Chevy Nova Forum banner

1 - 20 of 101 Posts

·
Registered
Joined
·
210 Posts
Discussion Starter #1
What is the need for different stall rpms? What does the stall mean? I feel like I should know this but I dont:eek: . Could some one please educate me on torque converters!
 

·
Premium Member
Joined
·
31,194 Posts
Stall RPM is the RPM the engine will get to before the t/converter will engage the trans an the car will begin to go.A 3500 stall will allow you to be on the line holding the brake and bring RPM to 3500 also known as preload without the car moving untill after 3500 RPM.It works like a manual trans rev it up and let the clutch out.The advantage is you can leave the line harder with RPM's up tipically the more gear the more stall can be used,but vehicle weight and max RPM all will predict how much stall you should use as well as a host of other things,but theat is the basics of a stall converter.At low RPM it feels like the trans is slipping untill it reaches it's stall then it feels pretty much normal.Hope that answers your question some of the drag racers here maybe able to explain it better.

Paul
 

·
Registered
Joined
·
9,967 Posts
The 'stall' in stall speed refers to the rpm's at which the motor stalls against the load of the converter and chassis. The same converter will stall the motor at different speeds depending on the applied torque and the load.
Stall speed does not refer to the rpm's you must be going to get the car to move, nor does it refer to some magic happening inside the converter causing it to suddenly work.
These sites have some pretty good explanations of stall speed:

www.converter.com
www.converter.cc
 

·
Premium Member
Joined
·
298 Posts
In simple terms Tax3 is right, it is the rpm the converter must be spinning before it engages the trans. But Goble is right too, this stall will change depending on the weight of the car and the load. If you chained your car to an achor in cement it is the speed that the engine would begin to feel the load of the car and in theory stall if the car didn't move. So it is somewhat dependant on load. But in simple terms if you held the input shaft of a converter in your left hand and the output in your right. Stall speed is the rpms you would need to spin the input shaft in your left hand before the output tried to turn in your right. This is the way I understand it, but I'm no professional.

The following website had the following definition (and a cool exploded view of a torque converter.

http://www.tciauto.com/tech_info/torque_converters_explained.htm

Stall speed --- the rpm that a given torque converter (impeller) has to spin in order for it to overcome a given amount of load and begin moving the turbine. When referring to "how much stall will I get from this torque converter", it means how fast (rpm) must the torque converter spin to generate enough fluid force on the turbine to overcome the resting inertia of the vehicle at wide open throttle. Load originates from two places (1) From the torque imparted on the torque converter by the engine via the crankshaft. (This load varies over rpm, i.e. torque curve, and is directly affected by atmosphere, fuel and engine conditions.) (2) From inertia, the resistance of the vehicle to acceleration, which places a load on the torque converter through the drive train. This can be thought of as how difficult the drive train is to rotate with the vehicle at rest, and is affected by car weight, amount of gear reduction and tire size, ability of tire to stay adhered to ground and stiffness of chassis. (Does the car move as one entity or does it flex so much that not all the weight is transferred during initial motion?)
 

·
Premium Member
Joined
·
31,194 Posts
Mike Goble said:
The 'stall' in stall speed refers to the rpm's at which the motor stalls against the load of the converter and chassis. The same converter will stall the motor at different speeds depending on the applied torque and the load.
Stall speed does not refer to the rpm's you must be going to get the car to move, nor does it refer to some magic happening inside the converter causing it to suddenly work.
These sites have some pretty good explanations of stall speed:

www.converter.com
www.converter.cc
Thanks Mike I just kept it simple wasn't sure how to better define it.
 

·
Premium Member
Joined
·
298 Posts
also relavant from same website listed above....this is probably more of what Goble was refering to.

"While referring to the resistance of the vehicle to move while at rest, the torque converter's stall speed and much of its characteristics for a given application are also affected by the vehicle's resistance to accelerate relative to its rate of acceleration. This resistance has much to do with the rpm observed immediately after the vehicle starts moving, the amount of rpm drop observed during a gear change and the amount of slippage in the torque converter (turbine rpm relative to impeller pump rpm.) A discussion involving how resistance to acceleration affects a torque converter involves more theory than fact and must involve all the dozens of other variables that affect rpm and slippage. The primary thing we want to remember about torque converter stall speed is that a particular torque converter does not have a "preset from the factory" stall speed but rather its unique design will produce a certain range of stall speeds depending on the amount of load the torque converter is exposed to. This load comes from both the torque produced by the engine and the resistance of the vehicle to move from rest. The higher this combined load the higher stall we will observe from a particular torque converter, and conversely, the lower the load, the lower the stall speed. Naturally, if the engine is not at wide open throttle we will not expect to observe as high a stall speed as we would under a wide open throttle."
 

