What torque convertor do I use?

Maybe some one can help me out here. I’m removing a 340 from my duster and installing a blueprint engines 408 stroker. The car currently has a built 904 trans.What torque convertor do I use? Can I use the current one or do I need a different torque due to it being an externally balanced rotating assembly.

Depends on a number of things.
Is/was your 340 a late '72 or '73 externally balanced engine? Since it's a 904 (not original to 340s in any form) it could be balanced by convertor weights or by the flexplate; which changes the needs for the new engine...
Is your new Blueprint 408 internally or externally balanced? I thought most of Blueprint's engines came through internally balanced...
If your torque convertor and crate engine are both internally balanced, bolt 'em together and go- if not, then even the two external balances would be different (the externally balanced 340 is unique and unlike the LA360 or 5.9 Magnum balance).
@Johnny Mac may chime in and shed some light on BP's balance requirements.

BluePrint Engines 408's are all "LA360" balanced on the REAR.
do not confuse that with the fact that we use the Magnum balance damper on the front.

As @Professor Fate mentioned, 340's are unique in balance.

it'll depend on if your previous 340 was stock balance, or had been rebuilt to be internal balance.

chrysler also balanced the converter, and used neutral flexplates, where most aftermarket companies build neutral converters, and use the flexplate to balance.

I would also doubt that your current converter would have the correct characteristics (Stall speed and such) for the bigger, torquier, cammed, 408. unless that 340 was a really stout piece.

you'll most likely want to call a good converter company like gearstar/yank (330) 434-5216, and have them build you one specific to the car. or...ship them that one and have them re-stall it, and balance it properly.

remember, converter is typically balanced as neutral when going aftermarket. in which case you'll just need an an aftermakret flexplate for a 904, with LA360 balance.

gearstar will set you straight on all that, and want to know your weight, rear gears, cam specs, etc.

thats my recommendation. hope it helps.

Johnny Mac said:

BluePrint Engines 408's are all "LA360" balanced on the REAR.
do not confuse that with the fact that we use the Magnum balance damper on the front.

As @Professor Fate mentioned, 340's are unique in balance.

it'll depend on if your previous 340 was stock balance, or had been rebuilt to be internal balance.

chrysler also balanced the converter, and used neutral flexplates, where most aftermarket companies build neutral converters, and use the flexplate to balance.

I would also doubt that your current converter would have the correct characteristics (Stall speed and such) for the bigger, torquier, cammed, 408. unless that 340 was a really stout piece.

you'll most likely want to call a good converter company like gearstar/yank (330) 434-5216, and have them build you one specific to the car. or...ship them that one and have them re-stall it, and balance it properly.

remember, converter is typically balanced as neutral when going aftermarket. in which case you'll just need an an aftermakret flexplate for a 904, with LA360 balance.

gearstar will set you straight on all that, and want to know your weight, rear gears, cam specs, etc.

thats my recommendation. hope it helps.

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Ok thanks guys I talked to the salesman and he said it was an externally balanced rotating assembly? The 340 is a 69 block according to the casting.stock other than a slight cam. I bought the car a few years back dont know too much about the engine other than that

Cardinge72 said:

Ok thanks guys I talked to the salesman and he said it was an externally balanced rotating assembly? The 340 is a 69 block according to the casting.stock other than a slight cam. I bought the car a few years back dont know too much about the engine other than that

Click to expand...

correct. "LA360" is external balance. just know that chrysler had more than one "external balance sb's" so "LA360" is important.

if you converter has a weight welded to it...it's not balanced for a 360, since it was behind a 340 originally. If it has no weight, it may be internal, however I still recommend buying a new one, or sending that one off for a re-stall/re-balance

In an application like that I think you'd be happier with a 727

Cardinge72 said:

Ok thanks guys I talked to the salesman and he said it was an externally balanced rotating assembly? The 340 is a 69 block according to the casting.stock other than a slight cam. I bought the car a few years back dont know too much about the engine other than that

Click to expand...

External balance 340s were limited to late '72 and '73 model year engines, if your '69 340 is a stock bottom end, it will be internally balanced.
Therefore, you should be able to bolt your torque convertor up with a B&M 10239 flexplate, which is made for the LA360 balance with a904 convertor. But as @Johnny Mac (who IS the Blueprint rep, btw) said, you would probably be better off with a better, tailored to your needs convertor with the new motor.