Adapting Suzuki Throttle Bodies to a Ford Zetec Engine
Step by Step

One of the many things that attracted me to this particular car was that it had Suzuki GSXR fuel injection. I had been considering doing this on my Seven, so by the time this Europa came up for sale, I had already done quite a bit of research on other installations, and on the MegaSquirt system. While reading discussions from others who have traveled this road, one concern was which Suzuki injection to use -- the GSXR 600 or the 1000. Throttle bodies from the 450 - 600 cc bikes have a throat diameter of 38 mm while the ones from 750 - 1000 cc engines are  42 mm.

So the common thought was to use the bigger 1000's probably because of the popularity of Weber DCOE  40s or 45s.  To me, this reasoning is faulty because it is the venturi size that really determines the flow rate. Venturies for a  DCOE 40 are replaceable, and range from 24 to 36 mm. Those for a DCOE 45 range from 28 to 40 mm. Given a 2000 cc engine with a red line around 6000 - 7000 RPM, a 40 mm venturi would work fine at WOT and high RPM but have poor tractable at low RPM and part throttle, which is where we drive on the street. So, for a street driven daily driver, you are likely to find DCOE venturis in the 28 - 36 mm range. That's smaller than the bore size of a GSXR 600.

Using this information, conversations with the previous owner, and my own driving experience with his installation, it looks as if the GSXR 600 throttle bodies will be more than adequate. Remember, throttle bodies don't have a venturi. That means the smallest part of the bore is 38 mm where the smallest part of a DCOE could be as small as 24 mm. In measuring my 600's the throat is 40 mm and tapers to 38 mm at the engine side.

So, all this sizing stuff deals with is maximum flow rate. The low speed drivability issue is a different matter. It deals with flow depression through the venturi, and low speed with big venturies normally don't get along because the flow depression is too low to draw and atomize fuel consistently. I don't know exactly where problems begin to occur with DCOE's, but drawing fuel isn't a problem with fuel injection so low speed tractability is not as much of an issue as with a carburetor. 

In theory, however, there could still be low speed issues with fuel injection if the bore is too big. That is because once the fuel is atomized, it needs to be swept into the cylinder. A large bore produces a slow moving column of air which may allow the injector to spray all the way across the bore, and wet down the far side because the slow moving air just can't accelerate the fuel droplets fast enough. This leads to unpredictable air/fuel ratio across the RPM range. The ideal size would fill the combustion chamber at high speeds while keeping a fast moving column of air at low speeds. The fast air flow helps combustion chamber turbulence which leads to more complete combustion and better economy.

So, what is the optimum size for a 2 liter Zetec? I don't know. I haven't put either 600's or 1000's on a dyno, so what I'm really trying to say is that 38 mm should offer plenty of flow to produce a bunch more horsepower. Besides, almost anything will be loads better than the stock setup. 

If you have the bigger bore GSXR 1000's, they'll probably work fine too because the computer should be able to compensate for the slow intake charge at low speeds. Just remember, as you read the rest of this article, any dimensions given here are for the GSXR 450 - 600. If you have bigger throttle bodies, you need to use this for concept and not a cook-book.

When I bought the 72 Europa TC, it already had a Ford Zetec installed. The previous owner had created a manifold using parts of the original Ford manifold, some metal tubes and rubber hoses. I've seen articles on the Internet where others have done the same, or similar sort of thing, which is probably where he got the idea. To my eye, this just didn't look professional nor did it look like well designed runners for the intake gasses to traverse. Besides, it wasn't pretty looking.

I was in the process of designing a different way to go when a member of the Yahoo!® Lotus Europa Enthusiasts group, Lee Brown , showed me what he had done for his Europa. To say it was a much cleaner setup would be an understatement. His work was so inspiring that I decided to mimic what he had done.  The following is how I went about replicating his idea.

First, of course, I had to separate the throttle bodies from the previous owner's home made manifold. That exposed a few things that weren't visible from the outside. I found bent tubes, and partially collapsed rubber hoses.

There were lots of opportunity for air leaks, and certainly it couldn't flow properly with the deformed tube and the constantly changing diameters (throttle body to hose, hose to tube, tube to 2nd hose, hose to manifold). 

The odd angles of the Zetec manifold, and differences in spacing between the throttle bodies and the manifold caused sever alignment problems. The left hose of the three in the foreground is an example. The sharp bend was simply more than the rubber hose could take without collapsing and over-tightening the hose clamp deformed the metal tubing. This design just had to go.

This is the intake manifold I selected. It is from Raceline (www.raceline.co.uk) and is designed to put twin Weber DOCE carburetors on a Zetec. In the next few photos, you'll see how fabricating a custom adaptor for the Suzuki throttle bodies gave them a straight shot at the intake ports. No pinched tubes or sharp bends in the rubber hoses.

This Photo taken from the Raceline web site. The one they shipped me arrived five days after I ordered it. Considering I'm in Oregon, and it came from the UK, that is fantastic. Plus, the quality is outstanding. I strongly endorse them. 