·
Registered
Joined
·
210 Posts
Discussion Starter #7
why then, do some camshaft grinds require a higher stall?
 

·
Registered
Joined
·
9,967 Posts
WhitecatD said:
why then, do some camshaft grinds require a higher stall?
As the duration and overlap of the cam increase the motor will produce less torque at low rpm's. In order to get the engine into the power producing higher rpm range you should install a converter with a higher stall speed. Again, the engine and load determine the stall speed, so what stalls at 3500 rpm behind a torquey 406 may only stall at 2500 behind a high rpm 283. This is why converter manufacturers like to know all about your engine, tranny and chassis.

To reiterate, stall speed has nothing to do with how fast you need to rev the engine to get the car to move. My converter stalls about 3600 rpm in my 406/700R4 setup, yet I can easily achieve freeway speeds without exceeding 2500 rpm's.
 

·
Registered
Joined
·
730 Posts
I kind of get it :D. Take a given car 3000 pounds 283 (200hp,150ft/lbs @ 2500rpm) with a 2500 stall. Lets say the car start to moves at exactly 2500 rpm. Now say theoretically the motor goes to 250hp,250ft/lbs .... what happens ... Does the car move at 2000rpm?


For a driver Im thinking what you don't want to happen is step on the gas .... torque converter engages and motor bogs ... on the other side you don't want to step on the gas .... wait until motor hits 3500 rpm ... and start to move (unless of course you are racing).

arndog
 

·
Premium Member
Joined
·
8,405 Posts
arndog said:
For a driver Im thinking what you don't want to happen is step on the gas .... torque converter engages and motor bogs ... on the other side you don't want to step on the gas .... wait until motor hits 3500 rpm ... and start to move (unless of course you are racing).
I think you are assuming a torque converter is like a centrifugal clutch on a mini bike. It's not.
Here's how they work:
If you take a running desk fan and point it at non running desk fan, the blades will begin to spin.
A torque converter works in much the same way. You have two sets of blades (curved to capture and direct the fluid) pointing at each other. Instead of air as the medium you have thicker transmission fluid. When the engine side (the impeller) turns, it causes the transmission side (the turbine) of the converter to spin also. There isn't a direct connection between the engine and the transmission and this is why the engine can still run while stopped at a light in gear. The torque converter can slip. If you take your foot off the brake the car will creep forward but not like you "let the clutch out" on a manual car. A higher stall speed will still creep but you'll notice the rpm goes higher when you accelerate.

There is a third component called a stator. This fits inbetween the impeller and turbine. It redirects and accelerates the fluid force leaving the impeller. This adds torque multiplication much like a lower gear ratio does.
Stall speed is adjusted by fine tuning the mechanical relationships of the impeller, turbine and stator. Some cheaper converters just cut open the unit, tweak the blades and weld it back up. More expensive converters are often more efficient and reduce power robbing high rpm slippage while maintaining the correct stall speed for the application.

All torque converters slip to some degree even when past the stall speed unless it's has a lock up converter which incorporates a mechanical clutch. The amount of slippage varies depending on the speed and load, not to mention the design and clearances of the torque conveter. A torque converter with less slippage at stall speed is more efficient. Slipping creates heat because the fluid is shearing between the blades. This is why you need additional cooling with performance coolers or when towing (increased load means more slippage).

I hope this explains it simply and accurately enough.
As far as what the driving sensation is with a high stall converter it's might be more educational if you just find someone with one and see if you can drive it. You'll then say "oh, I get it".
 

·
Registered
Joined
·
9,967 Posts
The converter doesn't engage and disengage. As soon as you drop your car in gear it is transmitting torque to the tranny. If you let your foot off the brake, your car will roll slowly. As you increase the rpm's the car will accelerate even though you are far below the stall speed. If you suddenly mash the gas from a standing start, the converter will flash up to the stall speed and you will be transmitting a lot of torque to the tranny.

Another feature of a torque converter is the torque multiplication at low stator speeds, usually about 2:1. This is why an automatic car can leave so hard.
 

·
Registered
Joined
·
218 Posts
So if you had a small cam(about [email protected])that makes good botom end torque with a stall that was too big it would act even bigger and you would never even see most of the power band because you spin the engine righ out of it? I think i have this problem right now.
 

