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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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#75 ·
holy crap that was a long thread and i actually read all of it since my 87 ported from Moe's is on its way and will get here weds. along with the vararam air grabber. hope this is going to get me movin again.:smiley_thumbs_up:
 
#76 ·
this is a fantastic thread, ive been doing lots of research on this topic and it was all wrapped up in a neat little package for me.

Im sorry in advanced if this has already been answered... im looking at an 87mm ported throttle body, is there a particular one that is better than the other??
 
G
#81 ·
Moe's 87mm Throttle Body

Installed 87mm Tb from Moe's on my 05 Ram and can't believe the difference it made. Revs are crisp and instantaneous.:smileup: Changed it's attitude completely. Old TB was kept clean so that was not a factor in the improvement I felt. Granted I already had Vararam, LT headers, dyno tune, etc, etc and this TB was just the cherry on top, but what a cherry. Highly recommend this mod, especially if you already have tune and headers.
 
#82 ·
So Im planning on a cat back and maybe a tuner in the near future so an 85mm ported looks like what I need. What would the drawbacks of an 87mm be for now if I were planning atleast headers down the road? Were talkin $50-75 more now instead of buying two TBs.
 
#84 ·
Also, Im not racing the truck, just looking for better performance, I do tow with the truck pretty regular so low end torque is definitely a plus for me. So would the 85mm be the better choice even if i did add headers later? Thanks
Honestly, to the avg person you will not truly notice the difference form an 85mm to 87mm.
Someone who is VERY inclined to this stuff will notice, but even to them it will be slight.
Where you will notice is if you were to drag your truck and it meant that 1/10 of a time
Forcing more air into your ram (via the 87mm) and not having the proper way to get it out quicker (headers, FI) will be your bottle neck.
But again, its not noticeable to the avg person or member here.
 
#83 ·
Also, Im not racing the truck, just looking for better performance, I do tow with the truck pretty regular so low end torque is definitely a plus for me. So would the 85mm be the better choice even if i did add headers later? Thanks
 
#86 · (Edited)
OK, have read thread from start to finish and still can't figure it out.
I have a 2013 Express Reg Cab SWB with Hemi and 3:55 gears. I put the Magnaflow 22" on and deleted the resonators. Have a K&N filter. Don't plan on anymore add ons.

I don't take it to track. I do tow a bass boat.

Now, what TB do I need and will it help gas milage and torque? Low end sucks!!
Also what brand do I need?
 
#87 ·
the only throttle body for the 2013's are the 87mm, no one is currently making an 85mm flavor for the 2013's as they showed little to no gain, we have 2013's ready to ship if you are interested let me know I would be more than happy to get on on its way to you this upcoming week, check it out http://bullysperformance.com/bullys...hrottle-body-87mm-2013-ram-5-7l-hemi-268.html, and if you purchase a hypertech interceptor/Bully Dog Hemi+/or Diablo Sport trinity t1000 we will take another $20 off of the price :) (note DiabloSport has not released support for the 2013's yet but it should be coming up right around the corner), we will also discount $20 if you purchase a Vararam Air grabber.

Thanks
Jeremiah
Bully's Performance
 
#92 ·
ok just read an article that explained everything about the stall.
There are different stall speeds, which you read as well.
If you tow quite a bit a higher stall is generally not recommended.
Those who don't tow and like their power and drag, will typically go with a if her stall speed.
But choosing the right stall for you is important and how you use your truck will determine this.
But a stall is definitely a great mod :smileup:
 
#100 ·
[off topic]

What RPM do most people spend driving? The answer is 2000-3500rpm. Where is our maximum torque? 2700rpm.

What RAM 5.7 hemi's come with max torque down at 2700? The one I bought is up at 4,000. As far as the rest of the technical description of induction science in the initial post, am I the only one who almost got a headache from noticing all the technical inaccuracies?
Why do I always have to be the bad guy who calls BS??:LOL:
 
#102 ·
Cdnoilram- thank you for the informative post! It makes me feel a little better about my $349 investment.
You name 'Moe's' and 'Fastman' ported TBs by name; what say you about your sponsor's PTB offerings, 'Bully's', to be fair? Is there a difference in their approach?:4-dontknow:
 
#103 ·
At the time of writing, Bully's was only just getting started and they didn't have a ported TB out yet. As for my opinion, all I can say is that I won't run a ported TB on my truck unless I knew that the backside of the butterfly had only been ported to match the inner diameter of my intake manifold, allowing it to maintain any velocity increase associated with a more gradual necking down from intake tube's inner diameter.
 
#104 ·
....... As for my opinion, all I can say is that I won't run a ported TB on my truck unless I knew that the backside of the butterfly had only been ported to match the inner diameter of my intake manifold, allowing it to maintain any velocity increase associated with a more gradual necking down from intake tube's inner diameter.
Okay, that's great, so what about Bully's TB's?
 
#105 ·
I had talked to the gents at Bully's about their profile during their development, but I don't know what the end result was, so I can't say. I haven't had the opportunity to inspect one of their profiles. If someone could vernier the rear portion of one of their TBs and the inner diameter of the intake manifold you'll know what my answer would be.
 
#106 ·
So I've read through this whole thread. Very informative. Thanks CdnoilRAM.:smileup: I've also read as many other threads as I could find. Grubrunners review on Bully Performance's new TB was a great read as well. Grubrunner went through a very detailed "break-in" schedule after his install. Is this common to all TB's post install? Does anyone else have any feedback on Bully's TB? Cheers to all! :smileup:
 
#108 ·
I was thinking the same thing. But might just hold off until the mods are done.

Magnaflow 16869
S&B CAI
ARH Headers

I dont think there would be much sense in getting a bigger TB until they are done, especially if I want to lift it and put 35's on there. Need the TQ in the low end to turn the tires.
 
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