Throttle Body vs. Low End Torque

Hi folks. I was discussing with a friend this evening about throttle body sizes and some negative effects they may have in terms of performance. This interests me cause I already installed in the past a larger throttle body (70mm vs. 65mm) and I ported and polished my plenum to increase air flow.

What my friend was arguing is that the larger throttle body decreases the velocity of airflow into the engine, therefore decreasing low end torque although allowing for a gain in the mid to top end because of a greater volume of air able to enter.

If the above is true, would that be true any time the throttle plate is open at any amount, or just at/near WOT, when the throttle body is almost fully unrestricted?

I am concerned with this because my car is a heavy car - it needs alot of power to get it moving. I am already making above stock torque, but still want to squeeze as much out of it as possible as this spring I intend to start attending some open track days and see what it's capable of.

Unfortunately when I installed the larger throttle body and plenum, I installed a larger airbox and MAF and computer tune so I can't compare before and after seat-of-the-pants power. Even worse, I don't have access to a dyno around here to verify what, if anything, has been lost or gained across the RPM range.

So I am curious to hear from others who may have more technical knowledge in this area or first hand experience on their cars.

Thanks!

I'm no expert here...I'm mostly good at theoretical bulls**t...but I'll offer my thoughts. The throttle body is generally just an air regulator and mixer with the fuel. The engine pulls air in at some flow rate (displacement x rpm ÷ 2...during the engine cycle half the cylinders are sucking air in and half are squeezing).

The velocity is created by forcing that mass to flow through a smaller cross-sectional area...through the intake runners, plenum, and throttle body. The point of smallest cross-sectional area would be the max velocity of the air. What is good about velocity is there is better mixing and atomization of the fuel when it is injected into the airflow. What is bad is that the restriction causes a vacuum or lowering of density of the air charge. An engine pulling 18" Hg of vacuum is getting only about 60% of atmospheric pressure, thus 60% of the potential air mass for that volume sucked in by the cylinders.

Therein lies the issue. If you increase the TB diameter, you can reduce the vacuum effect, giving more air mass, but then you also reduce the air mass' velocity. You need more fuel and higher fuel pressures to feed that air and maintain a fine spray. Otherwise you're wasting potential power.

I don't see how you would sacrifice low-end torque by increasing the TB diameter since this would decrease the vacuum and increase air density. But too large and you won't get the mixing that you need for complete combustion because the air velocity at low rpms is relatively low. Once rpms go up, the air gets alot more turbulent for good mixing, thus better high-end performance.

I may be way off, but at least it's a start for this discussion....

Thanks for your thoughts. Theory is always fun to hear, too!

So after reading what you said, would it make sense to you that if at partial throttle and low RPMs there would be no difference in airflow between a smaller and larger throttle body? Assume in both cases the throttle is opening enough to maintain the same RPM.

If so, then perhaps the only time I can [i]theorize[/url] that the air mass velocity would be too slow is when at a low RPM and WOT (i.e. floor it from a stop). At this time, vacuum drops to near 0" Hg for a brief moment due to the increase in air mass but decrease the velocity significantly, causing the engine to hesitate a bit. With that said, I don't see why a larger throttle body would make any difference over a smaller one.

I'm just speaking out loud (sort of) here and welcome any ideas. I'm very interested in this kind of stuff!

As a generalization, velocity makes torque, mass makes HP. So, all else being equal, a larger TB can decrease velocity and torque, but offer more mass which can result in more high end HP if the rest of the engine's tune can use it.

