SleeperZ. Question about modifing z31 AFM

[Guest Anonymous]

I like tinkering with things and have some extra parts (injectors, AFM) lying around. How did you determine the size of the new pipe for the AFM? I figured it was the same % of the differnce of the injectors. Or am I way off?

Example:

Stock injectors are 269cc

New injectors are 380cc

New injectors are about 1.4 times bigger so would the new pipe be 1.4 times bigger then the original.

The inside diameter of the original is 2 3/4 inch. So 3.85 (probably wont be able to find that size) would be the new diameter? Thanks for any help you can give me.

[SleeperZ]

You are close. That's how I calculated it though, just the same percentage of air as fuel. 3-1/2 inch diameter is the appropriate size, any fine tuning I performed was with fuel pressure adjustments. I am currently running 43 psi base fuel pressure.

[Bernardd]

So if I enter 30 as the base and 43 as the new fuel pressure and 35 lbhr as the injector rating here http://www.csgnetwork.com/fiflowcalc.html

you are running 41 lbhr or 440cc injectors with your new fuel pressure. Correct? Or are you setting 43 psi with the vacuum hose off which would change the base to 36 psi at idle? Do you have any idea what the injector duty cycle is on a wfo run? EGT's? Maf voltage? I've got my fuel enriched as much as possible and I still can't go over 15 psi without retarding the timing drastically. That's with 91 Octane crap in the tank.

Bernard

[SleeperZ]

I can't remember my exact calculations, but the 43 psi (base pressure) is actually exactly what I calculated. I used 260cc/min for my old stock injectors, and I flow tested the SVO units at the stock base pressure of 37 psi to 360cc/min. The ID of the MAF was close to 2.8" and the ID of the intake tubing (AWG16, 3-1/2" OD steel exhaust pipe) I used was something like 3.4". I don't know what mixture ratio I was running with stock injectors at WOT, but I believe it was leaning out on me at the top RPM.

 

After dyno testing on 41 psi, and getting a mixture ratio of 12.5:1 - 13:1, I increased my base pressure to 43 psi. I didn't get to check the results on the wideband. I need to make a harness for my portable scope so I can measure injector duty, and I still need to figure the wiring for a used HKS EGT I haven't installed.

 

All of my testing I had about 30% 100 octane mixed with 91, for safety, but I've run straight pump on 15 psi. I've not used up my last tank, so I haven't tried running 16 psi on the 91. I did detonate near the top of 4th gear (all 91 octane) when I'd forgotten to turn on the electric cooling fan, and my coolant temperature was around 200*. Talk about a close call!