Stall Converter Trans Brake Powerglide and launch rpm

[jreiley]

I'm confused about stall speed, etc. Given the above, what rpm do I want the engine to be at when I launch with or without the transbrake. Thanks.

[TheRabbit]

You want to launch "flash" your convertor 200 rpms over your peak torque on foot or t-brake. This alone is a good reason to put a motor on a dyno.

[jreiley]

I wish I could do that. No dyno's anywhere near me.

[dparker]

Need more engine info then......

[jreiley]

Ok. 509 Dart BBC, 871 Blower - 10lb boost - CompCams blower grind solid roller, Profiler heads, Advanced Transmission powerglide, new converter but I don't know what the stall speed is. I can find out easy enough, I just haven't asked yet. The old one on the non-blown engine was 3800.

[TheRabbit]

Hey guys when he threw the blower in I don't have a clue what to suggest other than they makes tons of low end torque but I'm lost on this one. Get the cam specs and convertor specs I could help a little more.

[hammertime]

Ok. 509 Dart BBC, 871 Blower - 10lb boost - CompCams blower grind solid roller, Profiler heads, Advanced Transmission powerglide, new converter but I don't know what the stall speed is. I can find out easy enough, I just haven't asked yet. The old one on the non-blown engine was 3800.

Stall speed is not something you would ask anyone its something you would find out one of these ways.

1. Have a playback tach, and when you launch the car takes off and the converter flashes, this point is called the flash stall. Also when you shift from 1-2 and the rpms drop this is called the stall recovery.

2. You can check it by pulling the launch chip and putting it up on the transbrake and see what rpms the converter goes to. This result is not the same as the track will be though.

[jreiley]

I am getting more and more confused. What does a stall converter do that a regular torque converter doesn't? And what is flash?

[FullTimeRacing]

SMASH on the brake,then start giving it gas,watch the tach,when the car tries to move remember that RPM.Then tell us :P

[TheRabbit]

I don't mean to pick or be rude, but I do have a ??. This seams to be a pretty big motor for somebody that doesn't know much about stall convertors. PLEASE don't take that wrong. I'm like everybody else and trying to figure out all the details so we can help. You're just a long ways into this to be asking this question. Again not trying to pick just trying to figure it out.

[hammertime]

What a Converter Does
Simply put, a torque converter is a hydraulic coupler between the engine and the transmission. It changes mechanical torque (engine torque) into hydraulic pressure before sending it back to the transmission. The converter also multiplies the torque at low speed or during periods of high engine load.

The insides of a converter consist of an impeller, a stator, and a turbine, all surrounded by transmission fluid. The impeller rotates at engine crank speed, acting as a fluid pump. The turbine is the output device hooked to the transmission input shaft. The stator sits between the two, acting as a torque multiplier when impeller speed exceeds turbine speed. When the converter reaches its stall, or lockup speed, the stator stops multiplying torque and the converter essentially acts as a fluid coupling. When the vehicle is coasting (no load), the converter directs torque back towards the engine, acting as a brake.


What is Stall Speed, Anyway?
The most misunderstood aspect of torque converters is stall speed. Many people think if a converter is rated at 2,500 rpm, their car will rev up to that rpm and then take off. That’s not how it works. Stall speed is a function of engine rpm. The more torque an engine makes, the higher the rpm the converter will stall, or lock up at, and transfer that torque to the transmission.

There are two types of stall speed—foot brake stall and flash stall. Foot brake stall (or true stall) is the maximum engine rpm achieved from a complete stop with the transmission in gear, the brakes apllie,d and the engine at full throttle. The rpm reached just before the vehicle begins to move forward is the true stall speed of the converter.

The problem with foot brake stall is that you will end up overpowering the brakes and suspension before you reach the converter’s stall speed. The only way to really measure true stall is by using a trans-brake. This will keep the vehicle from moving, allowing the converter to absorb 100 percent of the engine’s torque. Race classes that do not allow trans-brakes are often called foot-brake classes. In this type of racing, the rpm obtained when the brakes are applied and the vehicle is not moving is considered to be foot brake stall. When the brakes are released, the engine goes to full throttle and “flashes” the converter.