Its always a delicate dance. When you mash the pedal to WOT from a stop, you have full access to the full potential of atmospheric air that the TB can offer. If you've increased the size and flow potential by increasing the flow area with a larger TB, the air will be flowing slower. The secret is to choose a TB size that provides enough flow that supports the full peak HP potential, but not any larger so that it doesn't have the opportunity of reducing torque from idle-midrange. There is a magic velocity number which is actually somewhere around 300 mph. Let's say you're doing a dyno run which is at WOT from low RPM to high. As the RPMs increase, the velocity through those components increase. About the time those velocities get to around 300 mph, they will start to become more and more of a restriction - that is to say, it starts to take an exponentially larger amount of vacuum from the cylinder to increase its velocity past that point. You reach a point of diminishing return where the energy required to draw the piston down against the restricted flow becomes greater than the additional flow provided by the additional draw. The secret is to match the engine's mechanical potential (based on head flow, cam timing events, compression, etc) with where that velocity peaks. Choosing a cross sectional area of flow that makes the magic peak velocity happen at the same RPM that the engine peaks its HP is the ticket. That minimizes the trade off of being too big at low RPMs and not big enough at high RPMs.

The other thing to consider is matching velocity from one component to the next. Let's say you have a stock engine with a small cam and small intake runners and you increase TB size. There is a chance you won't give up much low end torque because the net velocity that ends up in the runners is the same, but, airflow doesn't like to change velocity. So when you open that massive TB you have slow flow through the butterfly that has to rapidly change velocity to enter the runners. Engineers try to make components that won't restrict on the top end and don't slow velocity too much on the bottom end, but the trade off is that they have no choice but to make a fixed-potential set of components that need to supply the engine through a wide variety of throttle positions and RPMs.

That is also the reason that high-strung race engines have high stall converters, high numerical gearing, and are used in light vehicles. The large components have been used to chase high end HP at the expense of low end torque. The gearing, converter stall, and other factors help the engine breeze past the entire low-RPM band where the engine is pretty useless.

There are very similar properties happening in the exhaust. If you have stock restrictive exhaust manifolds and just hook them up to a 3" exhaust, the high velocity gasses suddenly slow down when it hits the 3". It therefore loses intertia and energy. There will be no restrictions in the 3" which means it won't be the limiting factor in top end HP, but it also won't retain much velocity which means it won't help scavenge itself out of the exhaust (which is a different chat altogether)

The point is, they should all match. Sometimes the OE engineers drop the ball and you have one component that is a large restriction, but these days things are pretty darn well matched.

One thing to consider... many less-experienced drivers mistake a larger throttle body as giving more low end torque in the seat of their pants. This is rarely the case as we have discussed. What is usually happening is that they notice much more "oomph" with the same driving style. Given the same throttle pedal travel and the same opening, the new larger TB flows more than before. When they leave a stoplight and apply 10% throttle like before, they are now effectively flowing as much air as if they had opened the old throttle body 17%. They are mistaking increased torque for just more airflow at the same foot input. In many cases its enough of a difference that the computer needs to be reprogrammed because the throttle position sensor no longer tells the computer an accurate expectation of how much air its taking in.

I combatted that in my car by re-designing the progressive linkage on the throttle body. My LT1 came with twin 48mm and I upgraded to twin 58mm when I went to a larger cam, head ports, and exhaust. I noticed that it was "jumpy" right off the line and then progressive for the rest of the travel. I just designed it to be slower during 0-50% travel, then open faster for 50-100% throttle. It made regular driving around town so nice and easy to modulate, and the last 50% of the throttle really brings on the power fast.

Quick and dirty summary: Match it all together, velocity makes torque, flow makes HP, all a trade off, yes its possible that a larger TB can reduce low end torque while increasing high end HP, Led Zeppelin was the greatest band ever.

One other thing to add based on your second post...

Measuring engine output at anything less than WOT is difficult; partly because of that velocity change we talked about. If you opened that 70mm throttle body enough to simulate the same flow as the original 65mm throttle body, you wouldn't get the same dyno results as if you never switched - partly because you enlarged the plenum behind it and the airflow is now going from fast to slow.

There are other factors involved, but I don't know if you want to speak theory or practice... The engine's ECM is a big factor. In practice, if you hold your 70mm TB open enough to simulate the flow of the 65mm, the ECM is not at WOT fueling maps, but in theory you can get close to your old dyno numbers by simulating the flow of 65mm through a 70mm TB.

