What is your target A/F with nitrous?

[bobc455]

I've been leaving my FAST set for 12.2:1 on the nitrous, but I get the feeling a lot of people go richer (11.2 - 11.5:1) on the bottle. My engine didn't like 12.2.

Just as a general poll, I'm curious what other people set their target A/F for on the juice.

Thanks,
-Bob Cunningham
1969 Buick Special Deluxe 4-door w/ FI 455
bobc@gnttype.org

 

[bobbyp]

I noticed your post, no response, surely someone besides
you and me are using nos. I have not sprayed mine yet, but
hope to within the next couple of weeks. Do you have any
advice? I'll be running a wet system.
Thanks
Bobby Patterson

 

[HighPSI]

I start at around 10.0 and then slooooowly lean it out from there. These are cars with big fogger kits.

 

[bobc455]

Yeah, I was definately hoping for more of a response! I know that lots of people run nitrous with a FAST, but maybe they do like I did and keep the nitrous control away from the FAST so they don't have a seperate target A/F for nitrous. Or maybe they want to keep a trade secret.

Oh well, I guess I'll start out at 10:1 and see how it goes. I am guessing I'll end up around 11.5:1 or so, I'll let people know when I've arrived at an answer. Or maybe I'll only tell Bobby and High since they are the only ones who responded- I know there are others reading! REVEAL YOURSELVES!!!

-Bob Cunningham
1969 Buick Special Deluxe 4-door w/ 455
bobc@gnttype.org

 

[Craig Smith]

FWIW Bob, the method we have suggested is getting your baseline tuneup squared away (and I am sure you are more than set in that department) and adding the appropriate amount of fuel for the shot of nitrous you are adding. We generally recommend 0.5 lb/hr of fuel for every HP of nitrous you inject as a starting point. I really don't know how this impacts the a/f ratio. The one thing to keep in mind is that the more nitrous you inject, the more likely you will have errors in the a/f readings. An engine simply can't use all of a big shot of nitrous and a lot of it winds up getting pushed right out the pipes. When you shove raw nitrous across the O2 sensor it isn't reasonable to expect that the readings are still dead-on.

For this reason, I would pay more attention to how much fuel you are using in lb/hr, how the car feels, and looking at the spark plugs than an O2 reading in a nitrous application. I have a feeling that your end result would be similar if you did it this way or did it Cal's way, but because of the possibility of false readings from the O2 sensor AND the fact that nitrous is some scary $#!+, I would recommend to go by fuel consumption and plug readings. Make absolutely sure that your injector flow rate is properly entered in C-Com to ensure the most accurate results.

 

[bobc455]

Hmm, very interesting.

I would also assume from what you're saying that when using a "big" shot of nitrous (I typically spray 150HP), you should run in open loop. Otherwise the FAST might be trying to correct for a situation that doesn't exist...?

Unfortunately I run a wet shot of nitrous, so I can't really calculate how many #/hr I'm using. So I will have to adjust my fuel pressure on the nitrous enrichment until my plugs / "feel" are good.

I appreciate the information.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

A 150 hp shot is easily manageable. I would probably start by adding 75 lb/hr of fuel, but if you aren't using dry flow that is tough to do. I see now. I guess at this point I would revert to Cal's suggestion of starting at 10:1. I wouldn't worry too much about false readings with a 150 hp shot.

 

[HighPSI]

Apparantly, I have a different definition of a "big shot". I was talking about 300 hp fogger kits (that are wet kits)

 

[bobc455]

Well I'd say 300HP is a big shot- anything over 150HP gets "really serious" in my opinion...

I am equipped to step up to 300 or more eventually, but for the time being I am going to get very used to running a 150 shot and then later on maybe start jumping up to 200, 250, etc. However once I go over 150HP, I think I'm going to drop into open loop and go by plug readings, etc.

Part of the reason for my question is that I have been running the nitrous completley seperately from the FAST- I would just load in a setup with the timing backed off a bit. This summer I will probably be at least telling the FAST when the nitrous system is armed, and let it think that it's triggering the nitrous (even though it isn't) so it will automatically back off the timing and shoot for a new A/F ratio.

One thing that I found when running nitrous is that even after I had the FAST tuned perfectly (<3% correction) and I set up the nitrous according to the recommendation of the system supplier, I was running insanely rich- the O2 would just peg at max rich (9.1:1?) (I think that's the opposite of the symptom that Craig described) and even after backing the fuel pressure down on the nitrous enrichment it would still be very, very rich (even with -25% O2 correction). So I will be trying to tune all over again, and was curious where I might shoot for on a target A/F...

I am going to err on the side of running a bit rich, even if it costs me a couple of HP, just for safety's sake.

Thanks for the insight!

-Bob Cunningham
bobc@gnttype.org