# TH400 Converter



## jmurtap (Aug 11, 2011)

I'm running a 350 with a high rise cam, intake, and headers in front of a TH400. In a Leman's Sport Convertible, I'm not sure what size the cam is, but I'm wondering what stall speed converter would be best for performance. I'm not planing on racing just want to lay some rubber with ease. Any suggestions? It's got the high ratio rear end gears in it. I will probably will get bigger wheels to drop the ratio down.


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## Eric Animal (Oct 28, 2007)

To properly pick a TQ converter, you need to know: cam specs (powerband), rear gear ratio, and tire HEIGHT....these are the 3 most important pieces of info to be supplied to the converter builder.....Eric:cheers


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## Rukee (Feb 8, 2007)

:agree gotta have all your facts straight to make the right choice.


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## jmurtap (Aug 11, 2011)

Well I'm going to have to guess, I don't know how to measure the cam an I'm not going to pull the motor apart to find out, it was in the car when I bought it.. The gear ratio is 2.78 as it sits now it has 175/65 R-14 tires.


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## Rukee (Feb 8, 2007)

I wonder if you could put a dial indicator on the rocker arm and a degree wheel on the crank to measure intake and exhaust lift and duration??
Maybe Bear knows.


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## jmurtap (Aug 11, 2011)

Whats the equation??


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## Rukee (Feb 8, 2007)

no equation, just a matter of setting up the dial indicator on the top of the valve, set it to zero when closed, set up the degree wheel and pointer on the ballancer, turn the motor over till the valve just just starts moving and stop, reset the degree wheel pointer to zero, turn the crank till total valve lift is achieved stop and record dial indicator results, continue turning crank till the valve closes then record the amount of degree's the valve stays open. Just guessing, but because the crank turns 2 to 1 with the cam, you would cut the degrees by 1/2? Not sure how to compinsate for hydrolic lifters though if you would have to. Do it for both intake and exhaust. You might be able to rent the tools if you don't have them.
I'm thinking that would be correct, if not maybe someone can tell us how.


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## BearGFR (Aug 25, 2008)

Rukee said:


> I wonder if you could put a dial indicator on the rocker arm and a degree wheel on the crank to measure intake and exhaust lift and duration??
> Maybe Bear knows.


That'd get you very close. Since I'm assuming this is a hydraulic lifter motor you'd also want to do something to take the lifter plungers "out of the picture" - like perhaps removing the rocker arm completely, using a very long rod on the dial indicator and taking the readings directly fromt he top of the lifter. Otherwise the pressure of the valve springs will most definitely depress the lifter plungers a little as the valve starts to open and it will throw off your measurements.

jmurtap, the reason we'er asking about the cam is because knowing where the engine's power-band is matters a great deal when choosling a converter, and the cam profile is the main factor that determines the power band. As an alternative, you could just go spend some time (and money) running your car on a chassis-dyno and find out where it is. To achieve your stated goals you'd want to find out at what rpm your motor makes the most torque, then choose a converter that will allow the motor to hit that rpm "almost instantly" when you jump onto the loud pedal.

Also, if you're wanting a "lower" (numerically "higher") effective gear ratio with a tire change, then you want to go with smaller diameter tires than what you have now, not larger. Putting on tires that are larger in diameter than what you have now will have exactly the opposite effect of what you're looking for.

Bear


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## jmurtap (Aug 11, 2011)

Thanks Guys, what does a stock TH400 come with? On the tires that what I though, my buddy was convinced it was the other way, I didn't think it made sense but I've never dug this deep in a car before, and he has built one.. So I didn't argue. My dad and I didn't know anything about converters when we did the transmition many years ago we just put one on, It ran great, but I don't know what it was it took a a min before it would start to power stall. I'm rebuilding the tranny again, and I want to do things right this time.. The motor is still very strong and just didn't want to tear into it at all, but I guess a rocker arm isn't a huge deal.


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## facn8me (Jul 30, 2011)

Is this a daily driver? If so anything over 24-2600 and you will learn to hate driving it. But you will be able to watch the gas gauge drop so that will be fun.


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## BearGFR (Aug 25, 2008)

jmurtap said:


> Thanks Guys, what does a stock TH400 come with? On the tires that what I though, my buddy was convinced it was the other way, I didn't think it made sense but I've never dug this deep in a car before, and he has built one.. So I didn't argue. My dad and I didn't know anything about converters when we did the transmition many years ago we just put one on, It ran great, but I don't know what it was it took a a min before it would start to power stall. I'm rebuilding the tranny again, and I want to do things right this time.. The motor is still very strong and just didn't want to tear into it at all, but I guess a rocker arm isn't a huge deal.


Converter stall depends on two things: the converter, and engine torque. Put the same converter behind a little v6 and also behind a 455 and you're going to get two different stall speeds. The 455 makes enough torque to "force" the issue, the v6 doesn't. So.... the converter stall has more to do with which engine it's behind than which transmission it's in front of. GM used many different converters in TH400's, depending on the engine and application.

