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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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#36 ·
In my opinion, there was never any reason to apologize. The original post is a lesson in the science behind throttle bodies and to a lesser degree, the intake tract. And in true science fashion, it was done in a factual manner without opinion or conjecture. I for one used this post to help me match my throttle body to my truck and my style of driving/intended mod list.

Great resource and great post!
 
#37 ·
When you're contacted by 2 separate vendors that supply ported throttle bodies (both of whom I've dealt with on good terms for quite a while) and are pretty much given the "WTF MAN!", plus getting harassed by members over all the forums I frequent, it gets tired fast. The clarification was done to sort out the misunderstanding that came about by people not understanding how I was classifying the throttle bodies.

I'm glad people are taking the info from this and making informed decisions about the correct TB size required for their vehicle, that was the intention.
 
#38 ·
Strange, but I don't see where anything you said is derogatory to any specific vendor or ported throttle bodies in general. Heck, it actually encouraged me to buy one. There are very few modifications you can make that come with absolutely zero drawbacks. It's all about balancing your choices to get the positives working for you.

For me, a Moe's 85mm seems to be the right setup for my 2012 R/T with a Vararam and Hemifever DS as the sole engine mods.
 
#44 · (Edited)
There is only 1 reason you should not be getting the Biggest Baddest Ported throttle body for your truck.

Because someone who has not tried one tells you too.LOLOLOL.

The 4G motor is just short of a race motor,it is a high compression tricked out motor ,that was detuned for emissions sake.

If you think the motor will not benifit from Non OEM parts then please dont continue to read this post.

I would recommend anyone planning to add a CAI,or just want a little better MPG or performance should get the biggest Thottlebody they can find up to and including a 87MM model.

To say a 87MM throtlebody will not show a big improvement in throttle response and in performance is just crazy talk.
Maybe not so much in a say a Ford or Chevy v-8,but the Hemi motor is not even in the same ballpark.

I would be happy to swap out my 87MM Moes throtllebody into CdnilRams truck,we could do some 0-60 tuner tests,and some MPG testing.
Surely a real world test would benifit everyone looking at this mod.

Im surprised Matt at Moes can even keep these on the shelf.
I have know Matt for a long time,he is all about performance,hes not about selling something that does not live up to his expectations.
Thanks for a great product Matt.

This doesnt have to be a pissing match here,but the information posted here is rediculous.and I think everyone would benifit from some real world testing.

Heres my number.
Grant 403-617-8733.
Name the weekend and lets do some testing.
Im sure Steve at Davenport Motorsports would let us do a before and after test on your truck as well.
 
#45 ·
Actually Grant, after talking with Matt about all this and my build, he agrees that his 87mm wouldn't be a good thing on my truck and the 90mm that I have is the best option. With the volume demands my new engine has, the constriction of the 80mm butterfly would not be a good choice as it would choke out the motor starting in the mid-range.

Here's some of the PMs he sent me on another forum:

For the wild build you are planning, there is no doubt that a 90mm TB is in order. 95% of guys on the forums stick to the basic boltons and a mild cam, and that is exactly the build that we designed these TB's for. I would like to get another truck on the dyno to try both the 85mm and 87mm compared to stock, but the seat of the pants gains from the guys running these has been great. I appreciate yo utaking the time to hear me out on this. Good luck with your build. I'm excited to see how it goes.

BTW...we just came out with our 2013 TB's now as well. We tested these in both the 85mm and 87mm design, and the 87mm actually performed better, even on a bone stock truck. We just finished our first production run of them, so I will be posting pics of these later. I have a few on the way to customers now, so we should see some feedback from them sometime next week. None of the 2013's are modded yet, so this should be a true test as to whether they work or not. Our test truck dropped .15 at the track with the TB alone, so I'm expecting good things once the guys start running them.

Take care.

Matt
Thanks. The info was good in the original, but I think the way it was worded was just over most peoples head, so it was taken in a different way than you intended. In all honesty, aside from the guys with boost, a ported stock TB like the 87 is more than enough for most guys. The straight bore TB's from BBK and Arrington are nice because they flow more air, but with less airspeed. Better to pend power, loss of tq. Having said that, boosted guys arent too worried about losing a few tq down low, so it is a good compromise. On a bolt-on, or even mild heads/cam truck, the 87 has proven to be the way to go.
So, you can see that Matt and I agree on the need for intake velocity to increase/maintain torque. He even states that there are limitations to his TBs on bigger builds, which you will see I talk about in the end of the original post. Matt and I are on the same page and I still recommend his and fastman's ported TBs if they suit a person's build.

