Rebarreling and Home-Shop Machining

Quad Throttle Bodies and Trumpets Project on 2005 Ford Focus ST

Some of you may have noticed some projects done on this car earlier in the Journal.  The car has changed some since the last entry.  The suspension is now complete with new Ford springs and stiffer Eibach antiroll bars (pic # 7).  The car wears Motegi Rims with 205/45R x 16 Falken Azeni tires.  Cornering power is right at 1.0 g's continuous in a corner like a clover leaf turn. The balance is absolutely outstanding now.  The car will see some duty at MCR raceway during a test and tune session and at an auto cross.  Top speed is now about 130 mph.  Speed limiter is removed.  Rev limit is raised to 7000 rpm.  The new tire size is programmed into the computer and speedometer checked by radar to be accurate.  Now its time for just a little more power.

This project will probably take some time and may require some machining.  As can be seen in the third photo, this type of intake is available from SBD motor sports.  A similar system using barrel throttles is available from Cosworth.  Price tags are similar at about $3500.  Both systems require the use of aftermarket computers and wiring.  Neither attempts to be low emmision, but Horsepower can be increased to 200 to 225 HP with few internal engine modifications.

So here is the plan

Obtain some small throttle bodies as a base.  The Hyundai Accent has one that is 42 mm in diameter on the throttle plate.  It has been tested on this engine and it worked well.  A 35 mm body would be better.  (new text).   I found that Suzuki 42 mm throttle bodies are plentiful and would be a better fit to my bore spacing requiring fewer mods.  As a bonus they have a set of secondary throttle plates that open as the engine RPM and load increase.  They keep intake air velocity high at lower RPM.  The Ford Focus manifold has an actuating mechanism for the dual runner length butterflies that could be adapted to run these secondary throttle plates.  I plan to keep a set of inner butterflies on the Focus Manifold that improve emmisions just off idle.  They open on full throttle so should not hurt power much.

A large volume airbox will cover the trumpets.  Air will feed into this air box from the current Cosworth Cold Air Intake tubing which uses a stock mass air flow sensor.  With luck this and a retune will allow the use of the stock computer, Stock injectors with higher pressure.  The engine computer is very good and able to be adapted to supercharging and turbo charging. I don't see why it would not work here.  I found the Suzuki 750 air box to be a good starting point. See pic 4 and 5. 

The stock idle air control valve will be worked into the system.  The stock EGR system will be hooked up.  Emissions should be very good when tuned.  The car can be returned to stock with a manifold change.

So what is the advantage say over a Cosworth intake with a plenum.  It has to do with air velocity.  at part throttle the butterflies on the intake shoot a jet of air into the intake manifold runners.  The stock system has a jet far removed at the single throttle body.  The jet is well dissipated before it ever sees the manifold.  This jet is responsible for good part throttle performance.  On a road track that is what you want.  The stock manifold does have some inner butterflies that provide turbulence that help in this regard that are semiclosed at lower speeds.   Now because the throttle bodies are a little bigger when added up, Wide Open Throttle (WOT) performance should also be very good.  The trumpets will allow the manifold to be in tune over a wide range of throttle positions.