I know that a lot of people either have or are considering plenum spacers even for FI applications. Originally the spacers and aftermarket plenums were to counter the lack of air getting into the front two cylinders due to the angle of the stock plenum. But given that there is positive pressure with FI, isnt that problem negated? Theoretically, with positive pressure, the front two cylinders should get all the air they need. So is there any benefit or purpose to using plenum spacers with FI? Are there gains to be made with them?

 

I've been thinking about this as well. About 75-85% of the time I'm running under vacuum, so the blower isn't helping to overcome the airflow imbalance. Not sure if boost removes the imbalance but that's only a small percentage of my overall driving anyway.

 

Boost won't fix the imbalance, it only increases the total flow rate with the same imbalance.

The only way to fix the imbalance is to reduce the pressure drop above intake runners 1 & 2. (there is no significant pressure drop above runners 3 through 6.)

Interestingly enough, eliminating the pressure drop above runners 1 & 2 will help to balance the A/F mixture ratio distribution too. This not only allows for more power, but it also helps to reduce the tendancy for lean ping on the other cylinders.

There is a front to back mixture ratio gradient on the cylinders with the stock plenum set up. Any one of the spacers or plenums on the market will go a long way to fix it.