I'm wondering... I got my car all dialed in on my wideband N/A and now i'm gonna tune it this week on the juice..

I tuned the car to 13.0 N/A and I know that most people say to tune for like 11.5-12.1 on the juice... Well why not tune it to 13.0? I know that some people say it's safer to tune it richer but is that just an IN CASE THING?? I mean what if I did tune it to 13.0?

 

With that much spray, you had better pull lots of timing and shoot for 10:1 to 11:1. Unless you have engine rebuild plans in the near future.

 

Not enough info. What are the details of your combo? What cam, plugs, CR, etc. And most of all, what fuel are you using? But no matter what, to some extent this is an empirical exercise. If you have the time (and money to pay for broken parts) the best thing is to find out what works best on the dyno. Experiment with timing, AF ratio, etc. all while monitoring for detonation and EGT's.

The short answer to your question (why not tune it to 13:1) is: it is more likely to break than if it is richer. Keep in mind that running a bit on the rich side costs a few hp but running a bit on lean side costs big $$$. Wanting to opitmize your combo is a big part of this hobby, but unless you are making your living racing, running at the edge may not be the best choice.

Mine is tuned for 11-11.5:1 on the juice+boost.

Rich Krause

 

Good points. One thing to remember also is that richer tuning usually yields higher torque numbers, which are not a bad thing for drag racing.

 

Well, when I built the motor I gapped the rings for juice, I always run race gas on the juice, not to mention pulling timing of course. But that's why im trying to figure out what is a SAFE a/f I can go with and NOT cost me that much power. I know that people go richer to be safe but I don't really care about it because I don't ever miss shifts or have to worry about over spraying or any of that crap.. So If running the car at like 13.0 on the juice does it just make it bad all together? Like is running at that a/f on the bottle just more stressful on things to begin with?

 

The answer is "stoichiometry".

Because nitrous is richer in oxygen than air, it requires a much richer A/F mixture to achieve full combustion. The stoichiometric A/F ratio for air and gasoline is 14.7:1. You typically run 10 to 15% fuel in excess of stoichiometry to achieve peak torque and power. That yields an A/F raio for a naturally aspirated engine in the range of 12.8 - 13.4:1.

Nitrous achieves stoichiometric combustion at a much richer A/F ratio.... about 9.6:1. So.... unless you are running at least 9.6:1 on the "nitrous" portion of the fuel combustion, you are leaving unburned O2 that is going to drive the mixture lean. NOS recommends that the nitrous/fuel ratio be managed in the range of 5:1 to 6:1.

In effect, you are combining two different combustion systems.... A/F at say 13:1 for the naturally aspirated portion, and N2O/F at 6:1. For a system making approximately 3/4 of its HP on "air" and 1/4 of its HP on "nitrous" (example: a 150-shot on a 450HP NA engine), the end result is a combined A/F ratio of about 11.25:1. The greater the portion of the total HP derrived from the nitrous, the richer the mixture needs to be.

 

NICE, that's the scientific answer that I was looking for

 

Fred is like, smart and stuff

 

I don't think it works quite like Fred said. Ignoring stoich for N2O for a second, you need to think about how an O2 sensor works. An O2 sensor measures O2 concentration in the exahust by a chemical reaction that generates a voltage. When there is little or no O2 in the exhaust, then the engine has more fuel than needed (ie is rich). It has nothing to do with stoich for the particular fuel you are using, unless I am missing something. The O2 sensor doesn't care if you are burning gas, methane, hydrogen, or whatever. And it doesn't care how the O2 gets into the motor - from ambient air or from N2O injection. It just measures how much O2 is left. The AF ratio displayed by the meter is really just a an interpretation of the voltage signal which in turn reflects the O2 content of the exhaust stream.

The reasons you always want more fuel than air to generate max hp is that combustion is inefficient. You want to be sure that every O2 molecule that gets into the motor has a chance to react with a carbon atom from the fuel. If you ran at stoich, you would be "wasting" some of the O2 because it wouldn't have time to find a fuel molecule to react with. Excessively rich mixtures are needed for boosted and nitrous apps because they slow combustion and help prevent detonation. Water (or water/alcohol) does much the same and allows a leaner AF ratio in boosted or nitrous apps.

Rich Krause

 

To sum all that up:

Fuel makes the power, not the nitrous - nitrous simply allows the engine to burn more amounts of fuel thus producing more power

 

my brain hurts

 

Rich, what A/F ratio are you tuning at? I've been shooting for 11.5-11.8:1 with the nitrous on. Fred's numbers make perfect sense, but I'm just looking for a second opinion! Definitely don't want to be too lean...

 

Fred's numbers do make sense, but refer to how much fuel needs to go in on the intake side to have the proper ratio for combustion. What you measure with an AF ratio meter isn't exactly the same thing. It's how much O2 is left in the exhaust after combustion, if that makes any sense?

Anyway, for most street/strip setups the ~11.5-11.8:1 that you cite works fine, as measured by a typical wide-band O2 sensor. It may make a bit less hp than a leaner mixture, but is safe from what I have seen.

Rich Krause

 

Rich, I see what you are saying, but I'm a bit lost. If I confuse you, ignore this. But, how can an O2 sensor only measure the amount of oxygen and give you a correct reading. It has to measure the amount of oxygen compared to fuel doesn't it? Otherwise it wouldn't give you an accurate ratio of air-to-fuel right? In other words, lets say I have a stock motor, I run 13.0:1. There will be x amount of oxygen compared to x amount of fuel.

You decide to run your boosted/juiced stroker at 13.0:1. The meter will read mine and yours correctly, even though you have a greatly vaster quantity of fuel AND oxygen. The O2 sensor doesn't have a reference from the injectors or ecu to tell it how much fuel and air was initial taken in, it can only say the ratio left. If it were only taking the amount of oxygen, then wouldn't yours read a LOT richer than mine, because of the different amounts of fuel being used and amount of oxygen taken in?

Does that make any sense?

 

I understand your question. But the fact is that an O2 sensor simply generates a voltage that is in some way proportional to the amount of oxygen in the exhaust gases. Obviously, you don't care what this voltage is. You care what your AF ratio is. So, each different oxygen sensor must be calibrated to. I am not sure exactly how this is done. But I assume it's done by measuring the voltage at various known AF ratios.

IOW, what the actual AF ratio for optimal nitrous performance is not the issue. What we are talking about is "what O2 sensor reading should I aim for". We aren't really measuring AF ratios.

Rich Krause