Afr's at lower revs

[gav2612]

Hi guys, just looking for some advice. Have a 96 1.8 with JR. supercharger. Bought it in the summer for hooning about in and track day use. It's running smaller nose pulley, fmic larger front rad and oil cooler. It runs well other than some dodgy idling which I'm pretty sure a dual tb set up will sort in the future. Now my question : AFR's at higher revs and WOT are in the region of 11.5 however below 3500 WOT afr's are only in the region of 13.5- 14. Is this too lean for a boosted engine, even for one at relatively low boost levels? Fueling is dealt with by an emanage blue using MAF so I have the ability to modify afr's. I'll probably go MS sometime soon to get better control over fueling and timing and loose the MAF.
Any help appreciated

[Richyvrlimited]

gav2612 said:

AFR's at higher revs and WOT are in the region of 11.5 however below 3500 WOT afr's are only in the region of 13.5- 14.

Still at WOT? if so, then yes, too lean. If it's mid/partial throttle, it'll be fine, if it's light throttle then it's probably too rich.

[gav2612]

Thanks, Yes still at WOT. It's much richer above 3500 revs. Should I be aiming for similar WOT afrs lower down the Rev range as well then or slightly leaner than at redline?

[skinny]

what's your boost level?

you are quite susceptible to knock at high load, low speed areas of operation, and so chances are your timing is retarded from optimum and you'll need to chuck some extra fuel in to keep the exhaust valves cool - so yeah, 13-14 AFR probably too lean.

[gav2612]

Boost levels are low at around 5.5- 6psi. Was expecting a bit more with the smaller nose pulley and spent a bit of time making sure the belt was tight and replacing the long routed intercooler piping with an over the rad job. Net gain was virtually nothing or .5 PSI if anything. Its an old original JR charger and if I can believe the former owner(no receipts) the head has had some flow work done and it also has forged pistons fitted. The exhaust is a stainless performance manifold and system with de- CAT, so all of the above may well lead to a reduction in backpressure bringing down the boost levels. Have an MS P&P lying in the garage which I plan to fit soon which will give me some more control and also more accurate boost levels than the current anologue gauge. Thanks for the help.

[MX-5 Lazza]

Do you have an O2 clamp? Without it you won't have a lot of control of AFR at lower revs as the ECU will be trying to correct the fuelling.

[gav2612]

Will need to check out the o2 clamp, would it usually be wired at the O2 sensor end or nearer the ecu?
From my understanding, it was only necessary when the car was in closed loop when the EMB wouldn't be controlling the fueling. Would it be closed loop at 3000- 3500 rpm WOT?

[snotrag]

Yes, and I also suspect same as Lazza. I have emanage on a turbo car and low room, high load is exactly where it falls down. I built my own 02 clamp which works great.

Follow the 02 sensor wire all the way in, of you don't find anything spliced in you have no clamp!

[MX-5 Lazza]

Over certain revs (I think it might be 4k) and at WOT the ecu will run from built in maps. All the EMB does is intercept this signal and add/subtract injector duration or ignition timing based on the EMB maps.
Below that point the ECU will run from the sensors and will aim for certain AFRs. The EMB map might try to add injector duration to richen the mixture but the ECU will try to subtract duration again to hit it's AFR target.
An O2 clamp just sends a constant stoich (14.7) signal to the ECU. This leaves the EMB free to control the AFR at all revs.

[snotrag]

~4200RPM, but I think it varies a little with coolant temp.

Very very obvious in my experience when running without the 02 clamp.

[gav2612]

Thanks guys, think you might have hit on the issue. Can't find any spliced wires between the O2 sensor and ecu. My next question then is, how to make an O2 clamp, or where to buy one if I can't? Did a quick google search, but all I could come up with was a user on one of the miata forums in the US selling home made ones. It's either that or I start over with the MS p&p I have sitting in the garage, but to be honest if rather not have the hassle if I can get it running ok with the EMB and O2 clamp.
Thanks again

[MX-5 Lazza]

If you are running stock injectors or even a little bit bigger and aren't chasing big numbers then the EMB is fine.