951 boost response, whats normal?

paul r · 06-17-2001, 02:45 AM

Hi,
If I keep the gas pedal on the floor my 220bhp 951 builds up its boost pressure to a maximum value of about 0.8bar. With the gas pedal still down the car continues to accelerate, but boost pressure starts to decay. The decay happens well before the redline, and can be felt as a weakening in the acceleration. All I want to know is if this is normal for this type of car, and if not what could be the causes?
Thanks in anticipation
Paul R

Blackfoot · Blackfoot's Garage

that's most likely the wastegate's fault. the boost enhancer will partly remedy this, but if you want to do it right, consider replacing the wastegate. lindsey racing's wastegate upgrade:

http://www.lindseyracing.com/boost3.htm

Dave951M · 06-17-2001, 11:04 AM

Is that .8 bar relative or absolute. If you're reading it off the gauge, then it's relative and you may well have a deeper problem than just the wastegate. On the guage, you're not making boost until you cross 1 bar. Think of the stock gauge as a vacuum/pressure device with anything left of 1 bar being vacuum, no boost, and to the right of 1 bar, pressure, boost. If the guage is reading 1.8 bar and then falling, then your wastegate may be malfunctioning. A temporary fix till you get the Lindsey unit is to shim the wastegate. That will help some but isn't the preferred permanent cure. There are other causes, check for leaks on all rubber intake plumbing. You may find a small crack on an obscure section of hose is the source of most of your problem.

Dave951M

tt · 06-18-2001, 06:11 AM

Factory boost curve for 85-88 Turbo (K26/6) is 1.7 Bar at 3000 rpm and falls off to 1.5 around 5400 rpm. The larger turbo (K26/8) on the 88 S and later models allows boost to stay at 1.7 for the full rpm range. These pressure readings are the same as the dash guage in which 1.7 bar is actually .7 bar positive manifold pressure. If you are getting this kind of response in 4 and 5th gear, sounds like your car is OK (assuming completely stock setup).

BTW make sure you have plenty of road if you test the complete range in 5th gear.

paul r · 06-18-2001, 12:34 PM

I'm talking absolute pressure, ie what the gauge reads. tt what is the source of your good news?

tt · 06-18-2001, 12:51 PM

Factory manual for the 944 Turbo. I can look up the section and page number if you need it.

Got home and verified it. Was recalling the #s from memory earlier.

1.75 bar +- 10% ~ 3000 rpm falls off to
1.52 bar +- .03 bar ~ 5800 rpm

The later models only drop from 1.75 after 5800 rpm.

[This message has been edited by tt (edited 06-18-2001).]

paul r · 06-19-2001, 10:27 AM

tt,
Thanks immensley for taking a weight off my mind and leaving it in my wallet.
Paul R