Thread: 4bar/3bar k-value

Sorry if this info is covered, my PC is down and I can't search from my phone...while needing this info bad!

What's the proper k value setting for 3 and 4 bar? I'm running nismo 555's if that info is needed.

Thanks

That's very dependent on your setup.

First find out what CC your injectors run at when at 4 bar.

Then follow this formula:
(Old INJ Size / New INJ Size) * Old K Value = New K Value

I hate to sound like a noob, but how do I determine the cc at 4bar? I know it's 59psi and I have realtime...is it a monitoring option in realtime do you know? Are we talking WOT or idle?

Old k value? I assume you mean OEM? Would you happen to know that number?

Thank you

Fuel Injector Calculator from WitchHunter Performance

3 bar (43psi) = 555cc
4 bar (58psi) = 640cc

So it would look like this
(555 / 640) * Old K Value = New K Value

So what's stock k value? Damn my PC!!!

That depends on your tune. Are building from a stock tune or from the 555cc tune?

Stock values are these though 259cc/DE MAF: B13 K Value=31206, B14 K Value=298

ill be building off an original stock tune that has since been modded to suite 555's.
the original tune with the ecu was set for 555's from calum.

for the formula to work properly, wouldnt i want to input the stock k value? so like this:
259/555 x 31206 = 14,562!?!?!?!?! that doesnt seem right at all!!! or did i divide backwards and the new k would be 66,870? either way though those numbers dont seem correct. im so confused!!! :-( i have plans to tune and lost at where to set my base k value as a starting point. im attempting a fresh start with the tune.

and with the 3/4 bar equation, the number is still smaller than the original. if im flowing more fuel, why would the k value be smalller? i guess thats my confusion.

my k value is currently at 28750 and fuel pressure at 3bar with 555's. if i bump to 4 bar i drop my k value to 24,931? i guess im not seeing the logic.

Theoretically - Yes, 24,931 is what that value would come out to be with your injectors at 4bar based off your current K value to start. Obviously, this is just a ballpark value, you might find it to be a bit too rich or a bit too lean, so it will likely require a little more adjustment after that.

Increasing fuel pressure is essentially the same as putting in larger injectors - which lowers the K value.



The K value decreases with larger flowing injectors because that number represents a theoretical pulse-width (time frame) for the injector to be open allowing the flow of fuel. If you put larger injectors in, they will flow more fuel in the same amount of time as the stock injectors. For example, the stock 259cc injectors flow "X" amount of fuel at a given point - the 555cc injectors are going to flow over double that amount at that same point, if you do not adjust the K value. Therefore, you want to lower the K value to just over half of what it was so that the larger injectors are open a shorter amount of time (but still providing the same amount the stock injectors would have been in a longer time.)

Ahhh, makes sense now. Thank you! I'm the hands on learning type, I need a picture to visualize without the physical to see and touch :-)

What about the 14,562 k value? Apparently I can't quote myself from my phone either. PC should be back this weekend...

That value would be what you would want to use if you were all stock except the injectors you were adjusting for. The K value also needs to be adjusted for MAF changes. I looked up your build thread, you said you were using a Z32 MAF. You use the theoretical HP rating of each MAF, but the equation is reversed: (new MAF/old MAF). Since the Z32 is rated at around 500hp, and the stock DE is rating at 250hp, your equation would be (500/250), or 2. So after you adjust for your injectors you multiply it by 2 for the MAF change. That's why your final value is at 28,xxx. The full equation is: K value * (old inj/new inj) * (new MAF/old MAF).