1994-2002 [Subscribe to Daily Digest] |
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Before I get to that discovery I need to explain a few things. Like Dean I also have the tap for my MBC downstream from the turbo. My tap is located on the short "T" that connects to the BOV (BPV). While this gives me better spool up the extra intake length to the tap causes boost overshot.
Initially when I moved the tap I had to lower my MBC to 11 psi, boost spikes would then max at 15 psi and be under the fuel cutoff. Clearly some sort of valve was needed to vent the boost spike to allow for maximum boost without fuel cutoff.
I read about Dean's “Pop Off Valve” and liked the idea, but I couldn't find any suitable valves to use, at least not without a lot of modifications. Cost was also an issue, I'm a college student and performance funds are a little hard to come by. So I decided on a different approach; to limit the boost the Map Sensor "Saw". Coincidently enough the device I needed was made by Dawes. Same design as their MBC but it vents to the atmosphere, even the colors match. Cost was $25.
Tuning this device is more difficult because when installed the only feedback it gives is fuel cutout. I used a bicycle pump to set the relief valve to 15 psi and installed it just before my MBC and after my boost gauge so that I could still read the actual boost.
Time for a test drive.
Success, I set my MBC to 15 psi, boost spikes would hit 18 psi and settle down to 15 in about a second. Everything was great and I ran for several months like this without a problem.
Now that I have dragged you through all that I will get to the 22 psi story. I was driving on some bumpy Pennsylvanian roads on a nice 65 degree spring day and I came to a straightaway. I floored it and felt the best acceleration ever. I looked at my boost gauge and noticed that I read 22 psi. I had no time to worry though because at that moment my intake pipes separated in a noise so loud that I thought that I had just blown up my tranny.
After pulling over and fixing the intake I decided to try it again, the excitement got the better of my judgment. With a heavy right foot down the straightaway the boost rose quickly to 22 psi and held there. I listened for knocking and heard none everything was working beautifully. After several runs I realized that I needed to get to the bottom of this and pulled over. Checking under the hood I saw that the MBC had worked its way loose and was kinked just before the wastegate. I was running full throttle with uncontrolled boost and no fuel cutout and I lived to tell about it.
This is obviously not a good idea for the long run and I decided that I better get back to 15 psi; I fixed my MBC and continued on to my destination. It was been several more weeks since the kink and everything has been working great.
Out of this, I realized, has come some very good information. The stock ECU dumps in much more fuel then is needed at 15 psi. The stock turbo maxes out at ~22 psi and cannot create anymore pressure, only heat. Also, shifting at 22 psi and listening to it vent to the atmosphere is quite exciting.
Eric
1996 900 SE
3” Intake
K&N filter
MBC
Viggen Intercooler
Forge Valve
Ported TBTC
JT 3” Downpipe, no cat
Saab Sport Exhaust
Stock ECU
posted by 12.79.33...
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