The Stovebolt Site Forums The Shop Area Engine Shop TH400 from a 350, hooked to a 305 question

I've been running a TH400 behind a 305, RV cam, 600 Edlebrock, MSN ignition and a 9" Fjord rearend (3:35 gears) in my 51 Chev 1/2 ton.

This thing runs nice on the highway but is a hog off the line absolutely no chance ever of spinning the tires.

Is it possible that the torque converter from the 350 is wrong? Is there a different one I should use on the 305?

I would think that's it's unlikely that a 305 would be hooked to a TH400 in anything stock so that may the problem too ... maybe to much loss in the tranny for the 305?

Any thoughts or suggestions would be really appreciated.

The problem is the 305. Woof, woof.

There is more parasitic loss in a TH400 than a 350 (some folks say as much as 40 hp) but the TCs are interchangeable...and if they're both stock, there shouldn't be much difference.

If they ARE stock, the stall speed is probably around 1200-1800 rpm (if that high) and won't let the motor get wound up enough to produce enough power to light the tires.

There are a coupla ways around this...but none of them are easy: replace the torque converter with a higher stall speed unit (2400-2800 rpm) and/or replace the 305 with a more performance-oriented motor like a 350, 383 or 406. (406 is my choice...but I'm prejudiced cuz I have a couple of them.)

Seriously, you've got a low-power motor coupled to a high-power transmission....and you're seeing the results of that mis-match.

At least you don't have the opposite combination: a fire-breating rat motor and a TH200C....the symptoms would be a little different...like ATF all over the street when the tranny grenaded.
But then the solution would be evident.

There's nothing wrong with the 305, but it takes a little more effort to make it come alive than some of the bigger engines. I've got a friend in Portland, Oregon who's outrunning engines 100 cubic inches larger, and setting lap records with one. Your problem is the torque converter, and your rear axle ratio. Try a converter with a 2,500 stall speed or so, and a 3.55 or lower gear. Also, if you haven't done it already, try some flat-top pistons and 1.94" intake/1.6" exhaust valves, and a pocket-port job.

I'm running a destroked 305, with 250 cubic inches, that's producing around 400 HP, and it runs comfortably at 7,500 RPM. My competitors on the 5/8 mile asphalt track have seen nothing but rear bumper from my driver for quite some time!
Jerry

What Hotrod is saying about the differential ratio is the gospel. Performance from any horsepower is going to finally lead you to what ratio you need in the third member to a large degree. I am running a 305 with a Th250c Tranny and a 4:11 rear gear, stock engine w/2bbl carb in my 64. I don't compete, but it keeps up with the traffic and with the light weight rear short stepside, it will spin the tires very easily. Also, a stallspeed with any cam that has enough lope requiring fast idle is a must to keep from having to keep both feet on the brake at a red light.

Truck Tinker