Swapping the KA24DE Throttle Body onto the CA18DET Intake Manifold (The Quick and Dirty Way)


Side by side image showing the larger opening on the KA24DE throttle body.

Installing the larger KA24DE throttle body onto the CA18DET intake manifold is one of the rare free mods that has no drawbacks. Although a gain in horsepower is usually not evident when swapping a larger throttle body onto a pressurized (turbo charged) intake manifold, increasing the size of the throttle body will improve the motor’s on/off throttle response as well as boost response. The following is a step-by-step guide to installing the KA24DE throttle body onto the CA18DET.

Step 1: Remove the Upper Intake Manifold

To take full advantage of the larger throttle body, the intake manifold inlet will need to be port matched to the larger size of the KA24DE throttle body. Because of this, the upper intake manifold needs to be removed so that metal fragments do not make their way into the motor during the porting process. First, remove the two lower support brackets from the manifold. One runs from the vacuum block at the back of the manifold to the side of the transmission bellhousing. The second runs along the side of the manifold to the engine bracket just above the engine mount. Now remove the throttle cable and then the two bolts that hold the fuel line to the back of the manifold. Next, remove the 8 bolts and 2 nuts that hold the upper manifold to the lower manifold. (It takes a combination of 14 mm wrenches and sockets to successfully remove all of these so be patient) Proceed and remove the two brackets from the top of the manifold and the two grounding wires. Finally, loosen the two bolts holding the coolant and cold start assembly to the front of the manifold (just below the throttle body) and then the bolts holding any other metal lines (coolant or idle) to the manifold as well as any pertinent plugs. At this point the upper intake manifold should be loose and capable of pulling away from the lower manifold. The proper removal angle may take a while to find but keep in mind that if you encounter resistance, something is either still connected or the manifold is catching on something. (Also, make sure you place all of the removed items and hardware in a safe place so they don’t get lost. A labeled zip-lock back will do wonders)

Image with bolt removal locations labeled for the removal of the upper intake manifold from the ca18det.
My motor runs without a few of the brackets and auxiliary systems so this image should help clarify the mounting locations. 1 is the location of the rear support bracket. 2 is the location of the mid support bracket as well as coolant and air hard lines. 3 points to the two bolts that hold the fuel hard-line to the manifold. 4 shows the upper brackets, plugs and grounding wires. 5 points two the coolant bracket and cold start solenoid. 6 is the throttle cable.This is the upper intake manifold removed from the CA18DET
The upper intake manifold removed from the CA18DET.

Another image showing the CA18DET motor with the upper intake manifold removed.
Another reference image showing what the engine bay will look like once the upper manifold is removed.

Step 2: Port the Intake Manifold Inlet

To take full advantage of the larger KA24DE throttle body, the intake manifold inlet must be widened to the larger size. You will need an assortment of grinding and finishing tools as well as a dremel and/or drill. A boring stone could be used but I ended up using a brass finishing wheel (2.5 inch). First, remove the CA18DET throttle body from the manifold and set it aside. Go ahead and remove the old gasket from the manifold inlet and then scrap any excess off with a gasket scraper or razor blade. Loosely test fit the KA24DE throttle body and open up the throttle plate by hand. Look inside the manifold and note the excess material that now sits at the opening. Remove the throttle body and begin to grind the excess material from the manifold in a circular pattern. After every few passes, loosely reinstall the new throttle body and once again open up the throttle plate to check for over hang. Repeat this process until the intake manifold inlet matches or slightly exceeds the size of the throttle body. Once they match, smooth out the ground surface with the finishing tool.

The excess material needs to be removed from the ca18det manifold in order to port match the new throttle body
This is the first test fit of the KA24DE throttle body on the CA18DET manifold. Notice the overhang of material from the manifold.

This is the intake manifold after porting to prepare the ca18det intake manifold for the ka24de throttle body.
After porting, the transition from the throttle body is now smooth and nearly seamless.

Step 3: Additional Throttle Body Prep

Two more things need to happen before the throttle body will be ready to operate with the CA18DET. First, the TPS (Throttle Position Sensor) needs to be swapped over from the CA unit to the KA one. The TPS is the small black box that connects to the side of the throttle body. Before removing the two small bolts that hold it in place, take note of its location. You want to move the TPS over to the new throttle body as carefully as possible to avoid rotating the sensor. If the sensor rotates you will have to reset the TPS. This process is covered in the FSM’s. Fortunately, most TPS’s on these motors have never been moved so you can simply look at the corrosion on the adjustment bracket to determine where the sensor needs to rest.

After the TPS has been moved over, some sort of custom pulley will be needed for the KA throttle body to mate with the throttle cable. A lot of people simply enlarge the opening on the CA pulley so that it can be moved over to the KA throttle body. This is a very small area to work in and frankly, I find it a P.I.T.A. to get this hole just right. This is where the quick and dirty method comes into play. I simply cut the throttle cable connection off of the CA pulley and connected it to the KA pulley (at 12 o’clock on the pulley) with a few tack welds. This process took about five minutes total.

This is the segment that is needed from the CA18DET throttle body pulley.
This is the cut section from the CA18DET pulley.

The cut piece from the ca18det pulley is tack weld to the ka pulley.
A couple tack welds later and the CA and KA pulleys are combined to make a single functional pulley that lines up perfectly with the throttle cable.

Part 4: Cleanup

One final step needs to be taken before putting everything back together. Take an air compressor or whatever alternate method you have access too and blow out the inside of the intake manifold and the throttle body. Do a very thorough job to ensure that none of the metal fragments created during the porting end up in the motor.

Part 5: The Final Install

There are two options when reassembling everything. You can go out and buy a new throttle body gasket or you can use RTV silicone. I used RTV red silicone around the throttle body and then firmly connected it back to the intake manifold. Next, reinstall the upper intake manifold in the reverse order that it was removed. Note: When tightening the 8 bolts and 2 nuts that connect the upper manifold to the lower, use a criss-cross tightening pattern starting from the center.) Finally, when connecting the throttle cable to the new pulley, if the cable end will not go all the way in, use an appropriate sized drill bit to enlarge the hole on the now closed off side that faces the KA bracket. The cable end should now fit perfectly in the new pulley.

A thin layer of RTV is used as the new tb gasket.
A thin layer of RTV works great as a substitute throttle body gasket.

Additional Install Notes

The KA24DE throttle body should be installed in the same manner as the CA unit.  This means that the small vacuum port on the bottom needs to be capped or the throttle body will leak air.  Also, the coolant line that run through the unit may need to be bent downward if it contacts the manifold, preventing the throttle body from seating properly.  However, this line is unnecessary for all but the coldest climates and can likewise be removed altogether.  (Very few CA owners run the coolant lines through the throttle body)

By 240am

Chris Simmons is a race car driver, instructor, business owner and all around gearhead. His passion for motorsport started in the drifting community over 10 years ago and progressed into wheel to wheel racing of all kinds. When off track Chris can be found training and working on a myriad of projects. After a hiatus from writing, he is back to share his passion, knowledge and experience in regard to motorsports on and off the track.

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