Safe lean cruise.

[paulg23]

Any idea how lean I can run in closed loop cruise? I see people on other forums running 17:1 on cruise. Would this be an issue for te 6A13TT?

[foxdie]

I'm running 15.2 on mine with no issue whatsoever

I did some research and noticed the Evo boys are running 15.6, so I chose 15.2 as a safe middle ground. It's noticeably added a couple of MPG, how much I can't tell because I've gotten 250 miles out of a mixture of inner city driving, tuning the car at 1.2 bar boost (so lots of WOT) and about 75~ miles of steady motorway / dual carriageway driving.

[Nick Mann]

I had a wierd issue a couple of years ago where my car was leaning slowly on cruise. I never really fully understood why, but the bottom line was the car seemed to be coping fine at 17 AFR at 80mph. I noticed it going wrong at 18.something. I'd be happy to give it a go at 16.0 next time I have the chance to play, but I obviously can't guarantee what you'll get!

[paulg23]

Thanks for that. I can adjust mine from the dash but limited to a max of 16.7 at the moment. Will crank it up and see how she runs.

[adaxo]

What we should look at on lean AFRs ?? temps?? or overall performance?? mine is running at 15.2, 15.8 @ steady motorway cruise, I didn't change any settings on my MTX-L wideband, run like that since first day fitted. Drop to 11- 12 at WOT.

[Nick Mann]

I would imagine temps and knock, but I'm no expert. I'd think that lean burn gives a fast burning mix, with potential for more heat and detonation like burning. I've no idea how quickly you would notice the effects of all that - hopefully before the engine melts!

[adaxo]

That what I was thinking myself, and heat shouldn't be an issue on steady say 70 mph run?? or I'm wrong??

[paulg23]

I would imagine that the heat generated at cruise is going to be nothing compared to that generated at WOT. It may burn hotter running lean but the cooling system still wont have much to handle.

[lateshow]

It's a shame that im using PLX wideband as that one doesn't have the option to change the output signal so only thing I can do is try open loop. Would have to drive a full tank to see does this affect anything. I tried this once but idle was giving me problems. With stock injector scaling I was gettin ~15.5 @ cruise (stock fuel maps in that area) and +17 at idle. I even tried to adjust idle to 13.8 and still was getting 17. Strange. My Bosch wideband unit is now about 3 years old, I think it would need a new sensor. I still cant believe running lean gives much advance.....
My other car, the Gdi can get +23 AFR:s at idle and works nicely.

When datalogging with lean mixture I tried to see if there was difference in the fuel consumption but saw nothing.

[foxdie]

I would imagine that the heat generated at cruise is going to be nothing compared to that generated at WOT. It may burn hotter running lean but the cooling system still wont have much to handle.

Agreed, unless you factor in slow moving traffic, if you're driving steadily at 15-20 mph, the ECU is still going to try and achieve what AFR you set it to, and this may lead to overheating if you've got it set excessively lean (at a guess, 17+), but I believe the radiators will kick in long before any problems happen

Also, for those reading thinking "Don't engines go bang if they run too lean?", the answer is yes, but only under heavy load (such as WOT or accelerating up a hill), and in these circumstances the car changes to "open loop" mode where it uses a fuelling table instead of just trying to hit a target. Suffice to say, it's quite safe to lean out your cars cruising AFR by a small amount.

[paulg23]

I was surprised how little pedal movement it takes to drop out of closed loop. Driving at 70 mph on the A1 if you 'touch' the pedal to compensate for going up a slight hill it goes open loop. Are most cars this sensitive?

[Davezj]

If you can remap your ecu and know a lot of us can.
you can change what AFR you want at what RPM in open loop, so could you lean out the reagion at motorway crusing for a bit more economy.

does the fuel map take into account what gear you are in, with respect to the afr to target?
if so then 5th gear 2500-3000 revs AFR can be leaned out quite a bit.
but if it does not take into accout what gear you are in then doing this will be a bit dodgy.

it has been a while since i looked at me ecu map and you tend to forget how it is all linked together, which table affect what, etc, etc.

what do you think.

