Nissan CA18DET engine with RB26DETT Individual throttle bodies

The Nissan CA18DET comes from factory with a relatively good intake manifold with 39cm runner length. Heavily tuned engines have shown it offers enough airflow to even support 3 times the factory output of 165-170hp. Contrary to the later SR20DET, the successor to the CA18DET, noone currently sells an intake manifold with individual throttle bodies for this engine. With many people drifting the Nissan 200sx S13 the CA18DET was delivered with, there may be some demand for an individual throttle bodies manifold, in particular for a low price. As I had a set of factory individual throttle bodies from a Nissan RB26DETT engine I designed a manifold around them, and see if I could keep the complete building costs down to 600 euros (a conclusion to this is drawn at the bottom of this page). Nissan racing division Nismo had in fact developed a special manifold to convert the RB26DETT throttle bodies in the 90’s, but the manifold adds kinks to the runners and places the throttle bodies much further away from the cylinders than prefered. And this manifold alone does not make a complete setup, requiring vacuum/idle piping, levers/pulleys and an enclosed manifold collector.

nismoadapt

Nismo adapter manifold

The main difficulty of fitting the RB26DETT throttle bodies, and what compromised the Nismo design seen above, is that the cylinder spacing of the RB26DETT is wider. The intake runners on the RB26DETT are also straight, while on the CA18DET the runners of cylinder 1-2 and 3-4 actually move towards each other. As a result, the RB26DETT throttle bodies are 11mm too wide. I came up with an unconventional solution, that is cutting the throttle bodies in 2 and taking out exactly 11mm. To ensure both sides are perfectly straight & centered, I clamp the throttle bodies in the milling machine on both sides and mill them in 2 to a 11mm gap. The shaft is sealed in the middle by O-ring, for which an additional cutout was made.

Instead of using a conversion manifold, the old flange was cut off on the milling machine, and a new one (20mm thick) welded up against it and resurfaced. The ports were matched by hand from oval to round, the transition is smooth allowed by the thick new flange. The shafts from the RB26DETT throttle bodies have slits cut through them to hold the butterflie plates. With a hand saw and a thin file the slits were made to bring each butterflie plate 5,5mm inwards. In addition, the bolts that hold the butterflie plates to the shaft were adjusted.

The factory throttle position sensor (TPS) was mounted to the throttle body with a special conversion flange, and the throttle shaft that goes into it slightly cut down to correctly fit the TPS. It’s a job that has to be done faultless. Too tight and the TPS will jam, or be damaged, too loose and the TPS won’t follow the shaft movement precisely sending erratic signals to the engine control unit.

Pulleys were custom made, and fitted with stainless steel fittings and screws to allow for adjustment. At the time I had to decide if I would fit the throttle cable directly to the throttle bodies or via a lever system such as the RB26DETT has from factory. A lever system has 2 advantages, it allows for an easier fitting of the TPS (though I already solved this problem) and less force put upon the throttle bodies bearings. However, the disadvantage is that it requires a stable mounting with additional bearings, and there’s very little space to mount the levers with the throttle bodies so close together and a fuel rail on top of it. I decided to add a throttle cable fitting to the pulley on the rear side, and this pulley will pull the pulley for the front side along.

The pictures below show more details of the pulleys and springs. Nissan uses M5x0,5mm threads for the adjustment bolts/nuts, so I tapped this same thread in several other locations for a uniform layout and identical adjustment. The pulleys are bolted down to the throttle bodies shafts by socket screws. The springs and plastic rings that avoid frictional between the throttle bodies housings and the springs were recycled from the RB26DETT units. As can be seen, in particular the 3rd picture, removing material from the flange also removed large parts from the welds. This left these sections open to air leaks. Later on, these sections were welded shut again. The surface was checked again for straightness, which was not affected.

I used Jenvey bolt-on velocity stacks. As I learned from testing the 2 VG30DETT manifolds I produced on a running engine, the heat from welding loosens up the aluminium threads in the plate and creates air leaks. One individual leak does no harm but there’s 8 of them here. Therefore, with the manifold done I cover the thread ends by dome nuts with a good sealant for an airtight seal.

The manifold collector (named plenum by some) is strengthened by 2 rods that are welded first to the upper side of the manifold, and finshed welded to the bottom from outside. These rods prevent the manifold from expanding as it’s subjective to turbocharger pressure. The expanding does no harm initially, but it will create fatigue over time leading to cracks and the manifold blowing up. The rods impose an unmeasurable small obstruction to the airflow. By welding them on the inside for the upper side, the manifold keeps a clean look.

In order to have the factory idle air system operational I manufactured a vacuum box underneath the manifold. Some people use thin silicone hoses to supply air to the cylinders, but these will blow off quickly and deteriorate from heat cycles. I therefore designed a small sub-manifold. The 4 small hoses used are special Bridgestone hoses designed for high pressure air fittings. The complexity is the result of the required ability to assembly the manifold to the engine, and avoid contact with a coolant hose and crankcase venting pipe, the heat exchanger and oil filter. It looks simple from the pictures but it took me several hours to find the right shape, angle and location for all the combined idle air and coolant parts.

Installation;

The manifold was installed onto the same engine I rebuilt at the time I made this manifold. I adjusted & welded the intercooler pipe coming from behind the pop-up headlight to fit the new manifold. The engine responded very well to the manifold with an aggressive response, however vacuum leaks from in between the halves of the throttle bodies caused a high, lean idle. It was clear that the O-rings I used were not up to the job of sealing properly. Since I could not source the replacement seals on a saturday afternoon, I replaced the manifold by the factory item and the customer returned for the manifold later on.

In order to replace the leaking O-rings the halves of the bodies were carefully aligned into the milling machine. The shaft hole is first aligned with a long 10mm shaft into the bearing on the lower side, but because the hole in the middle is 0.40mm larger it has be gauged in addition by the 3D taster. The hole is cut to 14mm and the new seals pressed in.

Conclusion

The material and parts required for the manifold cost close to 300 euro, depending on where I source the throttle bodies and if I find a way to make the velocity stacks myself (the Jenvey stacks list 40 euro each). However, there’s well over 60 hours of labor involved in the production of this manifold. By using pre made templates and malls, I might be able to reduce the time to under 50 hours, but in addition to the hardware costs, a manifold like this would require a price of well over 1000 euro to become profitable.
The main issue has been the idle air setup and the additional modifications required to make the throttle bodies fit well, such as the shaft modifications being a very delicate job.

For sale
I’m selling this prototype manifold in a ready to swap configuration. It includes a re-route of the coolant hoses underneath the manifold and a (slightly) modified) stock fuel rail. Basically, all you’ll need is a new head-manifold gasket and a custom (76mm or 3″) intake pipe from the intercooler. I can supply the manifold with new paper gaskets in between the throttle bodies and the other sides of the manifold, though I recommend using the steel OEM RB26DETT gaskets. Price is 500 euro’s + shipping. SOLD!