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Understanding and choosing Throttle Body sizes

135K views 151 replies 78 participants last post by  Billymac53 
#1 · (Edited by Moderator)
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.

[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 :poopstorm:

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
 
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#2 ·
Brilliant write up.

Thanks for taking the time to do so.

Kudos.

A "sticky" if I've ever seen one.
 
#4 ·
Great write up bro. this is pretty much what we talked about the other night and is why i'm going with the 84mm fastman and i'm sure it will do everthing im looking for from a throttle body
 
#6 ·
Thank you! This has answered a lot of questions! I like the science behind it too! :smileup:

:rep:
 
#8 ·
I don't even have any interest in a TB but that was a very intelligent and concise write-up. ups to you. Gotta love cold hard facts/numbers/science.
 
#9 ·
heres what i got

very happy with ive done as far as mods go.
jba cat back 3" exhaust, s&b cai, moes 87mm TB, dorman e-fan.
trinityt1000 not hooked up yet.

the performanxce is much better. smoke tire from stand still. punch it @ 30mph rear tires break loose... with the trinity this thing is gonna be a lil ripper.

my take: i am very happy with my choice of going with a 87 mm TB!
 
#19 ·
They will all have a bottleneck, but the TRUE TBs have a more gradual taper and the butterfly valve is the size of the advertized diameter. Now, I don't know anyone that makes an 83mm TB, the smallest I have seen is the 84mm from fastman, but if anyone does find ones that I didn't mention let me know, I would love to find more sources.
 
#20 ·
So after we put the new throttle body do we have to reset anything?
 
#22 ·
All you guys talking crap need to try a tb before you say they don't work. I bought a 87mm from Matt at moes and it nets me better low end, better high end plus 1.5 mpg better highway.


So either buy one or quit saying they don't work.

P.S. Matt at moes offers money back if not satisfied.........
 
#24 · (Edited)
Who said they don't work, and nobody has talked crap about any brands, get over yourself. Matt's TBs still fall into the 80mm category due to their butterfly diameter, and as such they fall into the stock-moderate range that I posted about. With the port profile you get a nice velocity increase over the length of the whole body and the throat acts like more of a velocity stack than the stock throat does. The whole point of that long thread was the need to maintain flow velocity at lower rpm, which is what matt's do quite well from everything I've read.

Now if you need more air volume, such as with the moderate-heavy builds then you can not get away with a ported throttle body, irregardless of port profile because the butterfly itself becomes the restriction.

And if you must know, in my builds I have gone through 2 fastman of different sizes, a bbk 85mm, and just got a nitrous ported 90mm arrington, so tell me the vast range of aftermarket TBs you've experienced.


Fastnugly, a well worded post, I have talked with Matt about his TBs since posting this, and the same principles apply if you look at the intent of the post. No where did I say ported TBs are garbage, what I did say was educate yourself on the product and match it to your build, which is what you did.
 
#23 · (Edited)
Kurtis,

Thanks for your write up.
It seems very well written and almost too good....LOL (dont mean anything negative by this just :smileup:)

I can understand the science behind this based on reading this. Typically threads this long do not grab my attention but YOURS did ;)


This part is not directed to you Kurtis, rather those members reading this thread.

Now many of you on these boards know, I have a lifted truck. Everything I did slowed the truck down and turned it into a dog. During the entire process I have been trying to get my truck back to stock truck feel and power and MPGs
Of course it cost money to do this, but guys who lift their truck need to know that your MPGs will stick but also understand what you need your truck for and what you hope to get in the end.
Now I did gears (4.56) to move these damn tires, Its almost a must for lifted guys who still want the stock or better than stock like drivability and pull. My truck definitely improved. I also played with tires (MT's vs At's and then 37's vs 35's) to get to MY sweet spot of how I want my truck to drive.
I want a lifted truck, but still want the truck to be able to boogie and I also want MPG's when I cruise with the family.
I am there!

