[mrmark1957]

Can a THM 350 torque converter be substituted for a THM 400 converter to slightly increase stall speed ?

[Schurkey]

Yes.

Not much difference in stall speeds, at least on the few I've switched. I think the bigger difference is the lighter rotating weight of the inch-smaller 350 converter.

Really, not much difference at all. Consider getting a "real" converter.

[Half-Inch Stud]

Yez, 2-speed or TH 350 converter are an advantage due to light weight, tightness and higher stall behind a 455.

[mrmark1957]

How much rpm difference, 200 to 300 rpm ?

[25stevem]

It all hinges on how much off idle torque your motor makes.

One thing I would sure as heck not due for long is place a stock TB 350 converter behind a good running 455, on less a strong running one.

[Half-Inch Stud]

Yea, i don't know how or why but i suspect the ST-300 and TH350 converters may be weaker inside;

i suspect the Stator-sprag spline fit may be prone to strip-out due to exposure to extreme cold and hot cycles. Leading to pulverized aluminum bit trash in the fluid and a low stall pull away.

[1969GiPper]

Originally Posted by Schurkey

Yes.

Not much difference in stall speeds, at least on the few I've switched. I think the bigger difference is the lighter rotating weight of the inch-smaller 350 converter.

Really, not much difference at all. Consider getting a "real" converter.

Out of curiosity I weighed my GM L88 six lug THM400 converter and it was about 38 pounds. Probably about half full of fluid.

[PAUL K]

Originally Posted by mrmark1957

How much rpm difference, 200 to 300 rpm ?

Exactly..... Probably closer to 200. But with slicks and and exceptional torque maybe 300.

[Sirrotica]

Originally Posted by mrmark1957

How much rpm difference, 200 to 300 rpm ?

Yes, 2-300 RPM, I had swapped one in my 73 T/A behind a pretty much stock 455. Because the car had 3.08 axle in it, that little bit helped get it moving on the street, or autocross course. I used it because I didn't want to experience the slippage of a "real convertor" on my street car. It was also free, because I already had a used 350 convertor sitting in my garage.

A guy that worked for me, tried a Vega 350 convertor, in his GTO, really disappointing for street use. That's why I went with the full size 350, there was way too much slippage with the Vega convertor to suit me.

Back in the day, the Vega 350 convertor was the poor mans high stall convertor. Car magazines were the source of the information, but it was poor information in my experience.

[PAUL K]

Originally Posted by Sirrotica

A guy that worked for me, tried a Vega 350 convertor, in his GTO, really disappointing for street use. That's why I went with the full size 350, there was way too much slippage with the Vega convertor to suit me.

Back in the day, the Vega 350 convertor was the poor mans high stall convertor. Car magazines were the source of the information, but it was poor information in my experience.

Those Vega converters turned a good GM turbo trans into the same slush box transmission the Ford Granada used.

By chance you ever run your 73 down the drag strip?

[Sirrotica]

Originally Posted by PAUL K

Those Vega converters turned a good GM turbo trans into the same slush box transmission the Ford Granada used.

By chance you ever run your 73 down the drag strip?

No, Paul, I never did, I like turning corners lots more than racing in a straight line. I never got much thrill from drag racing comparatively.

I did however participate in a few impromptu acceleration contests on the public thoroughfares when the someone thought that their brand X was quicker than my Pontiac. I remember a SBC in a side piped corvette that got his ass handed to him, he was nice enough to acknowledge that my car ran well, after his loss..... In my experience, that's not your average corvette owner though.

Background on the T/A, 455 HO heads, stock compression ratio, 8.2 to 1. Just a bone stock 455 block, and pistons. Hooker Competition Plus round port headers, and a 268 single pattern Comp Hi Energy cam, 1.65 rocker arms, a pretty mild engine.

Factory transverse muffler and 2 1/4 pipes (not the best choice as far as restriction). As I already posted 3.08 axle, TH 400, with the 12 inch 350 convertor. Many of the Herb Adams low buck mods made to the chassis. L60 X 15 radials on the rear on American Racing 200S 15 X 8 wheels on the rear, G60 X 15 radials on the front, on the same American Racing wheels. Very well suited to spirited cruising, and occasional auto crossing. My signature black, with gold accents color combination, with a custom metalflake bird painted on the hood. Timeline, 1980.

[Schurkey]

Originally Posted by mrmark1957

How much rpm difference, 200 to 300 rpm ?

