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:
GM 8.2-Inch 10-Bolt
Casting # ED32118 = series 2
Casting # EDB30116 = series 3
No series 4
GM 8.5-Inch 10-Bolt
Casting # 410408N = series 2
Casting # 410409N = series 3
No series 4
Casting # ED32088 = series 2
Casting # 30140PM1 = series 3
Casting # EDB30174 = series 4
Is it normal to take 20 years to rebuild my 1970 skylark convertible?I didn’t think so. When it is finally running,thanks to all the free time due to covid I’ll be the one laughing.HA.Seriously I do want to run it in 2021. A 7004r with 3.73 posi in the rear should help.it has an old (grooved cam) 396
I really enjoyed your article on Wide versus Close ratio Transmissions. Years ago I ran a 1970 Corvette LT-1 engine with a wide ratio Corvette Transmissions, in a Camaro. It would Rev quickly thru 1st but seemed to fall on its face in 2nd gear. I had 4:11 gears in the rear end. Thanks for the explanation in your article ! Jim