TBI headache

[robertisaar]

crazy, huh?

[pocket-rocket]

Call me soopid, but what's BPC?

[Canyonero]

Base Pulse Constant, it's how you tell these ECUs the displacement and injector flow.

[pocket-rocket]

Ah, good deal, thanks I knew it had to do with fuel injectors, I just couldn't figure out the acronym.

[Canyonero]

I grabbed this off another forum and added the injector flows and put my math in bold.

Ok our BPC (base pulse constant) is basically an injector constant and engine size all rolled up into one. So to calculate it we need to know a couple of things and get them in the right units and then plug it into the formula below.

BPC = 1461.5 * (cyl size / gm/s of fuel)

For cyl size you find your engine size in Liters. 350 * 16.39 = 5736.5 cc or 5.74 liters Then divide this by 8 to get your cylinder size:
5.74 / 8 = .717L

2.8 / 6 = .467L
3.1 / 6 = .516L
3.4 / 6 = .567L
4.3 / 6 = .716L

Injector Flows (from TurboCity):
2.8-3.1L - 36#/hr - black/white
4.3L - 46#/hr - yellow/blue
5.0L - 55#/hr - white/green
5.7L - 61#/hr - orange/black
5.7L HF - 68#/hr - yellow/brown
7.4L - 75#/hr - red/blue
7.4L - 75#/hr - black/blue
7.4L - 46#/hr - pink/purple


For say a 65 lb/hr injectors:
65 / 3600 * 453.6 = 8.19 gm/s

36 / 3600 * 453.6 = 4.536 gm/s
46 / 3600 * 453.6 = 5.796 gm/s

So our formula works out to be:
BPC = 1461.5 * (0.717 / 8.19) = 127.95 (example)

BPC = 1461.5 * (0.567 / 5.796) = 142.97 (3.4 w/ 46#hr)
BPC = 1461.5 * (0.716 / 5.796) = 180.54 (4.3 w/ 46#hr)
BPC = 1461.5 * (0.467 / 4.536) = 150.47 (2.8 w/ 36#hr)
BPC = 1461.5 * (0.516 / 4.536) = 166.27 (3.1 w/ 36#hr)

So our new BPC is going to be 128. Now what happens if you don’t have stock fuel pressure? There is a formula for this as well.

SQRT(New PSI / OLD PSI) x (Injector lbs/hr) = New lbs/hr

Lets try it for 65 lb/hr injectors at 15 psi with an old pressure of 12 psi

SQRT(15 PSI / 12 PSI) x (65 lbs/hr) = 72.67 lbs/hr

So our BPC with this pressure would be 114.4 you can see how simply cranking up your fuel pressure causes big changes in fueling.

Ok XJ, you have a big throttle body, which injectors did you get with it? Keep in mind that holley rates their injectors at a higher PSI than GM. (20 psi I think, can anyone confirm?)

[pocket-rocket]

Makes you wonder who sits in a small white room with bright white fluorescent lights on with a big pot of strong coffee and a calculator to come up with these formulas. What gets me is the "1461.5" figure. Where did they figure that was needed for the equation, lol

[robertisaar]

trust me, go through a hack and you'll see enough of the wall numbers to make your head spin, especially once you get to the 16bit constants...

[pocket-rocket]