A more appropriate question for you is what rpm does -your- converter stall to for -your- engine now, and where do you want it to be?

Also agree that the higher a converter stalls to for a particular combination, the less pleasant it tends to be to drive on the street. You also have to be a little careful if you plan on any highway miles --- you want to make sure that your converter stall rpm is _less than_ your normal highway cruise rpm, otherwise the converter is going to be "slipping" all the time - building heat and generally adding stress to everything.

Bear


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## geeteeohguy (Feb 2, 2008)

When I did the TH400 in my '67 convertible back in March, I went with a GM HS converter which has a stall of about 1800-2000 rpm. I'm running a lazy 2.56 rear gear, and it really helped "out of the hole" and it does not slip at cruising speed, which is a bit over 2000 rpm. Really high rpm stall rated converters are best at the track, but as the other gents said, they can be a headache on a street driven car.


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## Eric Animal (Oct 28, 2007)

i probably shouldn't mention this, but...you could put a 200r4 tranny in it and get a lower first gear...and an Overdrive too....:cheers


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## jmurtap (Aug 11, 2011)

facn8me said:


> Is this a daily driver? If so anything over 24-2600 and you will learn to hate driving it. But you will be able to watch the gas gauge drop so that will be fun.


It will be more of a sunday driver, I want it to be mean enough to satisfy me, but still easy enough that my wife can drive it. (occasionally) lol


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## jmurtap (Aug 11, 2011)

BearGFR said:


> Converter stall depends on two things: the converter, and engine torque. Put the same converter behind a little v6 and also behind a 455 and you're going to get two different stall speeds. The 455 makes enough torque to "force" the issue, the v6 doesn't. So.... the converter stall has more to do with which engine it's behind than which transmission it's in front of. GM used many different converters in TH400's, depending on the engine and application.
> 
> A more appropriate question for you is what rpm does -your- converter stall to for -your- engine now, and where do you want it to be?
> 
> ...


I guess I never payed attention to what rpm, Its been so long since I've drove it can't remember if it has a tach in it. I can tell you it was pulling at a stop.. Dang its at my buddys shop or I'd go look..


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## BearGFR (Aug 25, 2008)

Instead of just throwing random parts (and money) at it, if I wanted to follow some sort of logical process here's what I'd do:
1) Be honest and decide what I want the car to be able to do and how it's going to be driven (i.e. street, highway, race, etc)
2) Take it up and run it on a chassis dyno to find out where the motor's torque peak is, and also what the general torque curve looks like.
3) Pick a converter that's going to put the motor at/near its torque peak, taking into consideration how it's going to be driven "most of the time".

(Hint: any honest vendor who's selling you a converter is going to also want to know something about your engine's torque/rpm/usage. If they try to sell you a converter advertised for a specific stall rpm without asking you any of that information, run away.)

Bear


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## jmurtap (Aug 11, 2011)

In theory of I had a 1500 stall and that's were it actually stalled at, would that mean that the car wouldn't move until the motor reached 1500 rpm?


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## BearGFR (Aug 25, 2008)

jmurtap said:


> In theory of I had a 1500 stall and that's were it actually stalled at, would that mean that the car wouldn't move until the motor reached 1500 rpm?


No, not exactly. What it means is that if you were to put the car in gear, hold your foot on the brake so it couldn't move, then stand on the gas - the converter wouldn't allow your engine to rev any higher than 1500 rpm. 

This is why the same converter will have different stall speeds depending on which engine it's behind. A big ol' meaty 455, making much more torque, would be able to "overpower" the very same converter and reach a higher rpm. In both cases though, without the brakes applied to hold the car back the converter will begin to move the car immediately. You don't have to wait for "stall speed" before things start to move.

Why it matters: for drag racing, let's say, at the instant the car launches you want your engine to be able immediately to get up into the rpm range where it's making lots of torque. So, ror this application you choose a converter such that _with your particular engine_ it allows the motor to "flash" up into it's power band rpm range almost immediately as soon as you hit it.

Bear


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## jmurtap (Aug 11, 2011)

Thanks Bear ( and everyone) for taking your time to explain these things to me. I think I have a good understanding now of how the thing works. I'm glad the forum exist and there are knowledgeable guys who use it and help the new guys out!
Thank You!!!


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## pontiac (Mar 6, 2011)

shift kit in trans will get your tires to chirp, a lot easier that the convertor, when you will not see the gain for the money.


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## jmurtap (Aug 11, 2011)

Yep going to put a kit in as well, But I have to buy a new converter anyways, just had to get an idea of what I was buying and how it might affect performance.


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## jmurtap (Aug 11, 2011)

Project has been driven, it a rocket ship in the gas, however takes more than expected (rpms) to get moving.. lol overall I'm happy with it


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