Swapping a Gen III HEMI into your classic Mopar or any other vehicle is becoming easier and easier. With companies like Edelbrock, Comp Cams, FAST and others designing specific HEMI swap parts has been a huge relief for us Mopar enthusiasts.
There are many options right now to power your Gen III HEMI. FAST EZ-EFI and Edelbrock Pro-Flo 4 are just a couple of the many options. One problem with a lot of these engine management systems is that they require a drive-by-cable throttle body. This can be a difficult task since majority of Gen III HEMI vehicles are all drive-by-wire. That is why FAST developed their drive-by-cable HEMI Big Mouth LT 87mm throttle body.
Above: What an outstanding piece from FAST. The Big Mouth LT 87mm throttle bottle looked as good as it performed.
If you are new to some of these terms and HEMI swaps, drive-by-cable is where a cable connects your throttle body to your gas pedal. It works just like a carburetor, you push on the gas pedal and the throttle body opens.
Drive-by-wire is sensor based, where your gas pedal sends a signal through a wire to the throttle body sensor that tells it to open or close. Majority of vehicles today work this way.
The FAST HEMI Big Mouth LT 87mm throttle body has been around a little while. It features an 87mm opening for increased air flow and horsepower over a stock Gen III HEMI throttle body.
Above Left: You can tell there is quite a bit of different from the factory throttle body and the massive 87mm opening of the Big Mouth LT throttle body. Above Right: This is everything that comes with the Big Mouth LT TB.
It also features a progressive throttle pulley which enhances drivability, especially at low throttle, while a double return spring provides a solid feel and increased safety.
We reached out to the guys at FAST during the start of our Gen III HEMI swap in Project Orange Crush. At that time our engine management system required a drive-by-cable throttle body (TB).
Above Left: A little lock tight on the throttle cable bracket screws to insure they do not come loose. Above Right: Make sure the bracket is installed pointing forward or it will not work.
Later on during the HEMI swap our engine management system allowed us to use a drive-by-wire setup if we wanted but we decided to stick with the drive-by-cable FAST Big Mouth LT TB for a couple of reasons.
First, it was a simple installation. Since Orange Crush already had a cable driven gas pedal, we were able to keep our stock pedal. Without modification, we were able to slip our new throttle cable in place of the old and remount the pedal to the floor of our Dart.
Above Left: We used a squirt of lubricant to help slip the IAC in its hole and not roll the o-ring off. Above Right: Here are the sensors all installed and ready to bolt on to our HEMI.
Secondly, we have found out that it is very convenient to be able to manually rev the motor while working under the hood. With a drive-by-wire set up you’d have to push on the gas pedal to rev the engine, you cannot rev it while you are working on it under the hood.
Lastly, it is a direct connection with the throttle body and your foot. Just like a carburetor, you push on the pedal that throttle body is opening whether it wants to or not. With a drive-by-wire setup, you might push the throttle but the computer might keep it shut.
Above Left: We used a t-handle allen to tighten the bolts to ensure we did not strip the 6.4 HEMI plastic intake. Above Center: We used a criss cross pattern when tightening the four screws. Above Right: The FAST Big Mouth LT throttle body is ready to roar.
The feel of FAST’s double springs on the Big Mouth LT TB feels just like a carburetor. We really enjoy the instant feedback from our throttle pedal, which you do not get with a drive-by-wire throttle pedal.
FAST Big Mouth LT TB comes in two different variations. You can order it with the IAC (idle air control) and TPS (throttle positioning sensor) included or not included. We opted for the one with the sensors, which plugged right into our harness.
Above: The FAST Big Mouth LT throttle body makes the 6.4 HEMI look a ton better and will gain us a few horsepower too.
Note that the IAC and TPS sensors that are included are GM style sensors. That is what our wiring harness needed and seems that many of the engine management systems use GM style sensors. You should check with your specific engine management system for compatibility.
The only small issue we had was our engine management system required us to re-pin the IAC connector on our harness. It was no fault of FAST, it was our computer and luckily we were able to switch the wires around in the connector and has worked flawlessly since.
Above Left: Here is our Amazon dirt bike clutch cable. Above Right: The cable was an easy fit on to our stock gas pedal and the Big Mouth LT TB.
The installation of the Big Mouth LT TB was very straight forward. We had to install the sensors and throttle cable bracket. All the screws are included with the throttle body. We did put a little lock tight on the throttle cable bracket screws.
We had the Big Mouth LT TB installed on our 6.4 HEMI intake but then decided to switch intakes later on in the swap. We settled on a 6.1 intake and neither of the intakes gave us any hassle with installation.
Once the throttle body was installed, we had to route our new throttle cable. We purchased a cheap 48” universal dirt bike clutch cable off of Amazon.
Above: We routed the throttle cable under the intake to keep it concealed. We also used a line clamp to keep the cable from moving around too much.
We went with a longer cable than we needed because we knew we could cut it down shorter if needed. We also wanted to make sure we had enough cable to snake it through the intake and hide it the best we could.
Be sure when you install your throttle cable that your gas pedal is fully synced with your throttle body. When you depress the gas pedal to the floor and it stops, your throttle body should be all the way open.
Above Left: We had to swap a couple wires around to allow our ECU to communicate correctly with the IAC. Above Right: It is difficult to tell in the picture, but we had to bend the gas pedal just slightly to gain a little more travel so that the throttle body would open all of the way.
We had to get some help to make sure our gas pedal was synced with the throttle body. We depressed the gas pedal and then had a friend manually rotate the throttle body shaft to see if it was open all of the way or not.
Although our gas pedal was on the floor and would not push any further, our throttle body was not all of the way open. We ended up slightly bending our throttle pedal to give it more travel which then opened the throttle body all of the way.
Above: You can see how we routed the throttle cable under the intake and back to the Big Mouth LT TB.
We have also seen where some gas pedals get bound up in the carpet. It looks fine, but when you would press the gas pedal to the floor, it would be prematurely be stopped by the carpet. You thought you had it to the floor but in reality the throttle body or carburetor was only 3/4 of the way open, which can cause some issues.
Once our HEMI swap was done and ready to fire up, we had to sync our Big Mouth LT TB up with our engine management system. We needed to adjust the throttle blade at idle a tad bit. It was very simple task using the adjustment screw on the side of the throttle body.
Above: We are very pleased with the performance and feel of the FAST Big Mouth LT 87mm throttle body.
After we adjusted the throttle blade, we were off and running. The FAST Big Mouth LT TB is still on Orange Crush currently and we love it. It looks incredible, works flawless and we know that we are getting maximum performance with its 87mm opening.