·
Premium Member
Joined
·
298 Posts
Wow, I think I had the wrong idea all along. I used to think that with a high stall converter, when you pushed down on the gas, the car wouldn't move until you hit the "stall" rpm. And then your car would lurch forward? People used to say "XZY stall gives a performance advantage but still allows for tolerable street driving" I used to think they meant it wouldn't yank your neck with the lurch when you were leaving every stop light.

Now I think I'm starting to understand that no matter what stall speed you have, your car still starts to pull away when you step on the gas??

And it sounds like in a way the torque converter is a multiplyer, like a gear, a higher stall speed increases the multiplication of the input torque, but also has greater slipage under high acceleration so you see higher RPMs? Is that right?
 

·
Premium Member
Joined
·
31,194 Posts
Oh yeah! the data posted is great,You really have to drive one to understand what it's doing.
 

·
Registered
Joined
·
96 Posts
just thought i'd pipe in and say this post has been super edjumacational. i've been meaning to ask how convertors work for a long time. glad someone finally brought it up. it's all getting a little clearer now.
 

·
Premium Member
Joined
·
11,072 Posts
The converter in my car goes to 6000+ on the transbrake, but I can drive around at 2000rpm with no trouble. With the little 350 in it now, it'll go to about 4500.
A few years ago I borrowed a converter from a blown big block powered car to try; it went to 6000 with the big block--when I put it in gear with my small block, it stalled the engine! After turning the idle way up, I drove it down the street, it felt like driving with a clutch! The engine just lugged down like shifting too early with a stick. I'd never noticed how little torque my engine produced, because it had always flashed right thru that area.
As for slippage with a loose converter, my expensive 8" piece slips less at low speeds than the average 10" 3500 cheapie-Most of them are junk-spend the money on a quality piece, you'll be happier in the long run. Most 10" cheapies will slip enough to actually slow your mph at the track.
 

·
Premium Member
Joined
·
991 Posts
I have run a small 8" converter on the street a few years now. It is rated 4800~5200 stall. It acts like any other automatic. When you put it in gear you will feel a load placed upon your engine. It won't be the 'clunk' you are used to, it will be milder. You will still feel it. The idle in drive RPM's won't change much, but the engine seems to idle easyer. At a traffic light next to a patrol car you can easily pull away with no drama. With a light throttle, the bugger lets your trans shift normal. You don't have to rev the pee out of it to get the car moving. BUT YOU CAN if you want to. The more you get into it, the more it will let you rev depending on your traction. Without traction, I would have to say stall speed is theory. The stall speed I spec'd earlier is with a trans brake. This means the car can not move when the trans brake is engaged. The car can not move because the trans is engaged in first gear and reverse gear at the same time as long as you have the trans brake buton engaged. The drive train from the trans back is at idle, just waiting. At this point you may be able to experiment with stall speed. Let go of the trans brake button and the reverse servo dumps, wham. Like dumping a clutch at (choose your stall speed). Build torque converters to make us happy.
 

·
Premium Member
Joined
·
832 Posts
Whitecat, lot of great posts here, but the why of the "stall" convertor seems cloudy. A 550hp motor is making very little hp at 1200 rpm. If you have a stock type convertor and floor it, it will accellerate the car briskly, if you however have a 4500 stall convertor and floor it from a standstill, the motor will rev up into a rpm range where the motor is producing very strong power and torque and the car will accellerate at an enormously increased rate, providing adequit traction has been allowed for. As with everything there is a price to pay, in mileage, heat, etc, but to really unlock the potential of a performance oriented car, a compatible convertor is manditory.
 

·
Registered
Joined
·
2,506 Posts
Please explain the use of a stall converter (merged)

I have a B&M holeshot 3000 converter which has minimal run time on it. However, I'm skeptical about putting it in my car. 1) I'm not sure if it will be enough stall. 2) I've heard nothing but bad things about those converters, plus it has no anti-balloon plate. I really don't want to put this in and have it blow up and then I'd have to redo my fresh manual valve TH350. A 383 with ATLEAST 450 hp is goin in, 10:8:1, custom ground comp solid, world heads, etc. It will have 28''-29'' tires and 4:10s. I'm just not sure 3000 stall is enough. I've heard NOTHING but good things about redneck converters so I figured if all else failed, I'd get one from them. What do ya think? How big of a converter/gear ratio do you drive daily and/or drive often?
 

·
Registered
Joined
·
7,257 Posts
I think 3000 is getting pretty close, but it should be okay if you aren't buzzing down the highway a lot and have a good tranny cooler. DriveWFO and others have had good luck with redneck, I think I may order my converter from them when the time comes for the PW Claimer 350.

Kev
 
1 - 20 of 101 Posts
Top