what is correct a/f number when nitrous hit off brake?

[572 BBC]

what is best way to apply nitrous in an air/fuel aspect when using a plate kit right off transbrake?ive got a big shot plate on bbc, big superflow pump with a bleed off line back into tank (on pump). then -10 line up to fuel log.2 magnafuel regulators.one for carb and other for nitrous gas regulator.then capped(dead headed)...when i set regulator for nitrous, i took plate off motor and activated gas selenoid,set to 6.25 lbs flowing pressure.then put back on motor and started fuel pump.regulator is up to as high as 11 lbs when selenoid is closed.takes a sec or two once selenoid is open to drop down to 6.25 lbs.we dyno tested car and on initial hit it, dyno could only register air/fuel as rich as 10:1...it was at least that or worse.then by end of pull,went to 10.5:1,thats with 91nitrous 93 gas pills

when nitrous is applied being a little rich then leaning out is that desireable? i asked magnafuel where to go from here they said the regulators i have are dead head regulators so if i wanted to i could run a -3 line with .025" pill inbetween fuel regulator of nitrous selenoid and the selenoid and return to tank.

dunno.....thanks in advance!!!!!

[John_Heard]

No, ass fat off the hit isn't really what you're wanting. With the jetting your using and psi, I'm sure it's quite fat.

In my opinion, the bypass .025 pill back to the tank is a good idea. That will let your regulator maintain your set pressure at the hit. There are varying opinions on this, some people want that extra pressure to help get the fuel there quicker.

First thing you need to do is back off on the fuel jet on the plate you're using. Perhaps the best thing to do would be to back off down to a 200 shot or so and get it running clean and mean, then start working your way up. With the hit you got on it now and that much fuel, it's going to be hard on the engine. Maybe try 60N/54F at 5.5 to get started with, get some test passes on it and look at the plugs, then see where you are and step it up from there.

[Badfish67]

I have almost the same fuel setup on my car and here is what I had to do...
Get the bleed off kit from Magna and add one bleed from the engine regulator and another bleed from the nitrous regulator. They make a fitting that attaches both bleeds into a 8AN. (I had an extra 8AN line from my old system that I can use as a return to the tank). The Y fitting was a 2- 3AN's into a 8AN. After you setup the bleeds, you will be able to really adjust your A/F because the pressure will stay constant.

[Craig W.]

A big shot plate will run very well with square jetting at 5 psi flowing through the fuel jet. I ran a long time at 5.5 psi and have started pulling it back to 5 psi to get more out of the tune-up. Plugs look awesome.

[572 BBC]

thanks so far!!!!!

is there any truth to going fat on initial nitrous introduction then leaning it out being the best?cause if so then i might wanna leave....------->?

or is it best to have constants being the most desireable?

again thanks in advance!!!

[John_Heard]

This could be a long discussion bleed vs no bleed... it's something you'll have to decide for yourself. I like having the bleed on my car for several reasons;

• Prevents pressure from over-ranging my 0-15 psi data logger sensor
• Allows me to look at my pressure gauge and know where the pressure is
• At launch on my car, I pull extra timing timing out, having a little less fuel at the hit makes sense

Without the bleed the pressure will start creeping up to pump pressure, which will peg your gauges and won't drop till you open the solenoid.

[572 BBC]

THANKS SO FAR JOHN. I ORDERED 3AN LINE AND FITTINGS,TO BE PLACED RIGHT BEFORE MY GAS SELENOID,NEXT QUESTION IS SHOULD I GO REGULATOR TO BLEEDER FITTING TO FUEL PRESSURE GAGE TO SELENOID OR GO REGULATOR TO BLEEDER TO SELENOID THEN FUEL PRESSURE GAGE???

BASICALLY GAGE BEFORE OR AFTER SELENOID OR SHOULD IT NOT MUCH MATTER???

BY THE WAY U GOT ONE STOUT NOVA JOHN!!! WHAT HAS THAT MONTER RUN IN QUARTER?

