The search function is still down, and I can't find this in the "V-AFC settings library".

When I put my S on a dyno to tune my V-AFC, what AFR should I be targetting? A friend of mine has an identical setup as me, and his stock air fuel graph ranges from 15 at 3500 RPMs to 11.5 at 8500 RPMs. It seems that if my AFR's are like his, I will need to richen up a bit at lower RPMs and lean out at higher RPMs.

 

Assuming you still run a catalytic converter and that you will be running the wideband O2 sensor in the tailpipe...

Shoot for an average A/F of about 13.5-13.7. No leaner than 14.0 (although a small spike is o.k., just don't let it stay there, especially at high rpms). When measuring A/F ratios after the cat, you'll be reading something that's actually 0.1-0.2 points leaner than the mixture before the cat (which is what counts).

I've seen some S2K's tuned to A/F ratios in the 14-14.5 range across the rpm band, and it makes an extra hp or two, but that's really too lean for extended running IMO, especially if you track the car. Plus, cars tend to run a bit leaner in street situations than on any dyno, especially under high speed, high load conditions.

For reference, the Mugen ECU equipped cars I've tested tend to run right up at 14:1 at lower rpms and drop down into the high 13's at higher rpms.

UL

 

[QUOTE]Originally posted by ultimate lurker
[B]Assuming you still run a catalytic converter and that you will be running the wideband O2 sensor in the tailpipe...

Shoot for an average A/F of about 13.5-13.7.

 

Yeah, except the Mugen ECU actually runs a bit too lean in some areas for my taste. It also runs too much ignition advance for 91 octane gasoline, and is a bit too tolerant of detonation. Finally, there's that $1200-$1300 price tag. For the extra few hp you get over the VAFC, the extra $600 you spend may be questionable for many.

No doubt that the Mugen ECU works, but at the price level its offered at, those in search of truly maximized power will probably look at standalone setups. For a production race series its probably the best option though.

UL

 

Thanks ... I agree the price is a bit high. Here in FL we can still get 93 octane gas, so it may help matters a little. I thought about getting a VAFC; however, there are not many shops in the area to tune at, at it sounds like it may take a few visits to the dyno to get it set up correctly.

 

UL,

Do you have any pointers that you could pass along to help tune the V-AFC quickly? For example, if my air fuel ratio for 7K RPMs is at 12.0, and I want to lean it out to 13.5, what change should I make to the correction factor at that RPM? I know there will be other factors, but I'm just looking for a starting point, so I don't have to buy so much dyno time.

 

Gernby, I like to look at it from a percentages angle.

If you have an A/F ratio of 12:1, you've got 12 parts air to 1 part fuel (by weight). If you're shooting for a 13.5:1 ratio you need to reduce fuel since the amount of air being ingested is (relatively) fixed.

A 13.5:1 A/F ratio is about 12.5% leaner than 12:1. If we reduce our fuel by 12.5%, we end up with a ratio of 12:0.875, or corrected it ends up being 13.7:1. So, you can see, just by looking at percentages on the A/F ratio, you can get pretty close. What I normally do is make a rough estimate of how much leaner the mixture needs to be and then change it by a little less. So, in the case of trying to go from 12:1 to 13.5:1, I'd reduce fuel by about 10%, which theoretically would give me a 13.3:1 A/F ratio.

With practice you get pretty good at making a very close estimate the first time out, assuming the ECU and engine respond as you expect them too (and that's not always the case). The key is to make sure that you control conditions. If you make a pass at 175F coolant temp and then make one at 188F coolant temp, the mixture will get up to a half point richer without making any changes to the VAFC, so you have to be consistent.

UL

 

Thanks! That is exactly what I was looking for. Hopefully, I can do a base run (or two?), let the car cool down while I calculate my corrections from the AFR plot, program my calculated fuel corrections, do another run (or two?), let the car cool while I fine tune my calculated corrections from the new AFR plot, program my fine tuned calculated fuel corrections, do another run ... etc. etc.

Does this sound right? How many pulls should be done between cool downs / calculating and tuning? When I look at my friend's plots (3 pulls separated by about 4 minutes each), it is apparent that the 1st pull was significantly leaner than the 2nd and 3rd pulls. At 6500, the AFR for the 1st pull was 13.0, 2nd pull was 12.0, 3rd pull was 12.0. What AFR would be used as my base. Would coolant temp change that much between 3 pulls in 8 minutes?

 

I created an Excel spreadsheet to calculate correction values using the actual AFR from the dyno plot and the desired AFR and UL's equation (Correction = 100 ( [Base AFR / Desired AFR] - 1)). I either screwed it up, or my friend's car is in SERIOUS need of a V-AFC.

Does this look right? (This car has an AEM CAI, hence the lean spike at 3500 RPMs)

 

Gernby, have a look at the VAFC Library, the "trend" seems about right (IE, about 10% leanning out at low RPM and 20% at high RPM) But I'm still questioning the wisdom of tuning towards 13.7 all the time.

My time of the dyno has shown that at some points, power is still achieved when tuning above 14.7 (stoichiometric) and others, power is lost once gone richer than about 12.7.

As my tuner says, every engine is different and you have to tune for the particular engine.