Cruise control performance + boost?

[2seater]

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     I didn't know quite how to title this, plus a little explanation. I rarely, if ever, have used my cruise control in the last decade or more. It seemed to perform poorly in a 1700 mile round trip many years ago, not holding speed, inability to adjust to rolling terrain, etc... I chalked that up to poor engine vacuum due to my first version of adding boost, a turbocharger, to the existing engine. Since that time I have installed multiple configurations of the basic 3800 I started with. Compression ratios ranging from 8.3:1 to 9.5:1, with and without turbo. Since I had installed a vacuum/boost gauge maybe 15 years ago, I have noted my steady cruise engine vacuum reading is always in the 10"-12"Hg range, at highway speeds of 55-75mph. This is on relatively flat but gently rolling terrain. It doesn't seem to matter, the engine wants what it wants. With the recent work on the red and black cars for my sons, I noted both were concerned that their cruise control would be functional. 

 

With the above scenario in mind, I started to try my cruise control over the last couple of weeks and I was pleasantly surprised it functions just fine. This is the original unit that came with my car more than 25years ago, so that hasn't been altered. I have relocated the servo to mount on the crossbar between the strut towers many years ago to allow more room for the turbo plumbing, but is original otherwise. It held speed +-1mph flat and up moderate grades and with the vacuum gauge, it is easy to see it working the throttle.

 

So this brings up some questions, primarily for supercharged engine installations and the cruise control performance. I am guessing the cruise control doesn't know what it is connected to so the calibration of response may leave something to be desired in a modified situation.

 

1- If you have a vacuum/boost gauge, what is your expected manifold vacuum reading under the above conditions?

 

2- With the turbocharger, I find the cruise control to be too aggressive in situations where it needs to compensate for a larger load or to get back to speed. What I found was, if I hit Resume and my speed isn't within a couple miles an hour of the setting, it will open the throttle fast enough that it may spike the boost to my 5psi limit before the trans. can even downshift. Curious if the supercharged installations do the same?

 

3- Will your cruise control work hard enough to get you into a boost situation?  

[padgett]

At a steady cruise in flatlands I would expect more vacuum than 10-12". Unfortunately the 3800 is a MAF engine so is no MAP reading, you must add a vacuum gauge. That said the black canister next to the cruise control is supposed to be a resovoir and have a one-way-valve to store vacuum. Is that working properly ? Also I've seen the valve on the brake pedal leak but that usually hisses. Just some thoughts.

[2seater]

Appreciate the comments. I don't know if altitude makes a lot of difference the vacuum reading, at approx. 750ft here, and traveling westward gradual rise in general altitude through the great plains. Even though above sea level here, normal barometer is fairly high, like 30.2"Hg at present. Great Lakes induced high?? The vacuum reading is a curiosity that I found interesting since the general range is in the same area no matter my engine configuration, N/A, standard manifold or TPI, full or half-turbo, compression range over one full number different. Kinda why I was asking and I figured the boosted guys might be the only ones with some form of gauging.

[D-a-n-i-e-l]

It is possible and not to hard to swap to a newer electric gm cruise control.

There are many write ups on fiero sites.

[padgett]

Or do like the turbo GNXs did - add a vacuum pump.

[2seater]

Hmm, interesting comments. I will look into a different style cruise, perhaps something that can be tailored? I thought previously I lacked vacuum but now I found it is almost too aggressive so I was questioning if the supercharged conversions do the same?

[padgett]

Any time you have boost, you lack vacuum. Fortunately we have electric brakes (88-90).