Manifold vaccuum, EGR & long duration cams

[Tony Bryant]

In an attempt to go from my very bad RPM/TPS system 
to a reasonable MAP system, I built myself an conventional
arrangement plenum chamber, and tossed the
DCOEs I was using as throttle bodies.

However, It doesn't idle at all well.

Setup: 72 degree overlap cam, largish plenum chamber, MAP system

Problems:   
  Won't idle below 1500 rpm. Simply dies if revs drop below 
  that. Starts easily though. No idea how well it goes under
  load at higher rpms.
  
  (used to idle at 500rpm on tps system or carbs)

  At 1500 rpm with spark advanced for maximum
  vaccuum (13" Hg), actual fuel requirement 
  (as determined by using a colortune windowed 
  spark plug) is 1/4 what it should be, according 
  to the MAP calculations.

Hypothesis:
  During overlap a large amount of exhaust gas is being
  sucked back into the cylinder & plenum chamber, effectively
  giving me the EGR system from hell. 

How to fix?

I've so far managed to stay away from 2D tables, and stuck to 
parameterised formulas. (For ease of tuning).

Do map systems generally cope well with high overlap?

Will I have to retard the ignition at idle to slow it down 
below 1500?

thanks...




$$$ MAKE CAR FAST $$$
bryantt at psc.fp.co.nz