I believe that's the downside of why that EFI unit failed miserably. They tried messing too much with the flow characteristics of that manifold instead of buying the 4.0L carb manifold designed for it. (Clifford's) That "neck" of the adapter sat too far into the intake, so I believe that created a restriction along with adding too much volume for the intake runners to handle. Proper intake design is crucial to how and where you make your engines power. Generally you use longer, smaller runners (the "tubes" of the intake) for low end power. And you use shorter, larger runners for higher RPM power (look at a NHRA or NASCAR intake). These guys were trying to go for High RPM power with a large air/fuel source and skinny runners. Granted, the EZ-EFI was capable of fine tuning but they got impatient instead of thinking things through IMO. That head is a 1k head but for that price, you can stroke the 4.0L or a 258 and get more power. I mean if they spent that on the head, the right manifold is only 390 bucks so just get one cause obviously money was not an issue.