Engine Mods

Perhaps the most common part of a Gemini that is modified from the factory offering, the engine has a fair bit to offer in terms of strength and general design, as the crank and rods are quite strong, and the ports in the cylinder head are a decent size compared to other engines of similar capacity.

There are however several areas where the factory design lets it down.  These are static compression ratio, standard Nikki carburetor, and the standard inlet and exhaust manifolds.  The stock cylinder head also has some restrictive valve shrouds in the combustion chambers, and there is generally a rough transition from valve seat to port, but this is usually the way all heads are manufactured.

A different camshaft profile will generally be chosen for more torque at the same and higher rpm.  Valve reliefs in the pistons may be required If the head has been shaved excessively in the search for more compression, or simply due to the specs of the cam.  Check out the valve reliefs in the pistons shown on this page to get an idea what they look like.

One of the most common modifications is a carburetor swap, with the downdraught Weber being a good choice.  This conversion is covered in detail in the Carburetor Mods pages.  Other common carby conversions include downdraught 2 barrel Holleys, and twin downdraught or side draught carbs.

Exhaust modifications are probably equally as high on the list of common mods, with a set of headers (either 4-2-1 or 4-1) leading to a more free flowing exhaust system.  For a decent power upgrade and exhaust note, a 2" system with a straight through muffler and straight through resonator gives good results for a mild 1600 engine.

After doing a few modifications to increase power, the standard fuel supply may become an issue.  Ways to upgrade this are covered in the Fuel Pump Upgrades section.

Ignition timing is also an area where more torque can be found at lower rpm, from idle to about 2500-3000rpm on modified engines with bigger camshafts. 

Fuel injection has gained popularity since the introduction of relatively cheap engine management computers to the market.  Good gains can be had by fitting an EFI inlet manifold as they flow very well, and the ability to finely program the engine's fuel and ignition timing with respect to load and RPM improves engine efficiency, so more power and economy.