Thread: Proper k value forula

What is the formula to determine appropiate k value? I can do the math, just need the formula. I'm running 555's. Will be tuning this week and need the proper base k value to then start adjusting the fuel map. Thanks

It's all in here.

http://www.plmsdevelopments.com/files/nissan_ecu_tuning_basics.pdf

No doubt, thanx. :-)

No problem, someone directed me towards that link once and it helped alot.

so is it old/new injector size x current k value = new k?

And when your doing 4 bar are you suppose to use the 4 bar number of cc or the 3 bar say 520s, at 4 bar are 600 so do I use that?

Yes.



I'm not 100%, but I believe that would be the way to do it. Say you wanted to only change your tune from 520's @3bar to 520's @4bar - assuming 600cc is an accurate flow rating - then the equation 520/600 = .867 x old K-value oughtta get you in the right ballpark. You'll want to adjust your K-value until you reach around 14.7-15 AFR at idle (in open loop). All the formula really does is get you close to your figure. You'll always want to use a wideband to dial it in, in my experience. But doing that sounds like it should be a good start.

I prefer tuning to where my idle is ideal and work from there. I've read some people like to tweak the K-valuve so their WOT is ideal. To each his own.

Well I have a decent tune now, just rich at times and I dyno tuned it and a not so loaded dyno so my a.fs of 11.7's on there is now 10.9s on the street haha. I just went from 370s at 4 bar to 520s at 4 bar.

im slightly confused on one part. the link article refers to stock k value being: 34,275. then it goes on to talk about going from 370's to 550's. is the stock k value refering to USDM N/A 259's, or "JDM" turbo stock being 370's? cause i went from 259's right to 555's. i went from bone stock usdm to built avenir. would i calculate from the stock 259cc k value or allow assumption and calculate from stock turbo 370cc? if 259cc, what is 259cc k value?

edit: and where the hell are they pulling 21,475 from? if its refering to stock k value for the equation, what happened to 34,275? why did it get replaced with this magical 21,475?

still, with my confusion, my current k value is pretty damn close at 28,750 to the example equation of: 21,475 x (370/550) x(550/290) = 27,400 being for injector and MAF. im running 555's and a z32 and at 28,750, im actually running a little rich still waiting to square this away and get out and adjust my fuel map with my buddy. ~thanks

Yeah, I always found the wording hard to understand in that article as well, and during the calculation part it does get a bit confusing. What it is good for though, is helping you understand how it all works and what the equation syntax is. Which you have correct - Old K-value X (old inj/new inj) X (new MAF hp/old MAF hp).

For the hp values, I've always referred to this page MAF Size, Part #s And WHP Output. I know the article varies a bit, but Andreas is a pretty trustworthy source.


Your equation should look similar to this - assuming you are starting with a B13 ECU. The stock B13 BIN that I have has 31,206 for the K-value.

31,206 x (259/555) x (500/250) = 29125.6.

I like to have pretty easy numbers to work out with, so I would either bump it to 29150, or just 29100 and then if you have a wideband starting playing with different values changing it a hundred or two at a time until you get pretty solid AFR at idle. Also, don't forget that when you change your MAF, you not only have to recalculate the K-value, but you have to replace your VQ map with the correct one.

So are my numbers right? If upgrading injectors only and the maf stays the same, going from 370cc inj to 460cc inj, the new K-value is 17273?