Spin to Win
Jim Klug; Milwaukee, WI: I have a ’71 Vega ex-bracket car that I am converting to a Pro Street–type car. My current combo is a 350 Chevy with 10:1 pistons and bowl-blended fuelie heads with 64cc chambers, a Turbo 350 trans, a Ford 9-inch rearend with 4.11:1 gears, and 33-inch-tall M/T Sportsman street tires. The converter stalls between 2,300 and 2,700 rpm. I am trying to use an Edelbrock street tunnel-ram with two 465-cfm Holleys. The cam is an Isky 280 Mega cam. The specs are 0.485-inch lift, with 232 degrees of duration at 0.050-inch tappet lift on a 108 lobe-separation angle advanced 2 degrees. How do I get this package to work? Do I have enough gear ratio? I am just interested in nuking the tires. This car will not be raced, only street driven. Right now the car drives great. The converter seems a bit tight, but OK. This car just will not smoke the tires. I have a couple of other camshafts including a 270 Isky Mega cam and a Comp Mutha Thumpr. I’m just really interested in having the car really sound wild. Also I have a 400 10:1 motor with Vortec heads and an Edelbrock Air-Gap intake. What would be the best bang for my buck?
The problem is it just doesn’t have the grunt to turn over these big old 33-inch tires. I even tried taking off the tunnel-ram and putting on a dual-plane LT1 intake with a 600 Holley. I also have 4.56:1, 4.88:1 and a set of 5.13:1 gears and a 3,100 to 3,500 converter to try. What gears, converter, cam, or 400 engine would make this work. I don’t want to use a grenade on it!
Vacuum-secondary carbs on a tunnel-ram can be made to work well. We discovered that smalle
Jeff Smith: Let’s see if we can help you out, Jim. Let’s start with effective gear ratio, because you are correct in thinking you don’t have enough gear. As the rear tires become taller, they reduce the effective gear ratio. So in your case, if we use a 26-inch-tall tire and a 4.11:1 gear ratio as our baseline, adding the taller tires reduces the effective gear ratio to a much more conservative 3.23:1. Turning the math around to get close to an effective 4.11:1 gear ratio with those 33-inch-tall Mickeys will require every bit of that set of 5.13:1 gears. To equal the effective gear ratio of a 4.11 gear with a 26-inch-tall tire, you would need a 5.20:1 rear cog. So one reason you can’t “nuke the tires” is you just don’t have enough leverage. Plus, keep in mind that in addition to their height, those massive Mickeys are also pretty wide, so you need the additional leverage that the gear ratio supplies to help strike the tires. But there’s much more to this equation.
Working our way up the drivetrain, torque converters play a big part in strong acceleration. There’s far more to converters than just the stall speed. Effectively, the stall speed merely raises the rpm where the converter hits. By raising the stall speed, you bring the launch rpm closer to the engine’s torque peak. For maximum acceleration, you want the stall speed at least above the peak torque rpm, which for a 350ci small-block like yours is probably around 4,500 to 4,800 rpm. So looking strictly at the stall speed, the loosest converter you have would also help since it would push the engine farther up into its torque range to help overpower those tires. But wait, there’s more. What creates torque multiplication is an internal component known as the stator. The stator consists of a series of blades located between the engine-driven impeller and the turbine, which is splined to the transmission input shaft. The stator redirects fluid from the impeller to change its direction, but at the same time it also multiplies torque. Maximum torque multiplication is achieved with the greatest difference in speed between the impeller and turbine, which occurs right near zero vehicle speed. This multiplication factor is in the range between 1.8:1 and 2.5:1 depending upon the style of stator used. With a 2.0:1 multiplication factor, an engine making 300 lb-ft of torque could produce as much as 600 lb-ft of torque for that instant of initial acceleration. Unfortunately, this only occurs at maximum stall torque ratio. As the vehicle moves forward and the speed difference between the turbine and the impeller is reduced, the multiplication ratio diminishes rapidly. This is important because additional torque will make the car launch much harder and, in your case, make it much easier to spin the tires. The best way to get this information is to call your torque converter manufacturer. A combination of more torque multiplication and a higher stall speed will put much more torque to the rear tires.
Of course, we can’t forget the engine. In terms of just making torque, the single, four-barrel, dual-plane intake package should make more torque than the tunnel-ram, but much of this also has to do with tuning. A poorly tuned carburetor on that dual-plane will not perform as well as a pair of small Holleys on a tunnel-ram. Several years ago, we did a story testing a Summit tunnel-ram package using a pair of 600-cfm Holley 0-1850s on a tall tunnel-ram (“Tunnel-Rams for the Street,” Dec. ’08). We learned that the pair of vacuum- secondary 600-cfm Holleys generated far more accelerator pump shot than the engine really needed, and it bogged badly right off the line because the engine went dead rich. By reducing the accelerator squirter size in both carbs down to 0.022 inch, we radically improved the engine’s throttle response. I used a pair of 0.050-inch squirters and filled the outlet holes with epoxy and then drilled them to 0.022 inch. Of course, a stock Holley 0.021-inch squirter (PN 121-121) would also work, but we didn’t want to wait, so we made our own. This might be a big part of your problem. If the carbs have been modified, return the primary jetting to stock and see how that works.
You didn’t mention it, but your initial timing is also critical for making low-speed torque. For an engine like this with a big cam, I’d set the initial timing at 18 degrees and (with the vacuum advance disconnected) check the total timing. The engine will want somewhere around 36 degrees or possibly more total timing because of those old cylinder heads. If the engine doesn’t rattle (detonate), then the timing should be pretty close for optimal power. All of this should be enough to seriously improve both the throttle response and the engine’s low-speed torque. Then you can spin the tires until the cords scream for mercy.
Holley Performance Products; Bowling Green, KY; 270/781-9741; Holley.com
This was one of the historic engines on display in the GM Performance Division booth at the Summer Nationals. In the ’70 Buick GS, these made 360 hp at 4,600 rpm and 510 lb-ft at 2,800 rpm, the highest torque rating at the lowest rpm of any GM production engine.
The holding area for the Miss Car Craft Summer Nationals contestants. I’m feeling a movie plot developing here . . . we make dreams come true!
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