Learn me some torque converter stuff.

So I know how they work  and I know that a high stall converter helps you launch with the engine being allowed to reach the torque band right away.

But what are the trade-offs and how do you pick a converter?

Context:

The Molvo is getting a C4 auto box. I have a transmission with a stock converter. Mrs NOHOME wants to drive the car so it needs an auto box. Well see...not selling the 5 speed and bits.

The engine does have a mild cam. Should I just send it with the stock 12" ish converter or take the moment to install something? else. 

Pete

I don't know if you'll find your answer here, but here's the record holder for quickest and fastest Challenge drag pass holding forth on torque converter science.

Cam specs, compression ratio, vehicle weight, final drive ratio, and head casting numbers will really help us nail it.

You have the basics.

More cam = higher stall
More weight = higher stall
More compression = slightly less stall
Higher numeric final drive = slightly less stall
Bigger head ports = less velocity = potentially  higher stall.

Above all, you need to get a good one, and I'm not well-versed in what brands are a good bet these days.  I've been out of the game too long.  A good converter will act much like a stocker until you wail on it.  It will still creep forward when your foot comes off the brake, and it will still "feel" the same in daily driving.  A cheap converter will act almost like the transmission is fried UNTIL you reach the stall.  There was a guy who drove under my window every day in Los Angeles with a rumpity cam, and he cheaped out on the converter.  It stalled at about 3200, so every time he went through my intersection at 3200 rpm just to get the car to move.

I'm going to guess you could go for a factory stall if the cam is the only change and it's a mild one.  Any idle lope and you would likely benefit from a higher stall.  Also keep in mind that you're likely well within the range of what was offered from the factory.  There isn't a need to buy a $400 aftermarket converter.  If you're just looking for a few hundred RPM more, don't hesitate to just use a TC from a 4.3L or a smogger 305 application.

Edit to add... stall speeds are a range.  If you put a 3000-stall behind a cammed-up 4.3L V6, it might stall at 2800.  The same converter behind a 502 BBC might stall at 3300.  A competent TC sales rep will steer you in the right direction.

To give more detail on the "stall speeds are a range": If you could clamp the torque converter output shaft and spin the torque converter, you could plot a curve of how much torque is needed to spin the converter at a given RPM. If you plot that curve and your engine's torque curve, the stall speed would be where those two curves cross. So a mod that gives more low end torque can push the stall speed up (a stroker crank, for example), while a mod that reduces low RPM torque (such as a more aggressive cam) can cause the stall speed to drop.

In reply to MadScientistMatt :

^ Wise words.

Well that is all clear as mud until I digest it all. But the immediate feel is that a 2700lb car with a 3:31 gear on 24" tires is not going to care all that much if I go from 1600 to 2200 stall. Keep $500 in my war chest.

The LAST thing I want is the "high stall lurch" from the drag-racing days where loading my friends 9 second drag car on the trailer was an exercise in not parking it in the bed of the pickup.

Thanks.

Pete

I had to go down the stall converter rabbit hole with my 302/C4 project.

What simplified it for me is learning that I don't want a stall speed higher than the car's cruising speed on the highway. Higher = slip, which = ruined transmission. High stalls are for drag racing, not for street duty.

ddavidv said:

I had to go down the stall converter rabbit hole with my 302/C4 project.

What simplified it for me is learning that I don't want a stall speed higher than the car's cruising speed on the highway. Higher = slip, which = ruined transmission. High stalls are for drag racing, not for street duty.

That is something I can relate to. Tells me I don't really want to go above 2000 to 2200 if I get a new converter. I realize that the lock up rpm is a funtion of torque, so leave some leaway in this matter.

Probably going to leave the 12' converter in there and see how it works. 

What is the C4 and converter out of? Unless it's from a Pinto the Molvo is probably lighter and will make the converter stall a bit lower. I'd be looking for about a 2000 rpm converter in it - maybe a 6 cyl Mustang one would stall around there behind a 302.

I've got an RV 440 in my Plymouth wagon with the stock RV converter and it's a dog off the line

ddavidv said:

I had to go down the stall converter rabbit hole with my 302/C4 project.

What simplified it for me is learning that I don't want a stall speed higher than the car's cruising speed on the highway. Higher = slip, which = ruined transmission. High stalls are for drag racing, not for street duty.

With a non-lockup converter a higher stall converter will always slip more in any given condition.  But there's no magic case of "it's 2200 rpm stall and the engine is over 2200 so there's no slip".  The only thing 2200 rpm stall means is that it will allow the engine to reach 2200 rpm at WOT with 0 output shaft speed.  This also isn't the kind of slip that kills transmissions, as it's not clutches slipping.  It does make more heat though, so a higher stall converter will need more trans cooling. And no matter what the stall speed is, the converter is never not slipping. 

In general, a too-loose converter sucks on the street, at least with a trans that doesn't have a converter lockup clutch, as it feels mushy, makes more heat, and will lead to more RPM variation with throttle change when cruising at a steady speed.  But there's not a magic "fine vs not fine" transition point. 

Can confirm cheap converters, or at least some of them, suck. I have some no name converter a shop put in when the transmission was rebuilt under the previous owner in my Suburban 2500 with the big block. It's about a 2400 rpm stall which gets it off the line pretty well even with 3.73s and the tall 4L85 first gear, but driving around down it's got a pretty bad rubber band effect since the converter isn't really transferring torque until over 2000 rpm. Around town it just sits at 2000-2200 revs and acts like a CVT, all while generating heat and absorbing torque. It's like I gain 20 hp when the converter locks as I'm getting on the highway  or whatever. Annoying.

My old GMT400 Silverado has a stall speed only a few hundred RPM lower but it is so, so much more responsive to drive when it isn't locked up and doesn't have the bump in power feeling when it does lock up.

Racingsnake said:

What is the C4 and converter out of? Unless it's from a Pinto the Molvo is probably lighter and will make the converter stall a bit lower. I'd be looking for about a 2000 rpm converter in it - maybe a 6 cyl Mustang one would stall around there behind a 302.

Depends what year we're talking. My six cylinder converter was completely different than the V8 one. The six is smaller diameter, and has the ring gear welded onto it. The V8 uses a toothed flex plate. And different starters.

If anyone wants/needs a six cylinder TC, bellhousing and starter for a 200/250 six, come pick them up for free. 

gearheadE30 said:

Can confirm cheap converters, or at least some of them, suck. I have some no name converter a shop put in when the transmission was rebuilt under the previous owner in my Suburban 2500 with the big block. It's about a 2400 rpm stall which gets it off the line pretty well even with 3.73s and the tall 4L85 first gear, but driving around down it's got a pretty bad rubber band effect since the converter isn't really transferring torque until over 2000 rpm. Around town it just sits at 2000-2200 revs and acts like a CVT, all while generating heat and absorbing torque. It's like I gain 20 hp when the converter locks as I'm getting on the highway  or whatever. Annoying.

My old GMT400 Silverado has a stall speed only a few hundred RPM lower but it is so, so much more responsive to drive when it isn't locked up and doesn't have the bump in power feeling when it does lock up.

 

That's also a good point.  Different converters will have different behavior and driving characteristics, even at the same stall speed.