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Modifying
a 931
There
are two versions of the European specification 924 turbo, the
series 1 made between 1979 and 1980 and the series 2 from 1980 to
1982. The series 1 produced 170 bhp, could reach 60 in 6.9 seconds
and had a top speed of 140 mph. The later series 2 had electronic
ignition (DME) a reworked KKK turbo, single throttle body,
increased compression from 7.5:1 to 8.5:1. This model had a
slightly slower time to 60 but a higher top speed of 144 mph.
One thing that the 924 turbo
lacked was an intercooler. The Carrera GT was fitted with an
intercooler mounted on top of the engine hence the huge bonnet
scope. This cooler can be fitted to a series 2 quite easily as the
throttle body is the same shape (round).
The series 1 would require a series 2 throttle body and
modifications to fit. In both cases you would need to cut a whole
in the bonnet and fit the Carrera GT bonnet scope. Carrera GT
intercoolers are expensive, over £1,300 plus VAT from Porsche!
Fitting an intercooler from a
944 Turbo
Another
option is the intercooler from the 944 turbo, mounted behind the
badge panel. Below are details and pictures of a conversion
carried out by Porschembi ( www.schembi.co.uk
) on a 1980 series 1 model. The car has been fitted with Leda
suspension, up-rated springs, 944 turbo torsion bars, roll cage,
bucket seats and a fibreglass bonnet with both a turbo duct and a
Mitsubishi Evo 6 vent to help keep the engine cool. Wheels are 6
by 16 inch Fuchs items supplied by Jasmine Motorsport ( www.jasmine-motorsport.co.uk
) which look great and are lighter than the original cross spoke
design.
951 front end
panels, intercooler and radiator header tank were sourced from a
Porsche scrap yard. Custom intercooler pipes and silicone hoses
were used to plumb it all together.
The bits required
from the 951 panels were removed in addition to the 924 turbo
badge panel.
The center section
was cut away to make room for the 944 turbo panels.
Panels were welded
in-place and painted.
The intercooler
was installed together with the intercooler pipes and 944
header tank (inside of the n/s wing).
To reduce the
weight of the car a fiberglass bonnet was fitted and to aid
cooling a 924 turbo style vent and an Evo VI vent were fitted.
The suspension
set-up was changed from standard spec to:
Front: Leda
struts, 300 lb springs
Rear: 25 mm torsion bars and Leda shocks
The car now sits considerably closer to the ground and handles
pretty well on the road.
Engine Rebuild
A compression check on the engine revealed a
problem with number 3 and this was probably causing the problem
where oil was coming out of the breather pipe.
I decided to remove the head to see what
needed to be done, I was hoping a new set of rings would do the
job but after removing the head I discovered the problem. Part of
a piston ring had broken and put a small hole in the side of the
piston and marked the bore. With this much damage,  Patrick
and I discussed options. A quick fix would be to fit a short block
from another car. Pro-9 had a unit but they could not measure the
wear on the bores but said we could return it if it was not to our
satisfaction. The next option was to rebore my block and fit
oversized pistons. This seemed a good idea until we tried to get
some prices on the pistons. Patrick’s normal sources revealed
they were unobtainable and Porsche/AFN wanted £255 plus VAT for
each piston! I called the Harveys who prepare a 924 turbo for the
‘Classic’ who said they had the last set of oversized pistons
and they were not for sale!
Patrick started to call some other contacts
and came up trumps with a set of Mahle 1mm oversized pistons for
the S1 (7.5:1 compression) from Germany that could be delivered
the following day at a fraction of the prices Porsche/AFN (sounds
a bit like Trevor Bakers recent running report). So we decided to
get the pistons and rebore the block.
Once the pistons arrived I started work on
removing the block, which took some time but was pretty easy
especially with the car on the lift. Basically, the radiator was
removed and the torque tube bolts and engine mounts were undone
whilst a trolly was under the engine. Once the engine was clear
from the shaft the car was lifted and the engine was free to be
moved to another part of the workshop to be stripped.
 The
block was sent to a local engineering shop for a rebore and new
guides for the head. My waste gate had a one inch crack in the
casing but luckily Patrick had a spare waste gate that just
needed new studs.
After a few days the parts were back and
Patrick set about rebuilding the block and head. I spent much time
cleaning
various parts to make the rebuild cleaner if not easier. The sump
took about an hour to clean and the end result was a gleaming
piece of alloy. Once the engine was built it was time to fit it
into the car and all the parts were refitted in reverse sequence,
which took me 2 days!
The car started after a couple of attempts
and ran very well. Just need
to run it in now....
GTS Lamps
 In
addition to the engine rebuild I also found time to fit a set of
GTS style headlamps from Club Auto Sport. The old lamps where easy
to remove but must be a pain to refit and align as there are so
many adjustments to the set-up. I needed to make-up some brackets
to secure the light in place which took a couple of hours. The
lamps look great and attracted much attention at Walton Hall.
Boost Gauge
Where to connect
the pipe for a boost gauge. See below where a brass fitting has
been soldered onto a pipe, connected to this fitting is some clear
plastic tubing which is fed into the cabin and connected to the
boost gauge.
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