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+5mph = -5mpg;vacuum gauge & adv;carb metering rods
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Author:  emsvitil [ Wed Dec 28, 2005 3:58 pm ]
Post subject:  +5mph = -5mpg;vacuum gauge & adv;carb metering rods
Big subject line, but they're all related....... :wink:

First the observation. (+5mph = -5mpg)

Went to my parents for Christmas weekend (Los Angeles to Bakersfield)
went a little faster than usual; tried to stay at an indicated 80mph, and 75 mph up the grade on I5 (4000ft pass and about 3600ft differential low to high). Needed full throttle to stay at 75 at times. (1bbl 225).

Filled up before coming home, and averaged 14.2mpg....... yuck!

So I dropped back to my normal speed for the ride home...... indicated 75mph, and 70mph up the hill (no need for full throttle).

Filled up again and now I was back to what I expected 19.7mpg...


So I lost over 5mpg with that slight 5mph speed increase....

now for my theory.........
Author:  emsvitil [ Wed Dec 28, 2005 4:02 pm ]
Post subject:  carb metering rods, and vacuum advance.
I think the loss of the mileage was a combination of:

No vacuum advance and I entered the power part of the metering rod...

To sustain that little bit extra speed, I had to give the engine enough throttle that I lost enough manifold vacuum to lose my vacuum advance, and most likely I went on the power portion of the needle (not enough vacuum to keep the cruise portion in the main jet)
Author:  emsvitil [ Wed Dec 28, 2005 4:06 pm ]
Post subject:  Vacuum Gauge......
So has anyone experimented with a vacuum gauge and a Air/Fuel meter to find out just how much manifold vacuum you need to keep the needle in the cruise position??????

Once you know that, I'm thinking that you can use the vacuum gauge to go as fast as you can, but make sure it stays above the transition point of the needle (or power valve if you unfortunately have a Holley) and also have enough vacuum for full vacuum advance.
Author:  emsvitil [ Wed Dec 28, 2005 4:08 pm ]
Post subject:  Metering rods again.
Has anyone played with different metering rods, metering rod springs, vacuum gauge, and Air/Fuel meter to get an nice lean cruise mixture, and the proper power mixture?????
Author:  sandy in BC [ Wed Dec 28, 2005 4:17 pm ]
Post subject: 
I dont think you can do the math when you have a 3500 ft elv gain in the mix.

I could gain 5 mpg just be coasting down the grade.

You can get the same thing with wind....I once burned 17 gals of fuel in 35 miles with my Sub pulling a loaded 24 ft trailer uphill into a 40 mph wind,,,,,,I could only manage 35 mph at best. Now if I was going down that hill with the wind behind me....I could have gone 90mph and burned 2 gals of fuel.


I really like O2 sensors (TBI injection for me) because you get the correct mix regardless of load or speed. Exception is at WOT. When doing the big ,fast grade thing I usually stay just off WOT to get better mileage.
Author:  emsvitil [ Wed Dec 28, 2005 4:23 pm ]
Post subject: 
Whoops.......

The hill is in the middle....... you go up from LA, then back down to Bakersfield........ The 3500 is the max altitude gain.

There's like 300-400 ft difference in altitude between LA and Bakersfield, and over 120 miles that won't make an 5mpg difference.....

And my best mileage to date was from LA to Bakersfield when I went between 70-75 indicated and 70 up the hill and got 22mpg. 22mpg (or 19mpg) to 14mpg is a big difference for that slight faster speed.
Author:  sandy in BC [ Wed Dec 28, 2005 5:05 pm ]
Post subject: 
OOps! I should have known that!....
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