Before ordering your Howell TBI conversion kit, there are a few important things to check on your vehicle to ensure proper installation and function.
Inlet manifold carburetor flange – Due to the many variations in design between carburetor manufacturers and vehicle models and options, there are several factory applications that will not work for converting to our TBI kits.
The first thing to look for before ordering is a 4-hole, 4-barrel factory inlet manifold flange. (photo 1)
(Square flange vs. spread bore flange differences discussed further down.) When the adapter plate for the 2-barrel TBI unit is placed over the stock carburetor flange, the bolt pattern may be the same, but the placement of the throttle bores is in the center of the flange, creating an obviously unusable restriction. (photo 2)
The easiest cure for this problem is either to find a factory 2-barrel inlet manifold to use, or an aftermarket 4-barrel manifold with a divided, 2-section carburetor flange. (photo 3)
This will allow for the proper air/fuel distribution into the manifold, and unrestricted airflow.
If you’re running a stock engine, going from a 4-barrel to 2-barrel manifold won’t usually affect performance in normal street or off-road driving. If you plan to run at higher rpm and power levels, an aftermarket aluminum 4-barrel inlet manifold is probably the better choice. You can closely match the cfm rating of the original carburetor by choosing a similarly rated TBI unit for your kit.
Another potential issue with using an aftermarket inlet manifold is the thickness of the flange/carburetor pad walls. On some manifolds, the thickness of the casting surrounding the carburetor pad is barely enough to allow the plate-to-manifold gasket to seal. (photo 4)
The material thickness varies between manufacturers, so you will need to check yours to be sure it has enough material to work with. On some manifolds, minor blending may be needed inside the bores to eliminate the step below the adapter plate into the plenum, as shown in photo.
Rochester 2G/Carter BBD 2-barrel carburetor conversions
There are 2 different sizes of Rochester 2G carburetors, referred to as ‘small bore’ and ‘big bore’. As of this writing, we have adapter plates available for the small bore version (and Carter BBD) only. To determine which version you have, use the small bore illustration below for measurements –
The material thickness varies between manufacturers, so you will need to check yours to be sure it has enough material to work with. On some manifolds, minor blending may be needed inside the bores to eliminate the step below the adapter plate into the plenum, as shown in photo.
Spread-bore 4-barrel carburetors
We currently only offer a spread-bore adapter plate for Rochester QuadraJet, Carter Thermo-quad, and Holley 4165 carburetors. (See illustration below for specs.) Other spread-bores, such as the Ford/Motorcraft/Autolite 4350 have a different bolt pattern, and also different throttle bore spacing measurements. We do not currently offer an adapter plate for these carbs. Most spread-bore kits are supplied with a Holley 670 TBI unit with 2” bores, though adapters are available for use with standard GM/Rochester TBI unit with 1.68” bores.
Unlike the adapter plates for standard square-bore carburetors, the spread-bore adapter plate locates the throttle plates of the TBI unit over the secondary bores on the manifold, blocking off the smaller primary bores completely.