This brings us to flash stall. It is the maximum engine rpm reached when you do a full-throttle launch with the transmission in low gear and no brakes applied. Flash stall is always lower than foot (true) stall because there is less load on the converter. Changing the load on the converter can change the flash stall rating. Additional engine torque, a higher(numerically lower) rear axle gear, or adding vehicle weight will increase flash stall. Less torque, a lower (numerically higher) gear, and less weight will decrease flash stall.


Camshaft selection is also critical to torque converter selection. On the street, many people will choose a cam that will put an engine’s rpm range 1,500 to 2,000 rpm higher than stock. Not only does that reduce bottom end torque, a higher stall converter will be required to match the new torque peak. Many people will get the recommended converter, but neglect to upgrade the rear axle gear to compliment the higher stall speed (more on gear ratios and tire sizes in a minute).

Say you built a small block V8 with a 235 degree (at .050)/.488 inch lift cam and added a converter rated at 3,000 to 3,500 rpm. To make the combination work properly with a minimum of converter slippage, you will need a 4.10 or higher rear axle gear with 26 to 27 inch tall tires. Illustration One shows you approximate stall speeds based on engine type, cam duration, and rear axle gear ratio.

Nitrous oxide and superchargers also affect converter selection. An engine with a power adder produces more torque than it would if it was normally aspirated. That means a nitrous or blown engine needs a converter with a lower stall speed range. Otherwise, the converter will stall too high, causing it to slip and eventually self-destruct due to the extra heat.

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[bbchevy]

Alls i can say,is with a Supercharger?You will want a Tight Converter!
No More than say 3000?That windmill on top of the INTAKE is the Torque Multiplier!!!
I always used a Munsinger 10"as Tight as they could make them 2500-3000.I would leave off of a Dead Idle,and Flat FOOT the Pedal,8000 as Fast as the Car would move.........?
Although i have heard of converter companies telling you to go with a Loose Converter,maybe if you have a WEEK-SUCK Motor???
Later
G 8)

[hammertime]

I dunno about supercharger but with a NA and bracket racing deal loose is the best and most consistent converter you will find.

[jreiley]

Thanks for all the info. It is verrry helpfull. FWIW, I quit racing 40 years ago when I was in the military and just couldn't do it. Now I have the time, a little more money, and a lot of desire. When I was doing this stuff as a kid, most of the stuff people use, i.e. stall converters, didn't exist. Tnanks again.

[TheRabbit]

Gotcha now. Sounds like you building a nice car. I quit for about 12 or 13 years and started back about 3 years ago. Boy had things changed. When I raced it was a 350, 454 or 460 with a roller cam and a little work done to the heads. Costing about 7k to build. Now everybody in mud racing has a 406+ small block or a 600+ big block average cost is probably more than 25k.

All I can say about a blower and a tight convertor is most blower guys (we have about 20) leave the line off a clutch at 7,000 and turn 'em to 9,500. Then again we only have 1 class that allows blowers and it's basically our Top Fuel class. Several of the guys are good friends of mine I just don't know squat about their cars.
Take a ride on Mud Missile @ http://www.probadd.com/videos.html
Pro Badd. ( B lown A lcohol D irt Dragsters.)
Check out the home page too. These are the fastest guys on dirt in the US.

[jreiley]

Thanks. I'm trying to build a nice and reliable car. As you can tell by my questions, I'm not totally knowledgeable, but I try. It seems that there are more "gotchas" in building this car than I ever imagined.

[TheRabbit]

As you can tell, a bunch of good guys on this forum and can probably help solve any questions you have. Good luck with the car. Let us now how it does when you get everything done!!

[TheRabbit]

Hey Reiley you know you gotta post some pics of that ride so we can see it!! If it goes fast I like it!!!