What The Hell Is Causing This?

[rev210]

Here's a tricky one;

A friend came over today to ask me about a recent tune on his car, having a look at the dyno I was struck by a very weird issue it seems to having. Apparently the tuner he used was unable to get rid of it nor suggest a cause.

The car is an R33 gtst with

turbo back exhaust

9psi std turbo

Auto

Exhaust cam gear

S-AFC

There is a rather concerning leaness occuring between 3,600 and 4000rpm.

It's been a while since I tuned a S-AFC but, I found the settings a little wierd, I am sort of the view that the high/low throttle transition calcs may be at fault but, before I go asking to borrow a wideband again I was wondering if we had any ideas.

I had a look at the S-AFC settings and found they had high throttle, adding about 8% from 2500 to 4000rpm. There were no adjustments at all to the low throttle settings for fuel and they left the hi/low change over at 10% & 50%

Any ideas?

[QWK32]

but at what throttle % is it leaning out at? and how do you know its leaning out?

the tune you have described is adding the 8% extra fuel after 50% throttle point. the SAFC will interpolate between the 10% low throttle and 50% hi throttle change over points. so it will slowly increase the % of extra fuel from the set 0% extra at low throttle to the set 8% at hi throttle. e.g at 30% throttle the SAFC may have interpolated a value of 3%.

there is no point trying to tune under 10% or low throttle because the car will be in closed loop so the ecu will try and correct itself using readings from the O2 sensor. if you know for sure the car is leaning out take it to another tuner? you should be able to get it if there is no underlying problem like low fuel pressure or maxing out injector duty cylces.

Edited June 4, 2008 by QWK32

[rev210]

Hi QWK32,

I understand how to tune the S-AFC I have ,done a few in the past myself.

The issue may actually not be related to the S-AFC at all but, I am stumped as to what else it may be. I am sure the tuner he took it to would have had a good crack at it but, apparently couldn't get rid of the blip. S-AFC's take bugger all time to setup on a dyno or even without one.

I am 'another tuner' that he's taking it to (rather reluctant one in this case as I need to scab a wide band). I'd rather not have to if there are some other areas to look at.

P

[STATUS]

what boost is it making at the peak of that lean spot?

[rev210]

only 9 psi.

[nizmonut]

3700, isnt that about the rpm that the nvcs kicks in on those rb25 heads? maybe taking a moment longer for the ecu to match the fuel. ??? just a suggestion.

[sl33py]

Yep, I was gonna suggest NVCS. My mechanic said even PFCs and electronic boost can sometimes give slight (though fleeting) strangeness at the point NVCS kicks in.