6. What’s the right size carb for my engine?
The basic formula for carburetor sizing and cfm consumed is cfm = ci × rpm × VE ÷ 3456). The main variable here is the Volumetric Efficiency, which is essentially a percentage of how much air actually is pumped into the cylinder when running versus off. With the piston at bottom dead center (BDC), atmospheric pressure at 14.7 psi will fill it 100 percent, however, multiple restrictions from the intake to the heads will prevent most engines from ever seeing that while running. In most cases a ballpark guess of the VE will suffice; figure on stock or very mildly modified cars being in the 75-85 percent range, well-built performance engines in the 85-95 percent range, and dialed-in race motors in the 95-105 percent range.
Of course, that’s assuming your engine is actually providing the required vacuum signal that will allow the carb to flow at its max cfm. Carburetor cfm ratings are assigned using an assumed vacuum drop of 1.5 in-hg for four-barrel carbs, but it’s not uncommon for engines to fall short of that ideal scenario even at wide-open throttle. Actually fully optimized high-performance or race engines ideally won’t see more than .75 to .5 pound of vacuum at wide-open throttle since the higher the vacuum, the lower the air density.
Basically that means your 650-cfm carb may not be delivering 650 cfm of air to your particular engine. However, that may or may not actually be a problem depending on the combination you’re running, how it’ll be used (cruising versus racing), operating range, compression ratio, how heavy the car is, and what kind of transmission and gearing. There’s a whole lot to consider if you want the perfect carb, so ideally you should work with a tech guy from the manufacturer of your choice to pick the right package for your particular application. Since a little bit too much is better than too little, our general rule of thumb for hot street cars is to use the formula for a starting point, then step up to the next logical cfm size and tune within an inch of its life with the aid of a chassis dyno shop.
7. What’s the easiest way to figure out my gear ratio?
If you’re lucky enough to still have the original rearend in your car and there happens to be a data tag on it, simply decoding that based on the manufacturer’s conventions will tell you what the vehicle was originally equipped with. Of course that’s assuming it hasn’t been altered at some point.
The quick and easy way to figure out what ratio you have now is to jack up the rearend of the car and securely place a jackstand at each end of the axle (assuming solid axle) that provides at least an inch of ground clearance for both rear tires. Using chalk, make corresponding marks on the rearend flange and the housing itself. Now make a mark on the tire that’s referenced to a mark on the ground, or somewhere on the body or chassis. All you have to do now is put the trans in Neutral, slowly spin the tire (or driveshaft), and count how many times the driveshaft rotates for each full rotation of the tire using the flange and housing mark. Obviously this works best with a friend to help turn the tire while you lay under the car and count, but with good placement of the marks it could be a one man job.
By the way, now you also know if you have an open or limited-slip (posi) style rearend; if the rear wheels spin in opposite directions, it’s an open diff. If they spin the same way, it’s limited slip.
8. When do I need a rollbar?
If you’re looking to construct a rollbar or rollcage that meets NHRA tech specs, it’s imperative that you get a current year NHRA Rulebook, and preferably an SFI Foundation one as well. That said, we can give you a few basic specs.
A rollbar is required in any convertible running 13.49 or quicker in the quarter-mile, and in other cars beginning at 11.49. An approved rollbar is accepted in vehicles running as quick as 10.00 (10.99 for convertibles) provided that the stock firewall and floorboard is intact and unaltered, with the exception of wheeltubs. The rollbar must be constructed of minimum 1.75-inch OD x .11-inch wall mild steel tubing, or 1.34-inch x .083-inch chrome-moly tubing.
If the floor and/or firewall has been modified, then a full rollcage is required beginning at 10.99. A full rollcage is required in any vehicle running 9.99 seconds or quicker and any vehicle running 135 mph or faster (regardless of e.t.). The rollcage must be constructed of minimum 1.625-inch OD .118-inch mild steel tubing, or 1.625-inch x .083-inch chrome-moly tubing. Additionally, the rollcage of any vehicle running 9.99 or quicker, or 135 mph or faster, must also be certified by NHRA every three years and have a serialized sticker.
Most autocross events will not have rollbar requirements, though most will encourage convertibles to have one. Open road race and track events vary with the speed of the car and track type, but most will require convertibles to have at least a rollbar.
9. What’s the best vacuum vs. mechanical advance for the street?
Though you can get away with running either, in general carbed cars that spend most of their time on the street will want a vacuum advance distributor. That’s because a vacuum advance adjusts timing based on the engine’s load while a mechanical advance reacts only to a change in rpm.
Vacuum advance was invented to provide optimal spark advance in reference to engine load and changing operating conditions mostly to deal with varying lean and rich fuel conditions during driving. Lean mixtures, like at idle and steady cruise when the butterflies are mostly closed, take longer to burn than rich mixtures. To compensate, during lean times timing needs to be advanced to create an earlier spark giving the fuel more time to burn. The opposite is true with fast-burning rich conditions, such as acceleration when the butterflies are open. At or near full throttle, vacuum advance completely drops out of the equation and the engine is responding to the initial static timing plus the mechanical advance. That’s why dedicated drag cars often don’t run them; they don’t have the same part-throttle concerns.
That said, if you’re running a big cam on the street that makes little vacuum at idle, your vacuum canister needs to be adjusted accordingly. Stock GM units aren’t fully activated until they see about 15 in-hg, and many aggressive cams don’t make that at idle. Swap to a unit that is fully activated with a few inches less vacuum and the engine’s street manners will improve.