Modifying the Sierra Plenum Chamber
For use in a Locost
The first thing to be done is to cut the plenum chamber 115mm from the base. I did
this by clamping the head face of the unit onto a bench and marking with a height
gauge all around the casting. It makes it easier if you roughly mark it first with an
indelible marker, and then the scriber mark shows up much better. I came by the
115mm measurement by trial and error; first I cut it too long and ended up with one
end a very weird shape! 115mm is the longest that you can cut using this method
believe me. The length of the intake tubes is inversely proportional to the revs at
which peak power is produced, so the shorter the tube, the higher the power band. I
think that a cam other than the standard road cam would probably be a bad idea unless
you want to spend a lot of money on your bottom end – Oh Er!!
I cut the aluminium with a hacksaw, the Robbing Hood website recommend an angle
grinder with a cutting disc; this could be a problem as ally is very sticky and will clog
the disc, over heat and may shatter it with nasty consequences. Ideally use a milling
machine with the casting securely clamped down, but a hacksaw with lots of cutting
oil is good.
I checked the evenness of the remaining casting with my pillar drill and a spot weld
removing drill in the chuck, working around slowly and shaving off any excess
I now got some 8mm ally plate of sufficient size to cover the holes, and TIG welded it
Another view before opening up the casting.
I then chain drilled out the other side of the plate and cleaned up the inside of the bore
with a die grinder carbide bit in the pillar drill.
I bolted the casting to the engine and made a mild steel plate to bolt onto it. I marked
the holes to match and again chain drilled them out to match the casting. You cannot
extend too far forward or it will foul the distributor (I didn’t think of this at the time,
and now have only just enough room for the dizzy!)
Now make up the rest of the new plenum chamber (henceforth to be known as the
‘box’). I made the outer edge of mine angled to match the angle of (quickly looks in
book to find letter!) tube J2.
You now need to fit the throttle body and the idle control valve, and the connection
for the fuel pressure valve. The throttle body is connected to the idle control valve by
a cast hole in the original, and you need to make connection in the new box. I used
13mm round ERW bent to shape, tacked in place and the trimmed and then full
welded. Both units just have holes drilled in the box to match the original. The fuel
pressure valve needs a connection to the box; I just welded a ¼ BSP steel airline
fitting in place for the attachment of a rubber air pipe.
The throttle body spindle needs sufficient room to swing properly; I nearly balls this
up, but noticed just before I started drilling!
I made the idle control valve horizontal, but if your design is different, I think this
should work at any angle.
I then joined everything together and trial fitted it to the car. I obtained some 2mm
stainless plate for the to of the box and fitted machine screws at regular intervals (the
top needs to be removable to give access to the bolt which hold everything together).
To support the fuel pressure valve, I cut off the original bracket and welded to the
box. Check this will fit under your nosecone before you confirm the position
I then used the Sierra, rubber induction pipe and fixed it to the air-metering unit and
used an air filter from a Weber fitted to an Audi 100, but I am sure that those nice
fashionable cone filters will fit OK.
To further reduce the height of the engine, I moved the oil filler pipe on the rocker
cover to the rear and connected to the recirculating pipe to the filler cap.
Your are obviously going to have to make up your own gaskets, I used heavy gasket
paper for all but the top cover, I used 2mm rubber sheet for this as the vacuum will be
quite powerful at this point, and there is a large surface area to seal.
At this point, the engine has not been started, so the system may not work!, but I think
there will be no serious problems.
If you want to contact me, email me on firstname.lastname@example.org
Mark Allanson 2004.