minimum amount of dry nitrous w/FAST

[JeffB]

I'm trying to add a small (50hp) shot to my blown 396 combo... the problem seems to be that in the nitrous fuel map, there is a minimum fuel value that i can add (i can't set it down to 0, about 22lb/hr or so is the lowest it will go), and for a 50hp shot its way too much fuel... when i hit the nitrous the AFR goes to 10.4:1 and i lose most of the gains of the nitrous by being too rich (50hp shot adds 10rwhp)... am i missing something here or was the system not designed to do anything below 75hp or so?

thanks,
Jeff

ps. i don't want to use the system to do wet nitrous because the fuel pressure is climbing as the boost increases so this would be very hard to tune unless i had a second fpr...

 

[TurboTR]

Well actually 25 lbs/hr is the right amount of matching fuel for 50 hp if you assume a bsfc of around 0.5. I've done the dry shot on my own setup and didn't have a problem, started at 50 shot and moving up from there. If you've verified that the fuel flow is bumping up by only 22 lbs/hr then it sounds like the naawwzzz isn't flowing as much as it should. What size jet? What bottle pressure? Was it firing cleanly on the dyno? What is your N2O target a/f ratio set to? How much timing are you subtracting?

TurboTR

 

[JeffB]

i realize 25lb/hr is the theoretical number... that's where i started and it was even richer... bottle pressure was like 850, a little cool, but i don't have a heater... .036 jet... firing clean, just not making much power... on boost alone i'm at 11.6:1 making around 830hp (flywheel)... so relatively speaking, the extra fuel from the 50 shot should be pretty small (percentage-wise) vs. the fuel for WOT boost... but yet the AFR drops from 11.6 to 10.5... the other issue is that the minimum fuel value in the FAST increases throughout the rpms, so at higher rpms, i can't even go as low as 25lb/hr... i'm guessing that the system wasn't intended for pee-wee shots?

 

[TurboTR]

There's a nitrous target a/f ratio in the setup; what do you have it set to? Also, how much of an air temp drop did you see with the gas on? Don't take this the wrong way, but are you sure the gas is flowing? Did you purge it before the tests? I could feel the 50 shot on mine with similar overall hp levels. A 75 shot feels even better

TurboTR

 

[JeffB]

i'm sure its flowing, you can see the nozzle ice up... i did purge before hand... plus you can hear the motor change pitch when the nitrous comes on, and there is a small jump in power... the intake air temp never seems to drop too much though... actually not at all... i was surprised about that because it is upstream from the intake air temp sensor...

as far as the AFR, that option says "N20 CL Target A/F Ratio" so i was guessing that it was only for correction... and i'm not getting any O2 correction during the run... since the N20 fuel table is in lb/hr i figured that was the total fuel amount, unlike the normal tables which are a combo of VE and desired AFR... did i misinterpret this?

 

[TurboTR]

You should be getting correction just like normal, assuming you're in closed loop. The target nitrous a/f ratio is the new target a/f ratio for when nitrous is on. Meaning the a/f ratio table is not where the system gets the target a/f ratio when nitrous is on. It needs to be properly set. Also, the total fueling when nitrous is on will be the usual speed density calc result + the additive amount from the nitrous fuel table. Easy enough to estimate.

My older sw version had this table in terms of pulsewidth, not lbs/hr. A little more difficlut to calculate, but still had it close enough right from the get-go. But then the a/f correction afforded by the wb O2 system really shines with the hose IMO.

PS I saw a 20-25 deg reduction in charge air temp with an 0.40 jet. Wonder why you aren't seeing any?

TurboTR

 

[JeffB]

what's the proximity of your nozzle and IAT sensor? mine are about 16" apart...

what kind of temps were you seeing? my intake temps are fairly low anyway (100 degrees F at the time the nitrous kicks on)... perhaps if your charge is much hotter the n2o would make a bigger difference?


as far as the C/L n2o afr, my wb02 sensor is screwed up... it reads 9.1:1 when the dyno's horriba is reading 11.6:1... because the FAST *thinks* i'm so rich, it isn't adding in any more fuel (o2 correction is 0%, i don't let it subtract any fuel at WOT conditions)... so if i'm seeing 0% o2 corr. throughout the pull, then this number isn't having any effect, right?

