We made 707 hp with an 850-cfm carb and a stock oil pan with our first 496 back in 2007. W
Comparing a single-plane intake on a 496ci big-block with the Air-Gap dual-quad package is simple enough that we really don't need to run 'em on the dyno. The dual-quad system will make outstanding torque-something you may not need more of with a 496. More importantly, the dual-quad system must sacrifice intake runner length to fit in that second carburetor. The problem back in the early '60s was that big carburetors didn't exist-so adding a second carb was the only way to add more cfm. Today, you can buy a 1,050-cfm 4150-style Holley, so cfm isn't an issue. However, if the appeal of dual quads is what you're looking for-then go for it. Just know that you're probably going to give up 10 to 20 hp compared with the single-plane, and whatever torque advantage the dual-plane offers will be in an rpm range that won't offer much (if any) advantage on the dragstrip.
Based on this and the sacrifice of runner length on the dual-quad Air-Gap, the result will be that a good single-plane like a Victor 454-R would be the wise investment, since you're going to be running brackets. Managing one carburetor will be much easier than juggling two.
You mentioned you'll be using a Comp XR286 mechanical roller. That cam specs out at 248/254 degrees at 0.050 with 0.653/0.660-inch lift with a 110-degree lobe-separation angle. This is a pretty big camshaft, although smaller than the one we used in the Mar. '07 issue (254/260 degrees duration at 0.050 with 0.660/0.666-inch lift). The CC 496 peaked at 707 hp at 6,400 rpm. With a slightly smaller cam, you can expect your 496 to peak at around 6,000 or 6,200 and make power of around 690 or so. We've already discussed in this column how much power is lost with chassis headers (even 2.0-inch primary size), mufflers, a 180-degree water temp, and a full accessory drive system that we didn't run on the dyno at Westech for that test. If you package your big-block in a car with at least a 3,000-stall converter and a little bit of gear, you can expect it to run mid-10s, assuming you can hook it to the starting line.
As for your question about carburetors-assuming we're talking about the single-plane intake now and not the dual-plane-the plan for our new 496 is to start testing with a 950 HP Holley and then try a 1,050-cfm Dominator. There is power with the bigger carb not necessarily because of more cfm but with better mixture distribution from the larger bores of the Dominator carburetor. Plus, the larger carb at max rpm will slow the air speed through the carburetor and allow the air more of a chance to make the change in direction from vertical flow to horizontal flow into the ports after leaving the carburetor. This is why spacer plates work so well at peak rpm, because they help the air and fuel generate a gentler 90-degree turn.
Tire diameter plays a big part in overall gear ratio. On our Orange Peel Chevelle, our str
Charles Wilson, Elmwood, TN: Several years ago, Car Craft had an article on how to figure out some of the math involved in building a street machine. There was a formula to determine rpm, final drive gear ratio, tire size, and mph by knowing three of the variables. I have since lost that issue but would love to know the formula again. Any help would be most appreciated.
MPH = (RPM X Tire Diameter) ÷ (Gear Ratio X 336)
MPH = (3,000 X 26) ÷ (4.10 X 336)
MPH = 78,000 ÷ 1,377.6
MPH = 56.6
RPM = MPH X Gear Ratio X 336 ÷ Tire Diameter
RPM = 70 X 4.10 X 336 ÷ 26
RPM = 3,709
Gear Ratio = (RPM X Tire Diameter) ÷ (MPH X 336)
Gear Ratio = (3,000 X 26) ÷ (70 X 336)
Gear Ratio = 78,000 ÷ 23,520
Gear Ratio = 3.31:1
Tire Diameter =MPH X Gear Ratio X 336 ÷ RPM
Tire Diameter = 65 X 3.31 X 336 ÷ 2,700
Tire Diameter = 26.8 inches
Effective Gear Ratio = (Old Tire Diameter ÷ New Tire Diameter) X Gear Ratio
Effective Gear Ratio = (26 ÷ 28) X 3.55
Effective Gear Ratio = 3.29:1
Jeff Smith: Here are the critical equations that relate to tire diameter, engine rpm, and gear ratio. One reason I never became an engineer was because of the math. That side of my brain suffers from multiple disconnections. So I rely on an HP book titled the Auto Math Handbook written by John Lawlor. You can find it on Amazon.com for around $15. The constant (336) in all these equations simplifies the math by reducing the conversion from hours (in mph) to minutes and miles to inches and also accounts for determining diameter from circumference with Pi (3.1415927).