Project Considerations

There are up sides and down sides to what ITBs do for an engine. Individual throttle bodies (ITBs) allow each cylinder to have its own throttle plate and runner instead of each cylinder pulling from a communal plenum. ITBs eliminate the plenum style intake manifolds that use 1 throttle body to feed all four cylinders ports. Plenum style intake manifold work well for commuter cars since power is desired at lower rpm. Having your intake ports fed through a communal plenum increases the torque of your engine at lower rpm due to a ram effect as air coming into the plenum is introduced. Once air is introduced into the plenum it gets draw to each cylinder port. While each valve shuts, the incoming air in each runner runs into its self and packs into the running increasing the pressure in the runner. Once the intake valve opens, the higher pressure air moves into the cylinder faster and more air makes it into the cylinder before the valve shuts again. ITBs do not draw from a communal plenum so you loose a lot of volume that can help increase the ram effect. Because of this you will loose some lower rpm power and torque but you gain a lot of horsepower due to the less restrictive intake route and much larger runners that are able to flow more air. More CFM means more revs required to move that extra air.

This means I will need to rev my engine much higher to utilize the new found horsepower gains that the ITBs will offer and the cost of this new power will be a loss in lower rpm torque. Idling is usually difficult at low rpms also since throttle tip in is so much steeper than with a conventional intake manifold. So I am expecting to have to increase my idle from 700 rpm to around 1000-1200 rpm. I will want to retain my factory cruise control, which is vacuum operated and the MAF sensor will no longer be able to be used. Because of this i will need to run a communal vacuum system that links all post throttle body runners together and have the MAP sensor hooked up to that source. The change from a MAF to a MAP sensor will require a standalone ecu so I am doing either an AEM EMS setup or a Megasquirt III setup. The computer will also use the TPS, cam sensor and intake air temp sensor to complete its calculations. This will allow me to ditch the emissions equipment and re-tune for 93 octane gas. The factory fueling system should be able to support my 150whp short term goal so I will not be touching it. The ignition system may need a bit of an upgrade however to help with the efficiency of the new found rpm and power. The higher rpms will be putting more stress on the bottom end and increasing the chance of valve float. The 2000 Miata engines do not start to valve float until around 8000 rpm. To battle the increase of heat in the bottom end I will be adding an oil cooler and because these engines have oil squirters to cool the pistons, this should help to deal with the added heat build up from the added rpms. The Miata and racing communities in general rave about Redline Oils. They even have perfect weights available for the Miata transmission, differential and engine. I am hoping to get away with using my factory clutch. I will give it a chance but it should hold my 150whp short term goal.