Cruise control for max MPG?

[supercrackerbox]

Is it possible to re-write the cruise control programming to read from the TPS signal instead of the VSS signal?

The thinking is this: Currently, the cruise maintains a constant speed by taking a reading from the VSS and modulating the throttle to match that signal. On anything other than a perfectly flat stretch of road, this constantly adjusts the throttle position, increasing/decreasing the injector pulse, and often putting the engine into boost. As we all know, more boost = more fuel consumption.

However, if the cruise system were reading off the TPS, I think it would effectively "lock" the throttle in a fixed position. This would eliminate the extra fuel pulses from the injectors, keep the engine out of boost, and should increase fuel mileage considerably. Yes, you would lose speed going uphill, but you would regain it on the other side, and your speed would average out over time. Wind resistance would also be another factor.

Case in point: When my Charger (no cruise) still had a functioning odometer, with mostly highway driving such as my commute to work, it would average right around 30-31 mpg. My best for the car was on the way back from SDAC-21 between west of Chicago and Iowa City, which is a very long flat stretch of road, where I managed 34.1 mpg without doing anything special other than maintaining speed. The following year, my Daytona (regularly 28-30 mpg hwy) reached that same mileage on that same stretch of road, with the cruise control on.


Thoughts?

[shadow88]

It sounds like you're looking for a throttle stop to maintain a set throttle opening. Maybe making adjustments using a vacuum gauge will get your goals, but I'm unsure how it can be worked into the cruise control.

[shackwrrr]

How about a cruise that has a set load(vacuum) that it wants to maintain.


I'm not a big fan of my cruise at all, I have to help it up any incline and it takes forever to engage. But I live in a hilly area where boost is required or you'd be doing 20 at the top of the hill.

[cordes]

In my shadow I have vac running directly to the cruise servo. It won't go higher than 0PSI that way. It'll slow down going up hills unless you give it some throttle, but it is actually really nice during those times the car would normally go into boost when it's not really needed. The Shadow has gotten over 40mpgs all freeway at low speed that way.

[bfarroo]

I think something that would limit the cruise output may be more desirable. Locking the throttle in place has no chance of holding a constant speed. I have my cruise hooked up like cordes and as he said the car will not go into boost that way.

[tommyturbo]

I actually wish the cruise for our cars would go into boost on hills, then I could actually take advantage of cruise control. I think of all the turbo mopar's I've had, only one will go into boost on a hill, all the others hit zero vac and that's it. I have a Daytona whose cruise decided to go wide open throttle a short distance from a chp car, still don't know why I didn't get stopped for blasting by him at 85 or so in a 65.

[cordes]

I actually wish the cruise for our cars would go into boost on hills, then I could actually take advantage of cruise control. I think of all the turbo mopar's I've had, only one will go into boost on a hill, all the others hit zero vac and that's it. I have a Daytona whose cruise decided to go wide open throttle a short distance from a chp car, still don't know why I didn't get stopped for blasting by him at 85 or so in a 65.

If you follow your vac diagram it'll go into boost when it needs to. There is a vac reservoir and a check valve that needs to be in place IIRC.

[tommyturbo]

could it be a bad check valve? all the vac lines are factory and correct. I had heard someplace that there is a circuit board inside the servo itself that go bad.

[cordes]

could it be a bad check valve? all the vac lines are factory and correct. I had heard someplace that there is a circuit board inside the servo itself that go bad.

I would think that if the servo failed it wouldn't work at all. If the check valve or extra vac isn't there it won't work in boost.

[tommyturbo]

Thanks for your advice