Impalas.net banner

Vacuum Readings @ Highway

2K views 8 replies 4 participants last post by  Big Dave 
#1 ·
Went for a cruise down the highway, in the Caprice. I was looking at my vacuum gauge that I have mounted in the dash. I noticed that it fluctuates cruising steadily on a flat road. If I blimp the throttle, it'll jump up to about 15" otherwise sometimes it'll be at 10. Obviously the more load, going up a hill, the vacuum drops and if I go down a hill and just let off I get about 18". With the stock 2.93:1 gears, and 255 70/15 tires, I'm doing about 1700 rpm at 110. Also, my transmission kicks into 4th too late for my preference, like 90-95km/hr. and 2nd at around 55-60km/hr. I'd like to bring it down a bit. Are these conditions normal? What's the average reading for highway driving?
 
#2 ·
Is this an OD tranny in fourth gear with 2.93:1 ring and pinion gears? If so you might need steeper gears (3.55:1 or even a 3.73:1). The corporate ten bolt came with a 2.56, a 2.73 gear or a 3.07 installed in it. I don't recall a 2.93 gear available except for a Ford nine inch rear.

The vacuum is constantly in flux in any motor. You try and hold the vacuum steady to maximize gas mileage. If it is running low you have either a big cam or your foot in the throttle all the time (or a vacuum leak if it is running poorly along with a high idle you can not get to run at low RPM).

Your 110 mph top end charge is probably the governor in the tranny if you are still running EFI. As it is limited at the factory by the speed rating of the tires that were ordered with the car (the governor is one of those parts that gets left out when I rebuild a 700R4; though it has no affect on a carbed car with a free standing HEI that is no longer computer controlled.

Big Dave
 
#4 ·
Yup he's a Canuck, and they started to adopt the Metric system like we did back in the sixties so all the speed signs in Canada are in clicks per hour. But gas is still sold by the gallon, and you can not get iced tea anywhere (thats how you know your in Canada and have crossed the border no ice tea).

Big Dave
 
#5 ·
Dave its a rebuilt 700r4 with "MD8" on the side, it has a custom 3000 stall converter. When on the highway, yes I keep it in OD, but to maintain speed, my foot has to be on the throttle most of the time so yes I get about 10" of vacuum, like 5 if I'm going uphill. I also don't have an EFI, I have an MSD HEI distributor with the advance blocked. And I looked up the RPO code on the back of the trunk, and if I'm not mistaken it is a 2.93:1 ratio in the diff. Dave, what you mean to say is, you guys don't have Tim Hortons doughnut shops down there :)
 
#6 ·
If you have a 3000 RPM stall converter then your transmission is slipping untill it gets above that speed. with your gears that will not happen at cruise speed. I have 4.56 rear gears an I cruise at 3,200 RPM at 70 mph with ny 4L80e OD transmission. That is why your vacuum readings are all over the place.

Big Dave
 
#8 ·
Jason, Lukes86, 1986 Caprice, theres no computer in my car. Even if my converter has lockup it will still slip? I'm not too sure, but I feel like it's going through a ton of fuel. I'm trying to optimize mileage a little, with the vacuum gauge installed, I'm trying to get a lot of vacuum. What's more efficient, High vacuum and high rpm or low vacuum and low rpm. I was used to the 305 5L though before. It is a 350 Vortec now.
 
#9 ·
With a lock-up solenoid installed in place of computer control activated by line pressure your lock up convert will not slip. Without the lock up feature you are throwing 15% of your gas mileage down a hole as the motor is super heating the tranny fluid (that energy has to go some where). That is why you need a school bus sized oil cooler whenever you run a high stall converter to cool the tranny fluid. The 350 will burn more gas than the 305 because there is more of it even with a functioning lock-up converter.

I get maybe four miles to the gallon (of high test gas added to ten gallons of Toluene octane booster in my twenty four gallon fuel cell) with my 3,600 RPM locking stall converter behind my 582 using 4.86 gears (if I nurse it while driving down hill with a tail wind).

But my converter will lock up; which is good, because I cruise at 3,200 RPM on the freeway and my converter wouldn't flash (which is different from a clutch actually physically locking the converter up) until 3,600 RPM other wise. I have broken two lock up converters by trying to pump over a thousand horsepower through them pushing my lead sled and my big butt down the strip with racing slicks. But I don't think you will have that problem so I recommend a locking converter to all street driven cars.

Big Dave
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top