107a or 109 pump- specs? / which to use?

[meanvw]

I am planning on picking up a rebuilt pump.

I have read most of the Hagar saga, and guy repeated over and over that the 107a was best pump for mileage.

I have both a 107a core to exchange as well as a 109 pump core which seems like may be a 1984 only pump?

Question is - what is difference between these pumps, and is there a site that shows specs?
Also if mileage is goal, is one pump better than the other?

My setup is 1.6td bottom end from quantum with squirters. N/A head with mechanical adjust valves.  Turbo off a 91 Eco diesel   And currently a 109 pump.   1.5l n/a injectors .

So what are the differences in the 2 pumps?  What would you recommend and why? 

[vanbcguy]

One thing to note is that hagar never played with the 109 pumps, so don't read too much in to that.  He was working entirely with older Rabbits that only ever had the 107's.  His preferred pump was the 107 'yellow dot' version that was meant to be used with significantly more static advance than the other pumps, and only ever came on 1.5L engines.

The biggest difference between the 107 and the 109 is the cold start / idle setup.  The 107 has a more basic idle control system.  The 109 has a separate idle spring in the pump and a fast idle when the cold start is pulled, making for easier starts in colder weather. 

Realistically I think you can get excellent mileage with any of the pumps provided you feel like putting in a little effort sorting out tuning.  The advance curve of a particular pump is based mainly on its internal pressure and the stiffness / shims on the advance spring.  You can change the pressure, the shims and the spring in order to change how much advance and when it starts kicking in.  Given that you've already got something made out of various bits that were never originally engineered together, you will basically be on your own for figuring out the 'best' settings for your particular engine.

Specs for these pumps are basically impossible to find.  Bosch does a very good job of keeping that information secret.  It's also essentially impossible to tell what a particular pump is doing without a pump test machine, though you can definitely make some guesses!  Generally speaking though over time you will probably find less and less dynamic advance built in to pumps as NOx production increases dramatically the more advanced things are.  To a point, the best mileage will come with more advance.