Jfrahm

I connected an aftermarket boost gauge to my Turbo S to check if I was getting full boost. My stock boost gauge never got much over 1.6 bar so I was wondering if I had a boost problem or what. I also do not know what excatly is in this car. It's a Turbo S chassis but the engine, turbo, and DME have been replaced.

I connected the aftermarket boost gauge to a 4 legged tee that I installed in place of the three legged one near one of the manifold taps by cyl. 3. and temporarily wired the gauge to the driver's windshield wiper so I could easily see it while winding it up. I have not tried to calibrate the boost gauge, I don't have another gauge I can swear was accurate at such low pressures.

Anyway the car idled at 10psi of vacuum and would hit 10psi of boost (I saw 11 once or twice but only breifly. The car would not hold boost to redline, but tailed off a little down to 8 or 9 psi. I did not look at the factory gauge while driving, but it read .4 BAR at idle while the aftermarket gauge read 10psi of vac.

So how do these numbers sound? .68 bar? SHould the S hold full boost until redline?

I may try again tomorrow with some octane boost in the tank to see if that helps.

Thanks,
-Joel

Jake951

What you are seeing sounds about right for a stock Turbo S. In third gear and above, a stock Turbo S should hit a peak of about 11 psi, maybe slightly higher. Peak boost will be a little less in the lower gears. It won't hold maximum boost until redline. Boost will start to taper down at around 5000-5500 rpm and drop off by about 2-3 psi at redline.

adrial

You should see more than 10 in Hg at idle...

16-20 is normal.

What do you see on decel? (foot off the gas in gear)

As for boost, 10psi sounds like a regular 951 not 951S. My 89 951 made 12psi when stock...(same specs as 88 951S)

Jfrahm

Hmmm, you are right the gauge is in in Hg. for the vacuum side (as is only fitting and proper.) I'll check the deccel vacuum in a bit.

Thanks,
-Joel

Jfrahm

OK I got some more readings:

10 in Hg at idle (about .4 bar on the stock guage, a little less actually.)

15-18 in Hg on decelleration

9 psi of boost in 2nd

I'm pretty sure in 3rd I got it to hit 11 psi again.

I normally see about 10psi as max on the boost gauge.

Do these cars determine max boost by gear or is that just a function of the rate of change of the RPMs?

Thanks,
-Joel

Jfrahm

Also, I'm on Boulder, CO (over 5000') but I have heard that does not affect manifold pressure, which is measured as gauge pressure (except for the dash gage, which is absolute?)

-Joel.

adrial

Joel, well thats why you're only getting 10 psi of boost...the altitude.

Basically, my starting point here at sea level is 0. Look at your gauge with the car off, it probably reads a bit below 0. So you're still making 12psi, just that you're starting with 2psi vacum.

As for vacum at altitude, I dont know how thats effected. I would check for vac leaks though.

Jfrahm

Well the dash gauge reads .8 bar with the engine off... so it reaching 1.6 bar makes some sense. However the aftermarket gauge is not an absolute pressure gauge so AIUI it should be reading boost over ambient... but if the rate of change of RPM is too fast in 2nd to reach max boost I really need to wind it up in 3rd or 4th.

-Joel.

Jake951

Factory spec for peak boost on the Turbo S is 11.5 psi so you're in the right ballpark and yes, you won't see that unless in you're in the higher gears.

Waterguy

Both the 951 and 951S have a maximum boost spec of 0.75 bar +/-10% (10.9 psi +/- 1.1 psi) over atmospheric pressure. Sounds like you are in this spec range, althoug on the lower side. The factory gauge should peak at 1.55 bar (0.8 bar atmospheric + 0.75 bar boost.) I also think it is pretty common for the turbo to not spool up to maximum boost in first or second due to insufficient time, rather than variable boost settings by gear.

The factory manual suggests testing in second gear while using the brakes to keep from accelerating. I have never had the nerve to try this. They also warn that you should limit testing to 3-5 seconds at full throttle/boost, and be careful to not overheat the brakes; no kidding!

The 951S is designed to hold a constant boost to 5,800 rpm, then start decreasing boost to the red line. This is controlled by the KLR and the cycling valve. If your car doesn't hold boost to 5,800 rpm, it is likely a weak spring on the wastegate. The standard 951 hits full boost earlier (3,000 rpm in the upper gears), holds this to about 4,000 rpm(?), then decreases the boost to 7.5 psi +/- 0.4 psi by 5,800 rpm.