In the search for more power, a bigger throttle body (TB) was determined to be the next place to investigate given that the intake, headers and exhaust had already been improved. Since I have access to a lathe and other machinery, the first thing I investigated was overboring the standard Honda throttle body.
Upon investigation, the standard second generation B16A throttle body looks like it can only be bored by about 1mm, perhaps 2mm before you breach into the cavity that the throttle position sensor (TPS) lives. Such a small increase in diameter for the amount of other work that is involved just didn’t seem worth it. So I decided to investigate the option of swapping in a completely different throttle body. Of course the best place to start looking is in the range of OEM throttle bodies that Honda produces. Unfortunately, the 60mm throttle plate in the second generation B16A TB is the largest size that Honda produces apart from the B18C Type R throttle body, which is only 2mm bigger at 62mm.
For all the potential hassle that could be involved with doing a TB swap, an extra 2mm just didn’t seem like enough. Mainly because I don’t like doing things in a small way. I figure that if you are going to do a modification, you might as well do it big to get a noticeable change. I started looking at what aftermarket throttle bodies were available to swap in for the standard B16A throttle body and found several:
The Spoon 70mm throttle body (despite its name) in fact only has a 62mm throttle plate. The 70mm refers to the internal measurement at the end of the machined taper intake. The RC Engineering overbored throttle body looks like a first generation B16A throttle body that has been overbored to 64mm. I am not sure how they got around the cavity breach problem underneath the TPS. I suspect they filled in the cavity with weld before overboring, which looks completely feasible (however, the MAP sensor vacuum pick up point may of had to be altered as well). The Erick’s Racing throttle body looked fantastic, but it cost way more than that I was willing to pay for one. Also any of these throttle bodies would have to come from overseas, which makes the prices even more ridiculously expensive when you are paying with New Zealand dollars and have to pay US prices.
I remembered reading somewhere that someone had managed to use a throttle body off a Ford Falcon somewhere else in New Zealand (the land of Kiwi ingenuity) to get substantially more air. I did some more research locally and found out that a Ford XF throttle body can be obtained with either a 65mm or 68mm throttle plate. They can also be obtained at a pretty good price at the local Ford parts wrecker. This option seemed to satisfay all my criteria: relatively cheap with a pretty good increase in size. The only downside was that some fabrication may be required, but this didn’t really bother me. After all, nothing comes for free. I think that these throttle bodies usually live on a Ford straight 6 engine.
Below is a table comparing the percentage air flow increase for all of the different throttle bodies that I found during my investigation. The air flow increase is based on the approximate difference in cross sectional area at the throttle plate (allowing for the size of the spindle) when compared to the first generation B16A throttle body.
|Model||Throttle Plate Size||Air Flow Increase|
|B16A G1||58mm||0% (all comparisons to this)|
|B16A G2 and G3||60mm||7.6%|
|B18C Type R||62mm||15.4%|
|Ford XF Falcon||65mm or 68mm||27.6% or 40.5%|
|Erick’s Racing||68-66mm or 70-68mm||40.5% or 49.4%|
So, I ended up purchasing a wrecked 65mm Ford XF throttle body. It was harder to find a 68mm throttle body, and the approximate 27% increase in air flow sounded more realistic for the 1600cc engine than the massive 40% you would get with a 68mm TB. Once I had the throttle body I started trying to figure out what fabrication would be required to get it to mate with the Honda intake manifold and various electronic sensors.
Below is a picture comparing the first generation B16A throttle body with the wrecked Ford XF throttle body. The Ford XF throttle body (on the left) is visually much much larger than the stock Honda one. There is also enough aluminium in the Ford throttle body to bore it all the way out to a 70mm throttle plate. Click on the picture to get a larger view of it.
It ended up being a bit of a mission to install the Ford XF throttle body. Now things are finally running fine with it and I think the improvements to the torque curve were well worth it. There wasn’t much drop in torque down low, and the increase in torque up high was reasonably substantial as shown in the latest B16A Dyno Runs.
Ford XF Falcon Throttle Body Installation
This section contains the installation details of the Ford XF throttle body into my Honda Civic (including photos). The Ford XF throttle body was obtained from a local Ford wrecker. It is from a straight 6 Ford XF Falcon.
Below is a table containing a series of dimensions used to compare both the first and second generation B16A Honda throttle bodies and the Type R throttle body with the Ford XF throttle body. The Ford XF throttle body is much larger than any of the stock Honda ones.
|Measurement||B16A G1||B16A G2||B18C spec.R||Ford XF|
|Opening - Front||62mm ID, 69mm OD||n/a||n/a||71mm ID, 81mm OD|
|Opening - Back||59mm||63mm||n/a||67.5mm|
|Butterfly||58mm||60mm||62mm||65mm (68mm available)|
|Butterfly to back||20.5mm||20.5mm||20.5mm||53mm|
Installing the Ford XF throttle body is by no means an easy task for the Honda intake manifold. If you do want to use a Ford XF throttle body I suggest getting a second intake manifold (preferably the second generation model), because there is a possibility that it could all turn to custard since there is quite a bit of welding and grinding involved if you want to do it properly.
