Thanks xr7g428,
I can see where the stall speed on the torque converter could really mess with it. My situation is a bit simpler with the manual 5 speed. With the 351C-4V and its liking for higher rpms I think I’ll be good with either a 3.5 or 3.7 axle ratio but still struggling with which is likely to be better.
Thanks mfc133,
I’m looking at both the 3.50 and 3.70 but trying to decide which might be better. What engine are you running?
I’ve got the 302-4V in my 68. I’m back and forth on those ratios myself but leaning towards 3.50 for better highway manners.
mfc133,
I’m leaning toward the 3.50 as well. It should have more than sufficient overlap between 4th and 5th to minimize downshifting.
That does match my experience.
Thanks bmwhd1,
Thanks to feed back on this post, I’m confident that I’ll be able to run my 351C-4V down to 1500 rpm or lower when cruising. That should give a good drivability range with a 3.5 ratio with reasonable overlap between 4th and the OD.
To make the gear choice, you need to know what the torque curve for your engine and tire size. The torque curve depends on the heads, cam, and carb used in your 351C.
In my 1970 351C-4V convertible, it has a 600 CFM 84010 Holley and an ISKY 262 cam roughly equivalent to a 1972 351CJ cam. With open chamber 4V heads, the low-end torque is poor but it runs on regular anywhere (by design). I run a 3.50 rear gear with a wide ratio T5 (3.35 first, .68 5th) and 235/60-14 tires. Since NM top freeway speed is only 75, the .68 is OK (~2300 RPM) but the big step is a pain for passing.
Thanks cougarsunlimited,
I don’t have a torque curve but expect/hope the '70 351C-4V closed chamber, 11.0:1 compression, with Holley Sniper EFI, and a mild cam (Melling MTF-2 chosen for better low end torque), will do a little better at the low end with the 3.50. I’ll be about 2100 rpm at 75 (or 3500 in 4th). I don’t expect much passing power in 5th. That’s with a .63 5th and 235/60-14 tires. I’m wanting to be able to cruise comfortably up to 85 (2500 rpm). What do you think of a 3.70 instead?
The Tremec Gear Ratio Calculator shows your speed will be 75 MPH @ 2500 RPM with 3.70 gears and 235/60-14 (25.1" diameter) tires. A better choice would be 3.25 gears with 84.5 MPH @ 2500 RPM. (stock for a 1970 351C-4V, competition suspension, w/o AC)
A simulation of your engine shows the peak torque band pretty flat between 3000-4000 RPM.
Cougarsunlimited,
Thanks for the torque curve. I was definitely going the wrong way with the 3.70.
The T5 I’m getting has a 0.63 5th gear. The 0.63 with a 3.5 rear is essentially the same as your 0.68 with at 3.25 rear. I think I’ll go with the 3.5 for mine.
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cougarsunlimited,
Thanks, That matches my spreadsheet with speed and engine rpm at several rear axle ratios.
At 70 mph, about 3300 rpm in 4th and shift to about 2100 rpm in 5th. Probably a comfortable acceleration. Could push it harder if need be.