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Cruise Control, base and full boost explained
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 31 Jul 2001 22:17:11 Share Post by Email
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A few posts back was a thread about folks getting full boost at high RPM with the cruise control on. I was dubious, but promised to look into it - here's the reply-

Background - When the cruise control is engaged, the turbo system is limited to base boost. This is a safety feature, to limit speed if the CC system runs away. Some folks report that when they hit the magic 4200 rpm, they get full boost. The implication was that the CC had an override feature at high RPM. Perhaps to climb hills.

OK - After checking through schematics, here's the scoop -
If the CC (cruise control) is controlling speed, you are limited to Base Boost. Period. No matter how fast the engine is going.

If CC is engaged and you press the gas pedal, and go faster than the CC set speed, you can get Full Boost. But only if you override the set speed with your foot.

Here's why:

The CC controls throttle position with a vacuum actuator. More vacuum, the more it pulls on the throttle, the faster you go. Less vacuum (closer to atmospheric), the slower. The CC reads road speed from the speedometer, and adjusts the throttle to go at the set speed. If the car is going faster than the set speed, the CC will reduce the vacuum, comanding less throttle, to slow down the car.

When power is removed from the APC solenoid, you get base boost. There is a vacuum switch in series with the APC solenoid, and that switch sees the output of the CC pump. If the CC pump is holding the throttle open with vacuum, the vacuum switch is open, and the APC solenoid is in base boost.

So - engage CC at a set speed, and the CC pump runs, creating a vacuum to hold the throttle open to keep the car at the set speed. Hit a hill, and you'll get more vacuum to increase throttle to hold speed. Go downhill, and you'll get less vacuum to slow down some.

If you press the gas pedal and override the CC, the CC will think "whoa, too fast!" and reduce the vacuum - it's trying to slow down. It has no idea you're pressing the pedal. As the speed stays above the set point, the CC will continue to reduce vacuum, until it basically turns the pump off. At some point of minimal vacuum (close to atmospheric), the pressure switch will switch on, and engage the APC solenoid - Now you have Full Boost.

4200 RPM is not a magic number. The CC DOES NOT get engine speed at all - all it sees is road speed. There is no engine speed connection to the CC controller. The response of the CC will be damped so it doesn't slam the accelerator around. It's my guess that most people will cruise around 3000 rpm, and they'll hit the gas pedal hard, and in the time it takes for the CC pump to stop and pressure to leak into the system, the engine has sped up about 1200 rpm to a little over 4000.

So - there is no magic override of the CC full boost cut-out. Simple test - select a set speed of 60 mph, then hit the brake and coast down to 40 mph. Put the car in 3rd gear and engage the CC. You'll go over 4200 rpm, but you'll never have more than base boost.

I hope that makes sense.

Regards-
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