Bruce ( Shrinker )

Hysteric said
Here's another interesting paper:

Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge

Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge on JSTOR

"However, with PFI( Port Fuel Injection)the fuel is exposed to the air for a short time before it is inducted into the cylinder. This could result in incomplete fuel vaporization, particularly with T90 fuels. On the other hand, with a carburetor and as with TBI the fuel has a long exposure to the air before entering the cylinder. therefore it is conceivable that the mixture is better vaporized and premixed prior to combustion with carbureted and TBI fuel delivery. More complete vaporization could result in better combustion and lower unburnt HC emissions"

There's obviously a lot going on in the intake manifold and port than most people would think. If the fuel required more "energy input" to improve the mixture quality the intake tract could be a good place for that to happen.

The unleaded fuel that we have to deal with today is difficult to understand when tuning with large cam street engines.I did one today,350 chev with world product cast heads and big cam with not enough compression and not enough spark energy.So it likes to run more on the light vapors.required specific timing curve.Had to be a bit richer to get it nice but thats what it takes sometimes.

Hysteric said
Here's another interesting paper:

Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge

Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge on JSTOR

"However, with PFI( Port Fuel Injection)the fuel is exposed to the air for a short time before it is inducted into the cylinder. This could result in incomplete fuel vaporization, particularly with T90 fuels. On the other hand, with a carburetor and as with TBI the fuel has a long exposure to the air before entering the cylinder. therefore it is conceivable that the mixture is better vaporized and premixed prior to combustion with carbureted and TBI fuel delivery. More complete vaporization could result in better combustion and lower unburnt HC emissions"

There's obviously a lot going on in the intake manifold and port than most people would think. If the fuel required more "energy input" to improve the mixture quality the intake tract could be a good place for that to happen.