Stall Converter Trans Brake Powerglide and launch rpm
07-10-2009, 05:54 PM
#11
Senior Member
RACING JUNKIE
Join Date: May 2003
Location: Coldwater, MI
Posts: 2,998
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.
[/quote]
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.
[/quote]
07-10-2009, 07:01 PM
#12
Senior Member
RACING JUNKIE
Join Date: Mar 2003
Location: Fountain,MICHIGAN
Posts: 856
Stall
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)
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)
07-11-2009, 04:09 AM
#14
Senior Member
MASTER BUILDER
Thread Starter
Join Date: Aug 2007
Location: Iowa - USA
Posts: 218
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.
07-11-2009, 05:06 AM
#15
Senior Member
RACING JUNKIE
Join Date: Jan 2007
Location: Augusta, Ga.
Posts: 2,276
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.
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.
07-11-2009, 05:34 AM
#16
Senior Member
MASTER BUILDER
Thread Starter
Join Date: Aug 2007
Location: Iowa - USA
Posts: 218
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.
