71ProjectJunk
Well-known member
- Joined
- Jul 27, 2021
- Messages
- 892
- Reaction score
- 666
- Location
- East Texas
- My Car
- 1971 Mustang Mach 1 M code 351 4 speed
The AOD or the 4R70W can both be run with 2.75 gears with no issues. The Fox Body Mustangs with the AOD came with 2.75 gears and the AOD, and they run fine. The issue with that high gearing is that the car will be a dog out of the hole. Those AOD 5.0 mustangs with 2.75 gears would loose a 1/4 mile drag race to a Honda CRX Si, trust me I was there, LOL. The issue with high gearing is what camshaft you limit yourself to when you are building your engine, besides how the car will perform. The 4R70W has an advantage over the AOD in that the first gear is a 2.84, vs the AOD 2.47. If you are going to build an engine with aluminum heads and try to get 500hp out of it, you really can't run 2.75 or 3.00 gears. It will be a dog out of the hole. To get to 500hp you need a decent sized cam, you will not get there with a small "RV" stock type cam or a little bigger than stock cam, which you can use with 2.75 or 3.00 gears. To make a engine make power AND the car perform, everything is a combination. At 400 HP or below, using a cam with at most 215 @.050 you may be able to get away with 3.00 gears with a good higher stall torque converter, but you are still loosing a substantial amount of performance compared with 3.50 gears or lower, not to mention with a bigger cam. Now you will probably not get to 500 hp, with 351 cubes with a small 215 @ .050 cam, even with the best flowing heads, intake, and exhaust. The bigger the cam, the more gearing it needs and the higher stall speed converter you will need to use. As you move up in HP your torque curve moves up in the RPM range, a 500 HP engine will probably make less torque that a stock 351C 2V under 2000 rpm, but from 3000 rpm on up it will be night a night and day difference. If you are looking at a real 500 HP normally aspirated 351, you really will need at least 3.50 gears, in reality you probably need more, and a 3000 RPM stall torque converter for the car to perform at its full potential.