I am right in the middle of a frame off restoration of my 72 Skylark Custom and will be putting in a 455 / THM400 as I put it back together. I would like to leave the 2.56 ratio gears in and spend my money on other things to get the car up and running again. My question is: will I be really disappointed with this combination until sometime down the road when I can swap the 2:56 open for a 3.42 posi? Does someone else have experince with these gears with a THM 400? Thanks Darryl
When we did the engine swap in my Skylark, I took it to the track with these gears. I went 13.5 @ 106 in 2nd gear crossing the line. I also overrevved and spun a bearing, but that's another issue. I did have a converter that stalled around 2600, so it was constantly slipping on the drive on the highway. It was better in 2nd gear at 60, but if you're not having a stall converter, you wouldn't have this problem. Or you may want to put it later if you do and use a stock converter. Depending on your tire size, you should be around 2000 RPM +/- at 60.
Garry, That gear ratio calculator works great. I like how you have the different trannys so you can see how the different reductions change RPM's at different speeds. Steve, 13.5 is pretty good with 2.56 gears. I think I will give it a try. How did you manage to over-rev it with these gears? Darryl
thanks Im glad you guys like it and find it useful and are getting some use out of it, ya its fun to see what combos do what. If you ever want more trannys or any changes just let me know and Ill add them or make whatever changes you think would be good.
The gears had nothing to do with too many RPM's. We had been modifying the governor to get it to shift around 53-5400 since it was a stock bottom end with a cam, good heads, intake, carb, headers. In our test runs, it was shifting around 5500. At the track, it shifted at 5900, which was just too much for it. The 60 ft times were only 2.31. My GS455 that ran the same ET at 99 mph had 1.85's with 3.42 gears. It was really slow out of the hole.
The abundance of (especially) low-end torque make it much more liveable to have numerically low (tall) gears like the 2.56. To be sure, it is a compromise on the performance end, but not like other engines with less torque that would have trouble at low RPM's. Among one of the more stupid things I did in my youth was to take my Dad's Stage 1 out to see how fast her top-end was. Dad had agreed to let me take the car to college for one semester and considering the drive was about 800 miles one-way, he opted to go to the parts bin and swap out the original 'OO' rear-end (w/ 3.42's) for a complete 2.93 (I think) rear from an old GS that had been parted out. Anyway ... on a long lonely stretch of interstate in a nameless Western state (with speed rated tires), the speedometer needle went all the way around and ended up somewhere between the "D" and the "N" on the speedometer face ... way more than 120mph. 2.56's? Top-end will never be your problem.
I've run 2.56 for years, and love them. I do a lot of highway driving, over 10,000 miles on the odometer this year. I finish the 1/4 in 2nd gear, but just think of it like a powerglide with 3.73s. (Note that my engine will rev to 6000+ RPM if I want to, so you may want to shift. On the other hand, if you enjoy the concept of 115 MPH in 2nd gear, I'll take you for a ride sometime.) This year I ran a 13.23 @ 108 in complete street trim. (1.99 60') -Bob C.
If you don't do a lot of highway driving, then they may not be for you. But if you decide you don't like them, you can always upgrade them in the future. -Bob C.
13.15 at 104 with 2:56 too....I am hoping to crack 12.9's in the next few weeks....2.0 60 ft is the best I can do though....ooops just realized you said turbo 400...I have a turbo 350......J
It sounds like the best of both worlds if I can get into the 13's and still get reasonable fuel economy... Thanks
I've had two high geared cars and loved driving them more than any old cars i've had. The 67 Skylark with 2.56 was good to go and sooo smooth. I miss that car. I've used about every gear from 4.10 on up the scale. If I had a performance car with a high gear like that I would get a Jim Wise 9.5 converter with at least 3500 stall and have the time of my life. His converters are tight at cruise yet will overcome the gear ratio enough give you a fun hole shot. Curve the distributor to be all in at 2500 but total advance at 34 or less. Use an aluminum radiator from Alumitech to prevent detonation during the long mid rpm range pulls. Keep the compression at 9.5 because the engine will rev slower with the high gear and could build more detonation prone combustion heat than a fast reving combination. Mill the block for .040-.050 piston to head clearance total(.010 deck height, .040 gasket). Do not mill the heads or detonation is more likely if the piston/head clearance is .060 or more. Tune it like a hot rod, but keep the engine cool.
