Recently there has been a mad rush of people going out and buying the largest throttle body they can find and I've put as much information as I can out there in a very sporadic and piecemeal fashion.
I will lay out what there is to know about throttle bodies and their flow capabilities as it relates to making power, which is what everybody want to focus on, but I will also talk about ideal application of throttle body size compared to modifications at the end, so if you don't care to read it all, just scroll down to the end for the [TLDR]
The first thing people need to know, is bigger is NOT
better, especially on a naturally aspirated motor. You also need to look at the difference between the horsepower and torque numbers when looking at flow characteristics and rates. Torque is used to get your truck going and is created by having a higher intake velocity; horsepower is what gets you moving faster once you're moving (in a very simplistic view) and this is created by flowing volume. Notice the difference? velocity is not the same thing as volume, especially in pipe flow, which is what our CAI tubes, TB, intake manifold, and intake runners all are.
This all relates to throttle bodies and is something that needs to be understood first because it will dictate the size of TB you purchase, and here's how: Flow through a tube is related by volume and velocity, in addition to flow state, density, heat, and pressure(or vacuum). Every pipe has a defined cross section and the inner wall characteristics is what I will focus on because it is what affects flow rates.
When a fluid (in our case air) flows through a pipe in a smooth (laminar) flow, it is affected by the friction against the inner wall of the pipe, and this friction is created by the no-slip condition (basically the air against the pipe wall is not moving) and this then slows all the adjacent fluid down as well. This boundary layer's thickness is determined by the velocity of the flow. This image shows you what the flow looks like based on velocity. All flow, given a long enough pipe will end up looking like the top parabola where you have reached maximum flow velocity but in a very small diameter of the pipe itself. The larger the diameter of the pipe, the longer it takes to reach this maximum flow velocity.
So, that's what the flow profiles look like. Now, all of this is determined from a large supply reservoir, which in the case of our engines, is the atmosphere, but also the intake manifold's plenum as well.
This is what most people don't think about. That nice hollow space in the intake manifold is used as storage of a volume of air, that is all. There are tubes going from the plenum to the intake side of the head, and it is THESE intake runners that actually increase the velocity of the air flowing into the engine. here is an image to give you an idea of what it actually looks like. Blue is low velocity and red is high velocity. This is an example of a long intake runner, which is excellent for generating low RPM torque.
I have talked a lot about flow velocity and its importance, but what creates flow velocity? Pressure differential, or in the case of a naturally aspirated engine, the vacuum created by the intake stroke of the piston in the combustion chamber. This vacuum is determined by the volume of the combustion chamber, and it is only created for a short period of time before the intake valve closes. Each cylinder will create the same amount of vacuum per cycle, but the vacuum that is measured is the manifold vacuum pressure which is dependent on the number of cylinders creating vacuum and the RPM. If you have an engine running at 1200rpm it will create less vacuum than an engine running at 4000 rpm because the cylinders don't need to be refilled as often per minute. So, it's this vacuum that determines the velocity of the air going into the combustion chamber from the intake manifold plenum. Remember this, if you remember anything: At maximum vacuum you need maximum volume to meet the cycle rate demands, but at low vacuum you need maximum velocity to meet the cycle's duration limitations.
Low-mid vacuum demand is 3500rpm or less, and high vacuum demand is 3500rpm or higher.
Now, how does this relate to your throttle body selection? Simple, the throttle body is what allows the air in the intake manifold to be replenished, and it's size determines how quickly it is replenished BUT
, the velocity of the flow into the intake manifold through the throttle body still translates into velocity of the flow down the intake runners when the throttle body is open. What this means is you want to maintain flow velocity through your entire
system. The throttle body is the natural choke point in our intake due to it's design and construction, but it also acts similar to a venturi, which is a device designed to increase velocity in a pipe in a way that does not create turbulent (unstable) air flows, which kill velocity AND volume.
Here is a picture of a common ported throttle body
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.
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
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