[T3] Lean Cruise

[Jim Adney]

On 7 Mar 2013 at 16:46, Max Welton wrote:

> Now that someone else in the group is going the Megasquirt route, this might be of interest on the list.
> 
> Some while back I was trying to explain my lean tuning strategy as a component of getting better than stock mpg. The point was made that my high MPG was due to the thin air here in Colorado Springs and that if I tried that at lower altitudes I would fry my engine. So I went off and thought about that and believe I have an answer. 
> 
> Last spring I reconfigured my MS tables to give a wide flat "lean" plateau below a certain MAP value (65 kPa) and above about 2400 rpm. On that plateau I set the target AFR at 16.1:1. Above the plateau I drop to 13.5. And since I want to minimize the chance of spending much time around Stoichio, I arranged for a very abrupt transition. 
> 
> It was a nice day today, so I brought the laptop to work and took this screen-shot on the way home. The stretch of road in this sample was undulating a little so to maintain a constant speed required a little throttle (and thus MAP) movement. This shows the relationship rather nicely.
> 
> http://www.maxwelton2k.net/squareback68/MST3/AFRStepLeanTune.JPG
> 
> So, why do I think I won't fry my engine?
> 
> When load goes up for any reason and I increase power, the MAP increase will cause the transition from lean (low 16s) to power (mid 13s). If I choose not to apply more power, the engine will remain lean, but the car will slow down. Eventually the falling RPMs will take it below the lean plateau.

There's a lot of interesting information there, Max. Is the coolant 
temp the oil temp? If so, that's pretty high, so where is it 
measured? Or is that the head temp sensor? 

Why did you want to minimize the time you spent at stocio?

Another thing that has lots to do with economy is the advance. The 
D-jet distributors give extra (10 deg) vac advance under your light 
throttle conditions. That gives 40 deg of advance when cruising, 
which is WAY more than you can tolerate with WOT, but perfectly 
acceptable with light throttle.

Does your system give you any control over the spark timing? If not, 
what distributor and timing are you using?

When I got my '72 many years ago, it had a number of FI problems, 
some of which led to lean running. In this case it was too lean all 
the time and didn't run well. When I took it apart, after fixing all 
the problems and rebuilding the engine, some of the pistons had 
little melted areas along the bottom edge of the front faces. I 
cleaned them up with a  file and reused them; they worked fine after 
that.   

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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