While I write, my car is in the shop getting my supercharger installed and I have a ? regarding the A/F mixture rates. I keep reading the importance og this yet I do not know what the correct A/F is and how do you determine it for the G35. I would appreciate any answer to this.

thanks,

DJ

 

A/F ratios from a chemistry standpoint would be ideal at 14.7:1. This is where a stoichiometric burn would exist where all fuel and air molecules would be spent. However, in reality the actual A/F varies dependent on many factors. Higher loads usually desire richer mixtures. This has to do with preventing detonation. Higher compression motors and forced induction motors are more prone to detonation. Higher octane fuels are desirable, but when working with the fuels available to us, a richer mixture can help reduce the chances of detonation. Heat being the biggest factor in detonation, controlling it yields good results. Intercooling helps to cool down the intake charge of forced induction by reducing heat. A fuel naturally does the same by absorbing heat when the fuel atomizes(evaporates from liquid to gas). Richer mixtures obviously have more fuel. when more fuel is atomized, more heat will be absorbed, lowering temperatures inside the combustion chamber.

That being said a slightly rich mixture is desirable to safely make more power and increase efficiency by being able to utilize higher compression ratios and more timing advance. This becomes more of an issue when you get into forced induction. Especially when coupled with relatively high compression ratios. Typically, I've seen anywhere from 12.0:1 to 13.5:1 in forced induction motors. But since each motor has it's own set of variables that need to be accounted for, there is no ideal A/F ratio. Even the climate you drive in can have an effect. Also keep in mind that going too rich can foul plugs and actually stall the engine. Fuel likes to be relatively warm to induce good ignition. Also, these A/F ratios might be what you see at Full throttle. Part throttle A/F's vary much more, and are more typically closer to stoichiometric as loads are lighter. Some modern engines are starting to run on the lean side of stoichiometrc at lighter loads. We'll likely see more of this as Direct Gasoline Injection motors become more widely used.

When choosing an A/F from a power standpoint, you want the leanest A/F possible without detonation. Keep in mind that this may vary from one RPM to the next. The conditions present inside a combustion chamber vary based on RPM.

 

thanks for the info. I appreciated.

DJ

 

"stoichiometric burn"......damn!

I think I got one of those from my living room carpet once......
my stoichio was sore for a week.......

 

I tried to keep mine under 12:1

www.kptechnologies.net/dynos/dyno.htm

for my dyno sheets (and to see why my supercharger blew my motor up)

 

13:1 is far too lean for a daily driven high compression stock internaled VQ.

For a non-scientific answer.

11.5:1 at WOT. Idle and cruise will be 14.7:1 or nearby.

The only way to measure A/F ratios accurately is with a dedicated wideband O2 sensor. AEM, Innovative, and others make good gauges.

Do not waste your money on a cheap narrow band gauge. They are worthless for tuning.

 

Hey gq, you bring up a good point. With us built internal people what is a good A/F goal? I am gonna play it safe and og with probably around an 11.5:1 but who knows at this point. (since my car is still in the freakin body shop)

 

Hey K,

With lower compression, you can run a slighly leaner A/F ratio safely...especially if you have access to 93octane fuel locally. Detonation can destroy forged internals as well...they are just less prone to the damaging effects of detonation.

But like you said, I would still play it safe and run about 11.7:1 as the target. Theoretically, peak power is acheived with a ratio between 11.7 and 12.3:1. If you go leaner that that, you will not make more power with most engines...and it becomes very unsafe.