One of the main benefits to ITBs is their very small throttled volume (that is the volume between the throttle plate and the back of the intake valves), which bestows them with excellent throttle response. Another benefit is that that they have a very large throttle area, which means they can flow a lot more air, without the negative consequences of a single throttle of similar area. You see, throttles must be sized appropriately for the engine. Like the fairy tale of Goldilocks and The Three Bears, you don’t want a throttle that is too big nor too small. Throttles control how much air can enter the engine and the velocity of the air that enters. Smaller throttle bodies allow the air to go faster, larger throttle bodies make the air move slower. A throttle body which is too small could limit air flow in the engine. If throttle body size is too large for a particular engine, the air will move too slowly and reduce power. It’s a delicate balancing act.
Fuel is generally not a limiting factor as it is always possible to install bigger fuel injectors and fuel pumps to meet the demand. The limiting factor is always how much air you can stuff through the engine. The more air that goes in per unit time means more power, all else being equal. The reason why forced induction makes more power is because you’re compressing the air so that it is more dense.
The stock 1.8 Miata throttle body has a diameter of ~55mm. We know that the area of a circle is pi*(r^2). Thus the stock 1.8 Miata throttle body has an area of 2,375mm^2. My TWM ITBs have four 45mm diameter throttles, one for each cylinder. Thus, the area of a single throttle is 1,590mm^2. But the total throttle area is 4 times that or 6,360mm^2. So my ITBs have >2.5x as much throttle area as the stock single throttle while being smaller, which means faster intake velocity. The stock throttle body is generally not a limiting factor until you’re well over 200hp. Most people are replacing them for reliability reasons (broke throttle shaft) not flow reasons. So my throttles are probably oversized for my needs and current power output. But that could change with a built head and E85 😆
All that being said, my ITBs need some acceleration enrichment to handle the massive influx of air that occurs when I open the throttles. With some “Accelerator Pump” style acceleration enrichment the engine revs a lot more cleanly now when stationary / no load (see video above) but I still have a lean stumble when I open the throttles slowly vs being fine when I open them more quickly. I took the car for a drive and it quickly became apparent that I now have way too much acceleration enrichment. I was auto-tuning so now my VE table is all messed up. This is why you always save your tune before making big changes so you can go back if need be. What I need to do is back off the acceleration enrichment and add more fuel to the VE table in the cells just beyond idle. I will study the data logs and make some educated guesses on how to fix this so I don’t have so much hysteresis as the AE overshoots making the mixture way too rich and then VEAL overcompensates and pulls fuel from the VE table. The acceleration enrichment settings and VE table are interdependent so it’s easy to end up chasing your tail. Basic tuning advice is not to touch AE until your VE is dialed in, but it’s not really possible to wait with ITBs. You kind of have to work on them both at the same time.