[John_Heard]

THANKS SO FAR JOHN. I ORDERED 3AN LINE AND FITTINGS,TO BE PLACED RIGHT BEFORE MY GAS SELENOID,NEXT QUESTION IS SHOULD I GO REGULATOR TO BLEEDER FITTING TO FUEL PRESSURE GAGE TO SELENOID OR GO REGULATOR TO BLEEDER TO SELENOID THEN FUEL PRESSURE GAGE???

BASICALLY GAGE BEFORE OR AFTER SELENOID OR SHOULD IT NOT MUCH MATTER???

BY THE WAY U GOT ONE STOUT NOVA JOHN!!! WHAT HAS THAT MONTER RUN IN QUARTER?

You want your fuel pressure gage on the regulator side of the solenoid. If it was on the nozzle/plate side of the solenoid you would not be able to see what the pressure is until you activate the solenoid.

Thanks.. I haven't ran my car in the 1/4 for a long time, it runs in the 5.20's 1/8th mile @136 - which is around 8.12 give or take some in the 1/4. I hope to improve that by quite a bit next time ol' Blue hits the track!

[572 BBC]

i have bleeder line on.set at 6.5 lbs flowing thru plate.when celinoid is closed its at 7 lbs pressure.i opened celenoid for 10 seconds(more than needed) and fuel pressure didnt move.(motor was not running)so for now i hope with wide open throttle going down track it reads same!

on big shot plate im gonna try to stick with 91 or 93 nitrous pills(260-275 hp shot) and probably start with like a 88 fuel jet and see where itll be on air fuel ratio or is that too aggressive(too lean)when motor only runs right at about 12.7:1 air fuel...it seems like i should be shooting for 11.5 to 12:1 air fuel ratio when on the bottle right?

thanks again sofar john!!!

[John_Heard]

While I would not suggest ignoring the AFR, what you want to watch closely is what the plugs are telling you in regards to rich or lean. That 91/88 at 6.5psi will be fat, need to start at 5.5 or put a smaller fuel jet in it. Even the 93/88 at 5.5 or so will be likely rich. Lots of things affect this like engine type, how rich the carb is, etc.

Starting out like this it would probably be better if you begin with a smaller hit then work your way into the bigger ones. With most of the ones we've been playing with lately we've been running them much leaner with plenty of timing out, get the plugs looking good then start putting the timing into it to get a decent looking mark on the strap. What other jets you got to play with that are smaller?

[572 BBC]

the kit came with91 then next size smaller is 82.im gonna pick up a few inbetween.maybe then ill go like 85 gas and try.(?) its the standard big shot kit

when i first started playing with setting the gas pressure i noticed a pulsation when it was flowing under 6 lbs so thats why i set at 6.5 lbs

i have motor pretty much running at 12.7 air fuel ratio so its just about where it needs to be right? and is 11.5 to 12:1 a good number on nitrous?

[John_Heard]

the kit came with91 then next size smaller is 82.im gonna pick up a few inbetween.maybe then ill go like 85 gas and try.(?) its the standard big shot kit

when i first started playing with setting the gas pressure i noticed a pulsation when it was flowing under 6 lbs so thats why i set at 6.5 lbs

i have motor pretty much running at 12.7 air fuel ratio so its just about where it needs to be right? and is 11.5 to 12:1 a good number on nitrous?

In my opinion, AFR is just a reference number. The car will tell you what it wants, it might want richer or leaner AFR's to run it's best numbers - the ET slip is the final word. I think Wide bands are great to see what's going on during the run, letting you see what parts might be too lean or rich - that's their big contribution to the program. But taking some other cars best AFR and trying to run your car at it will not likely be a perfect deal - your sensors might be a bit different, your engine more or less efficient, etc.

If you're motor is at 12.7 how do you know that's the best AFR? Yeah, it sounds good but it you were shooting for that and not the best ET/MPH you may have some more left on the table. In regards to what it wants on the spray, in my opinion ignore the AFR and read the plugs and get them looking great. Then start looking at your AFR for a reference and how the fuel curve is looking down track. But to answer your question, sure 11.5 - 12.1 is probably not bad, it's certainly not real fat at those numbers but some run them even leaner. But go by what the plugs look like.

