Cummins 4BT & Diesel Conversions Forums banner
1 - 3 of 3 Posts

·
Registered
Joined
·
131 Posts
Discussion Starter · #1 ·
I was trying to understand torque converters. I did a search and found this web site http://www.goerend.com/torqueconvertertech.php (yes I have found the search and figured out how to use it, I got lucky today dirty hairy :)) which has one of the best analogies explaining torque converters and how they work.

Anyway, so my question is why do diesels require different torque converters than gas motors? What would happen if I used a gas tc behind a diesel? What is the average stall speed of tc gas versus diesel? Finally would it be possible with a lock up tc to simply lock it when taking off and use it more or less like a clutch?

These maybe stupid questions, but I am just trying to find out options, and better understand motor to tc relations.
 

·
Registered
Joined
·
500 Posts
http://auto.howstuffworks.com/torque-converter.htm <--- good article with diagrams.


Gas engines have a higher stall speed because of their operating rpms. Say a diesel (stock 4BT) is 750-2500rpms. A typical V8 gas engine is 600-5500rpms. You can get a "fluid clutch" that works like a clutched automatic. My dad said a few guys used to run these on dirt-track cars with chevy Powerglide 2spds.
 

·
Administrator
Joined
·
1,447 Posts
Anyway, so my question is why do diesels require different torque converters than gas motors?
The difference between a "gas" or "diesel" torque converter is usually just one of stall speed.

The stall speed of a torque converter is selected based on the power curve of the engine it's used with. Most diesels make their peak power at lower RPM than gassers, so usually you'll use a lower stall speed converter with the diesel. But not always!

It is true that stock "diesel" torque converters typically are built heavier to handle the higher peak torque of a diesel.

What would happen if I used a gas tc behind a diesel?
If we stipulate that a "gas" torque converter is one having a higher stall speed than a "diesel" one:

Up to a point, if you increase the stall speed of the torque converter being used then you theoretically improve the vehicle's acceleration off the line, as the engine is working closer to it's horsepower peak. Somewhat like having higher ratio gears, except the torque converter offers a constantly changing ratio.

But.. You don't get something for nothing. Total power at the output of the torque converter is ALWAYS less than the input (except in a lockup converter). The loss shows up as heat, and can be QUITE a lot of heat.

What is the average stall speed of tc gas versus diesel?
I don't know that there's really an answer for this. I've seen stock stall speeds all over the map for gas and diesel. It's really a matter of an individual engine's power curve and the desired performance targets.

Finally would it be possible with a lock up tc to simply lock it when taking off and use it more or less like a clutch?
Possible? Yes. Desired? Well...

The ratios in automatics are set up with the "added ratios" of the torque converter in mind, so are typically not as steep as a comperable stick. Also, until recently, autos typically have fewer ratios (gears) than their manual brethren, and for the same reason. The torque converter adds to the total gear ratio to help get the load moving at a good rate, so 1st can be a lower ratio, and since it offers a variable ratio, it can help span the "gap" between gears. If you take the torque converter out of the mix, you will likely lug the engine quite a bit and performance will go down.

The other side of that coin is the fact that most of the stock lockup converters' clutches CANNOT handle the full output of the engines they're used with. If you locked the TC up at launch with full power commanded, even if you didn't stall or lug the engine, you'd probably wind up smoking the TC lockup clutch.

Now, one thing locking the TC as early as possible does do is to improve efficiency and reduce the heat produced.


Now, aftermarket stuff changes ALL of this around, but that's why it's there: If you change your engine, or your engine output, or where it makes that same power in the RPM range, then what was good for a torque converter before may not be right now as far as stall speed and power handling goes.
 
1 - 3 of 3 Posts
Top