break Specific Fuel consumption which indicates how much fuel in lbs an engine needs to produce 1 HP for 1 hours. The rest is pretty much mathematics involving how much air is required to produce 10 HP, how much fuel you need for that, how big your fuel pump will be and how big of fuel injectors you need accordingly.

The fuel volume you need will also change depending on the fuel you're running. Canada and the US I think both use E10 (I know thats what we get down here in the US, not 100% sure if that's normal in Canada) So at a given lambda, you will need a little more fuel volume than what someone running ethanol-free gasoline will need. That, and the fuel volume your fuel pump can deliver can both play a role in the injector duty cycle. Maybe take a data-log and see if your fuel pressure is dropping at any point during the run? if the fuel pressure is dropping/not rising, your ECU may be trying to compensate by holding the injector open longer. If the fuel pressure is rising with boost the way it should be, then you may just be out of injector. DW has an injector calculator to get you in the ballpark, and the 800cc injectors on a turbo 6-cylinder should be able to get about 800 crank HP, which is over 600AWHP at about 85% max duty cycle. I know this isn't a perfect calculation and it's making a lot of assumptions. However, that much duty on that much injector, to me, feels excessive.

One other thing to bear in mind is the Dyno brand - let's don't forget that they can register different whp number depending on particular brand so it can be 550 whp on one Dyno and 600 whp on another, creating a room for an argument.

You may be finding the limitation of the pump rather than the injectors as such.

Rail pressure at peak MAP and injector duty is one of the first things that you should be checking since you don't know the pumps provenance.

Hey Guys!

thank you so much for the inputs! it seems that the pump is the culprit here as you said!

FP drops on WOT and seems that the injector duty compensates for it. Am i reading into this right?

Raceworks and a few others like injector dynamics have online calculators that are super useful for this, that aren't 100% spot on, but are pretty close

https://raceworks.com.au/calculators/injector-hp-calculator/

https://injectordynamics.com/injector-selector/

From a quick look a this log I'd say you were missing a vacuum line to your regulator, the differential fuel pressure drops as soon as you see any positive manifold pressure

Also, there seems to be a Problem with battery voltage regulation.

Differential fuel pressure seems fine until ~65kPa - I'd investigate both the regulator and the pump if it were me.

With the increased voltage Dyno Dom mentioned I'd also be going over all your power and earth wires, especially those at the alternator