Newly built engines running Alpha-N or Speed-Density,a note.

[Turbo-Brown]

Just thought I'd share my findings as there seems to be a steady tide of people building engines and having aftermarket engine management systems fitted.

The point to note is that you really do need to regularly check your AFRs over the first several thousand miles once it's been initially mapped (unless you're able to run to a target AFR table that is)

What I found with mine is that when Ant kindly mapped it for me, it ran great for a while but slowly, as the engine loosened up, the fueling started to go out.

For instance, at 30mph when we mapped it, the engine needed say 25% throttle opening to maintain speed in 4th gear. However, as the engine loosened up, it only needed say 20% but as we'd not spent any length of time on that speed/load site it was too lean.

The point was raised again today as the car's got an MOT on tuesday so I thought I'd check the idle fueling. This was set at Emerald after about 800-1000 miles of running in.

At Emerald back in May, the idle fueling would've been set to around 14.7:1, possibly a little leaner. When I checked it today it was an eye watering 11.2:1 which is richer than you need even for full power!

It's nothing to do with the person doing the mapping I hasten to add, they can only tell the ECU to provide fueling relevant to the engine at the instant they're mapping, but it does raise the point that you need to check periodically that everything's as it should be.

It also explains why manufacturers will always need to use air measurement as main load sensing on production engines as it means you can get the correct fueling regardless of how new or old the engine is.

[Gunni]

isn´t it also a case of carbon build up on the cylenders ?
actually lowering the CC of the chamber.

[Turbo-Brown]

Guess there are a million and one things that can effect how much air enters the engine for a given throttle opening / crank speed combination (or manifold pressure/crank speed combination).

Point is that without actually measuring air flow or having a decent bum sniffer, the ECU has no way of knowing.

[215m3]

I had a wide band porbe and afr digital gauge in my S14 CF set up and kept my eye on it. It did need some tinkering, but was not way off.

I agree with you that these more highly tuned engines need to be monitored.

[Gunni]

On my XMS system I´m using a map sensor and tps only,
with weaker suction there should be more vacuum, and the unit should lower the fuel accordingly as the map compensation is used for multiplication on the fuel map, it was very easy to map and runs really well,

[jaistanley]

Guess this is another good reason to go for a wide band bum sniffer..... LOL

Jai

[Ant]

I think this may be more relevant to those installs using the TPS to drive the ECU as the primary load sensor, I know Emerald has a map trim but I'm almost certain the load sites below atmospheric are set to zero so the trim only comes into force under boost.

Another vote for closed loop idle fuelling, WB not required to enable this though its a good idea if the £Â£ can stretch to one, not the most reliable device long term as anyone with am 02 onwards VAG car may well testify.

Oz's 2.7 has had 3 rounds of tweaking on the idle fuelling as its bedded (motronic with AFM and no 02 )in so Point well made as usual Alex