I have decided to get a TWM throttle body for my upcoming FI conversion. The smallest size bore is 56mm which is supposed to be good to around 350Hp. I'm gonna start by injecting a 12A street port but will put in a 13B half bridge in about a year. If I get the injectors that max out at 350ish HP that should make the TB throat size and injectors matched. The other alternative is to match the injectors to my HP levels which will be quite a bit lower. Which do you guys recommend?

 

Seems kind of small considering that the injected 13Bs from '84 - FD used 3 throttle blades of about 50MM each.

Regardless I'd really recommend you maintain the staged port design of the original carb. Use a dual or quad throttle body and open up the primary ports first, then the secondaries. This is an arrangement Mazda has used from the original Cosmo Sport all the way up to the RX-8. The purpose is to maintain air velocity through the primary ports at idle and low load. Better chamber filling, more torque, stable idle, better fuel atomization from the primary injectors, higher fuel economy.

 

The TWM is an IDA downdraft type of throttlebody so it has two 56mm bores. For a piston engine it's supposed to be good to 350HP. The highest I'll ever run (half bridge 13B) should be about 280ish. It also uses 4 injectors. Since I have a 4port engine, any of mazda's Fuel Injected intakes will be difficult to impliment (sp?). I'm figuring that the IDA being a good carb for lots of rotaries should work well for me. Also the car has very low hood clearance and that's a factor too. I believe that one small hood scoop will work ok for me. Also, it's an autocross car so fuel economy doesn't matter a whole lot.

 

Size the injectors for the engine and HP levels you expect.