Thanks to all of the great info on this board, I have my 455 almost ready to drop in my 66 Skylark. My car currently has a decent Turbo 350. I plan to use it and the last part I need is a stall converter. I'd like to use my stock flexplate instead of purchasing one with a smaller bolt pattern. Do you guys see any issue with me purchasing a Turbo 400 converter so the bolt circle will match my 455 flexplate? Will the stall speed be different using a 400 converter in a 350?
I'm running a TH350 with a TH350 torque converter on my stock 455 flexplate. I'd have to go look at a spare flexplate, but I believe it's drilled with dual pattern bolt circles. It will probably stall at a little higher stall speed behind a big block if the torque converter was spec'd for a small block. Someone correct me if I'm wrong on any of my info.
There's always exceptions, but the OEM TH400 converters tended to be an inch in diameter larger than TH350 converters. Lots of guys went the other way--installing a TH350 converter on a TH400 for less rotating weight and maybe a tiny increase in stall speed. Of course, the "Switch-Pitch" TH400 converters, and the TH350C converters won't interchange with each other or any other "regular" 400 or 350 converter. The bolt circle diameter--and mounting method--will vary depending on what converter core and flexplate is used. "Chevy" applications tended to use a bolt-and-nut arrangement, the brackets on the converter allowed for a nut on the back side. Other GM divisions used threaded standoffs on the converter, no nut required. The Chevy style converters don't fit the OEM flexplates for some other divisions engine families, because the Chevy brackets won't clear the balance weight welded to the flexplates. Pretty sure Oldsmobile does this, but I don't think they're the only ones. You should look at your existing converter to see what the bolt circle diameter is, and how the converter is mounted to the flexplate.
Well, as mentioned you can do a "reverse" stall converter by using the T400 converter which are usually 13" units. But why do that when you can just use the T350 converter and actually pick up a little stall speed? As mentioned most BBB's have the dual pattern for both size converters. Easy to spot when looking at the flywheel. They are almost beside each other. A purchase of a new flexplate with the dual hole pattern is well worth the cash and it would allow you to use the T350 converter even if it is only a minimal stall speed increase. Do it and don't look back. Even if your current flexplate has the dual pattern holes make sure you inspect it for cracking around the holes where it bolts up to the crank and check the teeth for wear before you use it. I have used the higher stall T350 Chevy converters with the 455 flexplate. As mentioned the wider flange interferes with the balance weights but I ground off just enough weight to clear it and it worked fine with no extra vibrations. Just my way of thinking but there is no way I'd run a stock 13" T400 behind any kind of engine that I wanted for even the mildest performance use unless I was towing with it. The weight alone is not worth it.
