I'd like to share my findings about air / fuel ratios and how the S-AFC works on the G35. Apparently there is an automotive industry standard where most companies try to design their engines to a 14.7 (or Stoicheometric) A/F level.

If a NA engine wants to make more power, typically you can "trick" the ECU (and or MAF) into thinking the car is getting too much air. The ECU will then tell the car to add in more fuel (or Hydro-Carbons, HC). This in turn "richens up" the A/F mixture and creates more horsepower, but can also allow unburnt fuel to travel into the heads, cats and out the back of the exhaust. (Have you ever seen race cars that have fire coming out of their exhaust tips? This is because very hot,un-burnt fuel eventually hits the air and combusts.) The negative results of this mod are decreased gas mileage, and potentially contaminating the headers and cats with un-burnt fuel.

If you have a TT set-up on your vehicle, a more lean condition might be desired, but there are downsides to this condition. On a more lean side of things there is less fuel being sprayed onto the piston (I believe piston is the correct word). Fuel acts as a coolant to the top of the piston, and when there is less fuel, the piston can get a hot spot and eventually burn a hole completely through it and BOOM! I'll be the first to admit that I don't understand how the lean condition applies to TT setups, but I'm learning.

The following posts will be my attempts to show you some published information that shows how this "delicate balance" between performance (HC), fuel economy (NO), and emisions (CO) is obtained.

Self proclaimed member of the 300bhp club (soon to be even more [img]/w3timages/icons/smile.gif[/img])

 

The following comes from an Engineering book that a friend of mine used in his engine class while getting his masters degree. I can quote the reference upon request (I left it at work on the scanner).

Self proclaimed member of the 300bhp club (soon to be even more [img]/w3timages/icons/smile.gif[/img])

243041-air fuel ratio -1.gif

 

Here is the chart that shows how lowering the A/F ratio (increasing the amount of fuel) will help with an increase of hp, and also increase green house gasses and decrease fuel economy...


Self proclaimed member of the 300bhp club (soon to be even more [img]/w3timages/icons/smile.gif[/img])

243042-air fuel ratio -2.gif

 

But Neff this is implying that it's "the richer the better". Wheres the curve stop climbing? Eventually it will be too rich, and combustion will not be possible. There's an optimal power area. It's different for every engine. What's the optimal power mixture on the G, and why does Technosquare claim the G is too rich after 5000 RPMs? (or somewhere around there)? Thanks for the free research, I look forward to more!

2004.5 Coupe/5AT/Ivory/Everything but chin spoiler/Z Tube-Popcharger/Crawford V5 Plenum on the way

 

Because coolant system cannot handle full power for very long, plus the cats will over heat and melt......extra fuel above 12.0-12.5 AF is added to cool things down [a partially drowning effect]. to compensate for miniscule radiators and cat size.

After all the amount of air is the limiting effect on power production.

If anything you want less fuel from what oem software delivers at WOT........not necessarily during the acceleration but once peak rpms are achieved.

 

That's what I thought. So this device must be leaning it at higher RPMs and sacrificing cooling to achieve optimal power in the higher RPM range, right? Am I misunderstanding something? I think that's exaclty what Technosquare and JWT do as well, so what's this device doing?

2004.5 Coupe/5AT/Ivory/Everything but chin spoiler/Z Tube-Popcharger/Crawford V5 Plenum on the way

 

I wish Q45tech would "dumb down" his statements a bit more. I'm really interested in this subject, and my own research/question asking is reaching its end...

-fish, "where does the curve stop climbing?". From my understanding, you want to burn ALL of the fuel. At the point where you stop burning ALL of the fuel, this is a theoretical "end point" for the A/F ratio on a NA engine. The question should be, "Where does the G35 quit burning fuel?" This is the point where you are wasting fuel and killing the environment, not to mention your cats and possibly the headers. I guess you'd have to dyno tune the S-AFC to get an accurate # for the G35, however I can tell you that the Porsche GT racing numbers start at the mid to high 13's (under a yellow flag) and end around 12.5-ish (when going at it all out). After that point they are "waisting fuel" so there is no reason to go to an A/F ratio of 12.1 or 11.5.

I wish I understood more about how the SAFC mod worked with the G's, maybe Q45tech could enlighten us some more.

Some questions I'd like to know the answers to would be...

1. What is the "ideal" A/F ratio for the G?
2. Is there an "ideal" number or does the number vary over the rpm range?
3. What is the stock A/F ratio for the G?

Self proclaimed member of the 300bhp club (soon to be even more [img]/w3timages/icons/smile.gif[/img])

 

Isn't an A/F ratio of 12.7 optimal always? Don't our ECUs constantly monitor O2 levels in the exhaust gases to infer the A/F ratio at any given time, and either add fuel or reduce fuel on each cylinder charge in an effort to maintain an A/F ratio of 12.7/1 at all times? Isn't this true of all internal combustion gasoline engines?

'03 G35s 5AT, Garnet Fire/Willow
18" SSR GT1/Bridgestone PP S03
Eibach Springs/Sways, Stillen 2-pc BBK
Stillen SS Cat-Back Dual Exhaust System <P ID="edit"><FONT class="small"><EM>Edited by bullseye on 05/01/04 12:38 PM.</EM></FONT></P>

 

Neff,
Adding too much fuel will do more than kill cats and trees. It will reduce your power. The excess fuel will drag your HP down. In my piston airplane, I have a fuel totalizer, cylinder heat temp gauge, exhaust temp gauge, and a mixture lever. When I run too rich I can actually watch my fuel use go up, my temps go down, and my altitude or speed go down due to the decrease in power as a result. (you can also cool "lean of peak" too but that's a different thread.)

What Q is saying (I think) is that the G is rich at the higher RPMs. This is what Technosquare and Doug at Crawford have told me too. Leaning it makes a more eficient burn (more HP) but is riskier because Nissan has designed this engine conservatively by using fuel to cool since the radiator can't keep up at WOT.

I have seen Q45tech post on other boards citing that the only way tricking the voltage sent by the MAF was to trick it to run leaner. He was the source I was citing earlier so I'm glad he popped in. Sometimes you have to piece together several of his posts because he can be quite terse and forgot more this morning than I'll ever know about these cars.

2004.5 Coupe/5AT/Ivory/Everything but chin spoiler/Z Tube-Popcharger/Crawford V5 Plenum on the way