You have also made a very subtle change... We're discussing theory of going to larger intake passages, but in practice you probably haven't made any massive changes that have caused any "mismatches" like I discussed earlier.

In my hotrodding background, we typically discuss theory while hypothetically comparing a single 750 cfm carb with dual 600s. Then you're talking about massive differences and the velocity/flow potentials are vastly different.

... which brings up another point (and then I promise I'll shut up :)) MPFI versus TBI/Carb are a bit different. The TBI and carb mix fuel and air before the TB, so the mass of the airflow is much greater as it goes through the TB, plenum, and intake runners, while the port injection doesn't inject fuel until after all of that. An airflow change on a port injection engine has less impact since the inertia of the "dry" air is much less. A "wet" intake has several design factors to consider. Not only do massive changes in velocity tend to have larger dyno impact given the greater mass/inertia of the incoming charge, but they also tend to make fuel fall out of suspension and puddle.

Wow, that is alot of information to take it. Thanks for taking the time to type it all!

So I guess that settles the fact that a larger throttle body CAN cause a loss in low end torque, but depends on the mismatching of parts that can cause that problem. But in the end it seems like I shouldn't be too worried about my 70mm TB. I can't imagine losing more than 3-4 lb-ft of low-end torque.

Now with all of that you said, that is purely based on a naturally aspirated setup. If one had a supercharger that provided boost at lower RPMs (roots or twin screw type), I would assume that TB size does not matter so much as long as it is not a mass restriction since it would be before the forced induction compressor/pump. This make sense to you?

But I may do some SOTP experimenting just to satisfy my curiosities anyway. And Led Zeppelin is fantastic...but the best? Well, I will disagree with you there.

I agree with most of Curtis post. He is of course, correct that the bigger TB would alter the throttle characteristics. But I think his comments apply best to TBI systems (and carburetors!), but not to your MPFI engine.

For your Merc (multiport fuel injection) engine, torque and hp at WOT are a function of the intake runner and plenum design (as far as the intake is designed). If those stay the same, low end torque and hp will not change, because even the stock TB can flow enough air to meet demands. The larger TB will not change this, nor will it change the flow dynamics of the intake plenum and runners. Of course, you may get more peak power if the stock TB was the most restrictive part of the intake system.

Now if you were using a TBI (throttle body injection) system, I can see a larger TB would affect low end hp and torque. This is why Holleys aftermarket TBI systems come in 2 different CFM capacities, so it can be tailored to the engine fuel/air demands for best power characteristics.... and, for TBI systems (but not multiport FI) , I agree with Curtis explanation.

Ok I know this thread is old but I just need some answers as to why the throttle body design is always a larger input and a smaller output? For example I have a stock TB that measures 67mm input and 64.5mm output. Why is this? Why can't they just make 67mm input and 67mm output? Is there a specific reason as to why they design the way it is? What happens if you have a throttle body that has the same 67mm input and 67mm output? What's going to happen then? Please kindly advise, your information is very valuable to me thank you all!

As mentioned before, the reduction in the bore cross-section results in an increase in air velocity. The air velocity is needed to help fill the cylinders with fresh air when the intake valve opens. But this sucking in or air isn't constant. So, the air intake plenum and attached runners allow the engine to draw on a "store" of air that is at a more constant pressure (vacuum actually) and moves at a constant velocity. The TB bore is ideally matched to be able to replenish the plenum at the same rate and velocity as is needed.

Take the most extreme case of a racing engine that has no TB or intake plenum, only a set of velocity stacks atop the carbs. They are there to help mix the fuel and create that velocity to help pack more air into the cylinders.

Newer cars that have direct injection or port fuel injection (where the fuel is introduced after the TB) usually have TBs that don't have as large reduction in bore diameter. If they do, it is to support sensor function (MAF or MAP) that relies on being within a range of flow rates. For TBI systems, the reduction is for sensor function as well as for fuel mixing.

This is why that after the intake is usually changed past a certain point, the computer has to be reprogrammed to read the sensors correctly and/or the sensors need to be changed to match the flow that is expected through the bore