does a carb with mechanical secondary like a holly, typical have worse gas mileage than one with vacuum secondaries?
Mechanical secondaries get worse gas mileage. In theory, mechanical secondaries allow fuel to be dumped in before the engine really needs it all. Vacuum secondaries rely on engine vacuum to tell the secondaries when to open. That being said, it really depends on the application and I prefer mechanical. <!-- google_ad_section_end --><!-- / message -->
I would be inclined to say yes, if the engine is built aggressive enough and you womp on it its all 4 barrel right now and it will pull rather than bog. If its a mild street engine it most likely will fall on its face.
Not necessarily. There are too many other variables to answer the question. Maybe if you could tell us what it is you're working on...all the details...
it is a 300hp/327 out of a 62 impala that has been bored .40 it has an unkown mild cam and holly carb w/ double hump head(which i dont know what they are either). i am going to pull numbers off of the carb to find the cfm rating. this thing will fly, but gas milage ****s... i know cant have your cake and eat it too. this is a long way from my mopar inline 6 with 95 hp
lets see....... want to fast -"check" ...... runs like a "*****" - check ...... poor fuel milage ???? sorry , i must be buzzed .
On Holleys Anouther a thing to consider is a lot of mech 2ndaries are on double pumpers, which on acceleration give more gas due to the extra accelerator pump. I think that has to do with not having enought va***n to opperate the second pump properly really the deciding factor should be weather you roll the petal down or mash it to the floor with mech 2ndaries, because if you hit it too fast you will decrease va***n, and lose atomazation and siphoning of gas causing the motor to bog. Now my daily 283 t-400 powered 81 el camino got over 25 MPG on highways and had OK acceleration with a 450 mech secondary. After i wore the ****erflies out on that carb i stepped up to a 650 double pumper(cuz it's what i had) and an L79 cam and probably still get well over 20 mpg the main thing is conservitive jetting and carb size
Other than Holley produced carburetors, very few with vacuum secondary operation. Ford (Autolite, Motorcraft) 4-barrels were mostly vacuum secondary, but these were built by Holley. Carter built a very few vacuum secondary carbs (1957~1959) for Ford and Chrysler, but gave it up as a very bad idea. To my knowledge, Rochester has not produced a vacuum secondary 4-barrel; and I know Stromberg never did. Many confuse "vacuum secondary" with "demand" secondary. A vacuum secondary has a vacuum diaphragm which controls the opening of the secondary valve. A demand secondary has some control (weighted offset valve, or spring loaded valve) that opens with a preset demand my the engine. These valves, while a part of the complete secondary system, are generally referred to as auxiliary valve. A demand secondary is far superior (opinion) to a vacuum secondary. However, correctly sizing a carburetor to an engine, and calibrating the carburetor to the specific engine, transmission type, and m*** of vehicle PLUS the operation of the "foot on the petal" are more important factors in fuel economy than how the secondary operates. Jon.
100% true. Mechanical secondary carbs are usually set up with a progressive secondary linkage, you have to go about half throttle to start to get into the secondaries, so if you don't have a heavy left foot, then it shouldn't be too hard on fuel, Double pumper carbs are calibrated with a richer mixture, but that can be worked around as well. A buddy had a 351c falcon with a toploader that would get about 17mpg crusing main in town if it was driven real nice, that was with a 850 carb. It's all in the tuning of the whole engine, not just the carb.
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