How to determine best t/b size?

[stefan]

Like the title says

I see a lot of sizes around, I've read/heard that you can't go to big?
And that tapered t/b's are best, like 74 to 68mm wich would make nicer power in both low and high rpm?

Also... how about a t/b spacer, it does increase plenum volume, but what effect can I expect, better throttle response?

I was thinking about a set of a ported AEBS intake manifold plus a tapered 74-68mm t/b and a t/b spacer from DHR (Dave Hickham Racing), but I've read a horror story on H-T about a badly built engine by him, and not offering a solution (but that was back in 2005), all this would set me back about 450-460 dollar plus shipping, very affordable.
What do you guys think?

BTW, for application; it'll be on a stock B16A2 with i/h/e first, and it'll have boost later this year.

[suspendedHatch]

The stock throttle body will give you the best performance. Only a very highly modified engine will need to increase tb size. By highly modified I mean that you're running 9's.

Anything bigger than stock is too big. The first problem is that the ECU isn't going to respond correctly. It will hesitate and go lean when you punch the throttle. But even if you were to tune it with a standalone, the end result is that you lose power all the way up to about 5500 RPM and any small gain beyond that is not enough to warrant the power loss across the rest of the RPM band.

I tried a B18 tb on a D16Z6 but then switched back because I lost power. Later I found out why. Increasing the tb diameter decreases the speed of the air flowing through it.

A tb spacer will only change where in the RPM band that your power peaks.

There are formulas for calculating tb size but I am not a fan of "rule of thumb" information.

What I suggest is to 1) stop taking ebay ads and manufacturers ads seriously and 2) wait until you boost the motor, and since you'll be tuning it anyway, try a larger throttle body on the dyno. With boost, it may actually net you some power.