APC mod for 8valver [Archive] - SaabCentral Forums

: APC mod for 8valver

8th May 2002, 08:37 AM
I hopefully get a new APC-unit next week. It coming off a saab 9k, but as I have the newer (1986->) APC-unit on my 900T 8valver, the 9k's unit should fit.

Anyway, which mod is the best for a 8valver? I'm thinking something in the region of 1bar (15PSI) boost pressure. I was told the standard fuel pressure is sufficient for 1 bar of boost. This sould take me over 190HP.

I have the standard IC.


8th May 2002, 10:11 AM
Maybe John-w has something to say on this, but with my old 8 valver I always used to get a lot of pinking running that sort of boost.

One mod my be to add a bit more retard into the distributor actuator, some add a fifth injector or, as John-w has done, others add water injection.


8th May 2002, 10:27 AM
I'll have a look at the water injection, but I thought the IC helps a bit with the pinking. Anyway, I'm still looking for a tested APC-mod. No theory=) please.

Am I correct about the fuel pressure? I wouldn't like to run the car on too lean mixture.

Johann F
8th May 2002, 12:56 PM
Sustaining 1 bar of boost on an 8v will require water injection - no 2 ways about it. The heat generated by combustion at this amount of boost will quickly overheat the head - 8v 's have poor heat dissapaiting qualities. IC won't stop overheating and knock - this is a common myth - all it does is allow more mass of mixture per charge ( cooler is denser) therefore you will get more power for the same amount of boost - about 10bhp. Saab 8v's can ( and this goes for 16v's as well) always do with a bit more fueling than the stock set up. Just turn up the mixture on the meter head. I used to run at 2.5-3.0% co at idle as opposed to the 1% +/- .5% in the manuals. It ran much better and didn't seem to use any more fuel. You will have to check though if it still complies with your emissions regulations in your country. 8v's have a fuel enrichment system as standard over a certain boost level - I'm never happy about dumping loads of extra fuel though as overfueling actualy kills performance and can wash oil off the cylinder walls - in theory! The 9k APC should work - you can turn up the boost for short peaks or bursts of power but as I say it will soon start to knock and cut back the power.

8th May 2002, 03:07 PM
I'm running something in the region of 4,5% CO at idle. It is still in the legal limits, so this shouldn't be a problem. Now, here I'm glad I bought a 1986, since the 1987 model has to pass 3% (?) CO tests.

It's a pitty if I have to build a water injection system. I'll take my chances with retarding the ignition and setting the mixture. If that fails, then it's less boost for me. The stock produces something like 0,7bars (?) of pressure, but what would be a reasonable increase of boost pressure? 0,8 sounds a bit low, and the difference between 0,9 and 1 is very small.

I'm not too keen to drill extra holes to my intake manifold to fit a water injection system.

I'm sorry if I'm getting on your nerves with my stupid questions, but all input is appreciated. Blowing up my just rebuilt engine with excess boost doesn't sound too nice.

9th May 2002, 06:02 AM
You won't blow up your engine by doing the standard APC tweaks since knock protection is built in. Instead what you will find is that boost may peak at 15+psi but will drop very quickly when knock is detected.

Do try retarding timing by just a degree or maybe two or, better still, modify the actuator on your distributor to give you the extra retard. This will make a difference. Also running on the highest octance fuel you can get will help lots.

Final thing is, if you don't have an intercooler then definitely fit one and follow that up with an external intercooler water-spray. I made the water spray from a washer pump, a bit of tubing and a garden spray nozzle. It works well in keeping boost up in warmer weather.


10th May 2002, 12:17 AM
The 1986 8valver came with IC as standard. But I'm still a bit confused about the 15PSI problem.

Here in finland they race classic 900T 8valvers with a modified APC-boxes. They produce 1bar of pressure, and 200HP. With no modifications to the engine. No water injection systems, no nothing. And they have to use standard pump fuel.

So why the same doesn't apply with my car?

Johann F
10th May 2002, 04:10 AM
It's the difference between peak boost and sustained boost - they may well peak at 200bhp but they will not be able to hold it there for more than a few seconds. It must make an interesting race technique though - you only get one chance to pass at full power and if you don't make it you will have to wait a bit for things to cool off before you have another go!

Janne Selinummi
10th May 2002, 06:03 AM
The 900 Turbo Cup series racers also have big exhaust systems along with an improved turbo downpipe, and this helps lowering head temperature a bit. The cars must have a catalytic converter, but otherways the exhaust system isn't regulated much.

Also those cars have the injection and ignition set up for racing, since they don't have to worry about gas mileage and emissions in that respect. (No actual modifications are allowed, but mixture and timing settings can be changed)

10th May 2002, 09:53 PM
A water injection system can be setup in two ways. Both a high-pressure and a low-pressue system. There is little difference in their effectiveness'.
For a low pressure injection system water is injected before the compressor of the turbo.
With this sort of setup no holes need to be drilled in your inlet manifold, only the plastic low pressure tubing in the inlet tract.

Another point i wish to raise is the use of the word pinking rather than pinging.
My understanding is that both are different forms of pre-ignition.
Pinking- is cause by a hotspot in the combustion chamber causing rapid burning on exposure to fuel and air. The hotspots are usually caused by deposits of carbon or such.

Pinging- is caused by the sharp rise in temperature of a mixture due to compression. This can reach levels where it will combust uncontrollably.

This is my understanding of the two terms and would be interested in others ideas on the issue.

Johann F
13th May 2002, 08:26 AM
Water injection before the turbo is not a good idea - it destroys the blades of the turbo after a while. Otherwise it would seem to be the ideal method as there would be no need for high pressure pumps and atomising nozzels etc. Never heard of pinging and pinking? - your description of pinging is the classic pre-ignition/knock/pinking situation though.