Let me get back on topic :LOL:

Throttle Body
Every time I purchase something I take the time to do my research.
I HATE wasting money and I am sure many of you feel the same way.
But I am anal about researching to come to conclusion that what I buy is good and fits what I need.

I strongly believe in reading 'real world feedback' and Moe's TB's had nothing but GREAT feedback and seemed to come up a lot under google searches.
Even read feedback of guys switching from Fastmans 84mm, 85mm to Moe's 85mm/87mm and noticing improvements.
So I reached out to maybe 12-15 members who run the Moe's TB and ran others prior in either 85/87 and all love it. A few actually have real dyno numbers from before and after and etc, etc.
Im very hesitant in making purchases even though it may seem otherwise but he has a policy that if anyone was not happy with the results, he would take it back for a full refund.
So why not, I re confirmed with him (Matt) prior to my purchase and grabbed one.
Great guy by the way

Some of the research I found and would like to share with all members here when I compared the top choices for TB's along with Moe's and what led me to Moe's

On their (Moe's) 87mm, they opened it up to 87mm at the front of the blade, and then use a very gradual taper on the back of the blade so that it matches the intake manifold. If you went any bigger on the back of the blade, then you would lose velocity and it would also require you to port your manifold to match the TB. That taper is what increases airspeed and shows the gains in throttle response. Also, the reason why their TB's don't require custom tuning is because they dont touch any material at the blade. No high idle codes or any of that mess that many guys get with the BBK TB (I read countless threads on this). All together, they developed a way to offer the most airflow without any driveability or tuning issues to worry about.
All in all, the TB does work, and is not a restriction on airflow.
This is not to say it cannot be custom tuned....
I have also reached out the Hemifever as I wanted a custom tune (but I will now do it locally due to what I will be getting done) and he of course asks what mods you have. I mentioned what I had and this is along the lines of what Sean (Hemifever) said...he has been very impressed with the gains that the 87mm TB has been showing. He is doing custom tunes for many on the forums, and he even told me that he sent me a PM to Matt (1-2 weeks ago) saying how much better our trucks are responding with their 87mm TB. That says alot when the guy writing the tune approves of the part


Here is my real world experience on this.

I purchased and installed my 85mm (Moe's TB). Drove around for 20-25 miles and noticed my truck (remember, Im lifted with heavy wheels) felt lighter on its feet from stop to go, throttle response was better and I wasn't too sure if this was just me or was it actually for real.
Later that day I asked myself why did i just not go with the 87mm for the minimal increase in $$. Asked a few members here if they were interested and it was gone (to another member here, who may or may not see this thread).

Boxed and packed up the TB. The following day dropping it off at the postal office I noticed the lag of the truck and to my dismay what I felt the day prior was in fact not a figment of my imagination, rather it was actually true.

Fast forward 2 weeks later.....
I received my 87mm TB and reinstalled that and I did notice once again what I felt. Was it in an increase from the previous 85mm? That I do not know as I did not dyno it, nor removed the 85mm prior to installing the 87mm. But what I felt a few weeks ago with the 85mm was yet again there.

Now, I went with the 87mm because of what I will be doing over the coming 2-4 weeks.
But I can assure you my truck did not feel any slower IN ANY WAY
There are many members running both the 85/87mm TBs

This is my feedback on the purchase I made of the 85mm and 87mm Moe's TB.
I am a firm believer of sharing positive feedback and thought I would do that here.
 
#25 ·
Fastman's 84mm does not have a stock butterfly and I just put it on and it did help but like you said its not a game changer but I nice compliment for my other mods. so thanks for the right up.
 
#27 ·
We be cool Andrew! :smileup:

I've just been receiving lots of pms on the subject, some not so nice, and while I can't use ported TBs on my build because of the volume restriction, they will definatly be a difference on a properly ported one as long as it maintains flow velocity.

I suggested the fm 84mm to chris because it suited his build best with the mods he has and it's matched perfectly to the intake manifold diameter which will prevent a lip that can happen with an 85mm.
 
#33 ·
Great! This just got sticky status!
 
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