[lateshow]

Ok, some results. Drove today to work with closed loop and back home with lean settings. It goes around 15.2 when doing 80km/h, when doing hundred it approaches 14.8. When going downhill it leans out to 16 sharp and so on. It seems that momentary consumption can be lowered when going downhill. Datalogged everything and did some calculations. On the way to work my consumption was 10.38l/100km and back my consumption was 9.93 l/100km. The actual numbers are a bit lower. So yes, something can be done but needs a little tweaking. For example now im getting far too rich mixture with little boost pressures, however there are some parts where I need to make it richer. Heard some strange noise from the engine bay when accelarating on the way home, sounds like knocking but knock sensor picks nothing up (sensor is working however, got sometime 2 knocks when going with full boost, as usual) It goes 11-11.5 now boost.

The car feels somehow smoother and quiter (exhaust) because the mixture stays about the same ( lambda causes a huge loop)

What i've done so far: first took 5% off the injector scaling value
then leaned the map in power area to ~11.5
leaned engine temp vs fuel enrichement

[MackTheKnife]

Tried 15.7 for part of my trip to work today (75kms, mostly freeway). On an long uphill section of the F3 freeway I was losing power and the car couldn't even maintain speed in closed loop running.

Had to pull over and change the switch over point, decided to go back to 14.7 and see if there was any other issue. Once I did that it ran fine. I'll try some other settings to see what I can get away with on that hill, it seems a good tester. I'll have results of 15.5 tomorrow.

[Kenneth]

were you logging AFR and knock at the time? Without knowing exactly what was happening, it is hard to point to the AFR itself. If you were getting knock and the ECU was pulling timing you would rapidly lose power. Likewise if the ECU was making some assumptions on the AFR based on O2 feedback when the AFR was not actually 15.7 (Which narrow band would not be good at determining)

[MackTheKnife]

were you logging AFR and knock at the time? Without knowing exactly what was happening, it is hard to point to the AFR itself. If you were getting knock and the ECU was pulling timing you would rapidly lose power. Likewise if the ECU was making some assumptions on the AFR based on O2 feedback when the AFR was not actually 15.7 (Which narrow band would not be good at determining)

I was indeed, no knock whatsoever at the time. AFR fluctuated between 15.2 & 16, guessing the hill is a bit too steep for that afr.

I have a Zeitronix ZTX-3 in place of my narrowband and have the EvoScan GPS on my dash so was getting live data. I had no traffic around me so could keep an eye on things. Had a Knock Count of 2 at one point earlier in the trip, nothing showed up at the time.

The hill is a steep one, it needed 4th gear to keep 120 km/h in my 2008 Civic and my old 1982 Suzuki Sierra with it's 970cc engine needed 2nd gear to get to the top of the hill. The Legnum was always fine in 5th with tonnes of grunt to spare except for that one episode this morning. I'll keep experimenting, it seems the perfect place to test this out.

Granted that the Legnum is a daily driver that does about 600 km a week, a few MPG will make a difference.

[Kenneth]

And you are sure it was in closed loop? I would have thought the load required in that situation would put the car into open loop running.

You could be correct in that it is just a bit steep for that AFR. Try setting the switch point to 15.2 and see if that works. As I understand it, ~15.2 produces highest exhaust gas temps so should be best for efficiency.

[MackTheKnife]

And you are sure it was in closed loop? I would have thought the load required in that situation would put the car into open loop running.

You could be correct in that it is just a bit steep for that AFR. Try setting the switch point to 15.2 and see if that works. As I understand it, ~15.2 produces highest exhaust gas temps so should be best for efficiency.

Was definately in closed loop, AFR's went to low 14's/high 13's as soon as it went into open loop. Plus open loop had no trouble with the hill.

Will just change the switch point to 15.2 for a while, thanks Kenneth!

[lateshow]

This is why open loop works fine, I can tune it so that uphill takes it back to 14.7 and seems to work perfectly for me. Uphill loads are about 70-100 and downhill is 0-40. Downhill = near to 16, openroad = 15.2 uphill = 14.7 . It seems that it can create enough power downhill with much leaner mixture but uphill seems to be the problem point, i can now get momentary consumptions of 5-6 litres when going downhill. 8-10 open road and little over 12 uphill. Standard is about the same but you dont get so much benefits going downhill because mixture stays "too rich".

[MackTheKnife]

Ignore what I wrote above. Turns out my new fuel pump doesn't sit as low as my stock one. Looks like it may have been a mild case of fuel starvation.