Thread: MPG aids
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Old 12-13-2020, 11:04 AM   #17 (permalink)
Isaac Zachary
High Altitude Hybrid
Join Date: Dec 2020
Location: Gunnison, CO
Posts: 1,320

Avalon - '13 Toyota Avalon HV
90 day: 40.45 mpg (US)

Prius - '06 Toyota Prius
Thanks: 711
Thanked 385 Times in 297 Posts
When I had the VW Type 1 I installed a CB MagnaSpark electronic distributor. It had a built in MAP sensor, and I could hook up a laptop and see it work from inside the cab. I could also change both vacuum advance and RPM advance curves on the fly.

If I ever got a car with a carburetor and distributor again, I'd get a CB MagnaSpark box.

With the Bug I was able to advance clear up to a whopping 45 without any pinging that I could tell. Engine was peppy and I never got under 30mpg on trips. And all trips were over mountain passes.

One thing to note is that vacuum gauges and MAP sensor readouts aren't always what you think. Best fuel mileage isn't highest vacuum like some gauges tell you. It's not even highest spark advance. Best fuel mileage is usually the vacuum level right before secondaries open up and the carb goes rich. (I had a stock carb on the Bug that didn't have mechanical or vacuum secondaries, but I did also have an O2 sensor and AFR gauge to see when it would enrich for high load).

If you want the best fuel mileage with a vacuum gauge, first figure out how much vacuum it takes to open up your secondaries or power enrichment circuit. (If it's mechanical maybe you need a gauge that shows throttle position.) Mark a line there when the secondaries open and while driving open up the throttle to just before that line and get up to speed. Then let off the throttle, throw it into neutral, and shut off the engine and glide. Getting down to a slow speed, hit the clutch, put the stick back in gear and pop it and start all over again.
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