 

[TurboTR]

Find out what your nitrous target a/f ratio is set to.

The hoze nozzle and MAT sensor are in the up pipe in front of the throttle body. The hose nozzle is about 18"-24" or so ahead of the MAT. The charge air drops from about 100 deg to about 75-80 deg.

TurboTR

 

[JeffB]

its set to 10.5... but again, is this value really getting used in my situation? o2 corr. is 0%

another interesting thing... i'm running 8 75lb/hr injectors... but when the nitrous comes on the duty cycle jumps by over 10%... that's at least 60lb/hr, even though my n2o fuel table is set to 22lb/hr... what's going on there?

 

[JeffB]

ok, maybe i'm stupid... thinking about this some more, and re-reading what you wrote, it would seem that the "N2O closed loop a/f ratio" does more than just affect closed loop? for example, lets throw some numbers at this:

say i'm using a target A/F of 11.6 on boost alone... say that's using 480lb/hr of fuel... if my N2O AFR is set to 10.5 then it will increase the base fueling (the 480) by the extra amount needed to get to 10.5:1 and THEN add the extra lb/hr from the n2o fuel table? if that's the case, then i need to have the N2O c/l ratio set to the same value as my normal boost afr and then the only extra fuel will be coming from the n2o fuel table... does that sound right? i guess the "closed loop" in the description of that value is what threw me off... i was interpretting this as a value that would only be used for wbo2 correction...


addition: i think that's exactly what's happening... that explains me getting the huge amount of extra fuel (60+ lb/hr, vs the 22 i'm putting in the n2o table)... thanks for beating this into my head! sorry it took me so long...

 

[TurboTR]

Mmm- kinda. The system always needs a target for the a/f ratio. It usually gets this target from the a/f ratio lookup table. BUT when juice = ON it instead gets the target from the nitrous a/f ratio value, which you have set currently to 10.5:1. Which is also what you said the a/f ratio went to Sounds good to me. I tend to be conservative and prefer a bit richer than usual mix when juice is on. Like say 11.5:1 at 25 psi boost alone down to about 11.0:1 with hose on. Still experimenting with a/f ratios though.

And regarding the nitrous additional fuel table. Consider this- if you suddenly, drastically change the amount of O2 being introduced into the engine (via nitrous) then you can either a) try and wait for the system closed loop response to see the resulting a/f ratio error and catch up to the additional O2 and provide the matching fuel to achieve the target a/f ratio. It'll run out of correction range and won't be able to anyway. Not too mention run very lean for at least the first few engine cycles. Or b) provide an additional, additive matching fuel amount term for the system to be able to match the additional O2 coming in with the correct matching fuel amount. The nitrous additional fuel table provides that matching fuel amount term. Your job is to know how much extra O2 will be coming in advance and to program in the correct amount of matching fuel. So bam, juice gets switched on and in comes +50 to ?? hp worth of additional O2 all of a sudden, but no problem 'cause the system already knows to add the additional amount of matching fuel via the nitrous additional fuel table.

Hope that helps?

PS- what is your nitrous timing subtractive value set to? If it's set too aggressive then that might explain the lack of power increase you mention.

TurboTR

 

[JeffB]

for the 50hp shot i'm only taking out 2 degrees of timing (28 total vs. 30 on boost (15psi))...

right now my entire AFR table (above 100kpa) is set to 11.6... i think i'll put 11.6 into the N2O CL Target value too and then i tweak the fuel via the N2O fuel table only... I might run it a little fatter like 11.3 or something, but since its a smaller shot i wouldn't think it would be too risky to run at 11.6...