Though, if you are serious about the offer, I would love to prove the point when I get it all put together and onto the dyno at Davenport. I could honestly show the difference between a bone stock, ported 87mm, true 85mm, and a true 90mm on an engine that's built to flow volume. We've already seen the benefits of flowing a higher velocity on a stock and mildly modded engine, and I am genuinely interested to see how it would affect the torque and hp curves, just making the change to the volumetric flow rate.
 
#46 ·
For others, here's a good reference table from another forum that is quite accurate. I have bolded the 5.7L's stats.

Using the chart is simple. If peak power is higher than the rpm in the 1.2 “/Hg column, or peak torque is higher than the rpm in the 0.8”/Hg column, then you should really consider an upgrade. If peak power is higher than the rpm in the 0.8”/Hg column, or your shift point is is higher than the rpm in the 1.2”.Hg, an upgrade can be of benefit in an “max-effort” application, but isn’t absolutely necessary. I even included accompanying power outputs, so you can also make a selection based off of expected N/A power.

Throttle Body Selection Chart
-----------80mm ----------85mm ---------90mm
---------0.8-1.2"/Hg ----0.8-1.2"/Hg ----0.8-1.2"/Hg
345___5190-6360_____5920-7250____6700-8200
370_____4840-5930_____5520-6760_____6250-7650
392_____4570-5590_____5210-6380_____5900-7220
426_____4200-5150_____4790-5870_____5420-6640
440_____4070-4980_____4640-5680_____5250-6430
N/A hp____360-500_______440-580_______510-660
 
#47 ·
I dont think the 90MM throttle body will work without throwing codes but Im sure you will find out.
Im sure we can get something figured out with Davenport let me know.
Let me know if you need a hand.
 
#49 ·
I've been contemplating running my BBK 85mm for a while and holding onto the 90mm until I start running higher boost. For the 8-9psi a 90mm might be overkill, but I'll keep it just to have options. My engine won't like me either way, thank god for guys like Johan that can beat it into submission.
 
#48 ·
I put an 85mm on my Challenger and noticed great throttle response improvement. (other mods are LMI CAI, Flowmaster cat back and Diablo custom tune).
Dyno indicated about 4-5 ft lbs gained (when Tb was the ONLY addition) but that is throughout the low and mid-range which is where I wanted more power as that's where we drive the most...once I could tune it the guy that did my custom tune said that it was VERY beneficial and on a 392 he would suggest the ported 87mm but I didn't spring for the extra expense again for +2mm porting.

I plan to run whatever Matt suggests on my 2013 Sport CC 4x4 if Ram ever gets around to building it for me...ha. Probably the 87mm and an S&B intake.
 
#64 ·
Do you know what the good thing about turning forty this year is? I finally have the ability to admit when I'm wrong!

Ignore my post about the 85mm being for all N/A bolt ons, it's not. You guys were all correct when you said that as soon as you add headers, the 87mm is required. There was a mixup in communication between Matt at Moe's and myself. He's now sending me a new 87mm and he agreed that I had the wrong TB.
 
#65 ·
Anyone who needs a ported throttle body let either myself or Andrew know we will get you some killer pricing! and only the best quality, we use factory Mopar cores port them to 85mm or 87mm taper them to the original butterfly, and we do this on both sides of the throttle body.

Thanks
Jeremiah
Discount Auto Performance
 
#68 ·
I just installed my 87mm on friday, I have noticed some seat of the pants power. It took a couple key cycles to really start to feel it, now it seems to have a more effortless feeling under part throttle acceleration. I haven't really had a chance to go WOT yet, and I haven't made it to the track with it either. I do think once my tune is dialed in, it will produce even more gains. Throttle responce is better and my slight off idle stumble is almost gone, so it was worth it just for that reason, the extra power is a bonus at this point.
 
#70 ·
I have an 07 hemi ram that has a cai,electric fan, lt headers ,no cats , flowmaster dual exhaust, and a superchips programmer( thinking of selling and getting a different one) and a couple other bolt on's. I'm very interested in getting a ported tb and from what i.ve read I would need a 87mm with the mods I have. My only concern is my fuel milege. How would the TB affect the milege?
 
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