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"normal" boost 9 Saabers Like This Post!
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Posted by Monster (more from Monster) on Wed, 22 Aug 2001 16:33:56 Share Post by Email
In Reply to: how much boost is normal?, SKB, Wed, 22 Aug 2001 14:37:32
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The boost in these cars is regulated by the APC system, and the maximum you see will depend on a lot of factors. Basically, the APC monitors a knock sensor, and when it sees the onset of knock (predetonation, which spells disaster in a turbocharged engine) it sends a signal to that little APC solenoid on top of the radiator, which in turn, opens the wastegate to bleed off some boost. It's normal, therefore, to see the boost needle occasionally twitch downward under sustained boost as the APC controls the boost level.

Lots of things cause decreased boost, or increase the tendency to knock. The primary one is fuel octane rating. Other factors include air temperature and the health and tuning of your engine.

Here's a test you can try. Be careful, and don't get a ticket! Find a nice stretch of highway with a straight uphill run and no traffic. (Drive to the top, come back down to the bottom again, and make sure there aren't any radar traps!) At the base of the hill, with your car in 5th gear, and at around 3000 RPM (65 to 70 mph) push the accelerator pedal to the floor. Don't downshift. The hill will keep you from zooming up to insane speeds and give you a few seconds to look at sustained boost.

What you should see, if everything is running right, is the boost needle will go to the top of the yellow and hold steady in that region. If you approach 100 mph, you need to take your foot off the gas! While under sustained boost this way, you might notice a little boost twitch in the needle, as it drops back from the top of the yellow zone. That's normal, and shows your APC is working as designed to protect your engine. An unmodified car will not go into the redzone. If you get a lot of twitch, try it on a cooler day with premium gas, and see if you don't get better performance.

There are lots of aftermarket parts and tuning goodies that can be used to increase boost. Check out http://www.SPG9.com for ideas... I've got a lot of their parts and my Swedish Beastie sustains a steady 2/3 redzone boost when needed (I had to turn it DOWN to that level!) I use their modified APC box for tighter boost control, balanced/matched oversized fuel injectors and a Rising Rate Fuel Pressure Regulator (RRFPR) to prevent leaning at higher boost, their Advanced Ignition Diaphram (AID) to better control the ignition boost retard function, and modified tuning and other bits such as their wastegate controller and hooter valve for better performance. It was fun to do the mods and the car runs great now, better than when it was new. It cost me around $1100 for those goodies. There are cheaper ways to go, but I consider my money well spent.

It is not a good idea to try to make your car go faster by messing with the APC potentiometers and fiddling with the wastegate unless you know what you're doing. And then, you should ONLY attempt this when everything's running right in the first place. If you're not getting top-of-the yellow boost when you perform the test I've outlined, then don't modify the car until you fix whatever's wrong with it first.

Start with simple things like tracking down and fixing every vacuum leak you can find, clean the throttle body, check the plug gap and ignition timing, check and clean the injectors, make sure all your ignition parts (rotor, wires, plugs) are good, etc. Then see the performance board for some ideas on what to do next, if you want to take it further. Don't try to "soup up" a car that's running poorly. Get it to top-of-the-yellow zone boost first.

Good luck!
- = M = -
'87 900T 167K

posted by 208.63.20...
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