Here are the steps that we went through to install the Ford XF throttle body.
- Fill the existing TB mounting holes, and the existing air supply lines to the Idle Air Control Valve (IACV). You’ll need to know someone that is competent at welding aluminium for this step.
- Clean up the face of the intake manifold taking care not to mark the face too much. Otherwise you may end up with an air leak between the intake manifold and the throttle body when it is put back together again.
- Line up where the new TB will be mounted, taking care that you will be able to port match the intake manifold without breaching any of the existing cavities. On the second generation intake manifold the right hand side of the original hole was left and all of the material was taken out from the left.
- Mark the locations of the mounting holes and where the manifold has to be port matched to.
- Port match the intake manifold and drill and tap the mounting holes. You need to remove a lot of aluminium during the port matching because the back of the Ford XF throttle body is much larger than the stock Honda one.
- Drill a hole into the back of the intake manifold that will breach the old IACV air supply line. This is not required if you don’t mind the car idling low on a cold morning, or when the air conditioning is on. Weld on a spiggot that will be used to connect the air supply line from the existing Ford XF TB arrangement.
- Below is a comparison of the port matched second generation intake manifold compared to a first generation intake manifold (the one that is stock standard on the 1990 Honda Civic SiR).
- Now you have to drill a hole into the Ford XF throttle body that will be used for the vaccum line that connects to the Honda MAP (Manifold Absolute Pressure) sensor. We drilled a hole through the side of the manifold in the big square bit at the back and epoxied in a stock Honda vacuum line pickup that we pulled out of an old Honda throttle body that was lying around.
- The stock Ford XF throttle body contains two large air lines before the throttle body. One of these will be used to supply air to the IACV valve, but the other one is not needed. We just pulled out the brass pickup and got some aluminium rod the same size as the hole and then plugged it up.
- Create (or purchase) the gasket for the Ford throttly body and then attach it to the intake manifold. We just made a gasket out of some gasket paper. We also sprayed the intake manifold and throttle body with a high temp ceramic white paint in an attempt to prevent the intake from heating up through the conduction of the underhood air temperature.
- The next part was to get the throttle position sensor (TPS) sorted out so that it will register the correct voltage at closed, WOT and every where in between. Upon measuring the resistance of the Ford TPS at both the minimum and maximum rotation angles it was quite different to that of the Honda TPS at both minimum and maximum rotation. To prevent any problems with the TPS and so we could use the stock Honda wiring harness we decided to see if we could make up an adapter so that we could use a standard Honda TPS. The picture below shows the Ford XF TPS next to a chopped up Honda one.�
- To make the adapter we needed to remove the bit that contacts the Honda TPS from the end of a Honda butterfly spindle and attach that to the end of the Ford XF butterfly spindle. That bit is attached with a sleeve that slides over the Ford spindle and is secured using the standard split pin that normally turns the Ford TPS. We also needed to fabricate a block that could be attached to the Ford XF throttle body, but allow us to use the Honda TPS. The pictures below show all of the bits for the adapter, and the adapter installed in place on the Ford XF throttle body.
- The next step is to make sure that you get full throttle and adjust the butterfly stop if you are not. Note how much bigger the opening of the Ford throttle body is compared to a stock Honda one. Note also the hose that connects the air supply from before the throttle plate to the IACV valve.
- Below are some photos of the finished intake manifold compared to the stock second generation intake manifold and throttle body that used to be on the car.
After installing the new throttle body and intake manifold on the car, we had massive problems trying to get it to idle at anything below 2000rpm. It turns out the the Ford XF throttle body is not machined to similar standards as the stock Honda throttle body. After taking the throttle body off the car and looking at how well the throttle plate seals with the rest of throttle body, it was clear that there was probably enough air getting past it to cause the problem. We also checked the idle bypass screw and that too was letting quite a bit of air past when it was completely closed.
To remedy the problem the Ford XF throttle body was bored out to 66.5mm and had a new throttle plate created to fit the new surface. The throttle bypass screw was also altered so that it actually closes and shuts off (well almost) all of the air.
So, when purchasing a Ford XF throttle body, be sure to check how well the throttle plate shuts. This can be done by looking at the plate with strong light behind it. Any gaps between the throttle plate and the throttle body will be apparent as light gets past. Also check the idle bypass screw. This is a little harder to do, but can be done by trying to blow through the throttle body when it is closed and feeling for air escaping where the idle bypass exits on the other side of the throttle plate. A bit of moisture on a finger can help detect the air.
With the car now running properly and having dynoed the car, there were some pretty good gains. The car gained 5kW at peak and 10Nm at peak. The torque curve also looks a lot better and is shown in the B16A Dyno Runs section.