Lots of very positive stories have been posted about using ITBs on these motors (with a proper tune of course). They have always tempted me, but I am getting old enough that I don't feel like swapping stuff around every year or two for SMOG inspections. Now, I imagine that it depends on the shop to some degree, but I have passed SMOG with a MAF conversion, COP conversion, slightly hacked up stock air box and a suspicious looking vacuum line sticking out of the intake manifold. These cars are probably old enough that the guy at the station just wants to see a stock air box and a cast aluminum intake manifold.
So...
What about chopping up the lower intake manifold a bit, since it is almost completely obscured by the upper part and other adjacent stuff? You could either do some heavy mods with a milling machine & TIG welder to incorporate custom throttle butterflies into the original intake runners, or maybe chop parts of it out and bolt/weld in the throttles from some other ITB setup. Maybe get the upper & lower portions extrude-honed out first to open them up (at least for those of us with displacements > 1.8L) and totally bore out everything in the original throttle body so that it is just a "shell" to look stock.
I sort of like the idea of having a stock looking motor that is anything BUT stock, one because you don't have SMOG headaches, and two because the "sleeper" type of setup is always good times. Also, I think that the stock plenum dimensions are sort of important. The M42 is a small-ish displacement 4 cylinder with little enough torque as it is. The original intake system was designed to balance mid- & high-range torque, and the various resonances created by the stock runners & plenum are part of that. I know that ITBs really help with high end power, but I think that it would be nice to trade a little of that for more mid-range torque. Honestly, if I wanted to work that hard while driving, I'd just go buy a Honda S2000 and call it a day lol.
I have played with basic slide-throttle setups in CAD a few times, and that would be the easiest way (mechanically) to get the throttle as close to the head as possible. However, all of my reading about slide throttles indicates that they are extremely temperamental, and that they are almost guaranteed to get stuck open at some point unless you service them all the time. There's a reason that only race cars use them lol...when you tear a motor down after every race, you can use all sorts of temperamental stuff like that.
This isn't something I will be starting any time soon, but I would like to start the thought process on it. I have a custom motor coming in a couple of weeks, am going to Asia for work around the same time and have a wedding in October. So, maybe later this winter I will see about picking up a spare intake manifold setup and toying around with the possibilities.
I am interested to hear what people here think, and I am especially curious to see commentary from people with "proper" ITB setups. What differences did they make, how do you like them for street-use and what are your thoughts about the use of them with the stock upper intake manifold?
