Articles
SAAB 900: LPT to HPT
I have been running a 1991 900S Aero with a light pressure turbo since my previous 900S was written of in a rear end shunt. Not long after purchasing the car I heard that it was a relatively easy task to give it a bit more boost and with the gift of an intercooler the project began….
The LPT normally runs about 0.3 bar but to enable it to run at a higher pressure you need to fit an intercooler. (This gadget cools the compressed air for the turbo to work efficiently) I fitted one from a T16s (make sure you get all the hoses and parts to fit it including the plastic cowl for the top).
Ideally you would do the whole conversion in one go but mine was done in stages as time and finances permitted, so next an aftermarket fuel pressure regulator, also called a power boost valve, was fitted. I used a Malpasi rising rate fuel pressure regulator purchased from Larkspeed for £76. These valves have to be adjusted for the particular car they are fitted to and this was done at a later date using a fuel pressure gauge. The valve provides more fuel at the anticipated higher boost levels. I also fitted a free flowing air filter (a JR from Elkparts), which directly replaced the standard one in the air box.
The LPT is not fitted with a boost gauge as standard so to enable the proper setting up of the engine I needed to fit one. I could have fitted the instruments from a turbo but instead I fitted a 52mm Speedparts gauge (again from Elkparts) in place of the oil temp gauge in the auxiliary gauge panel already fitted. The benefit of using this gauge is that it is calibrated, enabling a much more accurate adjustment of the boost setting. The next step was the purchase of a 1 bar wastegate this was supplied by Midland Turbo’s but required a bracket to be made to fit it as well as extending the actuator so it would all fit.
It was at this point, after a fast trip to Brugge, that the car decided to blow its head gasket and give a good impersonation of a steam engine. The head gasket job took three evenings work. The head had to be Tig welded to repair some corrosion in the water galleys and then skimmed along with the exhaust manifold. The wastegate was also fitted at this time and set up using the boost gauge to a maximum of .9bar. Fuel pressure was checked and the over-boost switch under the dash adjusted. The initial test drive indicated much more power such that traction could now be a problem in the first two gears!
I have driven the car for a couple of months now and can report no problems except the one of keeping the car to legal speeds. I need to find how much more power I have - I think it must be up on a T16 now!!.
One more tweak has been added. The exhaust now features a Jetex system from the centre pipe back, ending in two nice stainless pipes giving the traditional T16 burble!
Stopping and Roadholding
The front brake discs have been replaced with some drilled and slotted ones and a pair of Spax adjustable dampers (an E-Bay bargain) were fitted at the rear to help the chassis cope with the new challenges resulting from the extra horsepower.
NB It is worth noting that my car has no equipment to detect knocking which can be very damaging to the engine. It is very possible that you could get pre-detonation after making the above modifications and it is important that you keep an ear open for that dreaded knock - go easy with the right foot if it is heard! Using high octane super unleaded can help to avoid pre-detonation. (I have heard it while towing a caravan up a steep hill but that was with normal unleaded in the tank)
Pete
(Original appeared in the June 2005 issue of the Club Magazine)
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