Originally Posted by PAUL K

Exactly..... Probably closer to 200. But with slicks and and exceptional torque maybe 300.

Originally Posted by Sirrotica

Yes, 2-300 RPM

I'm surprised it's that much. I'd have figured 100--150 as a guess. Of course, this depends on the engine's powerband, too; and mine were behind Chevy 350s or in a boat. In fact, I've never owned a true "400" converter in anything except for a switch-pitch converter in a Toronado. So I have nothing to compare against.

Originally Posted by Sirrotica

It was also free, because I already had a used 350 convertor sitting in my garage.

That's how it worked for me. I bought core transmissions, and got a core 350 converter for 'em. Had the converters flushed 'n' checked at the trans shop; rebuilt the transmissions myself. Total low-buck deals.

Originally Posted by Half-Inch Stud

Yea, i don't know how or why but i suspect the ST-300 and TH350 converters may be weaker inside;

i suspect the Stator-sprag spline fit may be prone to strip-out due to exposure to extreme cold and hot cycles. Leading to pulverized aluminum bit trash in the fluid and a low stall pull away.

I'm NOT a converter expert. The only splines that I've been warned about were on the switch-pitch units.

I just assumed that the 350 converters were essentially identical to the 400 converters except for the inch difference in diameter; in that some engine families got converters with flat brackets with a bolt hole, (Chevy) while some got welded-on oblong "nuts" that were threaded, to clear balance weights on the flexplate. Similarly, it seemed to me that some converters had roller thrust bearings while some had thrust washers. The thrust bearings might have been random, maybe a model-year change, maybe they were "big block" versus "small block", or maybe they were "Chevy" versus the more-expensive makes in the GM line-up.

Never gave any thought at all to the guts of a 350 converter being weaker than a 400, aside from the thrust washers/thrust bearings.

It'd be fun to spend a week at a converter specialty-shop, seeing how they do things and how the converters are built.

[mrmark1957]

I put a turbo 350 trans and a stock 350 converter behind a 502 Chevy crate engine. After five years of full throttle the converter deformed and the flex plate would rub the starter gear with any press of the throttle. I replaced it with a COAN 10" which has been trouble free. I dont think a stock Pontiac 400 motor would stress it too much, just want a little more converter from a stop without getting one with too much slip.

[PAUL K]

Originally Posted by mrmark1957

I put a turbo 350 trans and a stock 350 converter behind a 502 Chevy crate engine. After five years of full throttle the converter deformed and the flex plate would rub the starter gear with any press of the throttle. I replaced it with a COAN 10" which has been trouble free. I dont think a stock Pontiac 400 motor would stress it too much, just want a little more converter from a stop without getting one with too much slip.

How much stall did you get with that combination?

Stock TH350 with a factory converter.... the converter isn't the weak link.

[Half-Inch Stud]

I think most every manual fly-clutch-pressure plate deal will be rotating more METAL mass than the big converters.

Quote ""...my 6-lug conv weighted 38 Lbs Half-fulla fluid""

Converter stalls and tightness: just figure that GM et al, does have a method to match converters to the vehicle weight, engine and gearing.

Aftermarket Converter shops barely get that right. ECM must have found the real matching method because those ECM cars accelerate thru the mid-track like none others.

[Jay S]

I had a stock big car t400 with the bigger factory diameter converter (13”+)that stalled to 1600-1700 behind a 455. When we switched to a t350 stall the same engine pulled the stall to 2200. If you have the slightly smaller diameter converter(12”??) like was used in performance engines (Ram air GTO’s ect)with t400s that stall seems to be 1900-2000. Very little change from a t350 converter. The bolt pattern to the flywheel is smaller than the big converters..

[mrmark1957]

Originally Posted by PAUL K

How much stall did you get with that combination?

Stock TH350 with a factory converter.... the converter isn't the weak link.

I couldn't give you an exact number, as soon as the engine was against the brakes, it would overpower the rear brakes and the wheels would spin. It was low, maybe 2000-2200, car would move right off idle. The 10" converter in it now goes to 4000 rpm with any kind of throttle. I want to give this mild 400 Pontiac a little more help off the line in this heavy '73 LeMans without going to a high perf/high stall/expensive converter, being I have a 350 converter right here.

[Half-Inch Stud]

Fun fact: my K-B switch-pitch with 1600/4000 stalls ran the same 1/4 mile ET and MPH in either setting or timed at the 60-foot.

So left it in low-stall for best grip. 469 CID into 2.54:1 and 3850 Lb.