I have no financial interest in them or otherwise blah, blah, blah ... disclaimer.

The Suzuki Throttle Bodies have a secondary throttle valve where you might expect to see a choke on a carburetor. It is not needed for this application. The extra throttle plates had already been removed by the previous owner, but he left the throttle shaft in place. I don't like having unnecessary mechanisms hanging around, so out they went. The shaft holes were filled with 1/8" pipe plugs. The holes were tapped just deep enough so the pipe plug was flush with the inner bore.

Below you'll see how that cleaned up the throat:

The greater spacing of the Weber bores required the Suzuki throttle bodies to be spread out to match. With the GSXR 600 throttle bodies, the outside pairs needed 10 mm additional separation. The two pairs required a 14 mm spacer in the center. This separation made it necessary to weld extensions to the little tabs on the accelerator linkage (not shown in this picture). You'll notice that the tabs don't quite reach the spring-loaded synchronizing adjusters of the adjacent throttle body.

All four throttle bodies are held together with two long bolts that needed to be made 34 mm longer (the sum of the three spacers). That was accomplished by cutting, and welding a 34 mm extension in the middle. It was a little tricky to get perfect alignment and keep the long bolt straight enough to thread into the last throttle body.

The additional throttle spacing made it impossible to use the stock Suzuki fuel rail. Fortunately there are a number sources for custom fuel rail material because increasing the fuel capacity of a stock fuel rail is big with hot rodders and the after market supercharger bunch. This fuel rail came from RC Fuel Injection but other suppliers are on the web. Here it is being drilled 27/64" to match the stock hole within 0.001". Then it was chamfered and rounded to avoid cutting the O-Ring on insertion.

Notice the difference between the stock and the custom made:

This view is from the intake side. It is more or less the way you would see it when mounted on the engine. I like the clean, uncluttered look.

The fuel supply galley, running the length of the rail stock, is exactly the right size for a ¼” pipe tap. The AN flare fittings should make a much cleaner installation than the stock one.

I still haven't decided what to do about velocity stacks or air cleaners. That will wait until the engine is back in the car and I know how much room I have.

This shows the stock spacer that controls the amount of “crush” the fuel rail applies to the injectors. The spacers serve as the fuel rail mount, and fit between the tabs on the fuel rail and throttle bodies 1 and 3. Each tab is fastened to the fuel rail with four 6-32 stainless screws. Fortunately, the 1/8" thick tabs were exactly the right thickness to allow using stock spacers without modification.

This view is from the engine side. I'm not sure what Suzuki did with the two threaded holes on either side of each injector. I plan to use them to mount guides for the injector wiring -- still trying for the "clean look."

Look closely between each throttle body and you can see the tabs that were welded to extend the ones that synchronize throttle opening. They're black and hard to see.


Here, a CNC Vertical Mill at SD Machine Services, in West Linn, Oregon, is cutting the ID of an adapter from 6061 aluminum alloy. The portion being machined here, will be a perfect match (inside and out) for the Suzuki throttle bodies. The lower, large round portion will become a flange that exactly matches the Weber base.

Below is the prototype. Final design will have a slight groove to help hold the rubber coupler in place, and the flange will be a little thicker. I have no interest in SD Machine Service but recommend them.


Here the flange mating surface is being faced to assure a flat surface and a good seal against the manifold.
Since the Weber port diameter is slightly larger than the ID of the Suzuki throttle bodies, we tapered the bore to blend into a nice match. There is a good photo of the quality of this match a little further on.

 
Nearly finished. Photographing this process makes me want to run out and buy a CNC mill and lathe. SD Machine Service was very kind to allow me in the shop during this process. The photos really tell the story.

Here's a cross section with dimensions. The flange is standard Weber base, like below:

I would have liked a little more gradual taper on the blend, but the ID of the Weber opening is 1mm larger than the OD of the Suzuki throttle body. I added the shoulder, visible on the right, to allow the amount of blending you see on the left. The shoulder is the same height as the rubber coupler so it doesn't look out of place.

These are so pretty I almost hate to put them on an engine.

This shows the adapters on the manifold with the throttle body assembly ready to install. The clamps are 3/4" wide T-Bolt clamps. I selected these because one clamp would span the adapter and throttle body. That's only possible because they are both exactly the same OD.

The red rubber coupler is actually fabric reinforced high temp silicone hose.

Before

After

Here's a closer view of the couplers and the T-Bolt Clamps. They're more expensive than hose clamps, but this design only requires 4 clamps instead of the 16 required by the one it replaces. Much cleaner. I like it.

I need to use a better camera to take a photo of the way the adapter matches the manifold. This was taken looking into the engine side of the intake manifold with the adaptor bolted on. The camera wanted to focus on the stuff in the background so the walls are blurred. Until I can get a better photo, this will have to do. At least you can see that it's a straight shot. No unusual bends like before.