[darkostoj]

i got a 671 blower with dual holley double pumpers on it. The carbs have 50cc double pumpers on the primaries and secondaries. I tuned the cars with my wideband to get 16:1 cruise, 13.5:1 idle, 12.5:1 WOT, and 12.5:1 heavy acceleration in the primaries. timing is 22 initial 32 total

I have terrible gas mileage (10 or less) and was thinking that the 50cc pumps probalby have a lot to do with it becuase they really do shoot a lot of gas. I noticed if i'm cruising at 15:1 or so and nail the throttle while staying in the primaries it immideately drops down to about 10:1. I was thinking of swapping out all the 50cc pumps for 30cc pumps to see what happends....any thoughts?

 

[gearheads78]

Please tell me you are not worried about mileage with 2 holleys and a 671 ?

 

[blue66]

I don't see any change possible, unless you have a tremendous amount of idle to full throttle accelerations. Which would make for terrible mileage anyway.
The accel pumps should literally have nothing to do with the mileage.
At a 16:1 cruise something seems off @ 10 mpg.
Is 32* timing with vacuum adv. or just mechanical.
You really need to get your advance up around 50 at cruise.

 

[darkostoj]

Please tell me you are not worried about mileage with 2 holleys and a 671 ?

well when i read of people getting close to 20mpg with 871 blown big blocks with dual 850 cfm double pumpers, it makes me feel like i could be doing something differently to get better than 7-9mpg...

 

[curley8788]

well when i read of people getting close to 20mpg with 871 blown big blocks with dual 850 cfm double pumpers, it makes me feel like i could be doing something differently to get better than 7-9mpg...

dont forget they probably have OD transmissions too...

all you can do is buy the 30cc pumps and try it...

 

[darkostoj]

I don't see any change possible, unless you have a tremendous amount of idle to full throttle accelerations. Which would make for terrible mileage anyway.
The accel pumps should literally have nothing to do with the mileage.
At a 16:1 cruise something seems off @ 10 mpg.
Is 32* timing with vacuum adv. or just mechanical.
You really need to get your advance up around 50 at cruise.

well doesnt a 50cc pump deliver 50cc's of fuel compared to a 30cc pump delivering 30cc of fuel? Remember i have 2 carbs, so everytimg i go from idle to WOT i'm putting in 200cc worth of fuel with the 50cc pumps vs/ 120cc with the smaller pumps. With the smaller pump diaphrams i'd be cutting the fuel consumption in just the accelerator pump circuit almost in half. The 50cc's right now seem like they are richening up the motor too much anyway. My a/f gauge drops real rich, and I don't have that real sharp throttle response, its kinda lazy

 

[curley8788]

well doesnt a 50cc pump deliver 50cc's of fuel compared to a 30cc pump delivering 30cc of fuel? Remember i have 2 carbs, so everytimg i go from idle to WOT i'm putting in 200cc worth of fuel with the 50cc pumps vs/ 120cc with the smaller pumps. With the smaller pump diaphrams i'd be cutting the fuel consumption in just the accelerator pump circuit almost in half. The 50cc's right now seem like they are richening up the motor too much anyway. My a/f gauge drops real rich, and I don't have that real sharp throttle response, its kinda lazy

sounds like you just answered your own question.

 

[darkostoj]

sounds like you just answered your own question.

i was wondering if it is even worth the effort to swap everything over. Looking for real world experiences about how 50cc pumps affect mileage.

If i convert cc's to gallons, 200cc's(from going from idle to WOT) is .052 gallons from JUST the accelerator circuit.

 

[blue66]

I believe it takes 10 full pumps from each pump to delliver the 50cc.
Technically a 5cc pump.

 

[onovakind67]

I believe it takes 10 full pumps from each pump to delliver the 50cc.
Technically a 5cc pump.

You're right, it's 10 full squirts to get the rated cc's. Holley makes a chart that shows the volume associated with each cam.

 

[forcd ind]

the size of the squiters has more to do with it than the vol-use the smallest squiter you can w/o a boog-also the duration of the pump cam has an effect on it
the 30 vs 50 is only storage
size of the converter, rear ratio, cam etc all deal with gas milage-even a speedometer that is incorrect will not show actual milage
many years ago, i had a 57 chevy with a 454, 671, 2-750 holleys-i went on a trip with 5 other guys, most had street rods, and every time they stopped for gas, i was only 1/2 empty
i had stuck in 3:00 rear gears-i dont have any idea what my milage was, but it was dam good
you want milage and power, get a new vette, lol(or put a new vette motor in your chevelle, with a procharger and o/d trans)

 

[animal69]

well when i read of people getting close to 20mpg with 871 blown big blocks with dual 850 cfm double pumpers, it makes me feel like i could be doing something differently to get better than 7-9mpg...

You can lie about it ,too!:beers:

 

[Eric68]

Please tell me you are not worried about mileage with 2 holleys and a 671 ?

I think he's just messing with us . . . if not, maybe it's time to swap in an Ecotech.

 

[M.Maner]

Darko remove the accelerator pump cams from the primary side of both carbs,take note of there installed position 1 or 2 and read the ID number off of the cam. I suggest this only because if the carbs are older the plastic fades and it's not always clear which color it is. Let us know what you find.
You didn't make it clear whether you had vacuum advance. With the engine being blown it's doubtful that you can make use of the typical 50-52 degrees total timing at cruise that most people are able to use. Using whatever advance the engine will tolerate at cruise could be very helpful as far as mileage goes.
Mike