VACUUM SECONDARIES OR MECHANICAL?

grumpyvette · Feb 24, 2013

GRUMPY? IM about to buy a 750 cfm carb for my 383 in mt chevelle, the cars only going to be raced rarely if at all its my daily transportation and its got a auto trans with a 3000rpm stall converter, a 3.73:1 rear gear and a dual plane intake and a crane 113841 mild solid lifter cam and 10:1 compression, whats the best match,
VACUUM SECONDARIES OR MECHANICAL?

you should read this linked info
http://www.quickfueltechnology.com/tech ... econdaries

http://www.hipermath.com/engines/carburetor_cfm

IN the VAST majority of cases, in my experience, a street driven car benefits more from use of VACUUM secondary throttle bores, in the combo you posted Id bet the vacuum secondary would noticeably improve drive-ability and not hurt performance

http://www.summitracing.com/parts/hly-0 ... dia/images

here is holley's answer.
"QUESTION How do I know if a vacuum or mechanical carburetor is best for me?
ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes."

BTW that crane cam used to be VERY popular around where I was racing in many cars,with 350-406 displacement SBC engines its gives very good performance in combos similar to yours in most cases, without some of the problems ,its wide LSA and mid range in duration allow it to give both impressive performance and some drive-ability, its surely not ideal in that it gives up a bit of torque, that some of the larger duration cams with tighter LSA can give you, but those tighter LSA cams with longer duration,always seem to cause tuning and drive-abilty issues in street driven cars, its WIDE LSA bleeds off enough cylinder pressure even in a 10:1 383 so that you can get by on pump gas in most cases, just be sure the exhaust is not restrictive, actually measure back pressure of its over 3 psi at peak rpms, ID seriously consider an upgrade, low restriction exhaust.
I hope you have decent heads that flow at least 250cfm ay .500 lift, and long tube headers to take advantage of that cams mid and upper rpm potential in your 383.

you can tell a great deal about combustion chamber operational conditions,both about fuel/air ratio and ignition advance , by making a full throttle run on new spark plugs and then shutting the engine down, coasting to a stop, pulling the plugs and closely inspecting them.
combine that with a accurate graph of both plenum vacuum,readings and ignition advance curve and collector back pressure readings and if possible a fuel/air ratio meter reading from each collector and what an infrared temp gun shows your header temps to be and you get a very good indication of what might need to be changed.
you might be amazed at what a few tests with a fuel pressure gauge and vacuum gauge and timing light can tell you

http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

VACUUM SECONDARIES OR MECHANICAL?

grumpyvette

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