Learn me about torque convertors

Got the 700r4 out of the caprice and found out its about the worst 700r4 ever, 27 spline and an unmarked case.

My plan is to get a 30 spline from the junkyard but im wondering about the torque converter now. The engine it's gonna be behind is a stock 305. When I'm done it'll probably have some basic bolt on mods plus at max 125 hp shot of nitrious.

The stock torque converter has a stall speed of 1397 rpm according to the tag.....that seems low. Am I wrong ? I know I can't use the stock converter but I am using it as a guideline.

Well, if you increase the torque the stall speed will go up too, but that rpm sounds about right for a stock engine that makes 80% of its peak torque at that rpm and runs out of breath at 4000 rpm anyway.

There's a fair amount of voodoo in a torque converter, the stall speed is relative to the engine's torque but that also depends on the internal characteristics of the converter, some are really loose below stall speed and then get "hard" while others are a little more sleazy-sloppy, and there's a science to which type you want but, to be honest, I let wiser heads than mine figure that out. But a well matched converter is like magic.

A good way to cheat would be to find an OE engine with a similar torque curve and use its converter. Our trans shop has a big book of OE converters and the array of OE converters available is mind-boggling. I wonder if something from a 4.8l truck would be good for a mild 305. Similar displacement but near 300hp.

Rule of thumb is try too match your converter too your powerband and intended use. Most aftermarket converter companys are pretty good about helping you find the right size and stall speed.

The weight of the car and the gear ratio also come into play.

We use to run a V6 convertor in a C4 behind a small Windsor in a Courier. :^)

Stall speed looks about right for a stocker, but its rare that they put such a specific stall on a converter. That converter behind a V6 might stall at 1100, but put it behind a 454 and it might stall at 2200.

I would say that a good ballpark for you would be a good quality reman stocker. You won't be hitting the nitrous below stall speed anyway, so it won't factor into your stall needs. Stock 305, stock cam, stock converter. Done.

Knurled wrote: There's a fair amount of voodoo in a torque converter, the stall speed is relative to the engine's torque but that also depends on the internal characteristics of the converter, some are really loose below stall speed and then get "hard" while others are a little more sleazy-sloppy, and there's a science to which type you want but, to be honest, I let wiser heads than mine figure that out. But a well matched converter is like magic. A good way to cheat would be to find an OE engine with a similar torque curve and use its converter. Our trans shop has a big book of OE converters and the array of OE converters available is mind-boggling. I wonder if something from a 4.8l truck would be good for a mild 305. Similar displacement but near 300hp.

Pretty much where I'm at on the voodoo lol

curtis73 wrote: Stall speed looks about right for a stocker, but its rare that they put such a specific stall on a converter. That converter behind a V6 might stall at 1100, but put it behind a 454 and it might stall at 2200. I would say that a good ballpark for you would be a good quality reman stocker. You won't be hitting the nitrous below stall speed anyway, so it won't factor into your stall needs. Stock 305, stock cam, stock converter. Done.

I'm just going off what the stock tag says according to what I've figured out online.

I'm hoping to pull it out of another caprice but it looks like the yard has an excess of vans. For budget purposes I was planning on grabbing the stock converter too, should I stay away from a van with a 305?

Since you're power limited and actually trying to go sorta fast (otherwise why shoot nitrous) i would get the loosest stock converter you can find unless you don't plan to run sticky tires and want to street drive it all the time. Your goals aren't really clear to me.

Vigo wrote: Since you're power limited and actually trying to go sorta fast (otherwise why shoot nitrous) i would get the loosest stock converter you can find unless you don't plan to run sticky tires and want to street drive it all the time. Your goals aren't really clear to me.

It'll be a challenge car but since I am 2700 miles one way from Gainesville I need it to be decently fuel efficient on the trip down there, but still do ok in the challenge. Its sort of an odd build I guess

Antihero wrote:

Vigo wrote: Since you're power limited and actually trying to go sorta fast (otherwise why shoot nitrous) i would get the loosest stock converter you can find unless you don't plan to run sticky tires and want to street drive it all the time. Your goals aren't really clear to me.

It'll be a challenge car but since I am 2700 miles one way from Gainesville I need it to be decently fuel efficient on the trip down there, but still do ok in the challenge. Its sort of an odd build I guess

Assuming you're running that 700R4 in the challenge car, you should have a lockup converter. So a looser converter will only hurt your mpg a bit in the city, it won't matter on the highway (it'll be cruising with the converter locked).

