It is my understanding that one would go a size down on CFM from the math for a street car. In this example, a 650CFM carburetor.
The idea is better throttle response and sensitivity because of a greater vacuum signal.
Thoughts?
Update by Tim Suddard to the
Shelby GT350 project car
Aug 19, 2024
|
Shelby,
Holley,
Shelby GT350,
Carburetor
Photography Credit: Tim Suddard
When we first built our Shelby GT350, Holley asked us if we would try their new racing carburetor. This HP series carb seemed pretty perfect at first, and then winter came: With no choke assembly installed, we started to miss the choke, as well as the more street-friendly vacuum secondaries that are offered on standard Holley carbs.
When it came time to go back through our car, installing a new, more street-friendly carburetor was fairly high up on our list.
There is a common formula for determining the size carburetor you need: cubic inch displacement times the maximum rpm that you plan to run divided by 3456.
In our case, we have a 363-cubic-inch engine that spins to about 6500 rpm. That works out to a 682 cfm carburetor. Holley also has a carburetor selector on its website that allows you to type in a few parameters and then gives you a list of appropriate carburetors.
We tried both systems and decided that we needed something between 675 and 775 cfm.
The next step is to determine which carburetor you want or need.
The first thing you need to consider is if you are racing or driving your car on the street. An electric choke, if set correctly, makes cold starts a breeze. An HP, like the one we had on our Shelby, doesn’t have an electric choke nor has a way to mount one.
You also need to choose between vacuum or mechanical secondaries for a four-barrel carburetor common on most V8 engines. Again, vacuum secondaries are smooth and streetable, although still adjustable, and mechanical secondaries are better suited to a race track.
A standard cast carburetor is less expensive, heavier, less sophisticated and the tolerances are not quite as good as the Street Avenger and Ultra Street Avenger.
The difference between these two models is the Ultra Street Avenger provides glass sights so you can more easily adjust float levels, along with nicer billet metering blocks and a bit nicer finish that should hold up well.
With all this information, we decided we wanted about a 750 cfm, vacuum secondary, electronically choked Ultra Steet Avenger for our Shelby.
A no-frills (other than electric choke) 750 cfm Holley Classic carburetor with vacuum secondaries will set you back $430.16.
The nicer 770 cfm Street Avenger with electric choke runs $626.95. And, finally, the 770 cfm Ultra Street Avenger with electric choke and vacuum secondaries that we chose lists for $796.95. Summit Racing has most of the carburetors in stock. The carburetor we used, Part #0-867770BK can be found here.
The kit comes with gaskets and a fitting to mount your throttle linkage.
While we have not dynoed the car yet, with a few minor adjustments like idle, the engine ran great right out of the box.
It is my understanding that one would go a size down on CFM from the math for a street car. In this example, a 650CFM carburetor.
The idea is better throttle response and sensitivity because of a greater vacuum signal.
Thoughts?
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