Top feed fuel injectors are by far the most common, engines like the 4G63 use top feed injectors. Some of our favourite performance engines like early SR20, 3SGTE, 1JZGTE, RB25DET and EJ20 all come standard with side feed injectors.

So what is the difference?

Side feed injectors:

For side feed injectors the fuel enters through slots machined into the sides of the injector. The fuel rail is combined into the inlet manifold or cylinder head.

Top feed injectors:

As the name suggest fuel feeds into the injector from the top. 

Why change to top feed?

Side feed injectors are size limited. 1000cc is about the largest size available.

The flow rate of the integral fuel rail is limited.

Side feed are expensive compared to top feed.

Most performance inlet manifolds are made to suit top feed.

Top feed advantages

Cost advantage when changing to larger injectors.

Fuel pressure is generally more stable in a top feed fuel rail.

Top feed fuel rails usually flow more.

Top feed is simply more common.

Can use very large injectors 2000cc 

Top feed disadvantages

A conversation kit is required. Which may also require changing plug connectors.

Fuel flow does not cool top feed like it does on side feed.

Side feed advanatages

Some cooling effect offered by the fuel surrounding them.

Side feed disadvantages

Cost.

Limited range.

Restricted by size.

More prone to leaks.