[572 BBC]

my questions are:
1-lean is mean.if u run best mph with car it might be too lean then? can be a scary thought on a spray pass

2-my air fuel readings when on chassis dyno showed with carb set to run 12.7 thru whole operating rpms(motor only) when i let off the thing runs real lean(on gage shows like 15 or more to one)i wished i seen what it did on spray pass

so in short can plug readings might be a little false perhaps cause if its rich when getting off throttle motor will go lean and clean off plugs?
thanks!

oh u said start off small shot and work bigger.ok well if i find a good tune up on 175 shot then want to step up say 8 sizes on nitrous would u feel 8 sizes on gas would be ok?in short if i increase one does the other have to go up same then?big shot 4500 plate if i didnt say already

[John_Heard]

Yes lean is mean - sounds good but really it's the safest bet for your engine. Where people burn things up is when they have more timing than the engine wants. I believe that is the general consensus at this stage of the game. More timing is an easy trap to fall into as when you keep cranking more and more into the engine ET's drop and MPH climbs, but if you go too far you end up hurting parts. On a nitrous car it is essential to learn how to read a plug and recognize early signs of getting to much heat into the cyl.

In regards to what the AFR is doing in shutdown, sure it'll go lean. You've snapped the throttle blades shut and turned off the fuel. Intake vacuum shoots up and you're not drawing the same amount of air through the carb so it will be lean. It won't hurt anything because there is no heat.

Would you believe I've sprayed a 400 shot one time with no added fuel? It happened a couple years ago when we had a solenoid problem on the fuel side. The car slowed down a LOT, but didn't hurt anything. Got it back home and unhooked the fuel lines from the solenoids - it wasn't moving anything! Scary but true. What prevented it from burning up was it was super lean, but not hot because there was no extra fuel to burn. Cylinder temp and pressure is a parabolic curve, you can be on either side of max temp and max pressure (timing) and be safe, when you get towards the peak of the curve is when things start to get hurt at high cyl temps and high combustion pressures due to timing.

In regards to the false plug readings from shutdown, I don't think so because the cyl temp drops quickly when you pull the fuel out. Would you get a better read by clicking it into neutral and turning the ignition off? Maybe - but it's not a safe thing to do putting it in neutral at speed like that sometimes people end up in reverse - ouch, and if you have a Turbo 400 it can actually blow the transmission up sticking it in neutral.

When stepping up from a good known tune up you want to use the same n/f ratios as a starting point, not the same jet sizes. For example, say you were at 60n/55f at 5 psi, that would be a 6.36 N/F Ratio (950 psi bottle). If you added 8 jet sizes on both and kept the same pressures, it would go to 6.22 N/F Ratio (fatter). In order to match the N/F ratio, you would want 68n/62f at about 5.1 psi which would give you 6.38 N/F ratio which would be real close to where you were at on the smaller jets.

This is where my friend Dave Kohler's Nitrous Master software is real handy. You can stick your numbers in and see where the N/F ratios come out with a given jet size. I use that sucker all the time, highly recommended - http://www.koehlerinjection.com/Koehler ... master.htm

[572 BBC]

When stepping up from a good known tune up you want to use the same n/f ratios as a starting point, not the same jet sizes. For example, say you were at 60n/55f at 5 psi, that would be a 6.36 N/F Ratio (950 psi bottle). If you added 8 jet sizes on both and kept the same pressures, it would go to 6.22 N/F Ratio (fatter). In order to match the N/F ratio, you would want 68n/62f at about 5.1 psi which would give you 6.38 N/F ratio which would be real close to where you were at on the smaller jets..........

this is your above paragraph.when u say 6.38 and 6.22 n/f i didnt know what it meant.but i think i know what your saying about trying to shoot for same air fuel ratio weather if its 175 shot or a 375 shot.

so what your saying is when spraying lean it out till your car slows down,but be cautious of timing(ground strap radius is where i want heat line i know that).so if i do that i wont be in danger of aluminum specs of piston being burnt onto porcelin or any other damage then?



i highly recommend this site for people to check out your da man john!


also are nitrous jets created equal? like i said i have a big shot kit. my friend said he has every size jet for $5 a piece but not nos brand.if i could get nos brand ones at a comparable price id rather get brand named ones...what are thoughts on it?thank you!!!