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Mod Motor Throttle Body & Plenum Swap

The nice thing about intake upgrades is that they’re easy to do since the whole intake system is right there on top of the engine. Most enthusiasts start with a cold-air intake, or at least a K&N filter in the stock air box. When it’s time to get serious, you have to follow the airflow farther into the system to unlock some power. In this article we’re going to take a look at the 2-valve 4.6L Mustang’s intake system and install an upgraded throttle body and upper intake plenum.

Two-valve ’96–2004 4.6L Mustangs came with a plastic intake manifold, an aluminum upper intake plenum, and a 65-mm throttle body. There have been larger throttle bodies available from the aftermarket since the Fox-body days, but upgraded upper intake plenums are a relatively recent development. Though early non-PI 4.6Ls (’96–’98) come with a different intake manifold than ’99–2004 PI (Performance Improved) 4.6Ls, the aftermarket intake plenums and throttle bodies will work on either. If you do have a ’96–’98 Mustang GT, you can swap on a higher-flowing PI intake manifold for more power.

I was lucky enough to follow along while Mustang owner Kevin Van Bogart installed an Accufab upper intake plenum and 70-mm BBK throttle body onto his 2000 Mustang GT. Kevin has already installed a K&N cold-air intake, long-tube headers, a Mac Pro-Chamber mid-pipe, and a cat-back exhaust. Since the ’00 came with the PI 4.6L, Kevin’s engine already has the PI heads, cams, and intake manifold. This means that the rest of the airflow path through the engine (intake system-actual engine-exhaust system) is already pretty optimized. With no other corks holding the system back, the upgraded plenum and throttle body should make quite a difference. Installing these same parts on an otherwise stock car would probably yield less of an improvement.

This is Accufab’s aluminum upper intake plenum. It not only looks better than stock, it flows better too. Accufab claims 680 cfm, versus 505 cfm for the stocker. There are a number of upper plenums available from a variety of aftermarket manufacturers. 

This Tech Tip is From the Full Book, HIGH-PERFORMANCE MUSTANG BUILDER'S GUIDE: 1994-2004. For a comprehensive guide on this entire subject you can visit this link:


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BBK’s 70-mm throttle body flows better and looks sharper than the stock 65-mm piece. Many aftermarket manufacturers offer both 70- and 75-mm throttle bodies.

Alright, let’s get started. This 2000 Mustang GT has a K&N cold-air intake, so the removal procedure will be different if you have a different intake. Remove the two hoses from the intake pipe, unplug the air intake temperature and mass-airflow (MAF) sensors, and unbolt the mounting bracket near the air filter.


Loosen the clamp holding the K&N intake hose to the throttle body and remove the intake.


With the intake system out of the way, take a good look where everything goes. Remove all the hoses, sensors, etc., from the throttle body and plenum in no particular order.


Remove the red vacuum hoses and unplug the idle air control motor. We chose to remove throttle position sensor (TPS) since we’d be swapping it onto the new throttle body. You’ll need a 5/32-inch Allen-head bit, socket, or wrench.


The idle-air control (IAC) motor comes off next. It’s held in place by two 5/16-inch bolts. Be careful of the gasket, especially if you’ll be reusing it.


It takes a little elbow grease and 10-mm deep socket to get the exhaust gas recirculation (EGR) valve off. You may have noticed we put off removing the positive crankcase ventilation (PCV) hose. The truth is, it was stuck to the plenum pretty good and took more than a little sweet-talkin’ to come loose.


Remove the two 10-mm bolts holding the throttle cable bracket to the plenum with a wrench or socket.


The upper intake plenum is held in place by five 5/16-inch bolts—three in front and two in back. Be careful not to drop them down on to the intake manifold. (Yes, the throttle cable bracket is still in place. The truth is we switched around these two steps, but it’s easier to loosen the cable bracket bolts with the plenum still bolted down.)


It’s much easier to remove the throttle and cruise control cables with the plenum and throttle body free to move around. We stuffed some rags down into the intake manifold to keep anything from falling down there. The plastic cruise control cable piece just snaps off, while the throttle cable has to be maneuvered out.


Compare the Accufab upper intake plenum on the left with the stocker on the right. The opening on the Accufab piece is about 3 inches in diameter, while stock opening is less than 2-7/8 inches. The Accufab plenum is also taller, which allows the air to make a more gradual turn down into the intake manifold.


Here’s an alternate view of the Accufab plenum. The opening on the bottom of the plenum is the same size as stock, so it matches the factory intake manifold.


The first reassembly step is to bolt the BBK throttle body to the Accufab plenum. The reinstallation process should be the same regardless of which plenum and throttle body you choose.


Pull the rags back out and bolt the new plenum to the intake manifold. Make sure nothing has fallen into the manifold. We were able to reuse the gasket between the intake manifold plenum.


The gasket between the stock plenum and EGR valve was toasted, but fortunately the previous owner of this used Accufab plenum sent a good one along. Otherwise, the ERG valve goes on the same way it came off.


Reinstall the vacuum hoses, the PCV hose, and the TPS. Be careful not to damage the part of the TPS that mates up to the throttle blade shaft. If you try to tighten it down while the parts are misaligned, you will damage the plastic sensor.


Bolt down the idle air control motor and plug it back in. We were able to reuse this gasket.


Hook the cruise control cable and the throttle cable to the new throttle body. This may have been easier to do before we bolted down the throttle body and plenum. The throttle cable was actually a tighter fit into the BBK throttle body, and it did require some finesse.


Bolt the throttle cable bracket back down and reinstall the throttle return spring. We forgot about the spring until after we started it up, so don’t make that mistake.


Reinstall the rest of the intake system and you’re good to go. The whole project took us only a couple hours, but that was including time for photography. With the new throttle body and plenum, Kevin noticed an increase in throttle response and less of a drop-off in power at high-RPM.

Written by Travis Thompson and posted with permission of CarTech Books

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