Tuesday, January 24, 2012

Exhaust VANOS

It hasn't been a priority for me to get the exhaust cam working.  When I first started tuning the M54 I thought having control over exhaust timing was the holy grail of engine tuning, that great things would come from it.

After numerous trips to the dyno I gained exactly ZERO power with ANY amount of exhaust retard, anywhere.   I had best power by leaving it unplugged!

Like Spiderman's Uncle Ben said, "With great power comes great responsibility".   Use your exhaust powers wisely!

It wasn't until I installed the fantastic euro S50b32 headers that I found power by retarding the exhaust cam.

On to the nuts and bolts...

Datalog In is the only remaining input that is suitable for a +12v cam signal input.  As noted  by Megasquirt, the C12 cap is too large.  0.1uF delays the incoming signal by around .8ms which translates to around 4 degrees of error at 800 rpms.  What's worse is that the error grows significantly as the rpms increase and I saw the indicated cam position error increase by 15 degrees at 3000 rpms.  Unacceptable...  I removed this capacitor and the error disappeared.

Exhaust map.  Most retard at full load in the 3000rpm range.  Optimal retard highly dependend on your exhaust.
With my headers, I found that exhaust retard adds about 5hp in the 3000rpm area and needs to be advanced again at higher rpms. 

In theory, the exhaust can be retarded at lower loads to increase fuel economy but it won't increase power.

Megasquirt is programmed to target the minimum cam position + chart value so the "normal" position is 40.  25 on the chart is 15 degrees of retard.  I limit the advanced value to 38 to keep the integer value from winding-up and maxing-out the duty cycle to help the response time.  When the table is set to 40 the cam can't physically get there so the integer value keeps increasing to try to get it there.

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