You can see how it's has a nice 85 mm cross section, and very common for the 'ported' TBs, it maintains the stock butterfly valve diameter of 80mm. Now, you'll also notice that the butterfly diameter is not at a 90 degree angle to the TB's wall, and that is to increase flow in low vacuum situations. You'll also notice that there is a necking down and sudden narrowing of the cross section right at the butterfly's contact surface. So while the picture above is marketed as an 85mm, which granted, is its maximum diameter at one point in the TB, its still an 80mm butterfly.
[off topic] There are several manufacturers making TRUE diameter TBs like BBK, Arrington, some of the fastman TBs. The Moe's, modern muscle, and the rest of the fastman TBs, are all ported with some work done to the butterfly valve. Ask questions when buying a throttle body about what the butterfly diameter is, you might be surprised. [/off topic]
The necking down you see is so sudden that it actually slows down the flow velocity, which at low-mid vacuum is a bad thing. Ideally, you want a gradual reduction over the length of the intake side, and a gradual expansion on the outlet side to maintain flow velocity. Most ported TBs don't have this, but the bigger "diameter" numbers fool a lot of people.
Alright, after all that is said and done, here's the meat and potatoes of this entire thread. Thanks for bearing with me so far:
What RPM do most people spend driving? The answer is 2000-3500rpm. Where is our maximum torque? 2700rpm. What gets our trucks moving from a stop? Torque, not horsepower. Why is this important? Because it will determine where you want your power, up in the high RPM range, or down in the low-mid where you'll get the most out of it. If you do a lot of driving over 4000rpm, let me know where you live, because I want to move there and let my forged, supercharged, and nitrous blown truck do its thing all fuel tank long.
[TLDR:]
For a stock-mildly modded N/A engine (CAI, programmer, exhaust) a TRUE throttle body diameter of 80-83mm is ideal, it will allow a good velocity at low vacuum but still flow more than enough air into the intake manifold to keep up to the demands of the combustion chambers. An 85mm can be used and it will make a difference when at WOT, but you'll actually see a decrease in your low-end torque for getting off the line.
For a mildly-moderate N/A engine (above + headers, ported heads, cam, small nitrous shot) a TRUE throttle body diameter of 84-85mm will give near-stock low torque, but will flow significantly better at mid-high RPM.
For a heavily modded engine (above + BV heads, stroker, super/turbocharger, big n2o) an 85-90mm is a must or you'll be choking out in your mid-high RPM, and the more power generated, the more air needed.
What this comes down to is what is ideal for your engine and its use. Just because the heavier modded engines use certain sizes or products, doesn't mean they will make any improvements on a less modified engine. It may make you or the butt dyno feel better, but overall the performance will be moderate. Also, modifications work in conjunction with each other, so if you have a moderately modded engine and then slap a larger TB onto it, you'll see bigger gains, but if you put the same TB onto a stock motor, it won't make as big a difference.
I am just going to clarify something that seems to have been misinterpreted. I have been jumped on by people over several forums that are running ported throttle bodies of various brands who claim I'm full of
oopstorm:
No where in my post does it say that ported throttle bodies are evil, useless, a waste of money, or show no gains over stock. Everything about this post has to do with maintaining your intake velocity, and
I specifically talk about the butterfly's diameter when it comes to the TRUE diameter of a throttle body, irregardless of the opening or maximum port diameter. So in the final section when I talk about throttle body diameter vs intended use, I am speaking specifically about the butterfly diameter.
That said, for those people running ported throttle bodies such as the Fastman or Moe's that still maintain an 80mm butterfly, you will still see gains over stock, I did not dispute that... ever. Looking at my recommendations, they are still an 80mm throttle body, and as such, are geared towards a stock-moderate build, which incidentally is what 90% of the builds out there are and these TBs will still net you gains.
This thread is also intended as a road map for those who are doing more than just the standard bolt-ons. I could probably count the forum members that are doing insane build on my fingers and toes (that allows me to count to ten) that would require a true 90mm TB