Loosest stock converter was behind the 350 powered Camaros. Its rated at somewhere around 2300rpm, I think. Certainly available as a rebuilt unit.

I was told to think of it as riding a trials bike where slipping the clutch to keep the engine in the power-band is the norm.

Where that power-band is (unique to every build), what the gear ratios are in the transmission, how much the car weighs and what final gearing you run all makes a difference.

I do know from way back that driving a street car with a way loose converter (3000 rpm range) is not that much fun. Neither is loading said car on a trailer.

For the Molvo ( Mild 302 with an E-303 cam) I went with the stock 1800 rpm stall, an AOD gearbox and a 3.73 rear gear. Based on what I read in the Mustang forums, I might want a higher stall converter to improve launch, but since NOT RACECAR, plus the fact that I expect to weigh in much lighter than a Mustang, I will try this and see what I think.

Best bet is to find a build siimilar to yours and crib off the results.

Gearing also affects your converter needs. If you've got a short first gear, you can get away with a tighter converter as it'll spin up into the powerband faster, even if isn't there when you're loaded on the converter and just starting to move.

The 700r4 has a pretty low first gear for an auto.the car came with 2.73 rear end, not my first choice but great for the mileage side of things.

I think I'm over thinking this and will just stick with a stock tq

rslifkin wrote: Gearing also affects your converter needs. If you've got a short first gear, you can get away with a tighter converter as it'll spin up into the powerband faster, even if isn't there when you're loaded on the converter and just starting to move.

Unless you're drag racing, in which case the large gear drops mean you want a loose converter so that you don't fall into a torque hole after the shift.

Antihero wrote: The 700r4 has a pretty low first gear for an auto.the car came with 2.73 rear end, not my first choice but great for the mileage side of things. I think I'm over thinking this and will just stick with a stock tq

Sounds like a trip to Bonneville is in the cards.

With that ratio, I'd loosen up the converter some to get some jump out of the hole. Its a lockup, so it won't hurt gas mileage. Check for a rebuilt one- there is a remanufacturer in Saskatoon, so its got to be readily available.

Not sure its as big of a deal with lock up but a loose converter usually generates more heat. Learning that lesson cost me a c4. But that was a 2800 rpm non lock up converter.

Yeah, you really do not want to run a stall speed higher than your cruise RPM. The torque converter generates most of the heat in a trans...

Knurled wrote: Yeah, you really do not want to run a stall speed higher than your cruise RPM. The torque converter generates most of the heat in a trans...

Only matters if it's non-lockup. If it's a lockup converter, you can run a 3200 stall and cruise at 1800 on the highway with no issue.

Higher stall converters do increase the cooling requirements for a trans though.

The trouble with lockups is that they aren't locked much of the time.

The minute you hit the throttle it disengages.

You mean the minute you let off? They will stay engaged at any cruising-related throttle and only disengage when you're trying to accelerate hard, and then they will go to 'coupling' pretty quick which is not where they make the most heat. They are worse when the difference between impeller and turbine speeds is greater.

Streetwiseguy wrote: Loosest stock converter was behind the 350 powered Camaros. Its rated at somewhere around 2300rpm, I think. Certainly available as a rebuilt unit.

well most of ya'll got most of it right, this is the answer I would go with and get a new RE-BUILT converter they can be had for under a hundred bucks. THE BEST MONEY you can spend is to put a bunch of gear in it. 3.42 would be good without making 1st gear useless, I like a 200r4 myself and would use that behind ANY GM powered vehicle.put in a manual lock-up switch and don't worry 'bout Gas, kinda cheap right now. It ain't voodoo.

One last thing I'll add. If you do end up getting a higher stall converter (by that I mean non-stock) get a good one. A cheap, higher-stall TC does a poorer job at propelling you until it reaches close to the stall speed. By that, I mean: if you start out leisurely from a stoplight with a cheap stall converter of (let's say) 2500 rpm, the car will rev to 2400 while you accelerate. With a good stall converter, it will feel like a stock stall leaving a stoplight. It won't "flash" to the actual stall speed unless you give it more throttle.

I didn't explain that well, but does that make sense? There was a guy who drove past my loft every night with a Camaro that had a cam in the 245-degree range. (guessing) I could tell he had a cheap converter because when he left the stoplight the car revved to about 3000 while he went through the intersection.

It just makes a big difference in daily-driving situations where a cheap converter can be frustrating. If I have to go above about 2500 stall, I go straight to B&M. Pricey, but worth it.