We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we cover the difference between close ratio and wide ratio transmissions and provide casting numbers for GM differential carriers.
R.C. Elmore, AL
Q: I have a couple questions regarding driveline performance:
First, I have seen the terms “close ratio” and “wide ratio” used in reference to manual transmissions. What do these terms mean, and which is better for a quarter-mile race car that doubles as a daily driver?
Second, when preparing to upgrade the ring and pinion gears in various GM differentials, most of the charts I have seen make reference to changing to a numerically higher “carrier,” usually a series 2, 3, or 4 carrier. What is a carrier, and when salvage yard shopping, how can I quickly tell one series of carrier from another?
A: A wide-ratio transmission has a more drastic change in gear ratios between first and second, and third and fourth, with fourth gear being 1:1. A close gear ratio is more gradual, but again ending up at 1:1 in fourth gear.
The best transmission ratio depends on how much track time you’ll see and the ratio of your rear axle gear. With a street-friendly gear like a 3.73:1 or lower, go with the close-ratio transmission. If you’ve got a steeper gear and you’re less concerned about drivability, choose the wide-ratio.
The carrier is the center section of your rear end: your ring gear actually bolts onto it. You can determine the series of the carrier by reading the casting number stamped on it:
Casting # ED32118 = series 2
Casting # EDB30116 = series 3
No series 4
Casting # 410408N = series 2
Casting # 410409N = series 3
No series 4
Casting # ED32088 = series 2
Casting # 30140PM1 = series 3
Casting # EDB30174 = series 4