SGOGT4

I am finding conflicting opinions on what the appropriate (safe) AFR is for a 930. I know 14.6 is generally considered optimal but I have been told that it would be too lean on a high boost turbo car.

What do you all consider to be the upper bounds for a car running 1.0B with a big turbo (HFs)?

Also curious to see what you all set yours at for idle.

125shifter

around 12.0 on full boost, 11.5 to be extra safe

SGOGT4

Wow... lots of views but very few opinions... do that many of us NOT know where our motors should be running at?

David - That is a little more conservative than what I was told but much closer than the 14.6

JoeMag

agree with 124shifter. ...remember too that your dyno afr's will be slightly higher than real road driving.

38D

You absolutely do not want a 14.6:1 ratio on full boost. As others have said, something more like low 12 or high 11s. What you find is that the CIS cannot maintain a consistent AFR through the rev range, and will start to lean out at higher RPMs. So, you want to make sure that on boost the leanest AFRs are in the low 12s.

NSXTC

930LDR,

I think you're getting confused with the AFR settings at various states. The 14.5 everyone is refering to is at idle. At full boost, it should be low-mid 12s. During cruise, it should be around mid 13s.

Geoffrey

Ignoring the fact that CIS is not mappable, in an ideal world (EFI), your AFR will be 14.7-15.0 in the cruise range (light throttle, no boost). As you begin to add load to the engine you need the AFRs to be richer. By 1 bar of boost, you are safe at 12.0-12.2:1. Richer than that and it is a waste. At about 12.5:1 and 1bar of boost you begin to risk melting the engine down. Obviously things like compression, cams, etc. may change the requirements.

PorschePhD

I will second that notion. These car under WOT and full boost will live at 12.0-12.3 depending on mods, timing and so on.

A930Rocket

Geoffrey and Stephan,

Is the max 12:0 - 12:3 with a stock intercooler and 93 octane? Would occasional peaks to 12:5 with a kolkeln intercooler and 100 octane be OK? Or ia that pusing the envelope as well?

PorschePhD

No that would not be with the stock intercooler. The stock intercooler you should be runing a lower CR. On a 100 octane you can get away with a 12.5, with C16 even higher.

Geoffrey

The air fuel ratio is used to manage the heat generated during combustion within the cylinder. The higher the volumetric efficiency (load) on the engine (N/A or boosted), the richer the air fuel mixture needs to be. Different engines have different requirements and environmental factors such as intake air temp and engine temp will also have an effect on this requirement. The octane rating which is one of the most misused specifications of fuel will have little to do with cooling the combustion process but rather the latent heat of vaporization would be a better indication. It has been my experience that 12.5:1, even with 116 octane leaded fuel is too lean for 1bar of boost with the air cooled turbo engines. You will melt it down in a racing environment. On the street you may get away with it since you are never on boost for more that 10-15 seconds at a time with long periods of cooling down before another long boost run.

As long as your stock intercooler is able to maintain your intake air temps below 130F then you should be fine at 1bar with a 12.0-12.2:1 AFR on 93 octane fuel assuming of course your timing is appropriate for your engine combination.

SGOGT4

Geoffrey and Stephen -

Given your benchmark numbers at idle, cruise and WOT/Boost, how faw down (rich) is normal for the mix to drop when you go from cruise to full throttle while the boost builds? How low is too low? I am seeing 10.0 for a second or two before it leans out as the boost rises.

Thanks guys

PorschePhD

On a CIS car it is not uncommon to see the AFRs dump to the 10s. What are your AFRs up top? You can tune some of that out with playing with the WUR or installing a 7th injector type set up on the control line and mapping the control pressure via a 3D type map. Much like injection just controlling the control pressure to the head.

SGOGT4

Mid 12s on the top end. Actually I am playing with an adjustable WUR that allows for tuning the top, warm, and cold bands. Other than the fact that I have to take it off the car ot get to the Mid and Top adjustments, it is pretty cool and seems to work well. Unfortunately, due to the nature of the WUR (I believe) it seems that when you adjust one setting, it has a small effect on the others as well which means quite a bit of fine tuning. Beginning to believe CIS is as much art as it is science.