AFR's and manifold pressure at cruise...

[Quagmire]

OK, so I have my wideband sensor all calibrated and working nicely, let the tweaking of mixtures commence!

Couple of questions to point me on my way.

First up this is on a 9.35:1 comp 3.5 with 10 bolt heads and 4-2-1 cast Disco manifolds in a Landrover 90. Runs LPG most of the time. For tuning purposes and for this discussion I am talking in Petrol AFR's as people have more idea on them, and lambda is lambda right?

What manifold air pressure would you expect to see at cruise, or to put it another way, where does "cruise" technically begin from in terms of engine load? I guess in a Defender the engine load will be higher than most other cars for any given cruise speed due to the lack of aerodynamics.

Next is how lean can I realistically go? At the moment I am getting about 15-15.5ish on light throttle cruise. More fuel needed? Less?

Tickover is around currently between 13.5:1 and 14:1 it seems happy enough here.

What AFR should I be going for when giving it beans? I have 13.5:1 in my head for some reason- is this right?

Thanks in advance- and Merry Christmas!

Jamie

[kiwicar]

Hi
Firstly manifold vacuum, this is you need to monitor as an indication of how efficiently the engine is burning fuel so basically you are looking for the AFR at cruise that gives you the best fuel economy. Basically find a stretch of road and drive down it at a constant speed on a light throttle and tweek the AFR until you get the lowest manifold depression, pick another speed and load on the engine and doo the same again for a different set of map points, each time you will be tweeking a range of map points as to keep consant speed you should use less throttle as you get the engine running more efficiently. Light cruise should be in the region of 14.5- 17:1. Tickover for normal use should be whatever gives a nice smoothe idle, for the MOT you need it as leen as it will run even if you end up opening the throttle further. Full throttle will be in the region of 12.5 to 14 to 1, the rover needs a rich mixture as the chambers are not an efficient shape but if it is a well ballanced set up you should be getting 13.5 ish.
Best regards
Mike

[DaveEFI]

If it's any help, on my bog standard Vitesse EfI unit, 14.1 at idle gives a CO of just over 1.5% which satisfies the MOT man.

[stevieturbo]

Petrol or Gas AFR's, doesnt matter. The lambda sensor doesnt know or care what fuel is used.

They will only change from typical petrol numbers, if you change the lambda controllers output settings, which is a bit pointless.

So leave it set for petrol numbers, and tune for petrol numbers. ( or Lambda if you prefer, but I prefer AFR numbers. )

WOT anywhere around 13.0 is a safe starting point.

Cruise etc....you can pretty much go as lean as the engine will let you. Go too lean, and it will run bad.

Idle, whilst targetting 14.7:1 might be nice from an emissions cleanliness point of view. IMO run the leanest mixture you can that achieves a nice solid stable idle at all times.

[CastleMGBV8]

Jamie,

What they said, but one question, if your using gas all the time then a hike in compression and posibly ignition advance will be beneficial.

What head gaskets do you have, if tins that will give you approx .5/1 increase in compression.

The LPG guys can better advise on ignition advance settings.

Kevin.

[Quagmire]

Thanks for all the replies so far guys, appreciated

I have considered an increase in compression as its running on compositie gaskets, so could go to tin- but realistically this will probably wait until I get the EFi gear on that I have.

You may be wondering why I would bother fitting the EFi when I run on gas most of the time, but i figure I may as well have the thing running as efficiently as possible on petrol so that it doesnt break the bank when I need to use it. Unfortunately the two LPG stations near me have a habit of running out simultaneously, so I can sometimes go for about a week without being able to get any locally.

As for ignition advance, i think I have that pretty much sorted, as have been running Megajolt for quite a while now and have been able to tinker around with various maps.

My main problem at the mo is getting the AFR at WOT to go rich enough. See my post in the LPG section.

Cheers

Jamie