question on picking carburetor

[jveik]

hello, i have a question concerning picking the right carb for my application... first of all, ill run some specs by so you can get a feel of what the carb is going onto when i finally get the parts and build the engine...

stock brand new 4 bolt 350c.i. chevy smallblock v-8 bottom end, including dished pistons...

then some trick flow specialties heads providing about 9:1 compression with a thinner head gasket and 64c.c. chambers...

edelbrock performer RPM (maybe an airgap, i dont know yet) intake manifold...

long tube headers and turbo mufflers...

either the comp cams 262 hydraulic flat tappet cam or the 268 version, most likely i will pick the 262 for low end torque


ok... now that we have the engine described, with a potential for over 400 horsepower while still under 6000rpm, here is the most important part... the vehicle... it is a two wheel drive 1973 chevy truck that i think weighs in the neighborhood of 5000 pounds and has an automatic transmission that i might get a higher stalling converter for if needed (maybe a 2000rpm stall speed)


i know that i am going to go with a larger carburetor, such as a holley that flows around 750 cfm... but my main question is should i get a carb with mechanical or vacuum secondaries? i realize that a heavier vehicle may require vaccum secondaries added to the fact it is an automatic transmission... i just primarily want to know if a vacuum secondary carb can keep up with a mechanical secondary carb... im scared that i might loose a lot of power with the vacuum one ... any ideas? thanks for any imput...

[mazdatech177]

i had mechanical on my old chevelle and loved 'em. vacuum secondaries are for stock cars with stock engines in my opinion. just dont be concerned with gas mileage... cause you wont have any lol

[Black Lotus]

You didn't say what gearing you have.
If you looked in the how to hot rod XXX books, it'll probably reccomend a 600 CFM jobbie. Preferably a vacuum secondary one. The reasoning is that you engine is such a low-revver. If you have tall gearing, this is probably the way to go. Get a Holley or a Demon. That's my own opinion.
However, I'm a firm believer in a bit of over-carburation coupled with vacuum secondarys, so I'd probably try a 750 CFM Holley, as long as your gearing is reasonably short (for acceleration).
Use mechanical secondarys only if you have short gearing or a high stall convertor (you might have problems with pump shot duration, etc.).
Oh yeah, buy a book on the subject. Much better than scrounging for info on the internet.

[jveik]

i have heard about that problem with mechanical secondaries, apparently the accelerator pump shot wont sustain long enough for engine rpm's to raise high enough to bring in adequate flow...i bet that is why they reccomend them for lighter vehicles or high revving vehicles, since they will reach the higher rpm's before the accelerator pump shot is used up or whatever...

that prolly means i should go with a vacuum secondary model, its a 5500ish pound truck with a th400 3speed auto trans, which can do over fifty miles an hour in first gear without going over 4500 or 5000 rpm...

[mazdatech177]

accel pumps give squirts of fuel throughout throttle plate travel, and they are upgradeable with different capacities. the power valve in the carb is what gives you extra fuel for sustained high load conditions. once you stop pushing the pedal down the accel pumps stop squirting, then the power valve takes over due to the low vacuum condition of a high load engine. if you are camming this engine up you are going to have low vacuum to begin with, so youll be in secondaries just the same as with a mechanical version. get a "double pumper" mechanical secondaries and you wont have any issues.
plus the sound vacuum seconds make when you get on it... doop...dooooooooop.